Faraday's law of induction predicts how a magnetic field interacts with an electric circuit producing an electromotive force (EMF). This phenomenon of producing EMF is known as electromagnetic induction. Electromagnetic induction is a key phenomenon used these days in various electrical devices like inductors, electrical motors, transformers, solenoids and generators. For example, it is widely used in hall effect meters, induction cooktops, current clamps, induction sealing, etc.
Faraday states an EMF is produced, which is defined as the electromagnetic work done per unit charge when it travels along the loop of a conductor. This EMF varies with time.
Faraday's law of electromagnetic induction states that the electromotive force around a closed path is equal to the negative of the time rate of change of the magnetic flux enclosed by the path.
Faraday and Henry performed three experiments to prove electromagnetic induction. The three experiments are described below.
In experiment 1, Faraday connected a galvanometer and a coil. First, he inserted a bar magnet in the coil such that the north pole of the bar magnet is pointed towards the coil. Next, he moved the bar magnet inside-out so that the needle of the galvanometer gets deflected. This shifting of the needle in the galvanometer indicates the presence of current in the coil.
He also observed that the needle in the galvanometer does not move when the bar magnet is kept stationary. Instead, the needles deflect when the bar magnet is in motion. The motion of the needle depends upon the movement of the bar magnet. When the bar is moved inside the coil, the needle moves in one direction. When the bar magnet is taken out of the coil, the needle deflects in the opposite direction. Also, if the north and south positions are interchanged inside the bar magnet, the needle deflects in different cases.
The speed of the deflection of the needle also changes according to the speed at which the bar magnet is moved inside and outside the coil. All these effects are observed when the bar is kept stationary and the coil is allowed to slide over the bar. This shows that this experiment is related to each other. In conclusion, only the motion between the bar magnet and the coil is responsible for producing current in the coil and deflecting the needle of the galvanometer.
In experiment 2, Faraday and Henry replace the normal bar magnet with a current-carrying coil over the bar magnet. First, they connected the bar magnet with a battery to induce a current in the bar magnet. Then, they, later on, inserted the bar magnet inside the coil. After the insertion of a bar magnet in the coil, a magnetic field is induced in the circuit, which is analogous to experiment 1.
We will denote the bar magnet coil as the primary coil and the coil in which the bar magnet is inserted as the secondary coil. As we move the primary coil towards the secondary coil, the needle in the galvanometer moves due to the electric current in the secondary coil. Like experiment 1, here too, the speed of needle deflection depends upon the movement of the bar magnet inside the coil.
The direction of needle deflection also depends on the bar's motion, whether it is taken out or inserted inside the coil. Hence, this experiment proves that the current-carrying magnet can also generate current due to induction.
From the above two experiments, Faraday concluded that the motion of the primary coil, i.e. the bar, was responsible for inducing the current in the secondary coil in which it was inserted. However, experiment 3 showed that relative motion between the bar magnet and the coil was not necessary to induce the current in the coil.
Faraday placed two stationary coils connected to one other. He connected both these coils with an external power source, i.e. the battery. He then connected the secondary coil to the galvanometer and switched on the power supply so that it currently started to pass through the primary coil. He then observed that the needle deflected.
The deflection of the needle indicated the presence of current in the secondary coil due to the presence of current in the primary coil. The deflection in the needle was temporary and constant. There was no deflection in the needle of the galvanometer when no current passed through the primary coil.
This experiment showed that the relative motion was not necessary between the magnet and the coil. Therefore, only current can produce current in the other coil due to induction.