Perpetual Motion using Brownian Motion?

  • #1
Let me start off by saying that I very well know that PM is impossible. Thermodynamics aren't just good ideas-they're the law. :)

I have heard that Brownian Motion will go on infinitely, but you can't harness it and it is useless perpetual motion. Is this true?
 

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  • #2
Ryan_m_b
Staff Emeritus
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Let me start off by saying that I very well know that PM is impossible. Thermodynamics aren't just good ideas-they're the law. :)

I have heard that Brownian Motion will go on infinitely, but you can't harness it and it is useless perpetual motion. Is this true?
I'm not sure where you heard this, Brownian motion will slow down as the medium cools I believe.
 
  • #3
mathman
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Ordinary Brownian motion will go on indefinitely as long as the medium is in equilibrium. There is no way to harness the energy, since that would be equivalent to cooling the medium.
 
  • #4
726
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Well, to be fair, the motion of all atoms in the universe is perpetual by that logic, since temperature can never reach 0K.
 
  • #5
K^2
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There is an objective measure of "useful" energy, which is entropy. Energy with high entropy can only be used if you have a sink for the excess entropy. For example, you can use thermal energy of a hot object, only if you have a colder object handy to be used as a sink.

Random movement of pollen particles observed by Brown is not a violation of thermodynamics, because a) total energy is conserved, and b) the motion of pollen has high entropy, same as motion of individual molecules in the medium that the pollen particles are floating in.

This isn't an issue of laws of physics being violated. But I agree, a lot of thermodynamics books do a poor job of explaining all this, so the point on examining text books is valid.
 
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