AC and DC

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Summary

Appliances often have markings such as "240V AC" or "12V DC". This page explains the difference between AC and DC and why it matters.

Main section

An electric current consists of a flow of electrical charge around a closed path or "circuit" and so has a definite direction.

A battery or a solar cell creates a positive charge at one terminal and a negative at the other, amounting to a certain voltage (electrical pressure). This is more or less constant until the battery runs down (or the sun goes in) and is called Direct Current, or simply DC, since the direction is unchanging.

DC is what nearly all electronic equipment needs inside; whether you're trying to play a CD or browse the Internet on your computer, the electronics needs a constant and unvarying source of power in order to do its job. Imagine trying to drive a car if the engine kept cutting out every few yards!

By contrast, mains electricity reverses direction 100 times per second, that's to say it completes 50 forward/backwards cycles every second, and so is called Alternating Current, or AC, since the direction alternates.

So why would AC be used for power distribution, when many application will require the additional complication and expense of converting it to DC before it can be used? In fact, the earliest power distribution systems used DC. In fact, AC has several advantages.

The main advantage of AC is that it can be very easily converted from a high to a low voltage, or vice versa, using a transformer. Transformers don't work on DC. A high voltage (or electrical pressure) is very good for pushing electricity over large distances over which the resistance to the flow would mount up, but would be extremely dangerous if fed directly into the home.

For many purposes such as heating, and historically, for old fashioned filament lamps, the direction of flow is immaterial, so AC is just as good as DC. All but the simplest electric motors work just as well with AC as DC and some only work on AC, so when universal AC/DC motors were invented, the case for AC power transmission became compelling.

DC power is still used in a few specialized cases. For example, cross-border power sharing often uses DC as it would be impracticable to synchronise the cycles of AC between two independent power grids in different countries. For a similar reason, it is used in wind farms, since it would be almost impossible to synchronise all the individual wind turbines.

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