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Why is electricity transferred at high voltages when we use 120 volts?

There's a really good reason not to make voltages low until the point where they're about to be used, and to keep them as high as possible as long as possible.

If voltage is the electrical equivalent of water pressure, then current is the equivalent of how much water (say, how many cubic meters per second) flows down a pipe. Power, which is a measure of the rate at which you can get things done, is the product of voltage and current. That is, you take the voltage in "volts," multiply it by the current in "amps," and get "watts" - the unit used to measure how much power a lightbulb uses.

If you double the voltage, you can halve the current and get the same power. Similarly, if you multiply the voltage by 1,000, you can divide the current by 1,000 and get the same power. All this sounds like lots of choices, and there are, but the main push for lower current (and thus higher voltage) is because you have to carry electricity from place to place along wires.

Wires all have some resistance to the flow of electricity, and the more current that flows along a wire, the more energy is lost in the form of heat. If you have wire with a high resistance, you can use this phenomenon to good effect in a toaster. Usually though, it's a nuisance. To make matters worse, the rate at which energy is lost to heat in a wire goes as the current squared.

Let's see what this means in concrete terms: Suppose you want to send one amp at 120,000 volts. That's 120,000 watts, or enough to light up 2,000 bulbs each taking 60 watts. If you decide that 120,000 volts is a nuisance, since it has to be taken down to 120 volts for a house, that's fine, but it means that at 1,000 times less voltage you need 1,000 times more current, and that means 1 million times more energy lost as heat!

The 120 volts in a house is a compromise since higher voltages are more dangerous. If you wanted to run 12 volts around the house, you'd need 10 times as much current as you do now and would lose 100 times the energy in the wiring of your home.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

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