Faraday’s Law of Electromagnetic Induction
The magnitude of any induced e.m.f. (e) is proportional to the rate of change of magnetic flux linking the coil or circuit. This can be expressed mathematically as:
In words, this equation says that the induced e.m.f. is equal to the rate at which magnetic flux is cut by the circuit or coil.
The direction of any induced e.m.f. will be so as to oppose the change that it is producing it. Hence the negative sign.