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B Feeding on Entropy or Heat

  1. Aug 31, 2018 #1
    Carlo Rovelli described in "The Order of Time" that

    "Living beings are made up of similarly intertwined processes. Photosynthesis deposits low entropy from the sun into plants. Animals feed on low entropy by eating. (If all we needed was energy rather than entropy, we would head for the heat of the Sahara rather than toward our next meal)."

    Is there any exception? Are there living things or any object that feed on heat of the Sahara rather than low entropy?
  2. jcsd
  3. Aug 31, 2018 #2
    That seems to me to be a rather odd way of looking at it. The universe trends towards higher entropy just by statistics. Almost anything that has an order will correlate with entropy. So of course you can correlate the food chain with entropy. However to say that we consume entropy rather than energy sounds like pop culture goofiness.
  4. Sep 1, 2018 #3


    Staff: Mentor

    Living things need energy, but in maximum entropy states, the energy of the system is unavailable for work.

    For example, if we have a hot sink to our left and a cold sink to our right, that is a low entropy arrangement, and a cell can extract work from the flow of energy. But if we have the same temperature everywhere, that is maximum entropy, and we can't make a heat engine.

    I think that's what your question is about. Entropy is related to the availability of energy.

    We can imagine exceptions to the laws of thermodynamics, but we'll probably never find one in reality.
  5. Sep 1, 2018 #4
    Ok, I can buy that, I guess.
  6. Sep 1, 2018 #5
    Everything is relative in the end, if a living being is in "thermal equilibirium"(high entropy state) with its surroundings statistical mechanics dictates there is a high probability it will not suddenly jump into a low entropy state.

    Ive often thought however that there is a chance that after the universe reaches its maximum entropy state there is still a probability that eventually everything starts again from the low entropy state.
  7. Sep 1, 2018 #6
    So there is no example in nature where we can just acquire energy without it being in somewhat lower entropy state... how about when a solar cell is being charged from the energy of the sun... or Geiger counter clicking from exposure to radiation energy... ?
  8. Sep 1, 2018 #7


    Staff: Mentor

    No, there are no examples where the 2nd law of thermodynamics is violated. Those examples you cite seem wrong only because they wrongly define the "system boundaries". When done correctly, there is no violation.

    In fact, betting that the 2nd law will not be violated is one of the surest bets in the universe.
  9. Sep 1, 2018 #8
    Oh, I don’t know. I feel pretty confident betting Clemson will win the ACC again this year!
  10. Sep 1, 2018 #9


    User Avatar

    Staff: Mentor

  11. Sep 1, 2018 #10
    So how does the 2nd law works in the solar cell as it acquired solar energy? or the solar cell needing to feed higher entropy from the sun and not just feed energy from the sun without regards to entropy.
  12. Sep 2, 2018 #11


    Staff: Mentor

    To apply the 2nd law, you must include both the sun and the panel as parts of "the system."
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