Electromagnetic induction

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Concept Map

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Textbook

  1. Tenth Standart Science Textbook

Additional information

Useful websites

  1. http://en.wikipedia.org/wiki/Electromagnetic_induction
  2. http://www.youtube.com/watch?v=vwIdZjjd8fo
  3. http://www.electronics-tutorials.ws/electromagnetism/electromagnetic-induction.html
  4. https://www.google.co.in/search?q=electromagnetic+induction&oq=Ele&aqs=chrome.0.69i59l2j69i57j0l3.1895j0j8&sourceid=chrome&es_sm=93&ie=UTF-8

Reference Books

  1. Tenth Standard Ready Lesson CD from Education Department – Science Subject

Teaching Outlines

Concept #1 Introduction to electro magnetic induction

Learning objectives

  1. Recall the the properties of magnets
  2. Electromagnetic effect
  3. How emf induced
  4. Emf depends upon the number of turns and speed of the magnet
  5. Michel Faraday experiment

Notes for teachers

Electromagnetic Induction The magnetic flux developed around the coil being proportional to the amount of current flowing in the coils windings as shown.
Electromagnetism coiled wire.gifMicheal Farady.jpg

If additional layers of wire are wound upon the same coil with the same current flowing through them, the static magnetic field strength would be increased.

Therefore, the Magnetic Field Strength of a coil is determined by the ampere turns of the coil. With more turns of wire within the coil the greater will be the strength of the static magnetic field around it.

But what if we reversed this idea by disconnecting the electrical current from the coil and instead of a hollow core we placed a bar magnet inside the core of the coil of wire. By moving this bar magnet “in” and “out” of the coil a current would be induced into the coil by the physical movement of the magnetic flux inside it.

Electromagnetic Induction was first discovered way back in the 1830’s by Michael Faraday. Faraday noticed that when he moved a permanent magnet in and out of a coil or a single loop of wire it induced an ElectroMotive Force or emf, in other words a Voltage, and therefore a current was produced.

Faraday’s Law of Induction

  1. Increasing the number of turns of wire in the coil – By increasing the amount of individual conductors cutting through the magnetic field, the amount of induced emf produced will be the sum of all the individual loops of the coil, so if there are 20 turns in the coil there will be 20 times more induced emf than in one piece of wire.
  2. Increasing the speed of the relative motion between the coil and the magnet – If the same coil of wire passed through the same magnetic field but its speed or velocity is increased, the wire will cut the lines of flux at a faster rate so more induced emf would be produced.
  3. Increasing the strength of the magnetic field – If the same coil of wire is moved at the same speed through a stronger magnetic field, there will be more emf produced because there are more lines of force to cut.

Factors affecting the magnitude of induced emf: The magnitude of induced emf is equal to the rate of change of magnetic flux i.e., Induced emf= (Change in magetic flux)/(Time in which the magentic flux changes) i) the change in the magnetic flux and ii) the time in which the magnetic flux changes.


These are short notes that the teacher wants to share about the concept, any locally relevant information, specific instructions on what kind of methodology used and common misconceptions/mistakes.

Activities

  1. Activity 1 - Properties of Magnets
  2. Activity 2 - Electromagnet experiments
  3. Activity 3 - Electromagnetic induction using galvanometer
  4. Activity 4 - Factors influencing e.m.f.
  5. Activity 5 - Explanation of Michel Faraday experiment

Concept #2 Fleming right hand rule

Learning objectives

  1. State Fleming's Right hand rule of dynamo
  2. Know the main parts of dynamo
  3. Explain the working of AC dynamo
  4. Distinguish between AC dynamo and DC dynamo
  5. Application of dynamo

Notes for teachers

Direction of induced emf :
Fleming’s right hand rule :
Stretch the thumb, middle finger and fore finger of your right hand mutually perpendicular to each other as shown in the fig. If the fore finger indicates the direction of magnetic field and the thumb indicates the direction of motion of conductors, then the middle finger will indicate the direction of induced current.
Lenz’s law : It states that the direction of induced emf (or induced current) always tends to oppose the cause which produces it.
(b) When the north pole of the magnet is brought towards one end of the solenoid, the induced current flows in the solenoid in such a direction that the end of the solenoid, near the magnet and becomes a north pole so as to repel the magnet and thus opposes the cause producing the induced current. Therefore e the direction of induced current in solenoid at its end towards the magnet is anti-clockwise.
(d) When the north pole of the magnet recedes form the end of the solenoid, the direction of induced current in the solenoid is such that the end of the solenoid towards the magnet becomes a south pole so as to attract the north pole of the magnet and thus opposes the cause producing it. The current induced current in solenoid at its end clockwise. Lenz’s law is wider significance as it implies the law of conservation of energy. It shows that the mechanical energy spent in doing work, against the opposing force experienced by the moving magnet, is transformed into the electrical energy due to which current flows in the solenoid.
A.C. Generator:
An A.C.generator is a device which converts the mechanical energy into the electrical energy using the principle of electromagnetic induction.
Frequency of alternating current
In one complete rotation of the coil, we get one cycle of alternating emf in the external circuit. Thus the alternating emf has the frequency equal to the frequency of rotations of the coil.
If the coil makes n rotations per second. The magnitude of induced emf is given as
E=e0sin2nt And the current is expressed as i=i0sin2nt
Where e0 and i0 represent the maximum values of emf and current respectivily. Such a current is called an alternating current.

Activities

Concept #3 FLEMING'S LEFT HAND RULE AND WORKING OF MOTOR -

Learning objectives

  1. State Fleming's left hand rule of motor
  2. Explain working of motor
  3. Distiguish between dynamo and motor
  4. Application of the motor

Notes for teachers

Difference between A.C. and D.C Direct current (D.C.) Alternating current (A.C.) 1. It is the current of constant magnitude. 2. It flows in one direction in the circuit. 3. It is obtained from a cell (or battery) 1. It is the current of magnitude varying with time. 2. It reverses its direction periodically while flowing in a circuit. 3. It is obtained from A.C. generator and mains.

Advantages of A.C. over D.C.: In India, we use 220 volt a.c. in our houses and factories. The use of A.C. is advantageous over D.C. because the voltage of A.C. can be stepped up by the use of a step-up transformer at the power generating station before transmitting it over the long distances. It reduces the loss of electric energy as heat in the transmission line wires. The A.C. is then stepped down to 220 volt by the use of step-down transformers at the successive sub-stations before supplying it to the houses or factories. If D.C. is generated at the power generating station, its voltage cannot be increased for transmission and so due to passage of high current in the transmission line wires; there will be a huge loses of electrical energy as heat in the line wires.

A.C. Generator D.C. Motor 1. A generator is a device which converts mechanical energy into electrical energy. 2. A generator works on the principle of electrro magnetic inductions. 3. In a generator, the coil is rotated in a magnetic field so as to produce electric current. 4. A generator makes use of two separate coaxial slip rings. 1. A d.c. motor is a device which converts electrical energy into mechanical energy. 2 d.c. motor works on the principle of force acting on a current carrying conductor placed in a magnetic field. 3. In d.c. motor, the current from d.c. source flows in the coil placed in a magnetic field due to which the coil rotates. 4. A d.c. motor makes use of two parts of slip ring (i.e., split rings) which act as a commutatar.

Activities

  1. Activity 1 -

Concept #4 APPLICATION OF ELECTROMAGNETIC INDUCTION TRANSFORMERS AND INDUCTION COILS

Learning objectives

  1. Transformers
  2. Types of transformers Primary and Secondary
  3. Know the wokring principle of transformers
  4. Application of transformers
  5. Induction coils
  6. Know the wokring principle of induction coil
  7. Applications of induction coils

Notes for teachers

Applications of electromagnetic induction: Transformer: In our daily life, we use various electrical applications which require working voltage different form the mains voltage (i.e., 220V) e.g. a door bell needs 6V while a television needs several 1000V. To provide suitable voltage to different applications from the mains, we use transformers with them. Thus, Transformer is a device by which the amplitude of an alternating e.m.f. can be increased or decreased. A transformer does not affect the frequency of the alternating voltage. The frequency remains unchanged (=50hz) Principal : A transformer works on the principle of electromagnetic induction and make use of two coils having different number of turns. The alternating e.m.f. to be altered is applied across one coil. Which there is change of magnetic field line due to varying current in this coil the magnetic field line linked with the other coil also changes and so an induced varying current of the same frequency, but of different magnitude flows in the other coil. Construction : A transformer consists of a rectangular soft iron core made up from the thin laminated sheets of soft iron of T and U shape, placed alternately one above the other and insulated from each other by a paint (or varnish) coating over them as shown in the fig. so as to behave like a simple rectangular core. The laminated core prevents the loss of energy due to eddy currents in the core. On one arm of the core, a coil P of insulated copper wire is wound. This coil is connected with the source of alternating e.m.f. (i.e., at the ends of the this coil, the input is given). This is called primary coil. One the other arm of the core, another coil S of insulated copper wire is wound. The induced alternating e.m.f is obtained across the terminals of this coil (i.e., output is obtained at the ends of this coil). This is called the secondary coil. The ratio of number of turns Ns in secondary coil to the number of turns Np in primary coil (i.e., Ns/Sp) is called the turns ratio. i.e.,Turns ratio n= (Number of turns in secondary coil Ns)/(Number of turns in primary coil Np)

The advantage of using a closed core is that it gives a closed path for the magnetic field lines and therefore almost all the magnetic field lines caused due to current in the primary coil, remain linked with the secondary coil (i.e., the flux linkage is nearly perfect) and loss of energy is avoided. The core is made of soft iron so that hysteresis loss of energy in the core is less.

Working : When the terminals of primary coil are connected to the source of alternating e.m.f., a varying current flows through the primary coil. This varying current produces a varying magnetic field in the core of transformer. Thus the magnetic field lines linked with the secondary coil vary. The change of magnetic field line through the secondary coil induces an emf in it. The induced emf varies in the same manner as the applied emf in the primary coil varies and thus has the same frequency as that of the applied emf. The magnitude of emf induced in the secondary coil depends on the following two factors. (i) the ration of the number of turns in the secondary coil to the number of turns in the primary coil (i.e., turns ratio) and (ii) the magnitude of emf applied in the primary coil For a transformer, (emf across the secondary coil (Vs))/(emf across the primary coil (Vp))= (Number of turns in the secondary coil (Ns))/(Number of turns in the primary coil (Np))

((Vs))/( (Vp) )=((Ns))/((Np) )= is called Turns ratio 'n^' of transformer

The relation between current and voltage of primary and secondary coils of a transformer can be expressed as ((Vs))/( (Vp))= Ip/Is or IpVp=IsVs

Current x Voltage of primary coil = current x voltage in secondary coil Types of transformers : 1) Step up transformer : The transformer used to change a low voltage alternating emf to a high voltage alternating emf (of same frequency) is called step-up transformer. In a step-up transformer, the number of turns in the secondary coil are more than the number of tunrs in the primary coil. i.e., turns ratio n>1 or Ns/Np>1. Vs>Vp, but Is<Ip. More current flows in the primary coil. Therefore one must use thicker wire in the primary coil as compared to that in the secondary coil. 2) Step down transformer : The transformer used to change a high voltage alternating emf to a low voltage alternating emf (of same frequency) is called step-down transformer. In a step-down transformer, the numbers of turns in the secondary coil are less than the number of tunrs in the primary coil. i.e., turns ratio n<1 or Ns/Np<1. Vs<Vp, but Is>Ip. Here the secondary winding will require thicker wire due to high current. The use of thicker wire reduces its resistance and therefore reduces the loss of energy as heat in the coil. This energy loss due to heat is known as copper loss. Uses of Transformaers : i) Use of setp-up transforemer : a) In transmission of electric power at the power generating station to step up the voltage.b) with television c) with wireless sets and d) with X-ray tubes to provide a high accelerating voltage. ii) Use of setp-down transforemer : a) With electric bells, night electric bulbs, mobile phone, computers, etc. b) At the power sub-stations to step-down the voltage before its distribution to the consumers. Induction coil : You know that for discharge tube experiment high voltage is required. How to produce high voltage from a low voltage DC source? An induction coil is a device for obtaining a very high DC voltage starting from a low DC voltage. An induction coil consists of few turns of a primary coil ‘P’, of thick insulated copper wire wound over a soft iron core connected to a battery. S is a secondary coil of large number of turns wound over the primary coil. M is a make and break arrangement. When the DC current flows in the primary, ‘C’ gets magnetised. The iron head H of the switch M is attracted towards C so, the circuit breaks and the current in primary stops. C gets demagnetised and H comes back. The process repeats. During make and break circuit, large emf is induced in the secondary. The voltage depends on the turns ratio. Since the number of turns in secondary is very large, a large voltage in produced in the secondary. While using it remembers that the coil should be in the north and south direction. That means the needle and the coil should be parallel to each other.

Activities

Project Ideas

  1. List Scientist information of Faraday, Lentz, Fleming

Fun corner