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Homework Help: Ideal Gas Situation: What happens when P, V decrease?

  1. Dec 16, 2012 #1
    1. The problem statement, all variables and given/known data

    If the volume V and pressure P of an ideal gas decrease, which statement is true about the internal energy IE, temperature T and the intermolecular forces IF?

    (I am not gonna copy and paste the choices here in an effort to focus attention on the attempt of solution.) Would greatly appreciate if anyone can point out an error in thinking!

    2. Relevant equations

    Change in Internal Energy (I.E.) = Q (heat added) + W (done on system)
    PV/T = PV/T

    3. The attempt at a solution

    First: My final answer. (Incorrect)
    IE increases, T increases, IF are unchanged

    Since P and V decrease, by P1V1/T1 = P2V2/T2, the temperature of this ideal gas should decrease.
    Since Kinetic Energy is directly proportional to KE, the Internal Energy of the system should increase!
    (In an ideal gas: no Intermolecular forces, otherwise, the gas would liquefy!)
    So, Internal Energy is composed of KE, and since Temperature goes up, IE goes up, and IF is constant (or zero).

    Many thanks in advance !
  2. jcsd
  3. Dec 16, 2012 #2

    I like Serena

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    Welcome to PF, yaylee! :smile:

    That is correct: the temperature decreases.

    Huh? I'm assuming the with KE you mean Kinetic Energy, don't you?
    This doesn't make sense to me.

    Either way, the Kinetic Energy is directory proportional to the Internal Energy.
    And they are both also directly proportional to the temperature.
    So since temperature goes down, so do KE and IE.

    It's not true that the gas would liquify - that would depend on the circumstances.
    But yes, in an ideal gas there are no intermolecular forces.

    Hold on!
    Didn't you say before that the temperature decreased? :confused:

    Btw, you are right that IF is constant, moreover it is and remains zero, since we're talking about an ideal gas.
  4. Dec 16, 2012 #3
    Hello I like Serena !

    Thank you for the welcome.

    I am sorry for mixing up increase/decrease throughout the problem.

    In an ideal gas, As temperature DECREASES, Kinetic Energy decreases and therefore internal energy DECREASES.

    Also: Intermolecular forces are unchanged, or remain zero throughout.

    I am wondering why this is getting marked incorrect, oh well! Thank you once again.
  5. Dec 16, 2012 #4
    Ah, OK. There was a mistake with the scoring. Your answer was correct. Thanks for the help!
  6. Dec 16, 2012 #5

    I like Serena

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    As you just wrote it down, it is correct.

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