Perpetual motion

From Academic Kids

See also Perpetuum mobile as a musical term and Perpetual Motion as a solitaire game.

Perpetual motion machines (the Latin term perpetuum mobile is not uncommon) are a class of hypothetical machines which would produce useful energy in a way which would violate the established laws of physics. It is generally accepted that perpetual motion machines cannot exist. In particular, perpetual motion machines would violate either the first or second laws of thermodynamics. Perpetual motion machines are divided into two subcategories defined by which law of thermodynamics would have to be broken in order for the device to be a true perpetual motion machine. These two subcategories are referred to as "perpetual motion of the first kind" and "perpetual motion of the second kind". Scientists and engineers accept the possibility that the current understanding of the laws of physics may be incomplete or incorrect; a perpetual motion device may not be impossible, but overwhelming evidence would be required to justify rewriting the laws of physics.

Any proposed perpetual motion design offers a potentially instructive challenge to physicists: we know it can't work (because of the laws of thermodynamics), so explain how it fails to work. The difficulty (and the value) of such an exercise depend on the subtlety of the proposal; the best ones tend to arise from physicists' own thought experiments.

Because the principles of thermodynamics are so well established, serious proposals for perpetual motion machines are met with disbelief on the part of physicists, which makes a discussion of the merits (if any) of the proposal difficult if not impossible.

Serious discussions of perpetual motion usually occur only when dealing with the topics of open systems, aether theories, free energy, and vacuum energy.



Perpetual motion machines violate one or both of the following two laws of physics: the first law of thermodynamics and the second law of thermodynamics. The first law of thermodynamics is essentially a statement of conservation of energy. The second law has several statements, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known is that disorder (entropy) always increases; another statement is that no engine can be more efficient than a Carnot heat engine. See the respective articles, and thermodynamics, for more information.

Machines which claim not to violate either of the two laws of thermodynamics but rather claim to generate energy from unconventional sources are sometimes referred to as perpetual motion machines, although they do not meet the standard criteria for the name. By way of example, it is quite possible to design a clock or other low-power machine to run on the differences in barometric pressure or temperature between night and day. Such a machine has a source of energy, albeit one from which it is quite impractical to produce power in quantity.

Kinds of violations

First kind

A perpetual motion machine of the first kind produces at least as much energy as it uses. Therefore, once started, the machine could go on forever, which appears to violate the law of conservation of energy. Many of these designs utilize magnets as some kind of free energy source, and assume frictionless surfaces. However, while several of these designs are not perpetual motion machines, they often can run on their own for a long time, as long as it's not required that they do any real work to the external environment.

Second kind

A perpetual motion machine of the second kind is one which converts heat completely into other forms of energy. Such a device would violate the second law of thermodynamics (see also entropy) and would be viewed with great skepticism.

Thought experiments

Serious work in theoretical physics often involves thought experiments that test the boundaries of understanding of physical laws. Some such thought experiments involve apparent perpetual motion machines, and insight may be had from understanding why they either don't work or don't violate the laws of physics. For example:

  • Maxwell's demon: a thought experiment which led to physicists considering the interaction between entropy and information
  • Feynman's "Brownian ratchet": a "perpetual motion" machine which extracts work from thermal fluctuations and appears to run forever but only runs as long as the environment is warmer than the ratchet
  • "Cosmic background space drive": where redshift/blueshift of the background radiation is used to drive a rocket's engine

Inventions and patents

Main article: History of perpetual motion machines

The invention of perpetual motion machines is a favourite pastime of many eccentrics, who often come up with elaborate machines in the style of Rube Goldberg or Heath Robinson. These designs may appear to work on paper at first glance, but have various flaws or obfuscated external power sources that render them useless in practice.

This sort of "invention" has become common enough that the United States Patent and Trademark Office (USPTO) has made an official policy of refusing to grant patents for perpetual motion machines without a working model. One reason for this concern is that a few "inventors" have waved a patent in front of potential investors, who may believe that said patent proves the machine works. The USPTO has granted a few patents for motors that are claimed to run without net energy input. These patents were issued because it was not obvious from the patent that a perpetual motion machine was being claimed. Some of these are:

Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines.

Perpetual motion in pop culture

In an episode of The Simpsons, Lisa builds a perpetual motion machine for her science fair; her father, Homer, yells at her after seeing the machine: "in this house we obey the rules of thermodynamics!".

See also

External links




de:Perpetuum Mobile es:Mvil perpetuo fi:Ikiliikkuja fr:Mouvement perptuel lt:Amžinasis variklis nl:Perpetuum mobile ja:永久機関 pl:Perpetuum mobile pt:Moto contnuo ru:Вечный двигатель sv:Evighetsmaskin he:תנועה נצחית zh:永动机


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