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.
