Before the beginning of the 19th century, people used to believe that electricity and magnetism are two different phenomena until the observation of the fact that moving electric charge generates magnetic field. This observation was made by many scientists like Ampere, Oersted arrived at the conclusion that electricity and magnetism are somehow inter-related. The deflection of magnetic needle by a current which is placed at close proximity is one of those observation. Now the question which can arrive here that the reverse effect is possible or not. The answer is YES. The experiment related to this is was done by Michael Faraday and Joseph Henry which demonstrated the fact that electric currents were induced in closed coils when subjected to changing magnetic fields. Electro-magnetic induction is of great importance in day-to-day life.
Modern electrical instruments like generators and transformers are made up using the principles of electro-magnetic induction. The experiment made by Faraday was in three parts. In the 1st part of the experiment a coil C1 of wire connected to a galvanometer and a bar magnet with its N pole facing the coil is moved towards the coil C1. The deflection of galvanometer is observed as long as the bar magnet was in motion.
The galvanometer shows a deflection in the opposite direction if the bar was moved away from the coil. In the 2nd part, another coil C2 connected to a voltage source is moved toward the coil C1 and a galvanometer deflection is observed. The direction deflection is reversed again if the coil C2 is moved away from C1.
The 3rd part of the experiment shows that the relative motion between the two coils C1 and C2 are not the absolute requirement. Two coils C1 and C2 held stationary. Coil C1 is connected to galvanometer G while the second coil C2 is connected to a battery through a tapping key K. A galvanometer deflection is observed when the tapping key is pressed. When the key is released, the directional reversibility is observed. From the above three experimental observations, Faraday concluded that an emf is induced in a coil when subjected to time varying magnetic flux through it.
The magnetic flux through an area of A placed in a uniform magnetic field B as shown in the figure as Based on the observations Faraday gave Faraday’s law of electromagnetic induction which states that the magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit. Here the negative sign is due to the fact that the induced emf opposes the very cause to which it is due. If the coil has n number of turns then, .