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Change in thermal energy of gas

  1. Dec 1, 2009 #1
    1. The problem statement, all variables and given/known data
    A gas is compressed from 600cm3 to 200cm3 at a constant pressure of 400kPa . At the same time, 100J of heat energy is transferred out of the gas.

    What is the change in thermal energy of the gas during this process?
    2. Relevant equations
    Express the answer in Joules.

    Wgas=p(Vf-Vi)=p/\v


    3. The attempt at a solution

    400k(400cm.3)=p/\v

    I came up with a wrong answer of 160 J. Any help would be appreciated.

    Thanks,
    Jared
     
  2. jcsd
  3. Dec 1, 2009 #2

    kuruman

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    You calculated the work done on the gas not the change in thermal energy. What equation gives you the change in thermal energy of a gas?
     
  4. Dec 1, 2009 #3
    I found the correct equation to use is Eth=Q-Wgas.

    Alright. Eth=100J-160J = 60 J is that answer required to be in a positive or negative 60J?

    Thanks,
    Jared
     
    Last edited: Dec 1, 2009
  5. Dec 1, 2009 #4

    kuruman

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    Using the first law is the correct thing to do. It says ΔE = Q - W. You need to pay attention to the symbols and their signs. Is Q positive or negative? What about W? Positive or negative and why?
     
  6. Dec 1, 2009 #5

    ideasrule

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    BTW, the sign on W is purely arbitrary. Thermodynamics has adopted the convention that W should be considered from the point of view of the gas: that is, the work done by the gas is W.
     
  7. Dec 3, 2009 #6
    I am pretty certain the answer is -60J. Although, I don't feel confident that you can have a negative thermal energy change.

    @ideasrule- I don't quite understand what you mean?
     
  8. Dec 3, 2009 #7

    kuruman

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    Remember that the internal energy depends on the temperature only. A negative thermal energy change means that the temperature drops.
     
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