Direct current (DC) is a fixed amount of current per unit
of time, whereas the amount of an alternating current
(AC) alternates with time. The sine wave (or sinusoidal
wave) is the fundamental type of alternating current
and alternating voltage. Figure 3 is a graph showing
the general shape of a sine wave, which can be either
an alternating current or an alternating voltage. In
fact, when a sinusoidal voltage source is applied to a
resistive circuit, it will result in an alternating sinusoidal
current.
The current (or voltage) varies with time, starting at
zero, increases to a positive maximum, returns to zero,
and then increases to a negative maximum before
again returning to zero, thus completing a full cycle.
The time (in seconds) required for a given sine wave to
complete one full cycle is called the period (T).Frequency is the number of cycles that a sine wave
completes in 1 s. The more cycles completed in 1 s, the
higher the frequency. Frequency is symbolized by f and
is measured in units of hertz (Hz).