A coil with a changing flux has an induced emf. If it is connected in a circuit we generally call it an inductor, and the inductor can act like a battery much like a charged capacitor can. A capacitor acts like a battery with an emf of:

DV

=

Q C

=

I Dt C

When the flux in an inductor is changing because the current through the inductor changes, the inductor acts like a battery with an emf of:

e

=

– L

DI Dt

If the current is increasing the inductor tries to reduce the current and acts like a battery hooked up one way. If the current is decreasing the inductor tries to increase the current and acts like a battery hooked up the opposite way.

If the current is constant the inductor has no change to oppose and so it doesn't act like a battery at all. In that case it just acts as a resistor. Often the resistance of the inductor is negligible compared to that of other resistors in the circuit and we can treat the inductor just as a wire.