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Homework Help: Thermodynamics - Work Done in a Closed System (Polytropic?)

  1. Mar 3, 2015 #1
    Just want to check that I've used the correct method for this thermodynamics question i've been set. Any clarification would be greatly appreciated.

    So I have a closed system in which a gas is compressed from a pressure of 3 bar absolute to 5 bar absolute. The volume changes from 0.9m^3 to 0.6m^3 after compression. This is according to the law P1*V1^n = P2*V2^n.

    I've calculated the value n using transposition and logs.

    I need to find:
    b) The work done, stating whether it is done on the system or on the surroundings.

    2. The attempt at a solution

    b) P1 = 3 bar = 300000 n/m^2
    P2 = 5 bar = 500000 n/m^2
    V1 = 0.9 m^3
    V2 = 0.6 m^3
    Index N = 1.26

    I believe the formula that you are supposed to use is the polytropic process formula for work done:

    Work Done = (P1*V1) - (P2*V2) / N-1

    Work Done = (300000*0.9) - (500000*0.6) / 1.26-1

    Work Done = 270000 - 300000 / 0.26

    Work Done = -115385 Joules
    -115.4 KJ

    Because the work done is a negative value I believe the work is being done on the system.

    Now have I done this correctly or have I slipped up and used the wrong method? Cheers for any help!
  2. jcsd
  3. Mar 3, 2015 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Looks right!
  4. Mar 30, 2015 #3
    Thanks :)
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