March 28, 2008

Perpetual Motion

By John Ruch
© 2003

Q: Why don’t perpetual motion machines work, and why do people still believe in them?

A: The simple answer is that perpetual motion machines are impossible because they violate the first and second laws of thermodynamics.

This isn’t just dogma. The thermodynamic laws were largely written by the failure of perpetual motion attempts.

What alchemy was to chemistry, what astrology was to astronomy, perpetual motion was to mechanics. (A perpetual motion puzzle also led to the engineering principle of virtual work.)

Almost all perpetual motion machines have existed only as thought experiments on paper. Those actually built and tested have failed or have been exposed as frauds.

A perpetual motion machine is essentially a machine that will run under its own power, and only its own power, forever. More typically, it is also proposed to output enough excess energy to do useful work.

A classic early model was a pump or screw that would lift water, which would then power a turbine that operated the pump/screw.

Another, and the one most frequently built, was an overbalanced wheel with weights hanging off it on jointed arms, or with chambers inside containing small weights. The (wrong) idea is that the wheel, given a push, will spin forever as the weights fall on one side and rise on the other.

There are endless other varieties based on nearly every known physics principle. Generally, thermodynamics aside, they are based on misunderstandings and miscalculations of the machine system’s forces.

“Free energy” machines supposedly driven by an unknown force of nature are the modern euphemism for perpetual motion.

The first law of thermodynamics notes that energy cannot be created or destroyed, merely transferred. Thus, a machine’s energy output cannot exceed its input.

The second law notes that friction makes perfect mechanical efficiency impossible, so the energy output cannot even equal the input. And the waste heat given off by a machine cannot be fed back into it to do work without the addition of outside energy to the system.

Thus, a machine can’t run itself by itself, produce more energy than goes into it, and/or run forever.

Many “free energy” scammers claim their machines will run “until the parts wear out.” Of course, parts wear out due to friction and other heat-wasting forces, which are part of what makes perpetual motion impossible.

Perpetual motion today is largely a scam that exploits existing beliefs such as anti-government paranoia or environmentalism—and the hunger for a free lunch.

The idea first took root in early medieval Europe, where water-driven mills were economically important, then bloomed in the mechanically-inclined Renaissance. Leonardo da Vinci dabbled in it before concluding it was “among the excessive and impossible beliefs of man.”

On a cultural level, authors of that era made analogies to the alchemical idea of creating a self-perpetuating miniature world (a “microcosm”), or the legends of “perpetual lamps” supposedly found burning in ancient tombs.

The idea became a widespread mania in the mid-1800s, perhaps a natural response to the industrial revolution. It fed on the later discovery of radioactivity, which to some looked like “free energy.”

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