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I On the possibility of making an engine with efficiency 1

  1. Jul 3, 2016 #1
    https://www.researchgate.net/post/Gibbs_free_energy_U-TS_PVIn_a_high_pressure_environment_when_PV_equals_TS_can_we_actually_use_the_entire_internal_energy_of_a_system_to_do_work?_iepl%5BviewId%5D=EMvZ8y3kBTyHiPzK6A8YVYvn&_iepl%5BsingleItemViewId%5D=erF86LeZ0BmbObFXKscz92Ib&_iepl%5BactivityId%5D=725672637112320&_iepl%5BactivityType%5D=person_post_question&_iepl%5BactivityTimestamp%5D=1467455039&_iepl%5BhomeFeedVariantCode%5D=d_EU&_iepl%5Bcontexts%5D%5B0%5D=homeFeed&_iepl%5BinteractionType%5D=questionView [Broken]

    If so can an engine be devised to use its entire internal energy to do work
     
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jul 3, 2016 #2

    davenn

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    no such thing as a free energy engine
    please reread the PF rules that you should have read when you signed up

    discussion of these topics are not allowed
     
    Last edited by a moderator: May 8, 2017
  4. Jul 3, 2016 #3

    Drakkith

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    Indeed. Thread locked.
     
  5. Jul 4, 2016 #4

    Drakkith

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    @Useful nucleus contacted and convinced the mentors that an explanation of some basic thermodynamics would be useful to the OP and to others. As threads this close to being about PMM's tend to attract crackpots, the thread will remain locked but has been un-deleted. The following explanation was composed by Useful Nucleus and anyone with questions about it should start a new thread.

    Gibbs free energy G is not the available work. G is a state function describing a single state whereas work is a process from one state to another. What we can show is that the work delivered in a reversible process by a system in contact with a thermal reservoir and a pressure reservoir is equal to the decrease in Gibbs free energy. That is δW= - dG. Notice that the fact that process is reversible indicates that this is the maximum possible work delivered between the initial state and final state (Maximum work theorem).

    To be more concrete I adopt Problem 4.5-20 form the excellent Thermodynamics text by Herbert Callen.
    Suppose a system is in some initial state and is planned to deliver maximum work while going from this initial state to a final state. The final state is decided by that fact that it has to be in equilibrium with the ambient atmosphere which acts as a temperature and pressure reservoir (Tatm , Patm are fixed). To deliver maximum work the process has to be reversible. You can show that the maximum work is NOT -ΔU because there will be heat loss to the reservoir and even worse there will be work loss to the reservoir as well! So the maximum work is:

    W= (U0 + Patm V0 - Tatm S0) - (Uf + Patm Vf - Tatm Sf)

    The latter equation can be derived without any reference to Gibbs free energy and in fact historically it was the precursor to coin the term free energy.
    In summary it is not correct to think of G itself as the available work, rather one has to consider the decrease in G.
     
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