Physicists may try to test their knowledge of physics by proving, without using thermodynamics, that a proposed perpetual motion machine cannot work. Also, sometimes physicists will discover "apparent" perpetual motion in thought experiments. Such "paradoxes" expose misunderstandings of the meaning of accepted physical theories and are considered quite instructive.
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 of the proposal difficult if not impossible.
Serious discussions of perpetual motion usually occur only when dealing with the topics of open systemss, aether theories, free energy, and vacuum energy.
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2 Kinds of Violations 3 First Kind 4 Second Kind 5 Inventions and Patents 6 External links, resources, references |
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, perhaps the most well known is that entropy, or disorder, 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.
A perpetual motion machine of the first kind is one which produces power without energy uptake. Such a machine would, once started, operate indefinitely. This is forbidden by the law of conservation of energy.
Note that this explicitly prohibits the existence of devices which produce more energy than their input energy, as they can trivially be converted to a perpetual motion machine of the first kind by diverting part of their output energy back to their input.
Note that heat engines with an 'efficiency' greater than one do not violate this rule: the 'efficiency' in this case is defined as the ratio of heat output to work input -- the total energy input (heat + work) is still equal to the total energy output (heat).
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.
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 which 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.
The USPTO 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. These are:
An incomplete list of thought experiments posited by serious researchers:
Historic
Criteria
Kinds of Violations
First Kind
Second Kind
Inventions and Patents
Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines.
An incomplete list of proposed experiments:
External links, resources, references
Research
Patents