Perpetual Motion

  1. danago

    danago 1,125
    Gold Member

    Hi. In class we are currently studying electromagnetism. The unit we are currently doing is on generators and motors using faradays law of induced emf.

    While reading, i had a random thought:
    If a generator is set up, such that its output current is directed to a motor, which creates rotation, which in turn is used to turn the generator coil, will the system continue to operate forever, assuming resistance free wires.

    Ofcourse, the generator will have to be given an initial 'push' to get the current flowing.

    Now, im sure there must be some flaw in what ive said, or some physical problem with the setup, since as far as i am aware, perpetual motion is impossible.

    Ive just posted this really for the sake of curiosity, so if anybody is able to comment on what ive said, id be greatly appreciative :)

    Thanks,
    Dan.
     
  2. jcsd
  3. Danger

    Danger 9,878
    Gold Member

    To start with, there is no such thing as 'resistance-free' wires, unless you're going into superconductivity. If you can achieve that at room temperature, then you'll be so rich that nothing else matters.
    More to the point, though, are things such as friction and air resistance.
    No matter how you look at it, it ain't gonna work.
     
  4. danago

    danago 1,125
    Gold Member

    Ok, thats fair enough. But im just asking this as a theoretical question; its not like im actually going to go and try it.

    Lets say that it was possible to attain resistance free wires, and air resistance and friction etc. were negligiable. Would it work then?

    Thanks for the reply btw.
     
  5. olgranpappy

    olgranpappy 1,273
    Homework Helper

    You wouldn't get anything out of it. All the energy would have to be put back in to keep the thing running. Think about a frictionless ballbearing rolling back and forth in a dish. It can never get any higher than its original starting point.

    The above are statements about ideal systems with no friction and may be summarized as: "You can never do any better than breaking even."

    For the case of real systems (which you don't care about) there is an even worse rule which says: "You can't even break even!"
     
  6. NoTime

    NoTime 1,570
    Science Advisor
    Homework Helper

    You might consider that if you eliminate all air friction and mechanical friction then you don't need the wires.
    The rule is that an object in motion will stay in motion until an external force acts on it.
     
  7. Danger

    Danger 9,878
    Gold Member

    I gotta admit, No Time, that I never even thought of it that way. Great point!
    Danago, let's review the 4 laws of thermodynamics for future reference.
    1) No matter how hard you try, the best that you can do is break even.
    2) You can only break even at 'absolute zero'.
    3) 'Absolute zero' is impossible to attain.
    4) No matter how much you shake it, the last drop always goes down your pants.
    As Olgranpappy pointed out, the absolute maximum energy that you can attain under ideal conditions would be just enough to keep the system in motion. No useful work could be extracted from it. That applies to any perpetual motion device, regardless of what mechanism is involved.
     
  8. russ_watters

    Staff: Mentor

    Well anyway, yes, if you have a perfectly efficient motor and generator and no other losses of any kind, your system will not lose any energy and will run forever according to Newton's first or the law of conservation of energy...

    ...but that isn't a terribly useful thought experiment.
     
  9. HTS wire will carry a current forever? that's real...
     
  10. Danger

    Danger 9,878
    Gold Member

    'High Temperature' and 'Room Temperature' are worlds apart. I'm not sure what the cut-off point is, but I think that it's something like 50°K that is considered the 'HT' boundary.
     
  11. thats not the point, its a real system that breaks even and btw the record for highest temperature while still SC is 135K
     
  12. Danger

    Danger 9,878
    Gold Member

    True. The actual point is that you can't extract anything useful from a PM device. How long would the current in your SC loop keep going if there was a light bulb in the circuit?
    As to the temperature, I don't even try to keep track of what the record is. I was referring to the bottom limit of what is considered 'high temperature', and I really don't know what that is.
     
  13. russ_watters

    Staff: Mentor

    So is an orbit. The term "perpetual motion" isn't exactly the best term to describe the issue. People looking for perpetual motion aren't actually looking for an efficiency of 1, they are looking for an efficiency > 1.
     
  14. Danger

    Danger 9,878
    Gold Member

    Yeah. I never heard the term 'over unity' until I joined PF, but I like it.
     
  15. Hey Danger, I've heard all of those except the 4th one. I don't get what it is saying. What does it mean?
     
  16. olgranpappy

    olgranpappy 1,273
    Homework Helper

    urea-related humor

    I can't tell from your name--are you a guy or a girl? If the latter then it might help for you to think of this one from a guy's perspective.
     
  17. Is this suppose to be disgusting? If it is then don't tell me.
     
  18. Danger

    Danger 9,878
    Gold Member

    Thanks for the bail-out, Pappy. :biggrin:
     
  19. Danger

    Danger 9,878
    Gold Member

    Disgusting only because it's true. Why do you think that guys prefer dark trousers for formal wear?
     
  20. Never mind then. Forget i even asked.
     
  21. olgranpappy

    olgranpappy 1,273
    Homework Helper

    I forget

    Consider it forgotten--consider what forgotten?

    ... Exactly.
     
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