An inductor is an electronic component that stores energy in a magnetic field inside a coil of wire. A current-carrying coil of wire induces a magnetic field according to Ampère's circuital law. The greater the current I, the greater the energy stored in the magnetic field and the lower the inductance which is defined L = Φ Φ B / I where Φ Φ B is the magnetic flux produced by the coil of wire. The inductance is a measure of the circuit's resistance to a change in current and so inductors with high inductances can also be used to oppose alternating current.