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Direct and Alternating Current
There are two different ways that electricity is produced, and they are used in most cases for very different purposes. They can also be converted from one form to another, as discussed in this section.
The first and simpler type of electricity is called direct current, abbreviated "DC". This 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 somewhat 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.
The other type of electricity is called alternating current, or "AC". This is the electricity that you get from your house's wall and that you use to power most of your electrical appliances. Alternating current is harder to explain than direct current. The electricity is not provided as a single, constant voltage, but rather as a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats. A representation of an alternating current's voltage over time is shown in the diagram below.
While simple direct current circuits are generally described only by their voltage, alternating current circuits require more detail. First of all, if the voltage goes from a positive value to a negative value and back again, what do we say is the voltage? Is it zero, because it averages out to zero? That would seem to imply that there is no energy there at all. But imagine, if you will, a wave of water flowing across the surface of the sea. The peaks and troughs of the wave seem to "cancel each other out", but the wave clearly exists and has energy. The same is true of alternating current.
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.) Other parts of the world use different voltages ranging from 100VAC to 240VAC, and of course, heavy equipment anywhere can use much higher voltages.
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.
Why does standard electricity come only in the form of alternating current? There are a number of reasons, but one of the most important is that a characteristic of AC is that it is relatively easy to change voltages from one level to another using a transformer, while transformers do not work for DC. This capability allows the companies that generate and distribute electricity to do it in a more efficient manner, by transmitting it at high voltage for long lengths, which reduces energy loss due to the resistance in the transmission wires. Another reason is that it may be easier to mechanically generate alternating current electricity than direct current.
PCs use only direct current, which means that the alternating current provided by your utility must be converted to direct current before use. This is the primary function of your power supply.