A perpetual motion machine is a hypothetical machine that can do work indefinitely, without an energy source.
The term is used loosely, in physics, in science fiction, in conspiracy theories, and in scams.
One can classify these purported perpetual motion machines by the particular law of thermodynamics the machine purports to violate:
The following outline is loosely based on the Perpetual motion machine article from Wikipedia, Perpetual motion (Wikipedia)
A perpetual motion machine of the first kind
Produces work without the input of energy?
Violates the first law of thermodynamics!
Here is one design that supposedly runs forever, and produces more energy than it consumes.
The “motionless electromagnetic generator”
Look at the numbers: putting out more power than is being put into it.
Sounds awesome, right? Wrong – That just the sign of an engineer who doesn’t understand thermodynamics. Anyone can do this – because power is not the same thing as energy!
Sure, this device can temporally produce more power – as can any other device. But when you let it run for a while, all the energy drains and then it shuts down.
II. A perpetual motion machine of the second kind
Spontaneously converts thermal energy into mechanical work.
Now, 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
The giveaway is that there is only one heat reservoir involved:
It is spontaneously cooled – without involving a transfer of heat to a cooler reservoir.
This conversion of heat into useful work, without any side effect,
violates the second law of thermodynamics.
III. A perpetual motion machine of the third kind
Would supposedly completely eliminate friction, to maintain motion forever.
(Third, here, refers solely to the position in this classification scheme, not the third law of thermodynamics.)
Although it is impossible to make such a machine – dissipation can never be 100% eliminated- it is nevertheless possible to get close to this ideal.
Such a machine could not serve as a source of energy, but could work as an energy storage device.
This “perpetual motion machine” seems to use gravity and magnets to make a metal ball move forever.
Can you figure out how it is being faked? There are subtle clues in the video.
Why don’t perpetual motion machines ever work? – Netta Schramm
“Perpetual motion machines — devices that can do work indefinitely without any external energy source — have captured many inventors’ imaginations because they could totally transform our relationship with energy. There’s just one problem: they don’t work. Why not? Netta Schramm describes the pitfalls of perpetual motion machines.” Animation by TED-Ed.
APS Resolution on Perpetual Motion Machines
The APS (American Physical Society) Executive Board approved a resolution at its June 2002 meeting in Annapolis, MD, affirming the fraudulent nature of claims of perpetual motion machines.
The resolution was deemed necessary because of a recent increase in patent applications for such devices. Robert Park, APS Director of Public Information and author of the weekly electronic newsletter “What’s New,” reported that the US Patent Office has received several patent applications for perpetual motion machines during the first six months of this year alone. [Park’s 2000 book, Voodoo Science, devoted considerable space to the phenomenon of such devices throughout history.] The text of the APS resolution follows.
The Executive Board of the American Physical Society is concerned that in this period of unprecedented scientific advance, misguided or fraudulent claims of perpetual motion machines and other sources of unlimited free energy are proliferating. Such devices directly violate the most fundamental laws of nature, laws that have guided the scientific progress that is transforming our world.
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields.
Clarification Statements: Examples of phenomena at the macroscopic scale could include evaporation and condensation, the conversion of kinetic energy to thermal energy,
HS-PS3-4a. Provide evidence that when two objects of different temperature are in thermal contact within a closed system, the transfer of thermal energy from higher temperature objects to lower-temperature objects results in thermal equilibrium, or a more uniform energy distribution among the objects and that temperature changes
necessary to achieve thermal equilibrium depend on the specific heat values of the two substances.
HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
Influence of Science, Engineering and Technology on Society and the Natural World: Modern civilization depends on major technological systems. Engineers continuously modify these technological systems by applying scientific knowledge and engineering design practices to increase benefits while decreasing costs and risks. (HS-PS3-3)
Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. (HS-PS3-3)
Energy cannot be created or destroyed—only moves between one place and another place, between objects and/or fields, or between systems. (HS-PS3-2)
7.B.2.1: The student is able to connect qualitatively the second law of thermodynamics in terms of the state function called entropy and how it (entropy) behaves in reversible and irreversible processes. [SP 7.1]
– AP Physics Course and Exam Description