Science: Topics Electromagnetism

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 =



= Theme Plan =

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

= Curricular Objectives = = 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).
 * 1) The children are introduced to the following 	concepts:
 * 2) 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.
 * 3) 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.
 * 4) 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.
 * 5) 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.
 * 6) They will be introduced to the mathematical 	representation of Coulomb's law and the equation that describes how 	the electrostatic force acts.
 * 7) 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.
 * 8) 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
 * 9) 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.
 * 10) 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.
 * 11) 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.
 * 12) The children are introduced to properties of 	electromagnets, behaviour of current carrying conductors in a 	magnetic field and the operation of the motor.
 * 13) 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.
 * 14) The 	students must be able to understand the consumption of electric 	energy, units of energy consumed (kWh) and the cost of electricity.

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. '''



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' :



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.'''

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.

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:

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.



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*10 9 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

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

Additional web resources

 * 1) Basics of Static Electricity This is a good overview of the basics of 	static electricity.
 * 2) 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.
 * 3) 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.

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.

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.
 * 1) 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.
 * 2) 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.

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



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.



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.

'''V B = U B / 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, W BA , as the charge moves from A to B.

'''UBA= UB- U A = 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.

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:

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

Additional resources :

 * 1) Faraday's cage. This is a lecture by 	Walter Levin, Professor at MIT, demonstrating Faraday's cage
 * 2) MIT library This site shows you 	photographs of a Van de Graff generator
 * 3) 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
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.

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.



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.



Additional resources
= 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.
 * 1) Working of a cell This website has an interactive tutorial and an 	explanation of how cells work
 * 2) 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

The Chinese have known to use Magnetic needles for navigation on 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.

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.



Properties of Magnetic field lines
Magnetic field lines form closed continuous curves.

The following diagram depicts the magnetic lines of force between two north pole, two south pole; North-South pole.
 * 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.

'' 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:

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


 * [[File: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.

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 	declination (q)
 * Magnetic 	inclination (d)
 * Horizontal 	component (BH)

Magnetic declination is the angle between magnetic axis and the geographic axis.

 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.



Horizontal component

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



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  

where is the intensity of magnetisation,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.



The flux density produced by the magnetising field vacuum is proportional to the intensity of field.

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

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

[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.

5) The permeability of the substance, that is, mr &lt; 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

The vector sum of the individual dipole moments is no longer zero.

Some important properties are:

Examples include Aluminum, platinum, chromium, manganese, copper sulphate, oxygen etc.,
 * 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.
 * [[File: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 	&gt; 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.

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:

Examples are Iron, cobalt, nickel and number of alloys.
 * [[File: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&gt;1 	that is their permeability is 	greater than one.
 * [[File: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.
 * [[File: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.

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

 * 1) []
 * 2) []

= 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.



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.

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,

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.

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:

 * 1) Right 	Hand Thumb Rule: 	 The rule that gives the direction of magnetic fiel due to a current 	carrying conductors
 * 2) 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
 * 3) DC 	motor: 	 A device that would convert electric energy into mechanical energy

Additional Resources:
= 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.
 * 1) []
 * 2) []

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'.









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



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



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
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.

Mathematically,



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


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.



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:


 * 1) Changing the magnitude of magnetic field B
 * 2) Changing the area A
 * 3) 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.



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 , the loop moves a distance , then the decrease in the area of the loop = - lDx.



where



If R is the resistance of the loop,



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

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


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.



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 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.



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





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.



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.



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



(iii) Coupling is minimum



Mutual inductance of two long solenoids

Mutual Inductance of Two Long Solenoid



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



The flux f linked with each turn of S

= B1 x area of each turn

= B1 A

Total magnetic flux linked with S

f2 B1A x N2



Now f2= M12 I1

On comparing,



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.



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.



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

Now,



= 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.



= - NBA (-sin wt )w

E = + NBA w sin wt

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





 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.



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.



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



Since





For Step Up Transformers

Es &gt; Ep

i.e., K &gt;1 Ns &gt; Np

For a Step Down Transformers

Es &lt; Ep

i.e., K &lt;1 ns &lt; Np

If we assume there is no loss of power,

Out put power = Input power

EsIs = EpIp



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 :

Evaluation Questions:
 * 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.

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 :

Evaluation Questions:
 * 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.

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 :

Evaluation questions
 * 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.

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 :

Evaluation questions
 * 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.


 * 1) Why do some materials allow electric current 	to pass through them?
 * 2) What makes them to conduct electricity?

Activity 5: Conductivity
Procedure:

Locate the PHET “Conductivity” Simulation (either on a classroom computer or at [] )

 Part I--Conductors 

 Part II-Non-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 	-


 * 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 []
 * Click on electricity and magnetism sims.
 * Select the simulation “Magnets and 	Electromagnets.” It is at this link

Part II – Graphing relationships - Field Strength vs. Position
 * 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 III – Using the simulation to design an experiment.
 * 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.
 * 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.)

Field Strength vs. Number of Coils

Part IV
 * 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.


 * 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.


 * Click on 	[[File:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m7cdbf5e.png]]. 	Explain the two changes this causes in the simulation.
 * M[[File: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.

Then from 0V to ‘-’ 10V.
 * S[[File:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_m4de6ccf7.png]]elect 	‘Show Field Meter’ [[File: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 	[[File:Electromagnetism_%20Resource_Material_Subject_Teacher_Forum_September_2011_html_ma1f6eac.png]]. 	Observe the orientation of the small compass needles.
 * Click on the Electromagnet tab. 	[[File: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 ? [[File: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.

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

[]


 * G[[File: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 &amp; 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 &amp; 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 &amp; 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 &amp; 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 &amp; 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 &amp; 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.