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Is there any law that states something cannot have 100% efficiency?

  1. Dec 7, 2009 #1
    I understand that all real-world applications can never attain 100% efficiency, whether it be referring to something mechanical, electrical, or some thermodynamic cycle.

    While studying the Carnot cycle and learning about the upper limit of efficiency for thermodynamic cycles, I drew a blank when I tried to remember if there was anything that stated that the upper limit for ANY non-thought experiment was less than 100%.

    Is there any formal law that goes over this in depth? I was thinking that it may be tied into the second law of thermodynamics.
  2. jcsd
  3. Dec 7, 2009 #2
    Not even (real world possilbe) theoretical heat cycles can have 100% efficiency. For the carnot cycle to have 100% efficiency the low heat reservoir must be absolute zero.

    And yes it's the laws of thermo (1st 2nd and 3rd) that define why a system can never attain 100% efficiency. The thing that specifically prevents 100% thermal efficiency is entropy and the conpect was developed from work by Carnot and led to the 2nd law.

    Mechanical efficiency is something different altogther to thermal efficiency.

    EDIT: I've just read that back and don't feel i've explained it very well.
    Last edited: Dec 7, 2009
  4. Dec 7, 2009 #3
    Yeah I know that Carnot cycles' efficiencies can never be 100%, even under ideal cases. It's upper limit it set by 1-(T_Low/T_High).

    I was curious as to whether or not there was a formal law or proof that states all mechanical or electrical processes can never be 100% efficient. Or am I wrong in thinking that? I can't think of any case where 100% efficiency would be possible... but maybe there's some extreme case in a vacuum at absolute zero or something?
  5. Dec 7, 2009 #4
    Well for thermo, the law is entropy. Which isn't considered a loss.

    For mechanical, there are no laws against 100% efficiency. It's just practically you have losses, so friction, losing energy to heat, wear, etc etc.
  6. Dec 7, 2009 #5
    All you have to do to attain 100% efficiency is to:
    • Get a 1W electric motor. And,
    • Get a generator that generates 1W of electricity @ 1,000 RPM.
    Next and last, of course, is to connect the motor shaft directly up to the generator shaft, then get a drill or something to spin the common shaft to 1,000 RPM, and voila! The motor will spin the generator @ 1,000 RPM, which will generate the 1W needed for the elecrtic motor, which will in turn keep the motor spinning at 1,000 RPM, which provides the mechanical energy for the generator to generate 1W, which will in turn keep the motor spinning at 1,000 RPM, ad infinitum. :tongue2:
    Just kidding, guys! :biggrin:
    Last edited: Dec 7, 2009
  7. Dec 8, 2009 #6
    The Carnot engine is the theoretcial limit of HEAT engines. That is, if you are using a system that produces work by using a difference in temperature. The proof for this is in most thermo books. However, this is not the limit for any energy conversion device. Fuel cells aren't limited to Carnot, for example. The efficiency of mechanical systems is limited by friction and slipping. However, none of these will obtain 100% efficiency. Even in a fuel cell, as soon as current starts trickling out the losses begin to increase.
  8. Dec 8, 2009 #7
    Loss in efficiency is due to energy loss from effects other than the one desired, like friction, or heat radiation. Since we have no frictionless materials, perfect insulators, or ideal gasses, there will be energy loss.

    The one exception I can think of is superconductors. They conduct electrical current with absolutely no resistance, but one cannot make a whole machine out of superconductors.
  9. Dec 8, 2009 #8
    DyslexicHobo -> Carnot's efficiency is the highest achievable, even in principle if the 2nd law is to hold. If you combine a Carnot machine and an ordinary machine, the resulting one has 100% efficiency. (You should find the proof in most books on thermodynamics.) Which basically means you'd be converting all the heat (disorder) into work (order), that is, you'd lower the entropy of the universe.
  10. Dec 8, 2009 #9

    No you wouldnt. If you could build a carnot cycle engine, running with a lower reservoiur at absolute zero, you would have, at best, ZERO entopy change.
  11. Dec 9, 2009 #10
    There are lots of things that are close to 100% efficient. Superconductors, as has been mentioned. Hell, even a long rod. Push on it at one end, and you transferred practically 100% energy to the other end. However, even these examples are not 100% efficient. Maybe 99.999999999% efficient, but I don't believe anything could be truly 100%.

    Although I don't know...how about something in an absolute vacuum?
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