Perpetual motion refers to any device or system that perpetually (indefinitely) produces more energy than it consumes, resulting in a net output of energy for indefinite time.

Perpetual motion refers to any device or system that perpetually (indefinitely) produces more energy than it consumes, resulting in a net output of energy for indefinite time. The law of conservation of energy, which states that energy cannot be created or destroyed, implies that such a perpetual motion machine cannot exist. The most commonly contemplated type of perpetual motion machine is a mechanical system which (supposedly) sustains motion indefinitely, despite losing energy to friction and air resistance. A second type of impossible "perpetual motion machine" is one which does not violate conservation of energy, but produces work by spontaneously extracting heat from its surroundings, thereby cooling them down, and converting the heat energy into mechanical work. Such machines are forbidden by the second law of thermodynamics.

A perpetual motion machine of the first kind produces energy from nothing, giving the user unlimited 'free' energy. It thus violates the law of conservation of energy.

A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However it does violate the more subtle second law of thermodynamics (see also entropy). Such a machine is different from real heat engines (such as car engines), which always involve a transfer of heat from a hotter reservoir to a colder one, the latter being warmed up in the process. The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, as stated by the second law of thermodynamics. In contrast, a hot reservoir inside an internal combustion engine is created by a spark igniting fumes which contain stores of chemical potential energy. The temperature of the fumes increases above that of the surroundings. This is not a perpetual motion machine since the increase in temperature is a result of the release of a finite available amount of chemical energy, which is always much less than the total heat energy and mass-energy contained within the system. As explained by statistical mechanics, there are far more states in which heat distribution is close to thermodynamic equilibrium than states in which heat is concentrated in small regions, so temperatures will tend to even out over time, reducing the amount of free energy available for conversion to mechanical energy.

A great number of designs have been published that appear to allow work produced during the operation of a device to both maintain the motion of the device and provide excess work outside the system. Examples include springs, overbalancing wheels, and magnetic and electrical machines. These are considered impossible because the first law of thermodynamics states that the energy that is consumed by the device to cause its motion, is the maximum that could be delivered by that motion. Practically, because of frictional and other heat losses, the available energy would be less, causing the machine to eventually come to a halt.

Another kind of perpetual motion violates the second law of thermodynamics, which requires useful work always to be accompanied by an increase in the entropy of the Universe. Engines that are driven by the spontaneous evaporation of condensed vapors such as liquid ammonia are such devices, because the energy used to condense the vapor before the next cycle is greater than that obtained by harnessing its evaporation.

Sources

• Hibbert, Brynn (Lead Author); Cutler J. Cleveland (Topic Editor). 2007. "Perpetual motion." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth September 2, 2006; Last revised January 30, 2007; Retrieved March 12, 2010]. <http://www.eoearth.org/article/Perpetual_motion>
• Wikipedia contributors, Perpetual motion, Wikipedia, Accessed 12 March 2010.