MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Electomagnetic Induction Page 1

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1.

Experiment No. 1

• In the experimental setup, as shown in the adjoining figure, the galvanometer shows a momentary deflection whenever the magnet is moved towards or away from the coil.
• The deflection is more if the magnet is moved faster.
• If the magnet is stationary, galvanometer deflection becomes zero.

 Coil - magnet experiment

Experiment No. 2

• In the experimental setup, as shown in the adjoining figure, the galvanometer shows a momentary deflection whenever key k is closed or opened.
• If key k is continuously pressed, galvanometer deflection becomes zero.

 Coil - coil experiment

2. Faraday's Laws of Electromagnetic Induction

Statement:

1. An e.m.f. is induced in a circuit when the magnetic flux linked with the circuit changes.
2. The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux.
 Mathematically : e a df dt

1. Where e = induced e.m.f.
 df =  rate of change of magnetic flux in a circuit. dt

3. Lenz's Law

 Statement: The direction of the induced e.m.f. is such that it opposes the cause producing it. Explanation: Lenz's law is consistent with the law of conservation of energy. Suppose the flux of induced current due to induced e.m.f. assists the change in magnetic flux linked with the circuit. Then in this situation, the induced current and the change in flux causing it will go on feeding each other endlessly and electrical energy will be generated in the circuit without expenditure of any other kind of energy. This is contrary to the law of conservation of energy. Needless to say, it does not happen!

How Lenz's law helps to find direction of induced current (an example):

Consider the experiment of the magnet and the coil
(Experiment No. 1)

• When the magnet is moved towards the coil, the flux f through the coil increases. To oppose this flux f, the induced current i, must set up its own flux fi opposing the increasing f.
• Therefore, the magnetic field of the induced current must be directed opposite to the magnetic field of the magnet .
• If points towards left i must point towards right.
• To produce a field directed towards right, the current through the coil must be anticlockwise when viewed from the magnet side.
• When magnet is moved away from the coil, flux f decreases. Now, the coil must oppose this decrease in f.
• The field should now be in the same direction as , i.e. towards right. Therefore, the induced current in the coil, in this case, must be clockwise when viewed from the magnet side.

• A combined mathematical statement of Lenz's law and Faraday's law:

 as e a df dt
•
 \ e = k df dt
•
 In SI system, e is measured in volts and df in Wb/s. These units are such that k = 1 dt
•
 \ e = df dt

• Lenz's law can be incorporated into this equation by adding a minus sign as shown below:
 e = - df dt

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