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Homework Help: Thermodynamics Block of Copper Problem

  1. Apr 8, 2005 #1
    Hi, I need help with this problem for physics. I know it has something to do with Q=mcT. I can probably do parts b and c if I just get a start on part a. I'm just not sure how to approach it. Any help is appreciated. =)

    A 50g block of copper having a temperature of 400K is placed in an insulating box with a 100g block of lead having a temperature of 200K. a) What is the equilibrium temperature of this two-block system? b) What is the change in the internal energy of the two-block system as it goes from the initial condition to the equilibrium condition? c) What is the change in the entropy of the two-block system?

    Thanks in advance,
  2. jcsd
  3. Apr 8, 2005 #2


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    Actually part a) is the simplest.Compute the heat each body need to change (either accept or receive) in order to reach equilibrium.Then use the "insulating box" part...

  4. Apr 8, 2005 #3
    Well I just calculated that the initial heat of the copper is 7720J and the initial heat of the lead is 2560J, but how would I calculate how much heat they need to accept or receive in order to reach equilibrium?
  5. Apr 8, 2005 #4


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    U have a copper block at 400K and this cools till he reaches equilibrium temp:x (unknown).

    The heat lost is

    [tex]Q_{lost}=m_{Co}c_{Co}\left(400-x\right) \ [J] [/tex]

    The lead block heats from 200K till equilibrium temp."x".

    The heat received

    [tex] Q_{received}=m_{Pb}c_{Pb}\left(x-200\right) \ [J] [/tex]

    What is the connection between the 2#-s (i used no sign convention:both #-s are positive (i hate sign conventions in thermo & geom.optics)) ??

  6. Apr 8, 2005 #5
    Sorry I stopped posting after. I had to run out really quick unexpectedly. I did what you said and I got 794K as an equilibrium temperature, but that doesn't make sense. Maybe I did it wrong? You probably aren't there anymore but if you aren't, if someone else wants to help me that would be awesome.
  7. Apr 8, 2005 #6
    actually I take that back, I got 317K now. So then would that be the answer for part a?

    Also, when I do get the equilibrium temp. for part (a), for part (b) would I find the change in internal energy for each block and add the two values together? And as far as finding that, what would be the best equation to use?

    would I use an internal energy equation for an isochoric system?
    Last edited: Apr 9, 2005
  8. Apr 9, 2005 #7
    I'm just trying to get this thread back up higher since I'm not getting any responses. I still need some help. I'd really appreciate it!
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