Operation of power generators is based on the phenomenon of electromagnetic induction
whenever a conductor moves relative to magnetic field, voltage is induced in the conductor. Particularly, if a coil is spinning in a magnetic field, then the two sides of the coil move in opposite directions, and the voltages induced in each side add. Numerically the instantaneous value of the resulting voltage (called electromotive force, emf) is equal to the minus of the rate of change of magnetic flux ? times the number of turns in the coil: V=?N??/?t. This relationship has been found experimentally and is referred to as Faraday's law. The minus sign here is due to Lenz law. It states that the direction of the emf is such that the magnetic field from the induced current opposes the change in the flux which produces this emf. Lenz law is connected to the conservation of energy.