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        riddles >> hard >> Another Perpetual Motion Machine
        
(Message started by: Icarus on Jan 23^rd, 2008, 7:34pm)

Title: Another Perpetual Motion Machine
Post by Icarus on Jan 23^rd, 2008, 7:34pm

Inspired by a discussion in the Cavorite (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_medium;action=display;num=1200889166) thread:

Build a square of pipe of some insulative material. Cover three sides of the square with a conductor, leaving the 4th side exposed. The conductive material will act as a Faraday cage, protecting the 3 sides from any electromagnetic fields. Fill the pipe with an ionized gas and place a charged sphere of opposite polarity near one end of the exposed side.

The gas in the exposed section of pipe will be attracted by the sphere. The gas in the other three sides are shielded from the field, and so experience no force other than pressure. The force on the exposed side will create a differential in pressure along that side. The gas in the shielded pipe will move to equalize the pressure. The result is that the gas in the pipe will spontaneously circulate.

Obviously, this would violate conservation of energy (the circulating gas will create heat from friction as it moves through the pipe, but by the description above, it cannot stop circulating). So what is wrong with the description above? What really would happen with the gas?