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What is wrong with a perpetual motion machine?

  1. Oct 27, 2004 #1
    I'm just curious, why is it that anye perpetual motion machine that is thought up is discarded immedietly. I know that energy can't be created nor can be destroyed but then again, a few centuries we were absolutely positively sure that the Earth was flat! I mean anything can be wrong, why newton's law's itself was proved wrong and we still learn them in school! How can we be so sure that it can't possibly work!!! Can someone clear this out for me?
     
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  3. Oct 27, 2004 #2

    Integral

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    Every real mechanism involves some form of friction. Friction means energy is converted to heat and lost to the environment. Every mechanism requires some form of energy to operate, you cannot retrieve all of the input energy since there are unavoidable losses. Therefore it is impossible to get more energy out of a system then you put in.
    Since when? They still work the last I checked.
     
  4. Oct 27, 2004 #3

    russ_watters

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    The laws of thermodynamics have been observed in action for centuries. They have been observed to be correct to an extrordinary level of precision.

    And even if they are wrong (and there are several loopholes, mainly in quantum mechanics), that still doesn't show how perpetual motion can work. Its a burden of proof thing - something is only accepted to be true if it is proven true, not if it is not proven false.
     
  5. Oct 27, 2004 #4
    Because usually its though up by people who aren't very creditible and don't know what they're talking about. Other than that, name some ideas that have been prematurely discarded.

    Not necessarily. Read up in QM.

    We? Many peoples around the world didn't ever think the world was flat. Infact serveral cultures knew the world was round serveral milleniums ago. You're only thinking of a small portion of people in western europe.

    Correction: Newton was wrong. The only reason they're still used today is because they are simple and work well at low velocities. And yes, anything could be wrong. The only thing we have to go by is probability created by our observations, and probability suggests that a perpetual motion machine is either impossible or far ahead of our understanding.
     
  6. Oct 27, 2004 #5
    When was newton wrong? Not to challenge you entropy but i have never heard that and i am curious. And the perpetual motion machine, i was told by my physics teacher that it can never happen, due to the fact that no machine can , at this moment in time, have 100% effeciency due to energy converting into heat and so on.
     
  7. Oct 27, 2004 #6

    russ_watters

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    The Newton question depends somewhat on your perspective. Newton's laws do work in some cases. Just not every case. Are they wrong, incomplete, obsolete, limited, etc? You pick...
     
  8. Oct 27, 2004 #7

    arildno

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    As for the Newton question:
    A lot of the triumphs&predictions in QM are based explicitly upon using mathematical procedures where some terms are neglected or simplified.
    Does that make QM wrong?

    Similarly, you are fully entitled (in fact, obliged) to say that Newton's laws are simplifications (and hence, in a strict sense, wrong).

    Usefulness is perhaps a more interesting concept than "truth" by which to gauge the quality of some branch of science..
     
  9. Oct 27, 2004 #8

    T@P

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    hey i had an idea for a perpetual motion machine. what about two particles , say a and b, where a is attracted to be and be repels a (with the same force). first of allm is this even a perpetual motion machine? and secondly what is wrong specifically with this example? thanks in advance
     
  10. Oct 28, 2004 #9
    The theories stated in the original question could not be proven wrong, and in fact they were supported by the information they had at that time. Perpetual is not supported by our current information, and it is actually proven wrong by our current knowledge.

    As for T@P's question,

    First of all: it's not a machine, and it's not perpetual motion. It is not an example of something that can happen.

    Second of all: Also, attraction and repulsion are both forces involving two different objects/particles. Attraction is between two, and repulsion is between two. They don't exist independently. If particle A is net attracted to particle B, then particle B is net attracted to particle A.

    EDIT: Just for good measure, it was more than 'a few centuries ago'. In fact, by about 25 AD more than half of the earth's population generally accepted a theory of it being spherical.
     
    Last edited: Oct 28, 2004
  11. Oct 28, 2004 #10

    Mk

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    The Brownian Rachet is a perpetual motion machine postulated by Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962 as an illustration of the laws of thermodynamics.

    The device consists of a gear with a ratchet, that vibrates under Brownian motion (hence the name) in a heat bath. The idea is that motion in one direction is allowed by the ratchet, and motion in the opposite direction is prevented. Thus, it might be reasoned, the gear will rotate with a small force continuously in one direction.

    How does this not work?
     
  12. Oct 28, 2004 #11

    Chi Meson

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    THis would be a neat "heat engine" but the collisions of the molecules with one side of the rachet would take away the kinetic energy of those molecules. COnsequently, as the kinetic energy of the rotating rachet increased, the temperature of the gas in the box would decrease. It is not perpetual motion since heat would have to be put into the box for the process to continue. It is a clever idea for getting thermal energy to turn into kinetic, but it in no way violates the conservation principle.
     
  13. Oct 28, 2004 #12

    T@P

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    not to sound over persistent, but why cant two particles *theoretically* exist such that one attracts one and the other repels it? if they did exist then the two particle system would basically fly away on its own, and then even more *theoretically* one could harness such molecules allowing one to travel without working for it. Isnt that a perpetual motion machine?
     
  14. Oct 28, 2004 #13

    ZapperZ

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    You then first need to violate Newton's 3rd Law, because already you do not have equal but opposite force acting within the system. Such violation causes at least one symmetry-breaking of the underlying space.

    Zz.
     
  15. Oct 28, 2004 #14
    Well, thx guyz for posting your thoughts on it. And by the way, is there friction when a magnetic train floats over a magnetic rail? I don't know, that's why i'm asking you guyz. And i right of now believe that "friction" presists because our science is still "primitive", if it was absolute, we would be able to attain perfection and take off all the friction, sometime in the future i'm thinking, when computers are advanced enough to prevent frictuinal losses by perfecting the system for perfect efficiecy. I mean all our best engines right now can attain about 45% percent efficeincy, now that can be corrected can't it? Eitherway, the way i c it, there is still scope, physics being as crazy as it nowadays "god" ;) knows what will turn up! huh?
     
  16. Oct 28, 2004 #15

    ZapperZ

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    There's no fricton in a magnetic rail, but how you do propose to remove friction from the air surrounding these trains, or "friction" from normal electronic resistance, or even the energy loss in meandering vortices in a superconducting maglev? Even in an "ideal" engine, you do not get perfect conversion of energy - just look at the Carnot cycle.

    Zz.
     
  17. Oct 28, 2004 #16

    krab

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    Most PMM ideas are not really concerned with friction. Their promoters claim that they generate energy and so can easily overcome what little friction there may be. The 2 particles, one attracting, the other repelling, is a good example. If we want examples of machines with so little friction that they continue to move for long periods of time, we have many examples. The solar system is such a machine (at least over human time scales).

    So let's concentrate on energy-producing machines. Why do physicists dismiss them out of hand? Maybe an analogy will serve. Physicists have a close working relationship with the concept of energy; as close, I dare say as a layman has with a gold ring he is wearing. Imagine I approach this layman and tell him I have a very special box: if he puts his gold ring in it, wait for a day, and then opens it again, there will be 2 gold rings there. For free. Would this layman believe or be sceptical? Right. He would be sceptical. This is the same scepticism a physicist feels for spontaneously-produced energy.
     
  18. Oct 28, 2004 #17

    russ_watters

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    Great analogy, krab.
     
  19. Oct 28, 2004 #18

    T@P

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    and yet light moves on its own at an unchanging speed forever. doesnt this contradict alot of things? isnt it also a sort of pmm?
     
  20. Oct 28, 2004 #19
    Ok first of all newton was NOT wrong, it was just inadequate for the high relativistic velocities that is in QM. Newtons laws are taught in school because newtons laws are based on common sense, and it works for some everyda applications but not for QM.

    QM is just a refinement of Newton's laws
     
  21. Oct 28, 2004 #20

    Tide

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    Not at all. As long as it is travelling it is not doing work.
     
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