A inductor is a device which induces a voltage when the current through it changes. This ability is called inductance and this is measured in henrys.

Consider the following circuit.

The voltage source is a signal generator which generates a direct current square wave with a 6 volts peak.
The voltmeters V_{1} and V_{2} are in fact channels 1 and 2 of an oscilloscope.
Channel 1 measures the output of the signal generator, and
channel 2 measures the voltage across the inductor.
The difference between channels 1 and 2 will be the voltage across the resistor.
The oscilloscope's screen is shown below where the blue signal is channel 1, yellow is channel 2, and
red is the difference between channel 1 and 2.

When the blue trace moves from 0 to 6 volts the rate of change of current is controlled by the inductor (L) and the resistor (R). The voltage across the inductor can be calculated with

V_{L}= Ve^{-Rt/L}

In the case of the circuit given earlier and whose measurements are shown on the oscilloscope, L = 0.033, R = 10000, and V = 6.

In the other direction, when the blue trace moves from 6 to 0 volts, then again the rate of change of current is controlled by L and R. In this case, the voltage across the inductor is given by

V_{L}= -Ve^{-Rt/L}

The yellow trace clearly shows us that the voltage across the inductor does not immediately follow the signal
generator.
When the signal changes from 0 to 6 volts, the magnitude of
the opposing, induced, voltage is initially high but then it decreases as the current stabilises.
When this induced voltage drops to 0 volts, the current reaches its peak and the the voltage across the
inductor is 0 volts and all the voltage is across the resistor.
When the signal changes from 6 to 0 volts, the inductor *discharges* and the voltage across the
inductor slowly drops to 0 volts.

Energy is used to create the current through the inductor and this energy is stored in, and released from, the magnetic field. The amount of energy (U) held in the magnetic field at steady current I, is given by the following (i and v are the instantaneous current and voltage respectively):

v = -L di/dt vi = -Li di/dt (P = iv, and P is Js^{-1}) |U| = ∫_{0}^{t}Li di/dt di = L∫_{0}^{I}i di = 1/2×LI^{2}

Fischer-Cripps. A.C., *The Electronics Companion.* Institute of Physics, 2005.

Copyright © 2014 Barry Watson. All rights reserved.