Q. Why do we have AC electric power while Europe has DC?

A. Actually, the entire world uses AC, or alternating current, says Don Novotny, emeritus professor of electrical engineering at UW- Madison.

Europe, however, delivers its electricity differently: The frequency is 50 hertz versus our 60, and volts number 240 versus 120.

"So, indeed, people who go to Europe can't bring their appliances without some kind of converter," says Novotny.

He calls these differences a "historical accident." But there are good reasons why we mainly use AC over DC, or direct current, he says.

DC power - the type found in batteries - dominated in electricity's early days. Then a device known as the transformer came along.

Transformers can crank AC voltage up very high, which lets electricity travel long distances without losing much power. Today's biggest power lines operate at 500,000 volts or more, so before electricity reaches your home, transformers also take the voltage back to safe and useful levels.

In contrast, DC power, which gets transmitted at about the same voltage it's generated, loses much of its punch after just a mile or two.

But times are changing. Sophisticated power electronics for stepping DC voltage up and down are becoming less expensive. And when sending electricity a really long way, like between British Columbia and California, DC actually outperforms AC, Novotny says.

- Produced in cooperation with University Communications

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