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Zero Efficiency Machine

  1. Dec 13, 2011 #1
    Lets say if we have an induction motor or any motor whose rotating shaft acts as a source of alternating emf to another motor and rotating shaft of this motor again acts as a source of alternating emf to third motor and so on... We connect such a finite number of motors and we call it as a single "machine". At each step there will be some losses which will get added up at end and after finite number of such motors all the INPUT ENERGY will be converted into LOSS. The efficiency of such a machine will be zero. Is this possible?? I mean can there be a situation of zero efficiency ?? And if all the energy loss is dissipated as heat then ALL the electrical energy is converted to heat. Is this thermodynamically stable machine???
  2. jcsd
  3. Dec 13, 2011 #2


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    Efficiency of a machine is simply defined as the useful work out divided by the heat you had to put in. All you need to do to have a zero efficiency machine is make sure it does not useful work.
  4. Dec 13, 2011 #3

    Andy Resnick

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    Friction is an excellent zero-efficiency conversion mechanism.
  5. Dec 13, 2011 #4
    Put sand instead of gasoline in your car's gas tank and it will become a zero efficiency machine very quickly.
  6. Dec 13, 2011 #5


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    Not sure about the thought of the question, but the supply of power to the first unit would reduce through each of the next units, until the last one received so little power it would likely not turn, but would displace the energy as resistance loss.

    "thermodynamically stable machine", I can't quite see the value of that in a context. Can you add more about your thoughts ?
  7. Dec 13, 2011 #6
    I don't think it would have a zero thermal efficiency which is n=1-Tc/Th. If there was no change in tempature than it would be zero percent efficient... which seems rather obvious.
  8. Dec 13, 2011 #7
    The last motor will receive almost no power so it wont rotate. Energy at each motor is lost as heat. So if we use this "machine" as a heater, we will get a heater of 100% efficiency! as ALL the input energy is converted to heat energy (in form of loss). This will happen only and only when the efficiency of this machine with respect to mechanical energy is zero. But 100% efficiency is not valid in thermodynamics.
  9. Dec 13, 2011 #8
    I think its a known fact you can convert 100% of work into heat, but you can't convert 100% of heat into work.
  10. Dec 13, 2011 #9
    You can simply run a motor without a load excepting the winding resistance, bearing friction, the hysteresis losses, and other minor things. That's how to do it.
  11. Dec 13, 2011 #10
    You can't convert 100% of mass-energy into heat, nor vice-versa.

    Work is not something that "converts". Like heat, it is a process quantity, not a substance. "Work" and "heat" both act as verbs in thermodynamics.
  12. Dec 13, 2011 #11
    I can't look up a reference becuase I'm studying for a final right now, but it's something I heard a few years ago. I was in a physics lab and we were running an expiriment where a a rope turned a pulley when a bucket fell. The pulley had a friction bearing and the TA said this cuased the work to be converted into heat, that you could have 100% conversion of work to heat but not vice versa.
  13. Dec 14, 2011 #12
    Thermodynamics is the study of the inter-relation between heat, work and internal energy of a system.

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