Alternating Current

The circuits that you have been learning about so far have been direct current circuits. These circuits are powered by batteries, solar cells, and other dc (direct current) power supplies. Dc circuits are characterized by electron flow from the negative terminal through the circuit to the positive terminal. In other words, the electrons move in only one direction.

Alternating current, or ac, is quite different. The electron flow is constantly reversed from one direction to the other, many times a second. Ac is the type of current that is supplied by your local power company.

Ac is commonly produced by devices called ac generators. Alternators produce a current that builds up to a peak flow in one direction, then decays to a point where no current flows at all, then builds up to a peak flow in the other direction.

Figure 2.20

Figure 2.20 provides an illustration of alternating current.

The curve in the figure above is a sine wave, and shows that the amount of current varies from one instant in time to the next.

Since the current varies, the voltage (pressure) required to produce that current must also be constantly changing, as well as the power being consumed by the resistances.

The ac voltage, as depicted in Figure 2.20 is measured from Point A to Point B, typically referred to as "peak-to-peak." The peak-to-peak difference is twice the zero-to-peak value.

Fortunately, the standard ac watt is compensated. That means that if you plug a 100-watt light bulb into a 117-volt rms ac circuit, you will get exactly the same amount of light as you would in a 117-volt dc circuit.