Why does my cell phone screech when it is near my computer?

David Grier, chair of the physics department at New York University, dials up some possible answers to this mystery:

It sounds like a case of electromagnetic interference, or EMI: radio waves emitted by one device causing undesirable behavior in another. Virtually every piece of electrically powered equipment acts as a radio transmitter, whether it is supposed to or not; the changing electric currents running through these devices naturally radiate electromagnetic waves. This radiation is an inevitable by-product of harnessing electricity to do useful things, analogous to the clanking and clattering of traditional mechanical devices. Computers are particularly “noisy” because they rely on rapidly changing currents to act as clock signals that coordinate their calculations.

One possible explanation is that your computer unintentionally emits radio waves in the range of frequencies reserved for cell phone communications, typically around 800 megahertz (millions of cycles per second). If the signal coming from your computer were strong enough, your phone could mistake it for a cell phone transmission—albeit an indecipherable one.

Another possibility involves a deeper connection between your two devices. Just as changing currents generate radio waves, radio waves induce electric currents in conducting materials—which is how a metallic antenna allows a radio to detect signals transmitted by radio stations. The radio waves emitted by your computer may induce currents in the amplifier that drives your cell phone's speaker, which would cause it to produce random squeaks and squawks. (In 1975 computer pioneer Steve Dompier cleverly commandeered this effect, with more tuneful results: he programmed his PC, a MITS Altair 8800, so that its EMI would play the Beatles' “The Fool on the Hill”

There is no way to stop electrical devices from generating radio waves, but keeping spurious waves under wraps will curb EMI. Most electronic devices are housed in cases—either made of metal or coated with a conductor—that trap these electromagnetic waves, but holes in the cases and thin spots in the coating allow some waves to escape. Usually the leakage is so small that it just affects objects very near the source, which is why your cell phone only acts up right next to your computer.

How does the weight of CO2 released in combustion exceed the weight of the fuel burned? And by how much? —B. Easley, Jackson, Miss.

Susan Trumbore, chair of the earth system science department at the University of California, Irvine, replies:

Carbon fuels generally exist in reduced form—that is, the carbon atoms are attached mostly to hydrogen atoms. During combustion, the carbon becomes oxidized (combined with oxygen from the air) to make carbon dioxide (CO2). Because oxygen is far heavier than hydrogen, the product is heavier than what is burned.

Take gasoline, for example. One of its primary components, octane, is a molecule made up of eight carbon atoms and 18 hydrogen atoms. The weight of one mole (6.02 10<sup>23</sup> units) of octane molecules is equal to the weights of eight carbon atoms (at 12 grams per mole each) plus 18 hydrogen atoms (at one gram per mole each). Octane therefore weighs 114 (8 12 + 1 18) grams per mole.

The weight of CO2 is 44 grams per mole (1 12 grams per mole for the carbon and 2 16 grams per mole for the oxygen). If all the octane combusts to carbon dioxide, each of its eight carbon atoms becomes part of a CO2 molecule, yielding eight CO2 molecules per octane molecule burned—or eight moles of CO2 per mole of octane burned. Combusting one mole of octane, therefore, would produce 352 (8 44) grams of CO2.

Thus, the weight ratio of CO2 produced to octane burned is 352 to 114, or roughly 3 to 1. Actual ratios will vary, however, because gasoline is not purely octane.

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