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Alternating Current — AC

Tutorial Index

Alternating Current and AC Circuits

This begins our next section. We will use the same baby-step approach and assume the word AC is new.

A useful first picture

DC is like walking steadily in one direction. AC is like moving back and forth. The direction reverses again and again.

What is really happening?

With alternating current, voltage polarity and current direction vary with time. Utility power is generated as AC because rotating electrical machinery naturally produces alternating voltage, and transformers make AC voltage levels easy to change efficiently.

A little history

In the late 19th century, AC power systems became practical for transmitting electrical energy over useful distances because transformers could raise voltage for transmission and lower it again for use. The development involved many inventors and engineers, including Nikola Tesla, George Westinghouse, William Stanley, and others.

Worked idea

Watch the sine wave above. It rises positive, returns through zero, becomes negative, returns through zero again, and repeats. That complete pattern is one cycle.

Try these yourself

What makes alternating current different from steady DC? Its direction and polarity reverse periodically It has no voltage It cannot do work It always has zero current

AC changes direction and polarity with time.

A common household AC waveform is approximately: A straight line A sine wave A single pulse Always a square wave

Utility AC generators produce waveforms that are designed to be approximately sinusoidal.

When a sine wave crosses zero, is the circuit necessarily 'off'? Yes, permanently No; it is simply passing through zero on the way to the opposite polarity Only if resistance is zero Only in a battery

A sine wave passes through zero twice each cycle while continuing to alternate.

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