Back EMF

Key Stage 5

Back emf is the electromotive force (emf) induced in the spinning coil of an electric motor or in any coil where the current is changing, such as in the primary coil of a transformer. This back emf acts against the change of applied potential difference (pd).

Occurs due to Lenz's Law, which states that the direction of the induced emf is such that it opposes the change causing it.
In an electric motor, back emf increases with the speed of the motor.
It limits the current flowing through the motor as the speed increases.
In transformers, back emf is induced in the primary coil when the current in the secondary coil changes.
Back emf is crucial for the functioning of devices like motors and transformers by preventing excessive currents.
It is a key factor in the design and operation of AC and DC motors.

When an electric motor starts, the back emf is initially zero, allowing maximum current.
Back emf in transformers helps regulate the current and voltage during operation.