30.1: Induction

An emf is induced when the magnetic field in a coil is changed by pushing a bar magnet into or out of the coil. emfs of opposite signs are produced by motion in opposite directions, and the directions of emfs are also reversed by reversing poles. The same results are produced if the coil is moved rather than the magnet—it is the relative motion that is important. The faster the motion, the greater the emf. Additionally, there is no emf when the magnet is stationary relative to the coil.

A similar effect can be produced using two circuits, where the current in one circuit induces a current in a second, nearby circuit. For example, if a current-carrying circuit is moved toward or away from the other stationary circuit, then emf is induced in the other circuit. Additionally, if the current in the first circuit is controlled via a switch, then opening and closing of the switch induces an emf in the other circuit.

In all the above scenarios, the induced emf induces a current, called an induced current. The common factor in all these observations is changing magnetic flux. Here, the magnetic flux is changing, either because the magnetic field is time-dependent or because the motion of the circuit changes the magnetic flux passing through it. Induction occurs because of the non-static nature of the forces involved. Careful consideration must be given while analyzing static electric fields produced with charge distributions and non-static electric fields produced due to time-varying magnetic fields.

Electromagnetic induction is produced when an electrically conducting material is exposed to a time-varying magnetic field.

For example, consider a bar magnet placed close to a coil. The galvanometer connected to this coil shows zero current passing through it.

If the bar magnet moves toward the coil, a current flows through the coil. Furthermore, if the bar magnet is moved away from the coil, current flows in the opposite direction.

Similarly, if the bar magnet is kept fixed and the coil is moved in either direction, then current flows through the coil.

Suppose a bar magnet is replaced with a second coil connected to a battery. If one of the coils is moved, a current flows through the other coil. 

If a switch controls the current passing through the one coil, then an instantaneous current pulse is induced in the other coil while opening or closing the switch.

Here the current flowing through the other coil is called an induced current, and the corresponding emf is called an induced emf.