Perpetual motion machines*
(the latin term perpetuum mobile is not uncommon) are a
class of hypothetical machines which would produce useful energy
"from nowhere." The existence of a perpetual motion machine is
generally accepted as being impossible according to current known laws of physics.
In particular, perpetual motion machines would violate either the first or
second laws of thermodynamics. Perpetual motion machines are divided into
two subcategories, referred to as perpetual motion of the first kind and
perpetual motion of the second kind. There is a chance
that the accepted laws
of physics are wrong, but a lot of evidence
is needed to regeneralize these.
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
energy, and vacuum
Many say that perpetual
motion machines violate one or both of the following two laws of physics:
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,
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
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
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 and would be
viewed with great skepticism.
*licensed under the GNU
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material from the Wikipedia article "Perpetual