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Extracting Energy from the Casimir Effect

  1. Nov 2, 2006 #1
    Imagine you have two plates which can either be perfectly conducting or perfectly insulating by changing some conditions (like say, cooling an insulator down until it is superconducting).

    First, have the two plates be superconducting, and bring them close together so the Casimir effect is non-negligible. Extract work from the system by letting the plates come together. Now, turn your plates into insulators by raising the temperature (to make the material non-superconducting), and pull the plates apart (with less resistive force since the plates are no longer perfect conductors). Repeat the process, voila, free energy?

    This seems to violate conservation of energy, which doesn't make sense to me. Can someone explain to me why this can't be done and why energy IS conserved?
     
  2. jcsd
  3. Nov 2, 2006 #2

    turbo

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    How about the part where you increase the temperature of the plates to make the non-superconducting and then have to cool them to superconduction to repeat the cycle? You've got to put energy into this system in both parts of this cycle. Nothing is free.
     
  4. Nov 2, 2006 #3
    Yes, this does require some energy...however, the amount of energy required is independent from the Casimir cycle. Basically, with a superconductor, I just have to raise or lower the temperature beyond the critical point.

    Let me make sure I make this clear: I'm not talking about if it's feasible or if it generates a worthwhile amount of energy. I'm talking about whether or not energy is conserved.
     
  5. Nov 2, 2006 #4

    ZapperZ

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    What about producing a quantitative analysis to support your assertion, rather than simply doing a hand-waving argument? After all, you are arguing that you can produce MORE energy than what you put in. Without some realistic numbers, this is all meaningless.

    Zz.
     
  6. Nov 2, 2006 #5
    I'm not asserting anything here...I was asking someone else to actually go through the numbers and tell me why it didn't work because I don't think I'm qualified to answer the question.

    Assume that a material becomes superconducting below some critical temperature. Now lets say you have good enough equipment to adjust the temperature of the material by a tiny fraction of a degree (lets say a trillionth), just enough to flip superconduction on and off. In other words, ignore the energy required to turn the superconduction on and off because you can make the temperature change arbitrarily small.

    Now you can simply place the plates a small distance apart, let them be pulled together by the force, turn off the superconduction, pull the plates apart (with now less retarding force), and repeat.

    My question is what am I missing here?
     
  7. Nov 2, 2006 #6

    ZapperZ

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    1. Can you tell me what "superconducting material" has anything to do with this whole thing?

    2. What material actually has that small of a transition temperature? Look at the transition temperature range of real material, either via the conductivity data, or the magnetization data and figure out the temperature spread.

    3. Are you aware of the penetration depth of any EM fields close to the critical temperature? It is pretty significant. Right at Tc, there's almost no difference between the skin depth of the superconducting phase and the normal phase. This is because the superfluid density is horribly small at that temperature. So switching back and forth right at that point has no effect if all you care about is the EM field boundary condition, which is what is involved in a Casimir effect.

    Zz.
     
  8. Nov 2, 2006 #7
    This is what I was looking for...I was confused as to why the Casimir effect was always explained with the assumption of a perfect conductor. I had assumed that the conductivity of the material was a strong influence on the presence of the effect. If there is no significant difference in the effect on both sides of the critical temperature, then my apparatus would not work.

    Realize that I wasn't trying to be antagonistic here, I was just asking a question. I know people like to pounce on those who describe "perpetual motion machines" but I was just seeking an explanation as to why the apparatus I described would fail to violate energy conservation. I feel like the replies I got were kinda mean-spirited. :grumpy:
     
  9. Nov 3, 2006 #8

    ZapperZ

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    But this isn't unique to just Casimir effect. When you deal with waveguides and RF transport, you assume perfect conductor all the time. This is because in the SCALE of things, the small, finite resistive losses is insignificant. Including such small effects would only complicate the computation and produces no useful or measurable effects.

    This is why I asked for a quantitative analysis, because the scale of the numerical results here is central to any assumption one makes.

    Zz.
     
  10. Nov 5, 2006 #9
    Yes, Casimir and van der Waals forces are related to polarisability of media, which is affected by temperature. Very interesting. I am thinking about this and I would encourage others to do so as it is not that trivial.

    Take for example graphite planes. This form of carbon is semi-metallic and a good example of a solid state system binded by van der Waals. If there is a conductor, insulator transition I would suggest that the change in the free energy would outway any mechanical work derived from the system.
     
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