We now consider circuits where the currents and voltages may vary with
time (V=V(t), I=I(t) (also Q=Q(t))).
These lectures will concentrate on the special case in which the
signals are periodic, with time average values of zero ( 
).
Circuits with these signals are referred to as alternating current (AC)
circuits.
In general signals will have both DC and AC properties ( 
).
We will concentrate only on the AC components and assume that the DC
properties can be treated separately using the methods of the previous
lectures.
The algebraic equations representing Kirchoff's laws for DC circuits will take the form of differential equations for AC circuits. So now is a good time to review your differential equations and complex number theory because we will use it.