### Alternating Current (AC) varies sinusodialy
with time.

The voltage produced by a coil of wire which rotates in a magnetic
field varies sinusodially with time,

v = v(t) = V_{o} sin t
where
is the angular frequency of the rotation. All the other parameters or
characteristics of the circuit will also vary with time.

By *convention* or *custom*, we write
these variables as lower case letters to remind us that they are
functions of time. That is, we will write the voltage as v to
indicate v(t) and we will write the current as i to indicate i(t).
When we say the voltage varies sinusodially with time, we immediately
think of writing it as a sine function,

v = v(t) = V_{o} sin t
But, with a different choice of when we decide to set t = 0, this
same variation could be written as a cosine function,

v = v(t) = V_{o} cos t
There is no real difference in these two forms

Or we might have some arbitray choice of when we set t = 0 so that
the sinusodially varying voltage can be written as

v = v(t) = V_{o} sin (t
+ ) .
is
called the "phase angle" and is determined by when we decide to set t
= 0.

All the quantities involved with an **Alternating
Current** **(AC)** circuit vary with time; they are functions
of time. We usually indicate this -- and remind ourselves -- by
writing these time-varying quantities with lower case letters like
v or i. The value of a resistor does not change with time -- even
for an AC circuit -- so we write that with a capital R. When we
talk of power we usually mean the average value -- the rms value
-- so that,too, is written with a capital P
Quantities involved with a **Direct Current
(DC)** circuit have values that are constant over time.
Quantities for a DC circuit are usually written as capital letters
like V, I, R, and P.

(c) Doug Davis, 2002; all rights reserved