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B A simple question about reversible heat

  1. Jan 7, 2017 #1
    Is it possible to heat a specific amount of liquid water, say 1kg, (at 1 atm), let's say from 20°C to 21°C, without changing the universe's entropy?
    (Sorry but I have a little blackout on simple thermodynamics...)

  2. jcsd
  3. Jan 7, 2017 #2
    Ideally, yes. In reality, only approximately.
  4. Jan 7, 2017 #3
    Ok. Ideally how would you do, for example?

  5. Jan 7, 2017 #4
    Is this a homework problem?
  6. Jan 7, 2017 #5
    Not at all.

  7. Jan 8, 2017 #6
    The problem is I am discussing with someone with very strange ideas about time: he says it doesn't exist :smile:
    He thinks time could be replaced by entropy variation ΔS of a system (or with ΔS/m, m = system's mass), for example the ΔS of a specific amount of liquid water heated of a specific ΔT. I objected that, in case, an increment of time could have "something to do" with the universe's ΔS, and that of a finite system (like the said water) has no relation with the universe's ΔS (the last one, for example, can be made almost zero for an almost perfectly reversible system).
    But I know the real problem is the fact I would like to convince those kinds of people about the stupid things they write...

  8. Jan 8, 2017 #7
    Since the entropy of the universe can only get larger it is often argued that it is an indicator of the direction of time. However I think your friend goes too far when he argues that it is equivalent to time, and I think you have the right argument in showing an event that has a direction in time and yet does not change the entropy of the universe.

    Unfortunately it is hard to construct real world change that absolutely positively has no effect on the entropy of the universe. You can always find some little bit of friction somewhere or something. Otherwise a pendulum swinging in a vacuum would be a particularly apropos counter example.

    Even if you come up with a good example he'll just say it is the entropy of the whole universe that creates time and any isolated experiment can't change the march of entropy.
  9. Jan 9, 2017 #8
    Infact it's what he did :-)
    But processes like that, or others wich he would use to model a clock, can be made with different entropy variations; I mean, let's take a pendulum: if frictions are small, every single oscillation will be associated with a small entropy variation; if frictions are large, so would be entropy variation: the period of oscillation would be different in the two cases? It seems ridiculous to me.

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