1.3 Capacitors

A capacitor may be thought of as a pair of parallel metal plates placed close together. Since no current can flow between the plates they simply store an electrical charge proportional to the voltage across them.

When the voltage across a capacitor changes there is a flow of charge (a current) in or out of the capacitor. We may write this mathematically as

i = C dv/dt
where C is the capacitance in Farads.

For fluctuating voltages and currents this expression may be simplified by substituting previous expressions v = Vejwt and i = Iejwt to give

V = I/(jwC)
Verify this for yourself. By analogy with Ohm's Law we see then that a capacitor behaves as a resistor with a value 1/(jwC). We give this type of generalized resistance the name Impedance. Impedance will be discussed later, so keep in mind that it involves the fluctuating currents and voltages.