A capacitor can be made physically using two parallel conducting
plates which are held close together (but not touching). Electric
charge can be stored in a capacitor by applying a voltage across the
plates.

The defining equation of a capacitor is

(C.2)

where denotes the capacitor's charge in Coulombs,
is the capacitance in Farads, and is the
voltage drop across the capacitor in volts. Differentiating with
respect to time gives

where
is now the current in
Amperes. Note that, by convention, the current is taken to be
positive when flowing from plus to minus across the capacitor (see the
arrow in Fig.C.1 which indicates the direction of current
flow--there is only one current flowing clockwise around the
loop formed by the voltage source, resistor, and capacitor when an
external voltage is applied).

Assuming a zero initial voltage across the capacitor at time 0, we have

We call this the driving-point impedance of the capacitor. The
driving-point impedance facilitates steady state analysis (zero
initial conditions) by allowing the capacitor to be analyzed like a
simple resistor, with value Ohms.