Bouncing beads outwit Feynman: a machine based on a thought experiment performs work.
Researchers have built a machine that harnesses energy from the random motion of bouncing beads. The device, modified from a system dreamt up nearly a century ago, dances around physicist Richard Feynman's dictum that work can't be extracted from such a system.
In 1912, Polish physicist Marian Smoluchowski proposed a thought experiment in which tiny moving particles spin a windmill-type paddle, which then spins a toothed wheel. A pawl prevents the wheel from slipping backward, forcing the wheel to move in one direction only. But Feynman later pointed out, in his famous lectures on physics, that the original calculations missed something. If everything in the system was the same temperature, the pawl would occasionally slip off the wheel, resulting in no net movement, he showed.
By skirting some of the original rules, the new machine, described in a paper to appear in Physical Review Letters, keeps the wheel spinning in one direction. "It's an amusing play on a classical problem," says physicist Bob Behringer of Duke University in Durham, N.C. "By changing an assumption you can actually make this work."
In the new study, Devaraj van der Meer of the University of Twente in the Netherlands and colleagues designed a vigorously shaking platform that sends glass beads flying up like popcorn dancing off a popper. The beads smash into windmill like vanes, which start turning a rod, which rotates a sensor. If this spinning is directional, it can be put to good use.
If the paddles had the same kind of surface on each side, there was no net rotation--the machine swung back and forth evenly. After the scientists coated one side of each paddle with duct tape, though, the beads lost more energy when they hit the softer taped side of the vanes, causing the system to rotate in one direction.
As the vanes turned, they created a roiling pattern in the beads. This reciprocal give-and-take--beads moving the vanes and vanes moving the beads- could also happen for tiny molecular ratchets, such as those in the body, van der Meer says.
The machine requires energy for shaking and loses most of that energy through heat and sound. "It's an extremely inefficient device," van der Meer says. "In terms of the second law of thermodynamics, there's no problem whatsoever."
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|Title Annotation:||Matter & Energy|
|Date:||Jul 3, 2010|
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