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Why can't machines be 100 percent efficient?

  1. Dec 4, 2003 #1
    Why can't machines be 100 percent efficent[?]
  2. jcsd
  3. Dec 4, 2003 #2
  4. Dec 4, 2003 #3


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    For a heat engine to be 100% efficient, you'd need an infinitely hot heat reservoir, or a cold reservoir at absolute zero -- neither of which is physically realizable.

    - Warren
  5. Dec 5, 2003 #4

    Chi Meson

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    I'm assuming your referring to simple machines like levers, pulleys etc. To elaborate on the last two posts:

    friction is a force that acts between any two objects sliding against each other. In all machines there are at least two objects sliding against each other. The friction that is there will do worK while the machine is in use. This work acts at the molecular level causing the atoms and molecules of the two objects to vibrate faster. Faster vibrating molecules means more thermal energy which produces higher temperature.

    The thermal energy that is produced (heat) plus the useful work that comes out of the machine together must equal the work put into the machine. THere is no way to make the heat outpu be zero, so therefore less work will come out of the machine than goes in.
  6. Dec 5, 2003 #5
    Funny you should mention it, i just posted about E. coil in the general dicussion forum and how it amazes me that the torque they produce is almost 100% efficent from the current of protons. Anyways if your referring to heat cycles such as the carnot cycle then the maximum efficency is [tex]\eta=1-T_C/T_H[/tex] where [tex]T_C[/tex] and [tex]T_H[/tex] and the hot and cold reservoirs.

    Pretty much once you look at it everything is not as efficent as you want it to be except our bodies storing and using fat. :frown:
  7. Dec 6, 2003 #6


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    Out of curiosity, I realise that a 100% effecient machine is said to be impossible, but how is this principle prooved out in the case of superconductors? A superconducting wire is a machine that transports an ellectric current from one place to another without any loss. On the surface, this sounds like absolute effeciency, so is a supercunductor considered not to be perfectly efficient, or is it considered not to be a machine? If the former, then where is the energy lost, and by what mechanism? I suspect an answer, but would like to hear it from someone else.
  8. Dec 8, 2003 #7
    The energy "lost" in a superconducting machine is not the superconducting phenomenon itself, rather it is in the energy mechanism(s) utilized to cool the superconductor.
    In other words, though a superconductor allows zero-resistance to current flow, it takes dissipative energy(i.e. loss) to create and sustain the event!

  9. Dec 8, 2003 #8
    In a frictionless environment, and therefore gravity-less environment, is it possible for a machine to work at 100%, or are there other elements which prevent this?
  10. Dec 9, 2003 #9


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    Thanks, that was indeed the answer I suspected.
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