Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Faraday's Law of Electromagnetic Induction

Introduction

A time varying magnetic flux through a coil generates electromagnetic force (emf) in it. The magnetic flux through a coil can be varied by moving a magnet relative to the magnet. The magnitude of generated emf is proportional to the time rate of change of magnetic flux. The direction of induced emf is such that it opposes the reason behind change in flux (Lenz's law).

Apparatus

Galvanometer, coil, magnet

Procedure

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Connect the ends of a coil to a galvanometer. Since there is no current in the coil, the needle of the galvanometer will be at the zero mark. Now, bring a bar or cylindrical magnet towards the coil. The galvanometer needle will get deflected in a particular direction. When the magnet comes to rest, the needle comes back to its zero position, showing that the current in the coil has stopped. Now, move the magnet away from the coil. The needle will get deflected again, but this time, in the opposite direction. This shows that the direction of the current has reversed. You will also find that the direction of the current depends on the pole of the magnet facing the coil while the magnet is moving.

What happens if we move the coil instead of the magnet? Fix the coil to a wooden block, and move it towards the magnet. You will find that as long as the coil moves, there is a current in the coil, as indicated by the deflection of the needle of the galvanometer. If you move the coil away from the magnet, the needle of the galvanometer gets deflected in the opposite direction. There is no deflection, i.e., no current, when both of them are at rest.