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Thread: Thermodynamics

  1. #1 Thermodynamics 
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    Im reading the chapter about Thermodynamics from "Feynman Lectures on Physics" and there are some things I just cant understand.

    1) He states that no engine can do more work than a reversabile engine, I dont understand how reversability and efficiancy are corelated.

    2) He says that the amount of work that an engine can do when it assorbs a certain amount of Q does not depend on the design of the machine but is a property of the world, not of the engine. I dont understand this, because I imagine that its possible to design more or less efficiant engines.


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  3. #2 Re: Thermodynamics 
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Quote Originally Posted by Ultrashogun
    Im reading the chapter about Thermodynamics from "Feynman Lectures on Physics" and there are some things I just cant understand.

    1) He states that no engine can do more work than a reversabile engine, I dont understand how reversability and efficiancy are corelated.

    2) He says that the amount of work that an engine can do when it assorbs a certain amount of Q does not depend on the design of the machine but is a property of the world, not of the engine. I dont understand this, because I imagine that its possible to design more or less efficiant engines.
    As for the first statement I am sure he means something by that statement but without more explaination I am not sure what it is. It is a problem of translation. The real language of physics is mathematics and when things are stated like this in English they tend to become a bit obscure.

    In the second case you are correct. He should have clarified what he was saying. What he should say is that there is a maximum efficiency which depends only on the properties of the environment rather than on the engine. But I suspect that even this is not enough to make what he is saying absolutely true. I think there are too many assumptions behind his statement that are not being made clear (at least not in the version you are stating above). For example, I think what he says is only true of a steady state engine. A small engine with a large heat sink could exceed the limit on efficiency for a period of time, but the heat sink would have to be periodically replaced and so the overall efficiency when the replacement of the heat sink is taken into account would be below the maximum determined by the environment.


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