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Created page with "'''Scope of this document''' The following note is a background document for teachers. It summarises the things we will need to know. This note is meant to be a ready refer..."

'''Scope of this
document'''
The following note is a background document for
teachers. It summarises the things we will need to know. This note is
meant to be a ready reference for the teacher to develop the concepts
in electricity and magnetism from Class 6 onwards to Class 10.

This document attempts to cover all the topics
identified in the concept map. To plan the actual lessons, the
teacher must use this in connection with the theme plan.

 
 

 
 

= Concept Map =

 
 

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_17376afb.jpg]] 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

= Theme Plan =

{| border="1"
|-
|
'''CLASS'''

|
'''SUBTOPIC'''

|
'''CONCEPT
 
DEVELOPMENT'''

|
'''KNOWLEDGE'''

|
'''SKILL'''

|
'''ACTIVITY'''

|-
|
6

|
Charges,
static electricity, electric current, 
conductors, insulators,
simple circuits

|
Charge
is intrinsic to matter. There are two types of charges. The
charges at rest constitute static current, and the charges in
motion constitute electric current. Electricity needs a medium to
travel, which is called as conductor. The path of flow of charges
is circuit.

|
1.The
students recall the two types of charges. 
2.The students
recognise the accumulation of charges in the activity, 
3.The
students can recognise the necessity of a condctor to the flow of
charges. 
4.The students can differentiate between conductors
and insulators.

|
1.The
students can perform activities to accumulate charges. 
2. They
can identify the conductors and insulators in the given
materials 
3.They can draw the diagrams to represent simple
circuits.

|
ACTIVITY
1 
ACTIVITY 2 
ACITIVITY 3

|-
|
7

|
The
important effects of electricty, 
Electric appliances,non
contact forces-charges 
And magnets

|
When
current flows through the substances, it shows chemical 
magnetic
effect and heating effect. 
Magent attracts magnetic materials.
Force of magnets is 
non- contact force .

|
1.The
students identify the different effects shown by electricity in
some situations 
2.The students recall the different effects of
electricity 
3.The students give examples for magnetic
materials 
4.The studetns can idntify the non contact forces
such as magnetic force

|
1.The
students perform activities show the heating effect of
electricity. 
2.The students differentiate between magnetic and
nonmagnetic materials In the given objects. 
3.The students
can perform experiment to show the attractive propery of
magnets 
4. The students show the existance of non contact
forces

|
ACTIVITY4 
ACTIVITY5 
ACTIVITY
6

|}
 
 

{| border="1"
|-
|
'''CLASS'''

|
'''SUBTOPIC'''

|
'''CONCEPT
 
DEVELOPMENT'''

|
'''KNOWLEDGE'''

|
'''SKILL'''

|
'''ACTIVITY'''

|-
|
8

|
Magnets
and their properties, magnetic field lines 
Magnetic
materials,some fields where magnets 
are used,power and
electric power, 
sources of electricity,unit of electric energy
 
consumed

|
Magnets
attract magnetic materials,it sets itself in N-S direction when
suspended freely. It has two poles, like poles repel and unlike
poles attract, poles cannot be separated. Both poles are equal In
strength.The strength of the magnet is more near the poles, we
use magnets in many fields of life,power is rate of work done and
as electricity does work, electric power is rate at which it
does Work. Electricity energy consumed is the product of power
and time

|
1.The
students recall the properties of the magnets. 
2.The students
understand the applications of magents. 
3.They will be able to
identify the strentgh of magnets at various palces around 
the magents 
4.They will be able to recall definitions of work
and electric energy consumed 
5.They will be able to recall the
units of work and electric energy consumed 
6.They will recall
the different sources of electricity

|
1.The
students can perform activities to find out the properties of
magnets. 
2. The students differentiates between magnets
between the magnets and Non magnetic materials using the
properties of the magnets 
3. Can design simple devices which
work on the properties of the magnets 
 

|
ACTIVITY7 
ACTIVITY8 
ACTIVITY
9 
ACTIVITY10 
 

|-
|
9

|
Magnetic
field lines and the property of magnetic 
field lines,relation
between electricity and magnetic 
Field. Oersted's
experiment,pattern of magentic 
Fields in various
situations,elecric potential and 
Potential diference,
electric current necessity of 
electriomagnetic force in the
flow of current.

|
Magnetism
is due to charges in motion. Unlike charges, magnetic monopoles
do ot exist.

What
does a field line represent– it is not a force; but rather the
effect that a charged particle will experience. The magnetic
force is expressed in field lines, the field lines never
intersect.

Electricity
and magnetism are linked. When current flows through
conductors,magnetic field lines are produced which shows relation
between magnets and electric Current. Electric potential is the
work done to move charge infinity to to that point, when there is
difference in potential.

|
1.
The students identify magnetic field lines around diffetn shapes
of magents. 
2. The students recall the properties of magnetic
lines of force 
3. The students can identify the relation
between magentism and electricity 
4. They can recall Ohm's
law 
5. They can define potential,potential difference,electric
current and emf. 
 

|
1.
Calculate voltage, current and resistance using Ohm’s law. 
2.
Analyze circuit diagrams for series circuits and parallel
circuits. 
3. Can calculate the electric energy consumed in
various situations 
4. Solve equations that relate electric
power to current, voltage and electrical Energy. 
 

|
ACTIVITY11 
ACTIVITY12 
ACTIVITY
13 
ACTIVITY14 
 

|-
|
9

|
Ohm's 
law,
concept of resistance and resistivity, 
combination of
resistances,electric energy 
consumed,heat produced, steps to
save electricity.

|
The
flow in unit time is called electric current. In a conductor
the 
flow also depends on the resistance of the wire. The
resistance of a 
Wire is made useful in many
situations,specially to produce heat. 
As electric current is
the best form of energy, it should be saved.

|
6.The
students list the factors on which resistance depeds 
7.The
students will identify the heat produced when current flows
through resistances 
8.They describe resistivity. 
9.They
students briefly expain the methods to save electric current

|
5.
Describe devices and procedures for maintaining electrical
safety. 
6.They draw simple circuits . 
7. The students
perform activity to prove Ohm's law. 
8.They obtaining the
skill of calculating the problems on Ohm's law

|
 

|-
|
10

|
Electromagnetic
Induction,Faraday' laws,AC and DC, 
fleming's rules,AC and DC
dynamo, Types of 
Electromagnetic induction,Eddy currents and
Lenz' law.

|
When
magnetic field inked with the conductor changes an emf is 
induced
in thre conductor. The induced emf depends on rate of change
magnetic field and no of turns in the coil.

|
1
.The students observe the experimental evidence for
electromagnetic induction. 
2.They understand the
circumstances under which changing magnetic fields lead to
 
induced currents. 
3.They understand how the movement of a
conductor through a magnetic field leads 
to an induced emf.
 
4.They understand and use Lenz’s law for induced currents.

|
1.The
students can perform activities to prove Farady's experiment on
 
electromagnetic induction

|
ACTIVITY15 
ACTIVITY16 
ACTIVITY
17 
ACTIVITY18 
ACTIVITY19 
ACTIVITY20 
ACTIVITY 21 
 

|-
|
10

|
Induction,
solenoid, field lines in solenoid,motor and 
Its
applications.Domestic circuits, 
Steps to save electricity

|
The
direction of emf is 
perpendicular to magnetic field. The
advantages of AC in power generation; why we do not generate
using DC. The possibility of voltage step up and down.

When
current carrying conductor is kept in magnetic field it
experiences mechanical force.

As
electric current is the best form of energy, it should be saved.

|
5.They
use Lenz’s law and Faraday’s law to determine the direction
and size of 
Induced currents. 
6.They recall the laws of
electromagnetic Induction, fleming's rules. 
7. The students
differentiate between AC And DC; a motor and a dynamo 
9. The
students will mention the use of dynamos and motors 
10.The
students will be able to describe the methods to save
electricity.

|
2.
The students can draw the diagrams of AC dynamo,DC dynamo, 
3.
They can represent AC and DC with graph. 
4.They can construct
simple Dynamo 
5. They can construct simple motor 
6. They
can explain the working of Dynamo and Motor 
7. They can
perform activity to expain lenz's law

|
 
 

|}
 
 

= Syllabus =

# Insulators and Conductors – Meaning, types, functions and examples; will understand the concept of conductor.
# Pattern of domestic wiring and how it is done
# Electrolysis of water and diagram of an electrolytic cell
# Simple and complex electrical circuits and understanding safety measures
# Elecricity, Ohm's law
# Effects of electric energy
# Electromagnetic radiation, spectrum and uses
# Electromagnetic induction, Faraday's laws, understand the functioning of a DC motor and AC dynamo

 
 

 
 

= Curricular Objectives =

# The children are introduced to the following concepts:
# Forces can act at a distance, and the idea of force fields to explain forces act at a distance. The presence of four fundamental forces is introduced. They have already studied the gravitational force; now the electromagnetic force will be introduced.
# There is another intrinsic property of matter - that of charge. (Earlier, they were introduced to the intrinsci property of mass). There are two types of charges – positive and negative.
# Just like mass has two aspects – inertial and gravitational, charge also exhibits two kinds of effects – when they are stationary and when they are moving. Static charges result in electrostatic force and when a charge is moving, it exerts a magnetic force. They are also introduced to the fact that while gravitational force is always attractive, electrostatic force can be repulsive as well as attractive.
# Our understanding is based on the atomic structure - what constitutes positive and negative charge. Charge is conserved and is quantized. They will also learn about movement of charges, conductors, insulators and how to charge objects.
# They will be introduced to the mathematical representation of Coulomb's law and the equation that describes how the electrostatic force acts.
# The children are introduced to the idea of work done in an electric field and this is what we call as the potential difference between two points. They will learn about the electric potential at a point.
# Flow of charges is electric current and there is resistance to the flow of current in a conductor. They will learn the relationship between potential difference and flow of current in an ohmic conductor (Ohm's Law). They will also be able to calculate the effective resistance when resistors are connected in series and parallel
# They will be able to explain the working of a battery operated simple circuit. They will also be introduced to the idea of EMF and be able to distinguish it from the terminal voltage when there is a current flowing in the circuit.
# There are objects, called magnets, that have the ability to repel and attract other similar objects. Magnetic forces are also forces that act at a distance and we need the idea of force fields to explain magnetic forces. The properties of magnetic field lines and the Earth's magnetic field are discussed.
# The magnetic effect of electric current will be discussed and the objective is to introduce the idea that electricity and magnetism are not two different kinds of forces. Rather, they are different aspects of the same force. For example, a current carrying wire behaves like a magnet.
# The children are introduced to properties of electromagnets, behaviour of current carrying conductors in a magnetic field and the operation of the motor.
# The opposite effect is also explained – the change in magnetic field resulting in the generation of a potential difference – electro magnetic induction. This is at the core principle of power generation. They are briefly introduced to the idea of alternating EMF, peak voltage and transformers.
# The students must be able to understand the consumption of electric energy, units of energy consumed (kWh) and the cost of electricity.

= Electric Charge, Conductors and Insulators =

Charge is an intrinsic property of matter, just as
mass is. All matter is made up of charge, in fact a vast quantity of
it. The only reason we do not notice this charge is because there
are two kinds of charges and most objects contain an equal amount of
these two kinds of charges. These two kinds of charges are equal and
opposite in nature, such that an equal amount of these two types of
charges neutralize each other. These two types of charges are
called, rather arbitrarily, '''positive and negative charges'''.
(Benjamin Franklin, of lightning fame, coined these terms).

When these two charges are equal, the object is
electrically neutral and there is no net charge on the object. When
this balance is disturbed, a net charge develops on the object. In
one of the earlier discoveries, people noticed that when similar
objects with a net charge were brought together, they repelled each
other; while when dissimilar objects with a net charge were brought
together, they attracted each other. This led to one key finding
about electric charges – '''like charges repel each other and
unlike charges attract each other. '''

 
 

 
 

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_b641926.png]] 
 

This attraction or repulsion is a result of forces
that these charges exert on each other. This force, operating
between two charges, when they are not in contact with each other, is
called electrostatic force. The charges exert an influence on each
other through force fields; this explains how forces act at a
distance. This electrostatic force operates very similar to the
gravitational force, in as much that it is directly proportional to
the magnitude of charge and inversely proportional to the distance
between them. One key difference between gravitational force and
electrostatic force is that gravitational force is always attractive
while electrostatic force may be attractive or repulsive. This
follows obviously because there is only one kind of mass whereas
there occurs in matter two kinds of charge – positive and negative.

== Structure of the atom ==

The origin of these electric charges lies in the
atom.

The smallest particle of any element, and of all
mass, is the atom. An atom consists of three main sub-atomic
particles – electrically neutral neutrons, positively charged
protons and negatively charged electrons. The neutrons and
positively charged protons are held together in the nucleus (held
together by strong and weak nuclear forces) and the electrons revolve
around this nucleus in fixed orbits. The magnitude of charge on the
proton and electron are equal. The number of protons is equal to the
number of the electrons in an atom and this results in the atom being
electrically neutral.

The electonic charge has a magnitude of 1.6X10-19
C. This is the smallest amount of
charge that has been identified and all other charges are multiples
of this charge. Charge is not continuous but is quantized. Since “e”
is so small, 1.6 x 10-19
C, we see flow of charges as continuous.

'''''Charge is quantized as a multiple of the
electron or proton charge''''':

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m1ba34f6e.png]]

Electrons
are the smallest and lightest of the particles in an atom. Electrons
are in constant motion as they circle around the nucleus of that
atom. Electrons are said to have a negative charge, which means that
they seem to be surrounded by a kind of invisible force field. This
is called an '''electrostatic
field.'''

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7fb72f41.png]]Protons
are much larger and heavier than electrons. Protons have a positive
electrical charge. This positively charged electrostatic field is
exactly the same strength as the electrostatic field in an electron,
but it is opposite in polarity. Notice the negative electron
(pictured at the top left)
and the positive proton (pictured at the right) have the same number
of force field lines in each of the diagrams. In other words, the
proton is exactly as positive as the electron is negative.

To help express the mass of these subatomic
particles, we take the example of the simplest atom – that of
hydrogen. A hydrogen atom consists of a single proton and a single
electron. The hydrogen atom does not contain a neutron. The mass of
the proton and neutron are almost the same and is equal to the mass
of one hydrogen atom. The mass of the electron is negligible and is
equal to (1/1837)th of the mass of one hydrogen atom. The
contribution of electrons to the mass of the atom is negligible.

 
 

We saw earlier that like charges repel each other
and unlike charges attract each other. This property is at the core
of the forces that hold an atom together. The protons held in the
nucleus will try to repel each other. However, they are prevented
from being thrown apart by the strong nuclear forces which overcome
these repulsive forces and hold the nucleus together. These
electrons are also held in orbit around the nucleus by electrostatic
forces exerted by the positively charged nucleus.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_51ef531a.png]]The
electrostatic force exerted by the positive nucleus on the negatively
charged electrons is what keeps the electrons as a part of the atom.
Otherwise, the force that the electron will develop when it is moving
might take it outside of the atom. (This is, again, very similar to
the gravitational force because of which planetary objects remain in
orbits). However, if an electron is situated in an orbit far away
from the nucleus, then this electrostatic force is much weaker
(because it is inversely proportional to the distance). In such
cases, these outermost electrons can be readily “removed” from
the atoms and are called free electrons.

== Charging an object, flow of charges, conductors and insulators ==

In one of the early discoveries, some objects were
found to have developed attractive and repulsive properties when they
were rubbed with one another. It was discovered later that this
attractive or repulsive property was a function of charges
redistributing/ moving from one object to another.

=== Charging by friction ===

When two bodies are rubbed against each other, the
free electrons move from one object to another. They move from the
atom of the element where the electrostatic force on the electrons is
weak to the atom of the element with a higher electrostatic force of
attraction. This movement of free electrons is what constitutes
electrification of the body. In this process of charging by
friction, the object which loses electrons develops a positive charge
while the object which gains electrons develops a negative charge.
The magnitude of the positive charge will be equal to the magnitude
of the negative charge. It must be noted that only the electrons
move and not the protons – the development of a positive charge is
due to the deficit of electrons.

When a glass rod is charged with silk, the glass
rod loses electrons which are transferred to the silk. This gives
the glass rod a positive charge (due to deficit of electrons) and
silk a negative charge (due to excess of electrons). Similarly, when
ebonite is rubbed with fur, the ebonite rod gets negatively charged.
Can you explain in terms of flow of electrons?

=== Movement of charges due to free electrons ===

We talked earlier of free electrons. Free
electrons are free to move within the surface of the material between
atoms and are not bound to one atom; they “escape” the orbit of
one atom but generally drift around. Normally, these free electrons
move about in all directions randomly and have no net flow.

However, when there are a large number of free
electrons, they can be made to move in a particular direction by
applying a potential difference. In these cases, a large movement of
charges is possible. Materials where there are a large number of
free electrons are called conductors. It is, therefore, easier to
set up a flow of charges in a conductor. Insulators are materials
where the number of free electrons is less and consequently, it is
more difficult for charges to flow through them.

=== Ways of charging a conductor ===

An electrically neutral object may be charged (i.e., given a
positive or a negative charge) by conduction or induction.

Charging by conduction:

When two objects are rubbed together, charges,
electrons, move from one to another. This results in a deficiency of
electrons in one object which gets positively charged. The object
which received these electrons develops a negative charge. A
conductor can be charged only if it is mounted on an insulated stand.

Charging by induction:

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_f783dcf.png]]When
an electrically charged object is brought near an uncharged object
(the object must be a conductor), a distribution of charge happens in
the uncharged object. The end of the uncharged object which is
closest to the charged object will develop an opposite charge while
the farthest end will develop a similar charge to that of the charged
object.

For example, if a negatively charged ebonite rod
is brought near the a conductor, the end of the conductor closest to
the ebonite rod will develop a positive charge while the end of the
conductor away from the ebonite rod will develop a negative charge.

These separated charges in the conductor are
called induced charges.

The phenomenon due to which an insulated uncharged
conductor gets charged when held near a charged body is called
electrostatic induction and the charges so produced are called
induced charges.

In all of these processes, '''''charge is
conserved'''''. It simply gets redistributed from one object
to another.

=== Flow of charges and earthing ===

When there is a flow of charges, we talk of a
current. Current is nothing but a flow of charges. When two objects
are rubbed together, charges move from one object to another. But,
we cannot usually notice this charging if we hold the two objects in
our hand and rub together. While a net positive charge may be
created on the object, there will be a flow of electrons from the
earth through our hand to the glass rod, neutralizing the positive
charge. Similarly, the negative charges on the silk will flow
through our hand to the earth. In both the cases, the objects get
“discharged” by flow of electrons into and out of them. The
earth has acted both as a source and reservoir of charges.

The earth is always electrically neutral because
of the huge number of protons and electrons it contains (because it
is massive). If a few billion electrons are added or removed, it
makes but a small difference to the total charge of the earth.
Since, the total electric balance of the earth is not disturbed, it
always remains neutral and at zero potential.

=== Direction of flow of charges ===

Before the model of the atom was understood, when
an object was charged, the direction of movement of charges was not
clear. It was assumed that charges flowed from positive to negative.
The positively charged object was considered to be at a higher
potential. Flow of charges was defined as current and it was assumed
to flow from positive to negative. Positive charges were assumed to
be at a higher potential than negative charges. After the atomic
model was understood, it was clear that the flow of electrons was
what constituted current and led to the development of net charge.
In other words, electron flow is what constitutes current. While we
still show the direction of conventional current in a circuit as
positive to negative, the electron flow is in the opposite direction.

== Static Electricity in action ==

One of the most spectacular displays of static
electricity is during a thunderstorm. The cloud and the earth
surface develop opposite charges due to induction. During a
thunderstorm, electric discharge occurs between the negatively
charged clouds and the oppositely charged ground or between two
clouds which are oppositely charged.

When a charge build-up occurs between oppositely
charged surfaces, electric discharge occurs. To prevent this,
lightning rods are installed on tall buildings. Lightning rods,
which are metal rods, collects the electrons and thereby prevents a
large build-up of positive charge on the building. Even if lightning
strikes, the metal rod will conduct the electricity quickly down to
the Earth preventing any damage to the building.

== Electrostatic force and Coulomb's Law ==

We saw earlier that charges attract or repel other
charges due to the electrostatic force. This force can be defined as
follows.

The electric force acting on a point charge q1
as a result of the presence of a second point charge q2 is
given by Coulomb's Law, which is directly proportional to the
magnitude of the charges and inversely proportional to the square of
the distance between the charges.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m2d4a26f1.gif]]

where
ε0
= permittivity
of space.

The
unit of charge is the coulomb, abbreviated C. 1C is the charge
associated with 6.25*1018
electrons. The proportionality constant k has the value of 9*109
Nm2/C2.
If we had two 1 coulomd charges, they would exert a force on each
other equal mto 9*109
N. We do not see such charges in daily life.

== Inverse Square Law ==

Coulomb's
law of electrical forces, resembles the Newton's law of gravitation
which is used to calculate the magnitude of gravitational force
between two masses. Both are inverse-square laws, in which force is
inversely proportional to the square of the distance between the
bodies. Coulomb's Law has the product of two charges in place of the
product of the masses, and the electrostatic constant in place of the
gravitational constant. One important point of comparison is that
the the value of the constant in Coulomb's law (for force between two
charges of 1C separated by a distance of 1 m) is of the order of
magnitude 109,
which is 1000 billion
billion times more than the gravitational constant. This means that
the electrostatic force is a much stronger force than the
gravitational force. Can you imagine what the mass would have to be
for 2 masses at a distance of 1 m to exert a force of 9*109
N.

== Key Vocabulary ==

# Charge – Intrinsic property
# Quantization of charge – Charge occurs in multiples of electronic charge
# Conductors – Materials through which movement of charges is easier
# Insulators – Materials through which movement of charges is more difficult
# Conduction – Method by which a conductor is charged by touching it with a charged object
# Induction – Method by which a conductor is charged by bringing it near a charged object
# Earthing – Establishment of a path by which charges can be transferred to the ground
# Coulomb's Law – The law and the equation that describes the electrostatic force.

== Additional web resources ==

# [[Basics of Static Electricity]] This is a good overview of the basics of static electricity.
# [[Science Object - Electricity]] This isa very good interactive session on electrostatics and current electricity. You can register at [[www.nsta.org]] for free and view all these science objects and many free materials in your online library.
# [[How lightning strikes]] This page describes how lightning strikes and how lightning conductors work.

= Electric Field =

We already saw that electric field operates at a
distance, through a force field. Electric field has both direction
and magnitude. Electric field at any point around a charge is
defined as the electric force per unit charge. This is written as :
E = F/ q; where F and E are vectors; in the same direction.

The direction of the field is taken to be the
direction of the force it would exert on a positive test charge. The
electric field is radially outward from a positive charge and
radially in toward a negative point charge.

{| border="1"
|-
|
[[Image:Electron.jpg]] 
 

 
 

 
 

 

|
[[Image:proton.jpg]][[Image:proton.jpg]] 

|}
The strength of the field is given by the number
of field lines through a given point. The greater the number of field
lines, the stronger the field. And vice versa. The concept of
electric field is important in understanding what happens when
charges move. When a charged particle moves, it causes a disturbance
in the space and this disturbance is communicated through the field.
The information travels at the speed of light. This concept is at
the core of understanding the electromagnetic force. We will discuss
this more when we study the magnetic effects of electric current.

== Electric field in a conductor and shielding ==

Electric fields can be shielded by various
materials; this is an important differece between electric fields and
gravitational fields. For example, metallic conductors will
completely shield the field inside, regardless of the field outside.
Let us understand how this happens.

Let us see we have a charged conductor in
equilibrium; meaning the charges are not moving. This means that the
net charge on the conductor has distributed itself in such a way that
the replusive forces are all neutralized.

# In such a situation, there can be no field inside the conductor. Why? Because if there is a field in the conductor, the charges (electrons) would move to redistribute themselves. This violates the initial condition. Hence there can be no field inside the conductor.
# The second effect is that the electric field on the surface of the condutor is always perpendicular to the surface of the conductor. If this is not true, again there would be a movement of charges along the surface of the conductor. This violates our initial assumption.

These two results tell us that the field inside
a conductor is zero. This allows us to build a cage, a metallic
shield to keep out an electric field.

== Electric Potential ==

We
saw that the electric field indicates the force that will be
experienced by a positive charge. If “'''E'''”
is the electric field at a point, the force experienced by a charge
“'''q'''”
would be

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_4b56cbe9.gif]]

To move charge “q” from Point A to B in a
field work will have to be done. This will result in the change in
potential energy of the charged particle as it moves from A to B.
The energy possesed by a particle by virtue of its position in an
electric field is called electric potential energy. The difference
in electric potential energy between two points is represented as a
potential difference between the two points in the electric field.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m2f27def.png]] 
 

 
 

 
 

 
 

 
 

 
 

In diagram A, we have to do work against the
Electric Field, therefore, the electric potential energy of the
charge will increase. By similar reasoning, we will see that in
diagram B, the electric potential energy of the charge will decrease.

'''''Diagram B'''''

Let us say now we have A at infinity, where we
assume that the charge will experience no force due to the electric
field. Therefore, the potential energy of the charge will decrease
gradually from B as it moves to A; where A is considered to be at
infinity.

The
test charge in this case will have '''electric
potential energy at B, which is equal to U''''''B''' relative
to the zero potential energy at A. We must note here that what is
meaningful is the difference in potential energy between points B and
A. Work is done by or on the charge as it moves from A to B
(depending on the direction of the field) and this results in a
difference in the potential energy of the charge between these two
points.

Now the electric potential energy is a measure
dependent on the amount of charge. We will define a more useful term
called electric potential. This is the potential energy per unit
charge at a point in an electric field.

'''VB
=
UB
/
q'''

The unit of electric potential is joules/
coulomb and is given a special name, Volt, in honor of Alessandro
Volta. The volt is abbreviated to 1 V = 1 J/ 1 C.

Like we mentioned before, what matters is the
difference in potential energy.

The
difference in potential energy is equal to the negative of the work
done, WBA
,
as the charge moves from A to B.

'''UBA =
UB -
UA
=
WBA '''

Similarly we will be interested in finding the
difference in electric potential between two points in an electric
field.

'''VBA =
VB -
VA'''

'''''Why define electric
potential and what is zero potential'''''

We define the electric
potential because it is possible to assign a specific value to a
given location in an electric field whether or not there is a charge
present there. This is also useful when talking of voltages in a
circuit.

We also said that at an
infinite distance, the electric potential energy and hence, potential
is zero. Often times, we also take the ground or a conductor
connected directly to the ground to be at zero potential.

== Electric Energy Storage ==

=== Capacitors ===

We saw that a charged
particle has electric potential energy by virtue of its position.
Electric energy can be stored in a device called a capacitor. When a
pair of conducing plates is charged using a battery, it builds up an
electric field between its plates. The capacitor plates develop
equal and opposite and this acts as an energy storage device. A
capacitor is discharged when a conducting path is provided between
the plates.

=== Van de Graff generator ===

A Van de Graff generator is
a hollow sphere that can hold a charge generated by a motor drive n
belt. The charges lie on the surface of the sphere and this
generator can be build to very high voltages. These voltages are
used to accelerate charged particls that can be used as projectiles
for penetrating the nuclei of atoms.

== Key vocabulary: ==

# '''Electric Field''': Vector field – has magnitude and direction and gives the direction of force experienced by a unit positive charge.
# '''Electric Potential Energy''': The energy possessed by a charge in an electric field by virtue of its position in the electric field.
# '''Electric Potential: '''The electric potential energy per unit charge.

== Additional resources : ==

# [[Faraday's cage]]. This is a lecture by Walter Levin, Professor at MIT, demonstrating Faraday's cage
# [[MIT library]] This site shows you photographs of a Van de Graff generator
# [[Walter Levin]] explains how to build up charges in this video.

= Current Electricity =

== Rate of flow of charges, current ==

We have seen that in an electric field different
points will be at different electric potentials. When there is a
difference in potential, charges will flow. In this case, charged
particles, electrons will flow.

This can be compared to water flow from a
reservoir at higher pressure to a reservoir at lower pressure. Water
will flow as long as there is a difference in the water levels.

'''''Electric current is simply defined as the
flow of electric charge. The rate of flow of electric charge is
measured in amperes, A.'''''

We saw before that 1 C of charge carries 6.25*1018
electrons. So if we have a wire carrying 5 A, we have 5C of charges
passing in one second. A large number of electrons!

For charges to flow, there must be a potential
difference maintained. It is possible to maintain a potential
difference using two large charged spheres – one positively charged
and the other negatively charged. This will not work because once a
conducting path is provide, the charges will neutralize in one single
discharge. For continuous current flow we need to maintain a steady
potential difference.

== Voltage sources – batteries and generators ==

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_bb112d6.png]]A
battery or a generator does work to pull electrons from positive
charges. In a battery, this is done using chemical reactions; where
the energy of the chemical bonds is converted into electrical energy.
A generator provides this voltage by electromagnetic induction. We
will discuss this in greater detail in the section on electromagnetic
induction.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_70ab754b.jpg]]
The cell shown here uses dilute sulphuric acid as
the electrolyte. One of the electrodes is carbon ; the other is
zinc. The acid reacts with the zinc electrode which enters the
solution as a positive ion. The zinc electrode becomes negatively
charged and the electrolyte becomes positively charged. Electrons
are pulled off the copper electrode which becomes positively charged.
Thus a potential difference is maintained between the terminals and
current can flow when the circuit is completed; when the electrodes
are connected. If a charge is allowed to flow between the terminals,
after a while, all the zinc will dissolve and the cell will be dead.

== Electrical resistance ==

We have seen that a potential difference is
necessary for a current to flow. But there is one more factor that
determines how much current flows; that of the resistance. In the
case of the water reservoir

Let us think of a crowded railway platform. How
many children can go from one end of the platform to another in a
given amount of time depends on how long the platform is and how wide
the platform is. Why would the speed of the children be affected?
The children will face obstacles – benches, luggage, people, etc.
If the platform is wide, there will not be many collisions and the
children can move faster. Also, if is a long platform, they will
face obstacles for a long time and that will also affect the speed
with which they will move.

A similar analogy holds for electric wires. The
resistance offered to the flow of charges is due to the collisions
with the molecules in the wires. If the wire is of a smaller cross-
section, there are more collisions, and hence, higher resistance. If
the wire is longer, the electrons will have to travel a longer
distance and in that journey face more collisions. This also impacts
the resistance of the wire.

The resistance R can be written as follows:

R = ρ
L/ A

where L is
the length of the wire, A is the cross-section of the wire and ρ
is the specific resistivity of the material. Specific resistivity is
defined as the resistance offered per unit length per unit
cross-section.

Temperature
also increases the resistance of a wire, except for carbon. This is
so because temperature increases the movement of molecules in the
wire and this increases the collisions (with the moving charges) and
hence, the resistance increases.

The unit of
electrical resistance is Ώ.

=== Ohm's Law ===

For ohmic conductors, and for not very high voltages, the current
flowing through a conductor is directly proportional to the potential
difference across its ends.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m74e99ab1.gif]]

V = IR where R is the resistance of the wire

This holds for “ohmic” conductors where the
voltage is not very high.

=== Resistors in series and parallel ===

The combination
rules for any number of resistors
in series or parallel can be derived with the use of Ohm's
Law,
the voltage
law,
and the current
law.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m7ebc180f.png]]

=== Speed and source of electrons in a circuit ===

 
 

=== Additional resources ===

# [[Working of a cell]] This website has an interactive tutorial and an explanation of how cells work
# [[Introduction to current electricity]] This website gives an introduction to what typically happens in a wire when current starts flowing and explains very well all the elements of flow of charges – including drift velocity, resistance, Ohm's law and series and parallel circuits

= Magnetism =

From simple nails being drawn to a
magnet to surgery to circuit breakers, magnetism is everywhere. The
first magnetic phenomenon observed were those associated with
naturally occurring magnets, fragments of iron ore found near the
ancient city of Magnesia. These stones called lodestones These
attracted unmagnetised iron. The attraction was maximum at certain
regions of the magnet called the poles.

The Chinese have known to use Magnetic needles for
navigation on[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_494fe551.jpg]]
ships since the 12th century. In the 16th
century, William Gilbert, Queen Elizabeth's physician made artificial
magnets by rubbing pirces of iron against lodestone.

Since then, the magnetic materials have been
playing an increasingly important role in our lives. It's therefore
necessary to understand the structure of such material.

''Shapes of Magnets''

The natural magnets i.e., iron ore were irregular
in shape and weak. Later it was found that iron or steel acquired
magnetic properties on rubbing with a magnet. Such magnets were
called artificial or man-made magnet. These magnets have a desired
shape and strength.

{| border="1"
|-
|
[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m516b593f.jpg]]

 

|
[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m4b1bf843.jpg]] 

|}
== Electromagnetism ==

Magnetism and elecricity were being pursued as
independent subjects for a long time until a Danish physicist, Hans
Chritian Oersted discovered that an electric current affects a
magnetic compass. And this discovery was talked about in the
scientific circles and came to Sir Humphrey Davy to investigate.
Michael Faraday, a book binder by training was assigned this work to
investigate. Not trained formally, Faraday visualized the magnetic
force to be acting in the form of field lines.

Why should a current carrying wire deflect a
compass? To answer this, we must go back to the fundamental property
of charge. We said that charge is an intrinsic property of matter
like mass. And just like mass is displayed in inertial and
gravitational aspects, charge also possesess two properties – when
stationary and when moving.

When there is a stationary charge, it produces
around itself a certain effect – an information field called the
electric field around it. When the charge is moving it produces an
information field, called the magnetic field around it. We can think
of electric and magnetic fields as information. We can understand
this using an example:

Let us say you are standing on the bank of a river
and are giving instructions to someone in next to me to cross the
river. It does not matter which direction you are moving when you
give the direction. The person receiving the instruction will cross
the river.

Similarly, a magnetic field at a point gives
informtion to a moving charge to move in a particular way,
perpendicular to the velocity and the direction of the magnetic
field. So, charges exhibit electrical force when they are not moving
and magnetic force when they are moving. Both electrical and
magnetic forces are different aspects of the same phenomenon of
electromagnetism.

== Nature of the magnetic forces ==

Just like electric forces, magnetic forces were
also found to be attractive and replusive. The strength of the
forces depends on the separation distance between the two magnets.
Magnetic force can also act over a distance. It was found that
magnetic poles where magnetic property appeared to be “concentrated”
gave rise to these forces.

=== Poles of a magnet ===

If you suspend a bar magnet, it was found that it
always came to rest in the North-South direction. One end of the
magnet (the pole) was south-seeking and the other end of the magnet
(the pole) was north-seeking. These were simply called the North and
the South poles of the magnet.

When the north pole of one magnet is brought near
the north pole of another magnet, they repel. The same is true of a
south pole near a south pole. '''''Like poles repel and opposite
poles attract'''''.

This property is very similar to the forces of
attraction and replusion of electric charges. There is one
importtant difference though. An electric charge can be isolated -
for instance, just as a positive or a negative charge, whereas
magnetic poles cannot be. A magnet always has two poles – even the
atom. This suggests that atoms themselves are magnets.

=== Magnetic Dipole ===

The ordinary
magnetic effects in materials are determined by atomic magnetism. On
continuing to cut a magnet into its smallest bit, we reach the level
of a single atom. This is a tiny current loop in which the current
corresponds to the circulation of the electrons in the atom. To this
atomic current we associate a magnetic dipole moment. This tiny bit
cannot be further divided and hence the dipole is the smallest
fundamental unit of magnetism.

A magnetic material can be regarded as a
collection of magnetic dipole moments, each with a north and a south
pole. Microscopically, each dipole is actually a current loop that
cannot be split into individual poles.

=== Magnetic field lines ===

The space around the magnet contains a magnetic
field. They originate from one pole and return to another. By
convention, magnetic field lines were assumed to originate from the
North pole and end in the South pole.

=== The Bar Magnet ===

When a compass needle is brought near a magnet,
the needle always lies along the direction of the field. The figure
below shows the lines or pattern of the field, when the compass
needle is placed at several places.

These field lines are developed to visualize the
effect of the magnetic field. If we imagine a number of small compass
needles around a magnet, each compass needle experiences a torque to
the field of the magnet. The path along which this compass needles
are aligned is known as magnetic field lines.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_1e5941a4.jpg]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m78bd27f.jpg]]

=== Properties of Magnetic field lines ===

Magnetic field
lines form closed continuous curves.

* Outside the body of the magnet, their direction is from north to south pole.

* The tangent to these lines at any point gives the direction of the magnetic field at that point.

* No two lines can intersect each other. (Why?)

* The lines of force contract longitudinally and dilate laterally.

* Crowding of magnetic lines of force represents stronger magnetic field and vice-versa.

The following
diagram depicts the magnetic lines of force between two north pole,
two south pole; Nort[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m378bcdfd.jpg]]h-South
pole. [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_63b374f3.jpg]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_677bab2f.jpg]]

 
 

''Solenoid
as a bar magnet''

The field due to a current in a long coil
resembles that due to a bar magnet.

Inserting a soft iron core magnetises the iron and
produces an electromagnet. The electromagnet can be made strong or
weak by changing the current and the number of coils around the core.

== The Earth's Magnetism ==

A magnetic compass was used to help the sailors
for navigational purpose. But recently it has been discovered that
some migrant birds have magnetic sensors in their heads, which help
to guide them using the Earth's magnetic field.

William Gilbert suggested that Earth itself is a
huge magnet from various observations he had made:

* On disturbing a freely suspended magnet it returns quickly to N-S direction. The north pole of this huge magnet must be towards geographic south as to attract South Pole of the suspended magnet.

* Soft iron pieces buried under surface of Earth are found to acquire magnetic properties.

* On mapping magnetic field lines due to bar magnet, we come across neutral points. These points are those where magnetic field of the bar magnet cancel with that of Earth's field. But for the latter, we cannot obtain neutral points.

The exact cause of magnetic field of Earth is not
yet known but some important postulates are:

* [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m7c4d4e3d.jpg]]Magnetic field of Earth may be due to molten charged metallic fluid in core. This rotating fluid results in currents thus magnetising the Earth.

* Since every substance is made up of charged particles, these substances rotating about an axis is equivalent to a circulating current and hence is responsible for the Earth's magnetisation.

* As the earth rotates, strong electric currents are set up due to movement of charged iron (due to showers of cosmic ray). These moving ions magnetise the Earth.

=== Features of Earth's Magnetic Field ===

The earth's
magnetic field has an axis which is inclined 20o
west of the axis of rotation of

earth. The
point where this huge earth's magnet cuts the earth's surface are the
magnetic poles. A freely suspended magnet has its north pole pointing
towards geographic north; we therefore designate the earth's magnetic
pole close to geographic north as magnetic south. The same argument
follows for the south pole of the freely suspended magnet.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m1ef9ebaf.jpg]]The
magnetic equator divides the earth's surface into two. The field
lines enter geographic north and come out of the geographic south.

=== Magnetic Elements ===

The strength
of the earth's magnetic field is about 10-4
tesla or 1 gauss.

To describe the magnetic field of earth at any
place three quantities or elements are required. They are:

* Magnetic declination (q)

* Magnetic inclination (d)

* Horizontal component (BH)

''Magnetic declination (q)''

Magnetic declination is the angle between magnetic
axis and the geographic axis.[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m65aa48a2.jpg]]

''Magnetic dip or Inclination''

The angle
between the direction of total intensity of Earth's field with the
horizontal line in magnetic meridian. It is represented as d.

At poles, the
angle of dip = 90o
and at the equator, the angle of dip = 0o.

The dip at a place can be determined by an
apparatus known as dipcircle as shown below. The needle rotates
freely in the vertical plane of scale. The pointed ends move over the
graduations on the scale, which are marked 0-0 in the horizontal and
90-90 in the vertical directions.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_1f7d7930.jpg]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m3142907a.jpg]] 
 

 
 

 

Horizontal component

Horizontal component is the component of the total
intensity of Earth's magnetic field in the horizontal direction in
magnetic meridian.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m57cad797.png]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_38133c13.png]] 
 

 
 

=== Global and Temporal Variation in Earth's Magnetic Field ===

The dipole pattern of earth's magnetic field is
disturbed due to solar winds. Solar winds are a stream of charged
particles coming from the sun. These particles ionise the atmosphere
above these poles which display a light high up in the atmosphere.
This phenomenon occurring in the arctic region is called aurora
borealis or northern lights and in south it is called aurora
australis.

The earth's magnetic field is found to change with
time. The magnetic poles of earth keep shifting their position which
is short term change. Detailed charts are maintained and revised
periodically. The changes occurring over long term come from the
evidence of basalt. The basalt from volcano cools and solidifies and
provides the picture of earth's magnetic field. As the basalt can be
dated back, a clear picture of the earth's magnetic field has
emerged. The currents in the earth's core slow down, stop and pick up
in the opposite direction.

=== Magnetisation and Magnetic Intensity ===

The ultimate source of magnetism is
the magnetic dipole moment, associated with an atom due
to orbital motion and intrinsic spin. This suggests that all
substances possess magnetic property as energy material consists of
atoms having electrons revolving around the nucleus.

=== Intensity of magnetising field (H) ===

When a substance is placed in an external magnetic
field, the substance experiences a torque due to the field and aligns
in the same direction as the field. The magnetisation so produced in
the substance is called Induced magnetisation and strength of
external field is called intensity of magnetising field (H).

H is measured
in Ampere/meter or Joule/Tesla-m3.

=== Intensity of magnetisation (I) ===

This gives us the measure of the extent to which
substance has been magnetised under the influence of H-field and
depends upon the nature of the substance.

Mathematically ''' [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7e4eff4e.png]]'''

where [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_517f1c9.png]]
is the intensity of magnetisation,[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m702f710c.png]]net
magnetic moment, v the volume of the material.

=== Magnetic induction ===

The iron bar gets magnetised with north pole at B
and south pole at A. The field inside the specimen gets modified. The
magnetic induction at a point inside the magnetised specimen is the
total number of magnetic field lines crossing a unit area around that
point, the area being held perpendicular to the field lines.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_mdd4cec8.png]]

The flux density
produced by the magnetising field vacuum is proportional to the
intensity of field [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_ma7650ad.png]].

=== Magnetic susceptibility ===

This indicates
how easily the material can be magnetised. It is represented as Xm.
''' [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_3c447df9.png]]''' i.e., the
ratio of intensity of magnetisation induced in the material to
magnetising field (H).

Therefore Xm
is a number and has no units.

== Magnetic Properties of Material ==

On the basis of their magnetic properties
different materials are classified as:

* Diamagnetic substance

* Paramagnetic substance
* Ferromagnetic substance

== Diamagnetic Substance ==

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_6577ae4e.jpg]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m5788b4cc.jpg]]Michael
Faraday discovered that a specimen of bismuth was repelled by a
strong magnet. Diamagnetism occurs in all materials. These materials
are those in which individual atoms do not possess any net magnetic
moment. [Their orbital and spin magnetic moment add vectorially to
become zero]. The atoms of such material however acquire an induced
dipole moments when they are placed in an externalmagnetic field.

The diamagnetic materials are Type 1
superconductors as they exhibit perfect conductivity and perfect
diamagnetization when cooled to very low temperature. The
superconductor repels a magnet and in turn is repelled. Such perfect
diamagnetism in superconductors exhibiting the above phenomena is
called Meissner effect.

Some important properties are:

* When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown

2) In a non-uniform magnetic material, these
substances move from stronger parts of the field to the weaker parts.
For e.g.,. when diamagnetic liquid is put in a watch glass placed on
the two pole pieces of an electromagnet and current is switched on
the liquid accumulates on the sides.

[Note on increasing the distance between the pole,
the effect is reversed]

3) A diamagnetic liquid in a U shaped tube is
depressed, when subjected to a magnetic field.

4) The field lines do not prefer to pass through
the specimen, since the ability of a material to permit the passage
of magnetic lines of force through it is less.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_4e1ff244.jpg]]5)
The permeability of the substance, that is, mr
< 1.

6) The substance loses its magnetization as soon
as the magnetizing field is removed.

7) Such specimen cannot be easily magnetized and
so their susceptibility is negative.

Example: Bismuth, antimony, copper, gold, quartz,
mercury, water, alcohol, air, hydrogen etc.

== Paramagnetic Substance ==

Paramagnetic substance are attracted by a magnet
very feebly. In a sample of a paramagnetic material, the atomic
dipole moments initially are randomly oriented in space.

When an external field is applied, the dipoles
rotate into alignment with field as shown

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m1b102b1d.jpg]]The
vector sum of the individual dipole moments is no longer zero.

Some important properties are:

* The paramagnetic substance develops a weak magnetization in the direction of the field.

* When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.

* [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_33c219a8.jpg]]The lines of force prefer to pass through the material rather than air that is mr > 1 that is their permeability is greater than one.

* As soon as the magnetizing field is removed the paramagnetics lose their magnetization.

* In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).

* A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.

* The magnetization of paramagnetism decreases with increase in temperature. This is because the thermal motion of the atoms tend to disturb the alignment of the dipoles.

Examples include Aluminum, platinum, chromium,
manganese, copper sulphate, oxygen etc.,

== FerromagneticSubstance ==

Ferromagnetism, like paramagnetism, occurs in
materials in which atoms have permanent magnetic dipole moments. The
strong interaction between neighboring atomic dipole moments keeps
them aligned even when the external magnetic field is removed.

Some important properties are:

* [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m41e0c6c7.jpg]]These substances get strongly magnetized in the direction of field.

* The lines of force prefer to pass through the material rather than air that is mr>1 that is their permeability is greater than one.

* [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_33c219a8.jpg]]In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).

* A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.

* [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m1962e359.jpg]]For ferromagnetic materials mr is very large and so its susceptibility i.e., Xm is positive.

* Ferromagnetic substances retain their magnetism even after the magnetizing field is removed.

* The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418oF, which means above this temperature, iron is paramagnetic.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7825c906.jpg]]Examples
are Iron, cobalt, nickel and number of alloys.

== Hysteresis ==

Consider an iron being magnetized slowly by a
changing magnetizing field (H). The intensity of magnetization is
found to increase along OA. On decreasing H slowly, I also decreases
but does not follow AO. When H = 0, I has a non-zero valve equal to
OB. This implies that some magnetism is left in the specimen. This
value of I which is non-zero when H = 0 that is OB is called
retentivity or residual magnetism.

When the field is applied in the reverse
direction, the I decreases along BC till its zero at C. The valve of
H which has to be applied to the magnetic material in reverse
direction so as to reduce its residual magnetism to zero, is called
its coercivity. On increasing H further, I increases along CD till it
acquires a saturation at D. On changing the field, I follows a path
DEFA. This closed loop is called hysteresis loop and represents
cycles of magnetization a specimen has undergone. The hysteresis
therefore refers to lagging behind. Here I lags behind H.

The shape and size of hysteresis loop is
characteristic of each material, because of their difference in
retentivity, coercivity etc.

Materials for making permanent magnet should
possess high residual magnetism i.e., when the magnetising field is
reduced to zero, the intensity of magnetisation is high. Further, to
reduce the residual magnetism to zero, the magnetising field should
be applied in the opposite direction. The greater this value, the
magnetisation will be a long lasting one. This property of the magnet
is called coercivity. Examples of such substance are steel and alnico
(alloy of Al, Ni, Co).It is for this reason, that steel in spite of
its low residual magnetism has a high coercivity and so is preferred
for making permanent magnet

== Key Vocabulary: ==

Magnetic susceptibility: his
indicates how easily the material can be magnetised. It is
represented as Xm.

Magnetic declination (q):Magnetic
declination is the angle between magnetic axis and the geographic
axis.

== Additional Web Resources ==

# [[http://www.youtube.com/watch?v=uj0DFDfQajw]]
# [[http://www-istp.gsfc.nasa.gov/Education/Imagnet.html]]

 
 

= Magnetic Effects of Electric Current =

== Magnetic field around a current carrying conductor ==

We saw earlier that a moving charge produces a
magnetic field around it. It follows, therefore, that a current
carrying conductor produces a magnetic field around it that will
deflect a compass, an effect demonstrated by Oersted.

It is possible to demonstrate that the deflection
will be reversed when the direction of flow of current is reversed.
The direction of the field can be found through the right hand thumb
rule.

Imagine that you are holding a current carrying
straight conductor in your right hand such that the thumb points
towards the direction of current. Then, your fingers will wrap
around the conductor in the direction of the field lines of the
magnetic field. The strength of the fielsd depends on the current
flowing through it.

== Magnetic field due to a current carrying coil ==

We have seen how the magnetic field is around a
straight current carrying wire. The right hand thumb rule can be used
to find the direction of the field around a current carrying loop as
well. Each portion of the coil or loop can be treated as a conductor
and the field can be found out using the right hand thumb rule. We
can see that every section of the wire contributes to the magnetic
field lines within the same direction within the loop.

The magnetic field around a coil depends on the
number of turns in the loop. If there is a circular coil having “n”
turns, the field produced is 'n” times as large as that produced by
a single turn. The field produced by each turn has the same
direction and the field due to each turn adds up.

== Magnetic field due to a current in a solenoid ==

A
coil of many circular turns of insulated copper wire wrapped closely
in the shape of a cylinder is called a solenoid.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m78bd27f.jpg]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_677bab2f.jpg]] 
 

 
 

 
 

 
 

 
 

The magnetic field due to a solenoid resembles the
magnetic field produced due to a bar magnet. One end of the solenoid
behaves like the north pole and the other end behaves like a south
pole and the field is uniform inside.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_13941b10.png]]The
strong field produced inside a solenoid can be used to magnetise a
magnetic material. Such a magnet is called an electromagnet.

== Force on a current carrying conductor in a magnetic field ==

If
a current carrying condcutor can exert a force on a magnet, it must
be possible that a magnetic field will exert an equal and opposite
force on a conductor. Andre Marie Ampere suggested this would be the
case. Experimentally it has been found that this indeed happens and
the direction of force exerted on the conductor changes when the
magnetic field is reversed. The direction of force exerted on the
current carrying conductor can be given by Fleming's Left Hand Rule.

If
the index finger indicates the direction of the magnetic field and
the middle finger indicates the direction of flow of current, the
thumb indicates the direction of force exerted on the conductor.

This
force is called the Lorentz Force.

=== Electric Motor ===

An
important application of this effect can be seen in the electric
motor. An electric motor is a devide that converts electrical energy
to mechanical energy.

It
is based on the principle that when a current-carrying conductor is
placed in a magnetic field, it experiences a mechanical force whose
direction is given by [[Fleming's Left-hand rule]]
and whose magnitude is given by, Force,
[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_4f2315e8.gif]]

Where
B is the magnetic field in weber/m2.

I
is the current in amperes and

l
is the length of the coil in meter

The
force, current and the magnetic field are all in mutually
perpendicular directions. Thi s is a result of the cross product of
the length and magnetic field vectors.

If
an Electric current flows through two copper wires that are between
the poles of a magnet, an upward force will move one wire up and a
downward force will move the other wire down.

{| border="1"
|-
|
[[Image:motor.3.jpg]]

|
[[Image:motor.2.jpg]]

|-
|
'''Force in DC motor'''

|
'''Magnetic Field in DC Motor'''

|-
|
[[Image:motor.1.jpg]]

|
[[Image:motor.2.jpg]]

|-
|
'''Torque in DC Motor'''

|
'''Current Flow in DC Motor'''

|}
The
loop can be made to spin by fixing a half circle of copper which is
known as commutator, to each end of the loop. Current is passed into
and out of the loop by brushes that press onto the strips. The
brushes do not go round so the wire do not get twisted. This
arrangement also makes sure that the current always passes down on
the right and back on the left so that the rotation continues. This
is how a simple Electric motor is made.

== Key Vocabulary: ==

# '''Right Hand Thumb Rule:''' The rule that gives the direction of magnetic fiel due to a current carrying conductors
# '''Fleming's Left Hand Rule''': The rule gives the direction of force that would be experienced by a current carrying conductor in a magnetic field
# '''DC motor''': A device that would convert electric energy into mechanical energy

== Additional Resources: ==

# [[http://www.magnet.fsu.edu/education/tutorials/java/solenoidfield/index.html]]
# [[http://www.ncert.nic.in/html/learning_basket/electricity/electricity/machine/motor.htm]]

= Electromagnetic induction =

We
saw that a current carrying conductor produces a magnetic field
around it and that a magnetic will exert a force on a current
carrying conductor. Michael Farday and Joseph Henry examined what
would happen if a conductor was moving in a magnetic field. They
discovered, independently, that electric current can be introduced in
a wire by simply moving a magnet in or out of a coiled part of a
wire. The mechanical energy of movement of the magnet was enough to
'''induce '''an
electromotive force in
the coil when the coils are rotated in a magnetic field.

This
has led to the alternate ways of generating current. Till
electromagnetic induction was discovered only voltae sources were
those of chemical nature such as dry cells. The present large-scale
production, distribution is feasible because of this phenomenon of
electromagnetic induction.. Electromagnetic induction formed the
principle of two important electrical devices namely, electric
generator and transformer, which has revolutionized the life styles
of mankind.

== Faraday's law ==

The induced voltage in a coil is proportional to
the product of of the number of loops and the rate at which magnetic
field changes within those loops. The key concept here is that of
change in the magnetic field, that of magnetic flux.

=== Magnetic flux ===

Magnetic
flux can be defined as the number of lines of force passing through a
surface normally.

Considering
the surface 'Ds' in a magnetic field 'B'.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_8cba5b4.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_16468845.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m225205b8.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_6373b4c9.png]]

When a surface is a plane and has total area A
then

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m5e53150.png]]

SI unit of f
is weber and magnetic flux is a scalar quantity.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_23688484.png]]

Hence we find that the magnetic flux depends on

(i) the strength of the magnetic field.

(ii) the area of the surface.

(iii) the angle between the magnetic field vector
and the area vector.

Increasing the magnetic flux through a surface can
be done in 3 ways.

=== The Experiment of Faraday and Henry ===

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_55d6a46f.png]]Faraday
and Henry performed lots of experiments to learn about the connection
between electricity and magnetism. The results of these experiments
have led to the life styles of today, who made life easy by using
lots of electrical applications.

Some of the experiments are as follows

A solenoid is connected to a sensitive
galvanometer. On moving a magnet towards a coil, the galvanometer
shows a deflection. When the magnet is reversed, the deflection is
seen to be in the opposite direction.

Once the magnet is stopped, there is no deflection
in galvanometer. On moving the magnet faster towards the coil, the
deflection is longer.

Similar results are obtained when the magnet is
kept stationary and the coil is moved. It means that whenever a
current was induced in the coil there is a relative motion between
the coil and the magnet. The magnitude of the current depended on the
strength of the magnet and also on the magnitude of their relative
velocity.

Similar results were seen when the magnet is
replaced by as coil connected to a battery. Even without physically
moving the coils a current was shown in the galvanometer only when
the switch is on and when the current is put off i.e., when the
current is building up in the coil or when it reduced to zero the
galvanometer in the other coil showed a charge.

This current, which is produced in the coil
connected to the galvanometer, is called as induced current. The
induced currents direction, when the current builds up in the other
coil was opposite to that when the current reduced opposite to that
when the current reduced. The deflections were momentarily seen only
when the switch was opened and closed.

=== Faraday's laws of electromagnetic induction. ===

(i) Whenever the magnetic flux linked with a
circuit changes, an EMF is induced in the circuit, which lasts as
long as the change in magnetic flux associated with the circuit
continues.

(ii) The magnitude of the induced EMF is equal to
the rate at which the magnetic flux linked with the circuit changes.[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_1ac06770.png]]

Mathematically,

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_3e986b27.png]]

Faraday's laws of electromagnetic induction do not
say anything about the direction of the current. The direction is
given by Lenz's law.

=== Lenz's Law ===

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_72319496.png]]

The motion of the magnet in either direction
causes a change in strength of the magnetic field linked with the
coil and this causes a current to be induced in the coil. This
induced current opposes the change in the magnetic field by producing
its own magnetic field.

Whenever an EMF is induced, the induced current is
in such a direction so as to oppose the change inducing the current.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_497ddc16.png]]

The negative sign indicates the opposing nature of
the induced EMF.

=== Methods of producing induced emf ===

The three methods of producing induced EMF are by:

# Changing the magnitude of magnetic field B
# Changing the area A
# Changing the angle between the direction of B and the normal surface area

=== Motional e.m.f and Faraday's Law ===

Suppose
a uniform magnetic field B perpendicular to the plane of paper point
outward is represented in the region ABCD. A rectangular loop PQRS is
pulled such that it moves with a velocity V as shown in the diagram.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_2594d0ac.png]]

This
way the area of the loop inside the field changes. This induces an
e.m.f in the wire. If in a small time [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m4ce5d5c9.png]],
the loop moves a distance [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_45ce0699.png]],
then the decrease in the area of the loop = - lDx.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_4c38e4cd.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7de413e5.png]]where

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_45406d5c.png]]

If R is the resistance of the loop,

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m68155d35.png]]

The direction of the current is given by Fleming's
right hand rule. The induced e.m.f. Blv is called motional e.m.f.

Note:

The motional e.m.f can be understood by recalling
Lorentz force. When the loop moves, the charges inside it moves and
so experiences a force = q v b [as the loop is placed in a external
magnetic field]. The work done in moving the charge is q v b.l.

But e.m.f [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m598b6616.png]]

Motional e.m.f = B l
v

Similarly when a conductor is stationary, the
moving magnet or changing magnetic field produces an electric field
which forces the charges in the conductors to move thus inducing
current in the conductor.

=== Lenz's Law and Energy Conservation ===

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_2e54c387.png]]

If the north pole of the magnet is moved towards
the coil, the upper face(U.F) of the magnet acquires the north
polarity on closing the key between 2 and 3. [This is because the
current induced in the coil flows in an anti clockwise and this
produces a magnetic field with the upper face acquiring a north
polarity]. Therefore, work has to be done against the force of
repulsion in bringing the magnet closer to the coil. If the magnet is
moved away, south polarity develops on the same face. Therefore, work
has to be done against the force of attraction in taking the magnet
away from the coil. It is this mechanical work done in moving magnet
with respect to the coil that changes into electrical energy
producing induced current. Thus, energy is being transformed.

=== Fleming's Right Hand Rule ===

The direction of induced current can easily be
predicted using Fleming's right hand rule.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m5d4cf08f.jpg]] 
 

 
 

 
 

If we stretch the first finger, the central finger
and the thumb of our right hand in mutually perpendicular directions
such that first finger points along the direction of the field and
the thumb is along the direction of motion of the conductor, then the
central finger would give us the direction of induced current.

=== Eddy Currents ===

Induced currents are produced not only in the
wires, but also in the block of metals. If a metallic block is placed
in a continuously changing magnetic field, induced currents are set
up in the body of the metallic block. In the case of the wires the
induced current flows along the direction of the wire. How does it
flow in metallic blocks? They flow in a circular path by Lenz's law.
These current appear like eddies in a fluid and hence are called as
eddy current's.

Unlike the
metallic wires where the resistance is less metallic [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m19b8a684.jpg]]blocks
have larger resistance and hence the induced currents lead to large
amount of Joule's heat (H = i2k).

'''''Illustration'''''

If a bar pendulum is suspended between the pole
pieces of a magnet: Let us take another identical pendulum and kept
in a field free region. If we oscillate both of them with the same
force, it is observed that the one within the field damps faster.

The 'bob' of the pendulum consists of a copper
plate. The pendulum is made to swing between the pole pieces of the
magnet. Its motion is damped due to eddy currents.

Why does it happen? When the pendulum oscillates
inside the field it cuts the magnetic lines of force and hence
induces a current in the bar, that is, eddy currents. According to
Lenz's laws, the eddy currents oppose the motion and hence produce
damping.

Is eddy current advantageous or disadvantageous?

Eddy currents produce a large amount of heat,
which is undesirable in a number of cases like dynamos, transformers,
where the coil is wound on iron core.

How can eddy currents be minimized?

The solid iron core is divided into a number of
thin sheets. The sheets are electrically isolated from each other.
These sheets are so placed that the path of the induced eddy currents
is broken by the insulating material between the sheets. These are
called laminated cores. Hence, using laminated cores can minimize the
effects of eddy currents

=== Application of Eddy Currents ===

When a steady current passes through a moving coil galvanometer,
the coil undergoes a torque and does not come to equilibrium position
instantly. Hence the coil is wound over a metallic frame so that the
eddy currents produced in the frame can damp the oscillation and
brings the coil to the equilibrium position instantly.

=== Induction Furnace ===

Induction furnace separates certain metals from
their ores. It is done by heating the metal. The type of heating done
is called induction heating. This heating can be done using eddy
currents. The metal to be heated is placed in a high frequency
changing magnetic field. Strong eddy currents produced will give rise
to desired heating.

=== Electronic Brakes ===

Eddy currents can brake the motion of the train too. A metal drum
is attached to the train. To apply brakes a strong magnetic field is
applied across the drum. The eddy currents set up in the drum in a
direction as to oppose the change in the magnetic flux that is, it
opposes the motion of the wheel.

== Self Induction ==

When a current is established in a conductor, a
magnetic field is produced in its vicinity. We can visualize this
field in terms of magnetic flux. If steady current flows the number
of lines of force at a given place would remain the same. But if the
current changes with time, the flux associated with the loop changes
and hence an e.m.f is induced in the loop. This phenomenon is called
as self induction.

Self-induction
is the property of a coil, which enables the induction of an EMF in
it when the current in the coil changes.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m7d2a6274.png]]

Consider a
coil carrying a current I having N turns and lets the magnetic flux f
be linked with the coil. If the current in the coil is changed, the
flux link also changes. Thus, according to Faraday's law of
electromagnetic induction, induces an EMF on to itself. This EMF is
called self-induced EMF and this phenomenon is called self-induction.

It is found
that the flux linkage is proportional to the current through it.

i.e., f a I or
f = LI

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_195be383.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_36e3d4cc.png]]

Here, L is
constant and is called self-inductance of the coil or coefficient of
self-induction.

S.I., the unit
of self-inductance, is Henry

i.e., 1 Henry
= 1 Weber turns / Ampereor 1 Henry = 1 Volt / ampere/second

'''''Definition'''''

Self-inductance
of a coil is 1 Henry when a current changes at the rate of 1 amp/sec
through the coil induces EMF of 1 volt in the coil.

'''''Mutual
Inductance '''''

We know that
if a current builds up or varies in a coil, the flux change leads to
induced e.m.f in the same coil. This can happen event mutually
between two interacting coils are close together, and if current is
passed in one of them, it sets up a magnetic flux surrounding itself.
When the second coil is near the first coil, the changes in the
magnetic flux of the first coil produces similar changes in the
second. Thereby, producing induced e.m.f in the second coil. To
distinguish it from self-induction, it is called as mutual induction.

It is the
property of two circuits (or coils) by virtue of which each oppose
any change in the magnitude of the current flowing through the other
circuit by producing an induced EMF in it.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7c0bcdea.png]]

Consider two
coils P and S placed near coil P connected to a battery and key and
is called the primary coil. Coil S is connected to a sensitive
galvanometer and is called the secondary coil.

When the key K
is closed, the flux linked with the coil in the primary circuit
changes. This induces an EMF in the secondary coil indicated by the
deflection in the galvanometer.

When the key K
is opened, an EMF is induced in the secondary coil, but in a
direction opposite to that induced during the make, i.e., current in
S always oppose any change in current in P.

'''Note:'''The
mutual inductance of two coils depends on the geometry of the two
coils, distance between the coils and orientation of the two coils.

The following
diagrams indicate the maximum coupling between the two coils.

(i) Coupling
between the coils is maximum.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m661b5e87.png]]

(ii) Coupling
is less than in case (a)

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_4cc6f350.png]]

(iii) Coupling
is minimum

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7b27b130.png]]

Mutual
inductance of two long solenoids

Mutual
Inductance of Two Long Solenoid

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m2c4e32ec.png]]

Consider a
solenoid P within the core having N1 turns. Another
solenoid S having N2 turns is wound over the solenoid P.
Let 'l' be the length of each solenoid and let them have nearly the
same area of cross-sections A.

The magnetic
field B1 at any point inside P due to current I1
is

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m336837c3.png]]

The flux f
linked with each turn of S

= B1
x area of each turn

= B1
A

Total magnetic
flux linked with S

f2[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_1bf08989.png]]
B1A x N2

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m39eabe5a.png]]

Now f2=
M12 I1

On comparing,

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_12bed6e7.png]]

'''Note:'''If
the area of cross section was different from the area of cross
section of the inner solenoid, the smaller one is to be considered.

== Alternating Currents ==

Do we use dry
cells for operating electrical appliance? It is not impossible to tap
continuous supply of energy from electrochemical cells. Electrical
circuits in homes, factories and offices receive such energy form
local power companies. In most countries the energy is supplied via
oscillating e.m.fs and currents. These oscillating currents are
called as alternating currents, shortly as a.c.

Circuits
involving alternating currents are used in electric power
distribution systems, in radio, TV and other communication devices
and in a wide variety of electric motors. The designation
'alternating' mean current changes direction and value periodically
with time.

Can you guess
the frequency with which their direction is going to alter? In India,
the frequency of the alternating current supplied to homes is 50Hz.
What does this mean? The current flows along the length of the wire
in one direction and changes to the opposite direction, and this
happens at the rate of 50 times in one second. That means every 1/100
seconds, there is a change of direction. It is an amazing fact that
the charge carriers get this signal of direction change is propagated
at the speed of light.

What
difference does it make if direct current flows or an alternating
current flows in a conductor? As far as the heating effects are
concerned such as light bulbs and heaters the direction of current is
not important and the electrons transfer the energy to the device via
collisions with atoms in the device.

It does make a
big difference when the magnetic effects electric current are
concerned. As the currents alternate, the magnetic field surrounding
the conductor also oscillates. This makes possible the Faraday's laws
of induction. Moreover, alternating current is readily adaptable to
rotating machinery such as generator and motors.

One of the
methods of producing a sinusoid EMF is to rotate the coil in uniform
magnetic filed. Graphically, such a varying EMF or current is
represented as follows.

Note that Eo
and Io represent the peak or maximum values of EMF at a
particular time.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m3c925162.jpg]]

Therefore
induced EMF in a coil varies in magnitude and direction periodically.
Such an EMF is called alternating EMF. The corresponding current is
called alternating current (AC). The AC or EMF first rises to a
maximum +Eo or (+Io) in one direction and falls
to zero, the direction then quickly reverses so that the EMF and
current rise to maximum value Eo or (-Io) in
the reverse direction and again falls to zero. This completes one
cycle of AC voltage, the instantaneous value of EMF is therefore, E =
Eo sinwt and current is given by I = Io sinwt
where Eo, Io are the peak values of the EMF and
current and wt are the phase angles.

=== Average value of alternating e.m.f ===

Average value
of the alternating e.m.f over a half cycle is that steady e.m.f which
will send the same amount of charge in a circuit in a time of half
cycle as is sent by the given alternating e.m.f in the same circuit
in the same time.

Following the
above definition, it can be proved that the average value of the
alternating value of alternating e.m.f for positive half cycles is
0.637 time the peak value of the alternating current.

Why do we talk
about half cycle? What if the whole cycle is taken into account? Due
to positive half cycle it is 63.7% o and then due to
negative half cycle it should be -63.7 %o and hence for
the whole cycle the average e.m.f vanishes.

Then how to go
about full cycles? We define a new term called 'root mean square
value' of e.m.f (or) current.

The whole
process repeats once again. The energy of system oscillates between
capacitor and the inductor.

== AC Generator or Dynamo ==

An 'AC
generator' or 'dynamo' is a machine which produces AC from mechanical
energy. Actually, it is an alternator which converts one form of
energy into another.

=== Principle ===

AC Dynamo is
based on the phenomenon of electromagnetic induction. That is, when
the relative orientation between the coil and the magnetic field
changes, the flux linked with the coil changes and this induces a
current in the coil.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_2757b930.png]][[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_7ad1ee04.png]] 
 

 
 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

As the
armature coil rotates, the angle Q changes continuously. Therefore,
the flux linked with the coil changes.

Now,

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_26b78bd2.png]]

= NBA cos q

= NBA cos wt

where q is the
flux linked with the coil, N is the number of turns in the coil, A is
the area enclosed by each three of the coil and B is the strength of
the magnetic field.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_6102f53.png]]

= - NBA (-sin
wt )w

E = + NBA w
sin wt

e = eo
sinwt. This is the EMF Supplied by the A.C. generator

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_71aebc41.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_721eff61.jpg]]

'''''Construction'''''

Armature

ABCD is the
armature coil consisting of a large number of turns of the insulated
copper wire wound over a laminated soft iron core I. The coil can be
rotated about the central axis.

Magnets

N and S are
the pole pieces of a strong electromagnet in which the armature coil
is rotated.

Slip rings

R1
and R2 are two hollow metallic rings to which both ends of
the armature coil are connected. These rings rotate with the rotation
of the coil.

Brushes

Brushes B1
and B2 are two flexible metal plates or carbon rods. These
brushes are used to pass current from the coil to the external load
resistance.

'''''Working'''''

To start with,
suppose the plane of the coil is perpendicular to the plane of the
paper in which the magnetic field is applied, with AB at the front
and CD at the back, the flux linked with the coil is maximum in this
position. As the coil rotates clockwise, AB moves inwards and CD
moves outwards. According to Fleming's right hand rule, the current
induced in AB is from A to B, and in CD, from C to D. In the external
circuit, current flows from B2 to B1. After
half of the rotation of the coil, AB is at the back and CD is at the
front. Therefore, AB starts moving outwards and CD inwards. The
current induced in AB is from B to A, and in CD, from D to C. The
current flows from B1 to B2 through the
external circuit. We therefore see that the induced current in the
external circuit changes direction after every half rotation of the
coil, and hence is alternating in nature.

== Transformer ==

For a given
power requirement, one has the choice of the relative values of Irms
and Erms. That is, for the product to be a constant, we
can choose a relatively large current I and a relatively small
potential difference v or just the reverse. In an electric power
distribution system, it is desirable - both for reasons of safety and
the efficient design of equipment; to have relatively low voltage at
both the generating end and receiving end. But for transmission of
electrical energy from the generating plant to the consumer, we want
the lowest practical current so as to minimize the I2R
energy dissipation in transmission line. This mismatch between the
requirements for transmission and consumption calls for a device
which raises or lowers the potential difference in a circuit, keeping
the product IrmsErms essentially constant. This
device is a transformer whose operations are based on Faraday's law
of induction.

An electrical
device is used to change the AC voltage. A transformer which
increases the AC voltage is called a 'step up transformer' and a
transformer which decreases the AC voltage is called a 'step down
transformer'.

'''''Principle'''''

A transformer
is based on the principle of mutual induction.

'''''Construction'''''

It consists of
a soft iron core made of laminated sheets well insulated from each
other. The coils P1 P2 and S1 S2
are wounded on the same core.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_69835e94.jpg]]

The coil P1
P2 is a primary coil connected to AC source and S1
S2 is a secondary coil connected across a load resistance
R.

Working

As the current
in the primary varies, the flux linked with P1 P2
and hence S1 S2, changes.

If Np
is the number of turns in P1 P2, and Ep
is the alternating EMF fed to P1 P2 at instant
t under ideal conditions;

Self induced
EMF in P1 P2 at instant t = EMF fed to P1
P2 at this instant.

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m4d46ced4.png]]

Assuming there
is no flux leakage, the rate of change of flux through each turn of
S1 S2 is df/dt,

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_25234b7a.png]]

Since

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m2c99fa85.png]]

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_17793d1a.png]]

For Step Up
Transformers

Es
> Ep

i.e., K >1
Ns > Np

For a Step
Down Transformers

Es
< Ep

i.e., K <1
ns < Np

If we assume
there is no loss of power,

Out put power
= Input power

EsIs
= EpIp

[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m16701510.png]]

Energy Losses
in a Transformer

(i) Copper
loss is the energy lost due to heating of copper coils of
transformers.

(ii) Iron loss
due formation of induced current in the iron line resulting in lot of
heat.

(iii) Leakage
of magnetic flux. All flux linked with primary may not be linked with
secondary.

(iv)
Magnetostriction i.e., humming noise of a transformer.

 
 

 
 

= Activities =

== Activity 1: Repelling Strings ==

Principle: Electric charge, Static
Electricity

Procedure:

* Tie about 8 to 10 nylon strings to a rod.
* Rub the rod with fur or wool, and you remove electrons from the fur and deposit them on the strings.
* The strings will fly apart since they are all charged, and like charges repel.

Evaluation Questions: 

1. Why do the strings move apart?

== Activity 2: Charge of the light balloons ==

Principle: Electric charge, Static
Electricity

Procedure:

* Rub two balloons through your hair and you transfer some electrons to them.
* Suspend them by strings to show that they repel.
* You can illustrate polarization by showing that a charged balloon will attract an uncharged balloon, but once they touch and transfer charge, they repel.
* You can deflect a stream of water with a charged balloon because of the polarization of the water molecules.
* You can also "levitate" light strings and joke about snake charming.
* Sticking them to walls and ceilings is also fun.

== Activity 3: Rub the tube ==

Principle: Electric charge, Static
Electricity

Procedure:

* Rub a fluorescent tube with wool or fur and it will glow.
* Electrons are transferred to the glass from the fur, and some electrons dislodge and fly away from the other deposited electrons and excite atoms in the gas inside the tube.
* As the atoms de-excite, they emit ultraviolet radiation which is absorbed by the phosphor coating on the inside of the tube, which causes the phosphor, and the tube, to glow.

Evaluation Questions: 

1. Why do the balloons move apart?

2. Why uncharged balloon gets attracted towards
the charged balloon?

3. Why stream of water gets deflected away when
balloon is brought near?

== Activity 3: Electroscope ==

Principle: Electric charge, Moving
electrons

Procedure:

* You can construct a simple charge detector with a glass jar, aluminum foil, and some stiff wire.
* Choose a quart glass jar with straight sides and a plastic lid.
* Bend a small (~ 2 cm) sideways hook into a 25 cm piece of stiff wire (a stripped piece of coat hanger will work).
* Stick the unbent end of the wire through a small hole drilled in the plastic lid, and fix the wire in a position so that the hook is in the middle of the jar. A small glob of clay will work just fine for this.
* Cut the unbent end of the wire so that only a few inches of wire sticks up out of the lid. Once you have the wire where you want it, fix it in place and seal the hole with wax dripped from a candle.
* Hang two thin (~ 3 to 5 mm) aluminum foil strips from the hook so that they touch.
* Heat the jar so that it is dry and warm inside, then quickly seal the jar.
* Top the protruding wire with a ball of crumpled aluminum foil, and you are done!
* Bring something charged close to the ball, and the aluminum strips repel each other.
* This is because the wire and foil polarize when something charged is brought near.
* The opposite charge is attracted to the ball on top, and the like charge is repelled down into the strips, which then repel each other.
* Remove the charged object, and the strips return to normal. If you touch the charged object to the ball, you transfer charge, and the strips will remain deflected. Knowing the charge of one object, you can determine the charge of other objects with this device.

Evaluation
questions

1.
How can we test the presence of the charges in a body?

2.Can we transfer charges from one object to
another object?

== Activity 4: Conductors ==

Principle: Electric charge, Moving
electrons

Procedure:

* Make two 1 cm wide, 10 cm long aluminum foil wires by folding strips of foil 3 or 4 times.
* Tape one end of each to opposite ends of a D battery.
* Connect the other end of one wire to the side of a flashlight bulb with a clothespin.
* Tap the bottom of the bulb on the other wire and the bulb will light.
* You can have your students check various objects to see if they conduct by placing the objects between the loose wire and the flashlight bulb.

Evaluation questions

# Why do some materials allow electric current to pass through them?
# What makes them to conduct electricity?

== Activity 5: Conductivity ==

Procedure: 

Locate the PHET “Conductivity” Simulation
(either on a classroom computer or at
[[http://phet.colorado.edu/simulations/sims.php?sim=Conductivity]]
)

'''''Part I--Conductors'''''

* Check that the battery voltage menu is set to 0
* Under the materials menu, select metal. What, if anything, happens?
* Now, set the battery voltage to 0.5. What, if anything, happens? Illustrate with a diagram.
* The little spheres rotating around the ring represent electrons in a wire. Look at the battery. What terminal (positive or negative) is supplying the electrons? (hint: look for the side of the battery that has a “button”. That would be the positive terminal. The opposite side is the negative terminal).
* The battery and the wires form an electric circuit, that is, a complete path from the power source, through a wire and back to the same power source.. If an electric circuit is broken in any spot, the flow of electrons will stop.
* Adjust the battery voltage higher and describe the effect on electron movement in the wire.
* Adjust the battery voltage lower and describe the effect on electron movement in the wire.
* With the battery voltage at 0.5, Shine the light. What, if anything, happens?
* Set the battery voltage to zero
* Complete the following statement. Metals are conductors because they will allow a current of electrons to -------------

'''''Part II-Non-Conductors'''''

* Check that the battery voltage menu is set to 0
* Under the materials menu, select plastic. What, if anything, happens?
* Now, set the battery voltage to 0.5. What, if anything, happens? Illustrate with a diagram.
* Adjust the battery voltage higher and describe the effect on electron movement in the wire.
* Adjust the battery voltage lower and describe the effect on electron movement in the wire.
* With the battery voltage at 0.5, Shine the light. What, if anything, happens?
* Set the battery voltage to zero
* Complete the following statement. Plastics are non-conductors because ---------

== Activity 6: A current is a magnet; a magnet is a current ==

Principle:
Moving electrons. magnetism 
Prοcedure:

* Poke a hole in the middle of a piece of poster board and run a straight 60 cm large gauge solid wire (not twisted) through the hole.
* Support the wire and poster board so that the board is horizontal and at the mid point of the wire, with the wire perpendicular to the board.Place several small compasses on the poster board in a circle about the wire.
* Connect the wire to a 12 V lantern battery with allegator clip wires, and watch the magnets! Moving charges create a magnetic field in the form of circular loops perpendicular to the direction of their motion.
* For large classes, it might be more convenient to set the wire horizontal pointed towards the class, and trace the magnetic field loops with a dip compass.

== Activity 7: Electromagnet ==

Principle:
Moving electrons. Magnetism 
Procedure:

* Magnetic fields can be much stronger in materials than they are in air.
* Wrap an aluminum foil wire several times about a nail, and connect the wire to a D battery, and you have an electromagnet!
* With the wire looped, the "loops" of magnetic field produced by the moving charges all add up in the center of the wire loop, creating a much stronger field than what a single wire could produce.

== Activity 8: Make a magnet, break a magnet ==

Principle:
Moving electrons. Magnetism

Procedure:

* Magnetize a hacksaw blade by rubbing it in one direction with a strong permanent magnet using firm slow strokes, with the orientation of the permanent magnet the same at all times. Twenty to thirty strokes should suffice.
* Use iron or metal filings to show the location of the magnetic poles at the ends of the blade. Also demonstrate that the filings do not stick to the blade in the middle.
* Break the hacksaw blade in half, and now you have two magnets, each with a pole at each end!

== Activity 9: Totally tubular magnets ==

Principle:
Magnetism

Procedure:

* Magnetic fields affect moving charges, but not stationary ones.
* Similarly, a moving magnet (or a simply a changing magnetic field) can affect a stationary charge.
* Only relative motion is important (this is what got Einstein going).
* In other words, a changing magnetic field creates an electric field, and a changing electric field creates a magnetic field.
* As a general rule, the electric field created by a changing magnetic field will be oriented so that it could cause nearby charges to move in a manner that would create a second magnetic field directed to oppose the change in the original magnetic field.
* Electric currents created from this effect are called "eddy currents" due the circular motion of the charges.
* In other words, a moving magnet will create a "virtual magnet" if it moves near a conductor.
* The virtual magnet will be oriented to slow the moving magnet down.
* To illustrate this, drop a strong magnet down a copper tube, and show that it takes a lot longer to drop out the end than anything else of similar size and weight.
* You can also move a strong magnet past a non-magnetic conductor (copper is best) and feel the resistance.

== Activity 10 – PhET Magnetism ==

Principle:
To study magnetism, polarity

Part
I:

* Go to [[http://phet.colorado.edu]]
* Click on electricity and magnetism sims.
* Select the simulation “Magnets and Electromagnets.” It is at this link

[[http://phet.colorado.edu/new/simulations/sims.php?sim=Magnets_and_Electromagnets]]

* Move the compass slowly along a semicircular path above the bar magnet until you’ve put it on the opposite side of the bar magnet. Describe what happens to the compass needle.
* What do you suppose the compass needles drawn all over the screen tell you?
* How is the strength of the force/torque on the compass needle indicated?
* What are the similarities between the compass needle (magnetism) and a test charge (electricity)?
* Move the compass along a semicircular path below the bar magnet until you’ve put it on the opposite side of the bar magnet. Describe what happens to the compass needle.
* How many complete rotations does the compass needle make when the compass is moved once around the bar magnet?
* Click “flip polarity” and repeat the steps above after you’ve let the compass stabilize.
* Click on the electromagnet tab. Place the compass on the left side of the coil so that the compass center lies along the axis of the coil. (The y-component of the magnetic field is zero along the axis of the coil.)
* Move the compass along a semicircular path above the coil until you’ve put it on the opposite side of the coil. Describe what happens to the compass needle.
* Move the compass along a semicircular path below the coil until you’ve put it on the opposite side of the coil. Describe what happens to the compass needle.
* How many complete rotations does the compass needle make when the compass is moved once around the coil?
* Use the voltage slider to change the direction of the current and repeat the steps above for the coil after you’ve let the compass stabilize.
* Based on your observations, summarize the similarities between the bar magnet and the coil.
* What happens to the current in the coil when you set the voltage of the battery to zero?
* What happens to the magnetic field around the coil when you set the voltage of the battery to zero?
* Play with the voltage slider and describe what happens to the current in the coil and the magnetic field around the coil.
* What is your guess as to the relationship between the current in the coil and the magnetic field?

Part II –
Graphing relationships - Field Strength vs. Position

* Using the Electromagnet simulation, click on “Show Field Meter.”
* Set the battery voltage to 10V where the positive is on the right of the battery.
* Along the axis of the coil and at the center of each compass needle starting 5 to the left of the coil, record the value of B. Move one compass needle to the right and record the value of B. Repeat until you’ve completed the table below. NOTE: Be sure to take all of your values along the axis of the coil. You’ll know you’re on the axis because the y component of the magnetic field is zero along the axis.

{| border="1"
|-
|
Compass position (arbitrary units)

|
Magnetic Field Strength (fill in units)

|-
|
-5

|
 

|-
|
-4

|
 

|-
|
-3

|
 

|-
|
-2

|
 

|-
|
-1

|
 

|-
|
0

|
 

|-
|
1

|
 

|-
|
2

|
 

|-
|
3

|
 

|-
|
4

|
 

|-
|
5

|
 

|}
* What happens to the value of magnetic field strength inside the coil?
* Graph the compass position on the horizontal axis and magnetic field magnitude on the y axis. Print your graph. Make sure to label the axes and title the graph.
* Is your graph symmetric?
* Using your graph, what is the relationship between magnetic field strength and position? (Use the fit feature of graphical analysis to help you.)

Part III – Using
the simulation to design an experiment.

Field Strength vs. Number of Coils

* Design an experiment to test how field strength varies with the number of coils.
* Collect data in a table and graph your results.
* Field Strength vs. Current
* Design an experiment to test how field strength varies with the Current. (Recall that voltage is directly proportional to current….Ohm’s Law.)
* Collect data in a table and graph your results.

Part IV

* Test your predictions from part III using the electromagnet built in class and the Logger Pro sensor.
* Were your predictions correct? Explain.

== Activity 11: Simple Battery ==

Principle:
Moving electrons, chemical reaction

Procedure:


* Clean or brighten an iron washer and a copper penny. Soak a 1 inch square piece of heavy blotter paper or folded paper towel in vinegar.
* Press the soaked paper between the washer and the penny to form a simple battery.
* Measure the current with wires connected to the pocket current meter, with one wire pressed against the washer and the other pressed to the penny.
* This battery will not be strong enough to light a flashlight bulb.
* The vinegar induces a chemical reaction between the copper and the iron.
* Charged "ions" will flow through the vinegar from the copper to the iron.
* The reaction continues as long as one piece of metal can get rid of its excess electrons through the wires to the other piece of metal to balance the natural flow of charge through the blotter paper.

== Activity 12: Electromagnetism ==

Procedure: 

1.''' Go To:'''
http[[://phet.colorado.edu/simulations/sims.php?sim=Faradays_Electromagnetic_Lab]]

''Hint: Download the file using the ‘save’
option then run the ‘.jar’ file using Java. ''

2. Complete
the following tasks to help you investigate ''Faraday’s
Electromagnet Lab''. These tasks will help you conduct appropriate
experiments to answer the lab questions.

We will be using the '''''Bar Magnet''''' and
'''''Electromagnet''''' tabs for this activity and the
other tabs later in the unit. Click on the Bar Magnet tab.
[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_6bfd456c.png]]

* Click on [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m7cdbf5e.png]]. Explain the '''two '''changes this causes in the simulation.
* M[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_45126618.png]]ove compass to various locations around the bar magnet. Explain what orientation the needle takes with respect to the bar magnet.

 
 

* S[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m4de6ccf7.png]]elect ‘Show Field Meter’ [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_5242b6c6.png]]. The image below will appear. The meter can be moved to various locations and indicates the magnetic field strength at the crosshairs. '''Label:''' Total magnetic field, y-component of the magnetic field, x-component of the magnetic field, angle and units in the following diagram.
* You should be able to determine the '''direction '''of the magnetic field vector using the meter.
* Select [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_ma1f6eac.png]]. Observe the orientation of the small compass needles.
* Click on the Electromagnet tab. [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m276d0de7.png]].
* What is behaving like a magnet : The battery or The coils of current carrying wire ? [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m6b8ab73d.png]]
* Using the slider on the battery, change the voltage of the battery from 10V to 0V.

Then from 0V to ‘-’ 10V. [[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m42f9b776.png]]

'''Record''' the changes you observe in the
direction of the compass needle.

Select AC as your current source.

'''Observe''' and '''record''' the changes in
the compass needle.

3. '''Design '''and''' execute '''an experiment
using the simulation that will allow you to understand the ''direction''
and ''strength'' of the magnetic field around

(a) a bar magnet

(b) an electromagnet

You do not need to submit the procedure of your
experiment, only your results.

Questions

* Using diagrams and written explanation, explain the magnetic field '''direction''' and '''strength '''around a bar magnet, and an electromagnet.
* Explain the '''similarities''' and '''differences''' of a bar magnet and an electromagnet.
* Identify the characteristics of electromagnets that are variable (can be changed) and what effects each variable has on the magnetic field’s strength and direction.

== Activity 13: Faraday's Electromagnetic Lab – AC/DC Current and Electromagnetism ==

Procedure
'''–
'''do the
following activity using this web site

[[http://www.colorado.edu/physics/phet/simulations/faraday/faraday.jnlp]]

* '''G[[Image:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m519d161e.gif]]etting started.''' Open the website listed above and on the top of the screen select the tab marked electromagnet.
* '''Make observations & draw conclusions.''' Change the current source back and forth from DC to AC looking for how the electrons move in the wire. AC current is distinguished from DC current by the motion of the current. In this applet the current is represented by the balls moving in the wire. Based on your observations write a general rule for how current moves in AC verses how current moves in DC.
* '''Make observations & draw conclusions.''' Set up the applet so it is using a DC current and place a compass near the electromagnet. Your screen should look something like what you see to the right, on Screen 1. Using the slider on the battery, observe how changing the voltage changes the current flow and what happens to the compass needle. Write down your observations regarding the voltage, the current flow and the change in the compass. What does changing the current flow do to the magnetic field?
* '''Make observations & draw conclusions.''' Insert a field meter into your screen. Your screen should now look something like what you see to the right, Screen 2. move the battery slider back and forth and observe what happens to the strength of the magnetic field, the top number on the field meter. Write a general rule for how the voltage affects the magnetic field’s strength.
* '''Make observations & draw conclusions.''' Using the same setup as you used in step 4 change the number of loops and observe how this affects field strength. Write a general rule for how the number of loops affects the magnetic fields strength.
* '''Make observations & draw conclusions.''' Using the same setup as you used in step 4 move the filed meter from place to place and observe how the field strength changes. Write a general rule for how changing the distance from the magnet affects the magnetic fields strength.
* '''Make observations & draw conclusions.''' Use the same setup as you used in step 4 but change the source of current to AC. Your screen should look something like what you see to the right, Screen 3. Observe how the AC changes the compass and the magnetic field strength. Write down your observations regarding the change in the strength and direction of the magnetic field. Describe a way to get a DC supplied electromagnet to change the direction of the magnetic field, like the AC does.

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Changes

Science: Topics Electromagnetism (view source)

Revision as of 11:28, 24 August 2012

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