Mutual induction is the process by which a changing magnetic field in one coil induces an emf in an adjacent coil.
Mutual induction and
self-induction The amount of mutual induction depends on the rate of change of magnetic flux. The size of the induced emf can be increased by:
• coils being closer together • winding coils on a common soft iron core • increasing the number of turns on either or both coils.
Remember from Faraday’s electromagnetic induction formula that the number of turns amplifies the induced emf.
Mutual induction can be demonstrated by using and repeating the Faraday electromagnetic induction experiment shown earlier.
Self-induction occurs when a changing magnetic field in a coil induces an emf in the coil itself.
Another effect that can follow on from mutual induction is self-induction, which is a form of mutual induction in one coil. This occurs when the changing magnetic field in a coil induces an emf in itself, which opposes the changing current. In this case, this opposition emf is referred to as a back emf.
Self-induction can be seen in appliances all around you. For example, sometimes you see a flash (spark) when you turn off a hairdryer. This is the back emf opposing the current dropping off. The reducing current creates a magnetic field which induces a current that opposes it.
To demonstrate self-induction
1. Connect a bulb, solenoid, soft iron core and switch in series with a power supply as shown in Figs. 28.9a and b.
2. When the switch closes, current flows and magnetic field increases in the coil. 3. This induces an emf in the coil such as to oppose the change producing it (Faraday’s and Lenz’s laws). 4. This back emf delays the build-up of the current in the circuit but does not stop it. 5. The same applies when turning off the circuit. 6. All of this results in the bulb not lighting immediately but taking time to operate.
