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First law of thermodynamics, q=w for a reversible isothermal process

  1. Feb 18, 2010 #1
    according to first law of thermodynamics,q=w for a reversible isothermal process.this means all the energy absorbed is being used to to do the work ,but according to second law of thermodynamics,there cannot be 100% effeciency.please tell where i am going wrong
  2. jcsd
  3. Feb 18, 2010 #2


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    Re: Thermodyanamics

    More precisely, the first law says that the sum of the heat input ([itex]q_\mathrm{in}[/itex]) and the work input ([itex]w_\mathrm{in}[/itex]) equals the sum of the heat output ([itex]q_\mathrm{out}[/itex]) and the work output ([itex]w_\mathrm{out}[/itex]). The second law says that in a heat engine (where [itex]w_\mathrm{in}=0[/itex] and [itex]w_\mathrm{out}/q_\mathrm{in}[/itex] is a measure of efficiency), there must be some [itex]q_\mathrm{out}[/itex],* so [itex]w_\mathrm{out}/q_\mathrm{in}<0[/itex]. Does this answer your question?

    *(To remove the entropy brought in by the heat input; entropy can't decrease and isn't transferred by work.)
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