# Perpetual motion machines

Homework Helper
Gold Member
I found how to make a perpetual motion machine! (just joking, keep reading! :tongue2:)

We just saw what they are in my thermodynamics class, and if I understood correctly, it represents the only heat-to-work conversion machine not (theoretically) in violation of the second law. And this solution is that q=w=0 (the heat absorbed and the work done are both 0). So it's just this thing that gets back to exactly its original macroscopic state ($\Delta S=0$) after a certain "cycle" or "period".

But what goal does it have? It must have one if hundreds of people are submitting perpetual motion machines ideas to the patent office every year.

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A perpetual motion machine is impossible because its heat cycle converts mechanical energy to heat and/or vice versa with no loss of energy, which is impossible. Its like having a water pump pumping water up a hill where the pump uses no energy.

its the thermodynamic equivalent of forcably creating a convservative system but using forces that arent conservative, thus is impossible

HallsofIvy
Homework Helper
The cutest idea for a perpetual motion machine I have seen is a wheel with weights on its spokes. On one side the weights are clearly labeled "9" (kg) but as they rotate to the other side, turning over, the "9" becomes a "6" and so one side is always heavier than the other!

Danger
Gold Member
From the 'Hanna-Barberra School of Physics'? Love it.

My physics teacher told me the answer to any supposed perpetual motion machine is simply "No." End of story. Even if someone says "Wait, wait! I finally have it!"..."NO!" or "This is it, the long anticipated perpetual motio...." "STOP RIGHT THERE, NO!" Basically the world will stop when that happens (right after the Second Coming).

Homework Helper
Gold Member
Yeah but none of the above post answers my question. Why do people try to construct a perpetual motion machine? What purpose will it have?

Gokul43201
Staff Emeritus
Gold Member
There mostly 2 kinds of PMMs - and they're called PMMs of the "first kind" or "second kind" depending to which law of thermodynamics they violate. Besides these, there's the trivial PMM which is essentially the equivalent of say, a "pendulum" swinging through a "vacuum" (the kind you describe). This third kind is not the type that fills up patent office dumpsters.

Yeah but none of the above post answers my question. Why do people try to construct a perpetual motion machine? What purpose will it have?
At a time when we are clearly overstretching the earth’s natural resources and polluting our environment, it should be very apparent that a perpetual motion machine would be the Holy Grail of any inventor, and it has been for centuries. At the same time, since we here are all supposed to be well versed in physics and mathematics, we KNOW that such an over-unity device is absolutely impossible. But maybe we need to pause and give some equal time to the lunatic fringe once in a while, at least if they are acting fairly reasonably and not foaming at the mouth. I can remember years ago, when I was working with cryogenically cooled low noise amplifiers for space and satellite communications, that we had some fluky amplifier chips come off the assembly line that exhibited very low noise qualities without cryogenic cooling. At first, they were a curiosity and even an annoyance! Eventually, though we learned about the doping qualities which were happening by accident and managed to get those same qualities by design. Today, such low noise non-cooled amplifiers are common-place and cryogenically cooled LNAs are now very rare. Many advances in science happen by accident and are counter-intuitive. I must admit, however that I do get extremely frustrated discussing over-unity machines while secretly hoping against hope that maybe the lunatics are right!

HallsofIvy