1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A possible case of an irreversible process in which total entropy decreases

  1. Aug 12, 2011 #1
    When a hot stone is dropped into a lake the change in temperature of the lake is negligible, but the stone cools down and so its entropy decreases. Is this therefore a case of an irreversible process in which total entropy decreases?

    This isn't for homework but for revision towards my resit. I just can't find anything in my notes which is helping me understand.
     
  2. jcsd
  3. Aug 12, 2011 #2

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hi ticktock, welcome to PF. No, this is not an example of total entropy decreasing. The entropy increase in the lake is finite (and greater than the entropy decrease in the stone) even if you consider the temperature increase in the lake to be negligible.
     
  4. Aug 12, 2011 #3

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    When an amount of heat |dQ| moves from the stone to the lake, the decrease in entropy of the stone is[tex]dS_\mathrm{stone} = -\frac{|dQ|}{T_\mathrm{stone}}[/tex] and the increase in entropy of the lake is [tex]dS_\mathrm{lake} = \frac{|dQ|}{T_\mathrm{lake}}[/tex]The total change in entropy is dS = dSlake+dSstone. Given the relative temperature of the lake and stone, what can you say about the sign of dS? Does assuming Tlake is a constant make a difference?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: A possible case of an irreversible process in which total entropy decreases
Loading...