Friday, March 26, 2010

Electricity: Direct Current (DC) & Alternating Current (AC)

Direct current (DC), illustrated above, is the type of electricity that is produced by batteries, static, and lightning. A voltage is created, and possibly stored, until a circuit is completed. When it is, the current flows directly, in one direction. In the circuit, the current flows at a specific, constant voltage (this is oversimplified but good enough for our needs.) When you use a flashlight, pocket radio, portable CD player or virtually any other type of portable or battery-powered device, you are using direct current. Most DC circuits are relatively low in voltage; for example, your car's battery is approximately 12 V, and that's about as high a DC voltage as most people ever use.

Alternating current (AC), illustrated below, is not provided as a single, constant voltage, but goes from a positive value to a negative value and back again. The way the science world measures the energy in an AC signal is to compute what is called the root mean square (RMS) average of the voltage. In simple terms, the RMS value of an electrical current is the number which represents the same energy that a DC current at that voltage would produce; it is in essence an average of the alternating current waveform.

Whenever you see an AC voltage specification, they are giving you the RMS number unless they say otherwise specifically. So for example, in North America, most homes have 115 VAC electricity. This is AC electricity equivalent in energy to a 115 V DC circuit. (This is an approximate number, and standard household electricity in North America is also sometimes called 110VAC or 120VAC; it's the same thing.)

The other key characteristic of AC is its frequency, measured in cycles per second (cps) or, more commonly, Hertz (Hz). This number describes how many times in a second the voltage alternates from positive to negative and back again, completing one cycle. In North America, the standard is 60 Hz, meaning 60 cycles from positive to negative and back again in one second. In other parts of the world the standard is 50 Hz.


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