An inductor can be made physically using a coil of wire, and it
stores magnetic flux when a current flows through it. Figure C.2
shows a circuit in which a resistor is in series with the parallel
combination of a capacitor and inductor .
The defining equation of an inductor is
where denotes the inductor's stored magnetic flux at time
, is the inductance in Henrys, and is the
current through the inductor coil in Coulombs. Differentiating with
respect to time gives
is the voltage across the inductor in
volts. Again, the current is taken to be positive when flowing
from plus to minus through the inductor.