Production of Induced emf in a Coil

# Our Objective

To demonstrate the production of induced emf in a coil due to the movement of (i) a magnet towards and away from it (ii) similar coil carrying current towards and away from it.

# The Theory

Michael Faraday discovered law of electromagnetic induction

## Law of electromagnetic induction

The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.

ε = - d Φ/dt

ε - Induced emf

Φ - Magnetic flux

Φ = BA cosθ

B - Magnetic field

A - Area of the surface

θ - Angle between the magnetic field and surface

Magnetic flux can change by varying Magnetic field, Area of the surface, angle between the magnetic field and surface

The negative sign indicates the direction of e and hence the direction of current in a closed loop

Michael Faraday conducted different experiments to reach electromagnetic induction.

## Production of induced emf in a coil due to the movement of a magnet towards and away from it

Fig: When the bar magnet is pushed towards the coil, the pointer in the galvanometer G deflects.

Connect coil with galvanometer.  Move magnet towards coil. Pointer in the galvanometer deflects. It indicates the presence of the current. The pointer in the galvanometer deflects in the opposite direction when the magnet moves away from the coil. Deflection in galvanometer happens when magnet moves towards the coil or away from the coil. You can see the deflection of pointer in galvanometer in the opposite direction if you move opposite face of the magnet towards coil. Direction of deflection of the pointer in the galvanometer will be different when we bring north pole of the magnet towards the coil and when we bring south pole of the magnet towards the coil. Deflection of the pointer in galvanometer happens when we move the coil or magnet

Relative motion between magnet and coil induces electric current on coil with galvanometer. Deflection of pointer in galvanometer increases as the speed of the magnet increases.

When we bring magnet towards the coil magnetic field through coil increases. It causes emf in coil. Current induces in the coil. When we bring magnet away from the coil magnetic field through coil decreases. It causes emf in coil. Current induces in the coil. Direction of the current will be different from previous case.

## Production of induced emf in a coil due to the movement of similar coil carrying current towards and away from it.

### Case 1

Replace the magnet with coil C2 which is connected to battery. The pointer in the galvanometer deflects when coil C2   brings towards or away from coil C1 with galvanometer. Deflection of the pointer in galvanometer happens when we move the coil C1 or coil C2

Relative motion between coils induces electric current on coil with galvanometer.

Fig: Current is induced in coil C1 due to motion of the current carrying coil C2.

When we bring coil C2 towards the coil magnetic field through coil C1 increases. It causes emf in coil C1. Current induces in the coil C1. When we bring coil C2 away from the coil magnetic field through coil Cdecreases. It causes emf in coil. Current induces in the coil. Direction of the current will be different from previous case.

### Case- 2

Fig: Experimental set-up.

In this experiment coil C2 is connected to battery through key. The pointer in the galvanometer deflects when we close the key and open the key. The direction of deflection of the pointer in the galvanometer will be different when we close the key or open the key.

When we close the key current increases from 0 to maximum value. Current through the coil C

Increases. Magnetic field through coil C2 increases. It causes emf in coil C1. Current induces in the coil C1. When we open the key current decreases from maximum to 0 value. Current through the coil C2

decreases. Magnetic field through coil C2 decreases. It causes emf in coil C1. Current induces in the coil C1. Direction of the current will be different from previous case.

### Case-3

You can repeat the experiment by connecting the coil C2 through rheostat. Vary the current by moving slider of rheostat. The pointer in the galvanometer deflects when we vary current through coil C2. The direction of deflection of the pointer in the galvanometer will be different when we increase and decrease current.

When current through coil Cchanges magnetic field through the coil C1 changes. It causes emf in coil C1.

### Case-4

You can repeat the experiment by changing the common area between the coils C1 and C2. The pointer in the galvanometer deflects when we change the common area between the coils C1 and C2. The direction of deflection of the pointer in the galvanometer will be different when we increase and decrease the common area between the coils.

When we change the common area between the coil changes magnetic flux through the coil C1 changes. It causes emf in the coil C1. It causes the current in coil C1.

Direction of induced emf can be found using Lenz’s law-

Lenz's law states that “the polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.”