Answer:
Faraday's law of induction
Step-by-step explanation:
Faraday found that a when a conductor is placed in a varying magnetic field, an electromotive force is induced in the conductor. If there is a closed loop, a current is also induced.
Induced emf is proportional to time rate change of magnetuc flux.

Motor and generator are based on this principle. Motor converts electrical energy to mechanical energy where as a generator converts mechanical energy to electrical energy.