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Monday, October 11, 2010

Perpetual Motion

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled The Definition of Entropy.

It is a consequence of conservation of energy and the second law of thermodynamics that it is impossible to build a perpetual motion machine. There are many types of proposed perpetual motion machines.

There is a post that goes into a lot of detail of the various sorts of perpetual motion machines by Kevin T. Kilty, entitled Perpetual Motion.   Rather than go into arcane detail about different types of perpetual motion machines, I think it suffices to refer the interested reader to Kilty's post.

No machine can generate more more energy than put in (first law of thermodyanics, conservation of energy, Noether's theorem).  The first law of thermodynamics states that work can be converted into heat, and heat can be converted into work, but that the sum, the so-called internal energy (E or U) is a conserved quantity.

Conservation of energy means that if one could build a perpetual motion machine that it could not be used to do anything useful as all the energy would be required to keep the machine running. Conservation of energy alone does not forbid a perpetual motion machine, however.

One can imagine a machine that converts heat into work and converts that work back into heat and keeps running forever. It is the second law that forbids such a machine.  No machine can covert heat into useful work with 100% efficiency.  The consequence is that all real world machines (and processes) require some source of energy input.

Even today, people are still trying to sell perpetual motion machines for just one example see:
A magnetic motor (or magnet motor) is a device which converts power of or relating to or caused by magnetism (e.g., "magnetic forces") into mechanical force and motion, with no other input. It usually provides rotary mechanical motion. The machines that utilizes the properties of a magnet for mechanical energy. (Source)
The site acknowledges:
Conventional physics says that it is "impossible" for magnets to provide a primary energy source. Yet thousands of researchers worldwide have been pursuing the task of building a working magnet motor. Many claim to have achieved this objective. None has reached the marketplace yet.
Anytime someone claims to sell something that defies "conventional physics," one ought to hold on to one's wallet.

Magnets and Nonsense

Magnetic nonsense is a special kind of nonsense.  I have in mind a plan to write a blog post dedicated to the the mysterious powers attributed to magnets.  Because electromagnetism is a complex subject, I am not going to delve deeply into the matter here; however,  it is worth quoting David Griffiths for some context:
Magnetic forces may alter the direction in which a particle moves, but they cannot speed it up or slow it down.  The fact that magnetic forces do no work is an elementary and direct consequence of the Lorentz force law, but there are many situations in which it appears so manifestly false that one's confidence is bound to waver.  When a magnetic crane lifts the carcass of a junked car, for instance, something is obviously doing work, and it seems perverse to deny that the magnetic force is responsible.  Well, perverse or not, deny it we must, and it can be a very subtle matter to figure out what agency does deserve the credit in such circumstances.(Griffiths, p. 201)
We cannot get something for nothing.  We must put energy into a system to get work out, and in fact, we must put more energy in than we get work out.  There are some systems that have potential energy in them that we can access to do useful work (for example, a battery), but we must eventually run it down.

The next post entitled, The Hydrogen Economy, looks at a popular proposed solution to our energy needs, and shows how it is sometimes misconstrued to offer more than it really does.

  • Atkins, P. W. Physical Chemistry, W. H. Freeman and Company, New York, 3rd edition, 1986
  • McQuarrie, Donal d A., Statistical Thermodynamics,  University Science Books, Mill Valley, CA, 1973 
  • Bromberg, J. Philip, Physical Chemistry, Allan and Bacon, Inc., Boston, 2nd Edition, 1984
  • Anderson, H.C., Stanford University, Lectures on Statistical Thermodynamics, ca. 1990.
  • Wikipedia: Perpetual Motion 
  • Kilty, Kevin T., Perpetual Motion 
  • Directory: Magnet Motors 
  • Griffiths, David J., Introduction to Electrodynamics, Prentice Hall, Englewood Cliffs, NJ, 2nd edition,  1989.

  1. Introduction
  2. What the Second Law Does Not Say
  3. What the Second Law Does Say
  4. Entropy is Not a Measure of Disorder
  5. Reversible Processes
  6. The Carnot Cycle
  7. The Definition of Entropy
  8. Perpetual Motion
  9. The Hydrogen Economy
  10. Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner
  11. The Second Law and Swamp Coolers
  12. Entropy and Statistical Thermodynamics
  13. Fluctuations
  14. Partition Functions
  15. Entropy and Information Theory
  16. The Second Law and Creationism
  17. Entropy as Religious, Spiritual, or Self-Help Metaphor
  18. Free Energy
  19. Spontaneous Change and Equilibrium
  20. The Second Law, Radiative Transfer, and Global Warming
  21. The Second Law, Microscopic Reversibility, and Small Systems
  22. The Arrow of Time
  23. The Heat Death of the Universe
  24. Gravity and Entropy
  25. The Second Law and Nietzsche's Eternal Recurrence
  26. Conclusion

1 comment:

Anonymous said...

A magnet certainly can do work if it is not required to go through a complete cycle or path. No different than gravity.