Math 2.0

Classical thermodynamics, heat transfer Definition

Perpetual motion

Builds on Thermodynamic cycleThermodynamic workHeatFirst law of thermodynamicsKelvin–Planck statement of the second law

Definition

A perpetual motion machine is a hypothetical device that operates in a thermodynamic cycle and is of one of the following kinds.

  1. Of the first kind: in each cycle it does positive net work but takes in no net energy, as heat or otherwise.
  2. Of the second kind: in each cycle it turns all the heat it takes from one reservoir into work, with no other effect.

Perpetual motion is the running of such a machine.

Remarks

A machine of the first kind would violate the first law of thermodynamics, since over a cycle the net work equals the net heat taken in. A machine of the second kind would violate the Kelvin–Planck statement of the second law. Example: a ship engine that cools the sea water and uses all the extracted heat to drive the propeller would be of the second kind. Non-example: a real steam engine is not of the second kind, because it gives part of the heat from the boiler to the colder condenser. A frictionless pendulum in vacuum moves forever, but it does no net work, so it is not a perpetual motion machine of either kind.

Reading path

To understand this page from the ground up, read these 12 pages in order. The foundations come first.

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Heat, concept, no evidence levelHeatconceptHeat engine, concept, no evidence levelHeat engineconceptInternal energy, concept, no evidence levelInternal energyconceptState function, concept, no evidence levelState functionconceptTemperature, concept, no evidence levelTemperatureconceptThermodynamic cycle, concept, no evidence levelThermodynamic cycleconceptThermodynamic system, concept, no evidence levelThermodynamic systemconceptThermodynamic work, concept, no evidence levelThermodynamic workconceptFirst law of thermodynamics, statement, no evidence levelFirst law of thermody…statementClausius statement of the second law, statement, no evidence levelClausius statement of…statementKelvin–Planck statement of the second law, statement, no evidence levelKelvin–Planck stateme…statementPerpetual motion, definition, no evidence levelPerpetual motiondefinition
Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
  • no evidence level
  1. Heat concept
  2. Heat engine concept
  3. Internal energy concept
  4. State function concept
  5. Temperature concept
  6. Thermodynamic cycle concept
  7. Thermodynamic system concept
  8. Thermodynamic work concept
  9. First law of thermodynamics statement
  10. Clausius statement of the second law statement
  11. Kelvin–Planck statement of the second law statement
  12. Perpetual motion definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Perpetual_motion
About this page
TagCB8RMF4358
URIm2://CB8RMF4358
Kinddefinition
WritingW0 stub
MSC80A05, 80A10
DefinesPerpetual motion
UsesThermodynamic cycle, Thermodynamic work, Heat, First law of thermodynamics, Kelvin–Planck statement of the second law
Changed3 November 2026

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