Alternating Current (AC)

In this lesson we will define Alternating Current (AC), explain AC timing parameters, explain the meaning of root mean square.

The Basics

DC Current
AC Current

Unlike a direct current (DC), which flows in only one direction, alternating current (AC) flows one way, then the other way, constantly changing in magnitude and reversing direction.

As a result, an AC voltage is also continually changing (alternating) between positive (+) and negative (-) polarities, and varying in amplitude with time.

When plotted over time, an AC signal takes the shape of a sine wave, crossing the zero line every time the direction of current flow is changed.

An AC signal does not always take the shape of a sine wave.
For example, an AC waveform may be in the shape of a triangle or square wave.

Period and Frequency

A cycle is a single repetition of "back and forth" alternating current flow.

The time it takes for one complete cycle of the AC signal is called the period.

The unit of measurement for the period is seconds (s).

The frequency of alternating current signal is the number of cycles in a single second.

Frequency is measured in hertz (Hz).

In North America, the power line frequency is 60 cycles per second, or 60 Hz.

In the rest of the world 50 Hz is more common.

And if we know the frequency we can go back and calculate the period.

For example, using 60 Hz...

Amplitude and Peak

Amplitude is a measurement of the intensity of an AC signal and is used to determine the voltage of the signal.

The amplitude is determined by the peak height of the waveform.

This is known as the peak or crest value of an AC waveform and is usually shown as (Vpk).

Another way to measure amplitude is to measure the total height between opposite peaks.

This is known as the peak-to-peak value of an AC waveform and is usually shown as (Vp-p).

Root Mean Square

In practice, both peak (or crest), and peak-to-peak forms of amplitude measurement are rarely used.

Almost exclusively, AC voltage is expressed in Root Mean Square (RMS) values or (Vrms).

Vrms tells us what the equivalent direct current voltage would be to our existing alternating current current signal.

The power or the energy of a signal can be represented by the plotted area.

On the graph you can see a representation of AC Vrms (the DC equivalent).

But what is the relationship between Vpk and Vrms voltage?

This calculation will apply only to the most popular sine wave AC power signal.
RMS value for other waveforms will be different and vary depending on the form of that signal.
10 Vrms
5 Arms
2 Ω
50 W
10 VDC
2 Ω
50 W

These two lamps offer the same resistance (2 Ω), dissipate the same amount of power as heat (50 W), and give off the same amount of light.

One lamp is powered by AC and the other by DC.

Because the AC voltage source is equivalent to a 10 volt DC battery, we would call this a "10 volt" RMS AC source.


If the Vpk of a waveform is 55V what would the Vp-p be?

What is the frequency of the AC signal shown?

What does this graph represent?


  • Alternating Current reverses it's direction of flow
  • Frequency, measured in hertz, is the number of cycles in a single second
  • Amplitude determines the voltage of an AC signal.

We hope you enjoyed Alternating Current (AC)

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