Imagine a bar magnet placed along the axis of a conducting loop containing a galvanometer. There is no current in the loop and correspondingly no deflection in the galvanometer. If we move the magnet towards the loop, there is a deflection in the galvanometer showing that there is an electric current in the loop. If the magnet is moved away from the loop, again there is a current, but the current is in the opposite direction. The current exists as long as the magnet is moving. Michael Faraday studied this behavior in detail by performing a number of experiments and discovered the following law of nature:
‘Whenever the flux of a magnetic field through the area bounded by a closed conducting loop changes, an EMF is produced in the loop. ‘
The law described here by Faraday is called The Law of Electromagnetic Induction. The flux may be changed in a number of ways. One can change the magnitude of the magnetic field at the site of the loop, the area of the loop or the angle between the area-vector and the magnetic field. In any case, as long as the flux keeps changing, the EMF is present. The EMF, so produced, drives an electric current through the loop. The EMF developed by a changing flux is called induced EMF and the current produced by this EMF is called induced current.
One of the ways to find the direction of the induced current in the loop is to use Lenz’s Law. The current is induced by the changing magnetic flux. The induced current itself produces a magnetic field and hence a magnetic flux. This magnetic flux may have the opposite sign. It strengthens the original flux if it has the same sign and weakens it otherwise. Lenz’s Law states:
‘The direction of the induced current is such that it opposes the change that has induced it. ‘
If a current is induced by an increasing flux, it will weaken the original flux. If a current is induced by a decreasing flux, it will strengthen the original flux.
Let us imagine a situation where a magnet is brought towards a circular loop. The North Pole should face the loop. As the magnet gets closer the loop the magnetic field increases and hence, the flux of the magnetic field through the area of the loop increases. The induced current should weaken the flux. The original field is away from the magnet, so the induced field should be towards the magnet. Using the right hand thumb rule, we can find the direction of the current that produces a field towards the magnet.
When a current is established in a closed conducting loop, it produces a magnetic field. This magnetic field has its flux through the area bounded by the loop. If the current changes with time, the flux through the loop changes and hence an EMF is induced in the loop. This process is called self-induction. The name is so chosen because the EMF is induced in the loop by changing the current in the same loop.