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Thermodynamics problem! How to find the temperature of a gas.

  1. Dec 8, 2011 #1
    Thermodynamics problem! How to find the temperature of a gas. Please Help!

    1. The problem statement, all variables and given/known data

    In an engine, 0.45 moles of gas at 1050 degrees K in a cylinder expand adiabatically against the piston. The gas does 3200 J of work against the piston. What is the final temperature of the gas?

    2. Relevant equations

    What formula do I use?

    3. The attempt at a solution

    The only formula I can think to use is

    W=(-P)(delta V)

    But pressure and volume aren't given, and I can't find them with the given information.
     
  2. jcsd
  3. Dec 8, 2011 #2

    ehild

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    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    Assume that the gas is ideal and apply the ideal gas law together with the First Law of Thermodynamics.

    ehild
     
  4. Dec 8, 2011 #3
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    How do you apply the first law of thermodynamics?
     
  5. Dec 9, 2011 #4

    Andrew Mason

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    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    First law: Q = ΔU + W where W = work done by the gas.

    Here you are given Q and W. You first have to determine ΔU.

    In order to find the change in temperature, however, you have to know the relationship between ΔU and ΔT. That depends on the gas. Are you given the Cv of the gas?

    AM
     
  6. Dec 9, 2011 #5

    ehild

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    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    What have you learnt about adiabatic expansion of a gas? What is the relation among pressure and volume during such a process?
    Anyway, you need to know the ratio Cp/Cv. You do not need the initial volume and pressure to solve the problem.
    Collect all equations you know and are relevant: The ideal gas law, the adiabatic equation of state (you can derive it using the First Law if you do not know it) and the expression of work done by the gas.

    ehild
     
  7. Dec 9, 2011 #6

    Andrew Mason

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    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    I don't think the adiabatic condition applies here since there is no indication that it is a quasi-static adiabatic expansion. Besides, one would need to know either P or V as well as T. However, we do need to know Cv to solve the problem

    AM
     
  8. Dec 9, 2011 #7

    ehild

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    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    You are right, we do not need to assume a quasi static process. But we need to know if it is an ideal gas and the number of atoms in its molecules, or Cv for the internal energy. .

    ehild
     
  9. Dec 9, 2011 #8
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    Sounds like you have to assume ideal gas
     
  10. Dec 12, 2011 #9
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    Thank you for your help!!!
     
  11. Dec 12, 2011 #10
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    No problem - guess you got it :-)
     
  12. Dec 12, 2011 #11
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    I sure did!! :) Can you help me with my new problem? lol
     
  13. Dec 12, 2011 #12
    Re: Thermodynamics problem! How to find the temperature of a gas. Please Help!

    point me to it
     
  14. Dec 12, 2011 #13
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