The phenomena of electromagnetic induction are described in two laws.

The first of these Faraday's law, states: "when the magnetic flux through a coil is made to vary, a voltage is set up. The magnitude of the induced voltage is proportional to the rate of change of flux".

The second law , known as Lenz's law concerns the direction of the induced voltage. " A change of flux threading a closed circuit induces a voltage and sets up a current ; the direction of this current is such that its magnetic field tends to oppose the change in flux"

The opposition to the change of flux through a circuit depends on the presence of current in the circuit; if the coil is open-circuited, the induced voltage cannot set up a current and there is no opposition.