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Perpetual Motion Machine

  1. Aug 11, 2008 #1
    A heat engine is combined with a heat pump so that the entire system is an isolated system. In the heat engine, energy shifts from the hot reservoir so that some of it is used for work and the rest is sent to the cold reservoir. The work, however, is used entirely for the heat pump, in which the heat pump reverts the energy from the work and the energy exhausted to the cold reservoir back to the hot reservoir. Technically, a machine like this is useless (its performing work on itself)...but, while ignoring friction, conduction, or any other unnecessary dissipation of energy, wouldn't this be a perpetual motion machine?
  2. jcsd
  3. Aug 11, 2008 #2


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    Homework Helper

    Gee if you are allowed to ignore things like dissipation of energy or inefficiency, when do you think a spinning top would stop?
  4. Aug 11, 2008 #3
    Hmmm...true...but altogether, it is as claimed (while ignoring the impracticality of no dissipation or inefficiency)...right?
  5. Aug 11, 2008 #4
    You could do the same with a superconducting motor and generator with frictionless bearings and sealed in a vacuum. The motor, turning the generator, powering the motor.

    If your machine had no losses, then yes, it would run perpetually. But the idea of a perpetual motion machine is usually one where you can get it to run forever, AND extract usable work from it. A machine that spins in the empty void of space without friction is useless. In the case of the motor/generator, if you tap their circuit to a switch and light bulb, as soon as you flip the switch to energize the light, all of the energy of the system quickly radiates away as light and it slows to a halt.
  6. Aug 11, 2008 #5
    I see...thanks for clarifying things.
  7. Aug 11, 2008 #6


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    A heat pump is nothing special. It is a thermodynamic device that takes energy in in one form and uses that energy to perform work of some kind (in this case, the work is moving heat energy, not converting heat energy to mechanical energy), with a certain efficiency, just like any other. It is not a violation of the 1st law of thermodynamics as the OP suggests.

    Don't confuse a C.O.P of greater than 1 with an efficiency of greater than 1. More energy can be moved around than is being input, but it is the same as moving around a large mass: the larger it is, the less it can be moved with a certain input energy. What that means in a heat pump, is the higher the COP, the lower the delta-T on the output side. So the more efficient the heat pump, the less efficient the heat engine attempting to use that energy. Mathematically, that is easy enough to see: the efficiency equations are inverses of each other. http://en.wikipedia.org/wiki/Heat_pump#Efficiency

    So the bottom line is that a theoretically perfectly efficient heat pump combined with a perfectly efficient heat engine (both running at Carnot efficiency) would produce exactly as much energy as is put into it. But we all know thermodynamic cycles run nowhere close to 100% efficient...

    Perpetual motion machine speculation is a non-starter, always based on a conceptual error. Thread locked.
    Last edited: Aug 11, 2008
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