ELECTROMAGNETIC INDUCTION

Can a magnet produce electricity? Oersted’s experiments showed that electric current produces magnetic field. Michael Faraday discovered that when a magnet and a coil of wire are in relative motion wrt each other a current is induced in the coil. This phenomenon is called electromagnetic induction

Faraday’s Experiment A coil of wire is connected to a galvanometer. A magnet is moved into the coil and taken out. The galvanometer shows deflection.

Faraday’s observations Induced current is produced- whenever there is relative motion between the coil and the magnet. The strength of the induced current depends on the rate of relative motion. When the coil and magnet are at rest relative to each other there is no induced current.

What is the cause of induced current? Whenever there is relative motion between the coil and the magnet there is a change in the magnetic flux linked with the coil. When current through a coil continuously changes also a current is induced in a neighboring coil The change in flux is the cause of induced current in the coil.

Faraday’s Laws of Electromagnetic Induction 1.Whenever there is a change in the magnetic flux linked with a coil a current is induced in it and the induced current exists as long as the change in flux exists. 2. The strength of the induced current is proportional to the rate of change of flux. I α dФ/dt

Direction of induced current The direction of induced current is so as to oppose the change in flux. I = - dФ/dt The direction is given by Lenz’s Law For a coil of N turns induced current is I = - N dФ/dt

Lenz’s Law and conservation of energy Whenever the magnet is moved wrt coil work is done against either a force of attraction or repulsion. This work done is converted to Electrical energy. When the N pole of magnet is moved towards coil induced current is anticlockwise.

METHODS OF PRODUCING INDUCED CURRENT Induced current is produced whenever the magnetic flux linked with a coil changes.The change in flux can be brought about in three ways. The magnetic flux linked with a coil is Ф = B A sinθ. Flux changes when (i ) B changes ( ii ) A changes ( iii ) θ changes. (θ is the angle between B and the outward drawn normal to the plane of the coil). When there is relative motion between coil and magnet B changes.

Induced e.m.f by changing area of coil linked with a magnetic field PQRS is a coil in a uniform magnetic field B. Side PQ is movable. Let PQ move with a velocity V through a small distance x. dA/dt = l dx/dt = -lv The e.m.f induced in the coil is E = - dФ/dt = - B dA/dt = Blv If the resistance of the coil is R then induced current is I = Blv/R.