Chapter 28: Electromagnetic Induction

Electromagnetic Induction

The induced current or e.m.f. can be induced across the ends of the conducting wire in two ways:

  1. Move the wire through a magnetic field.
  2. Move a magnet through a coil of the wire.

Faraday’s Law

The magnitude of the e.m.f. induced in a conductor equals the rate of change of flux linkage or the rate at which the conductor cuts a magnetic flux.

Imagine a straight piece of wire of length l is moved through a magnetic field at a velocity v. if the wire is moving at right angle to the field lines an e.m.f. is induced. The e.m.f. is given by;

ɛ= NΔØ/Δt

For one loop of wire, ɛ= ΔØ/Δt and flux is given by Ø = BA which gives ɛ= ΔBA/Δt
B is constant so ɛ= BΔA/Δt and we know A = l × vt
So ɛ= (BΔl vt)/Δt since the length and velocity of wire are constant so it becomes
ɛ= BlvΔt/Δt Which cancels to ɛ = Blv

Lenz’s Law

The direction of the e.m.f. induced in a conductor is such that it opposes the change producing it.

Solenoid/ Right hand grip rule

A solenoid with a current flowing through it produces a magnetic field like that of a bar magnet. By using right hand grip rule, if our fingers follow the direction of the current through the coils; our thumb points out of the North Pole.

When we push the north pole of the magnet the induced current in the solenoid flows to make a North Pole repel the magnet. When we pull the North Pole of the solenoid the induced current flows to make a south pole to attract magnet.

Fleming’s Right Hand Rule

If we are moving a straight wire through a uniform magnetic field, the direction of induced current can be shown buy this rule. The first finger points the direction of the field from North to South, thumb points in the direction the wire is moved and middle finger points the direction of the conventional current.