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Homework Help: Problem with ideal gas law and a spring! Help please

  1. Apr 11, 2007 #1
    Problem with ideal gas law and a spring! Help please!!

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

    We have a box divided in two parts by a piston without friction, and in one part there are n moles of an ideal gas and a Spring orf constant K and natural longitude L which keeps the piston in equilibrium. According to this, and knowing that the temperature of the gas is T0, find the amount for K.

    2. Relevant equations


    3. The attempt at a solution

    I know its (nRTo)/6L^2 because the book has the answers, but I dont know where it came from! Please help!
  2. jcsd
  3. Apr 11, 2007 #2
    what do you know besides eqn. How do we link the two systems?

    The common part to both is the piston, so Pressure of the gas multiplied by its area exerts a force on it.

    This force must be the same as that exerted by the spring in order for system to be in equilibrium, that is piston is not moving.

    Does this help?
  4. Apr 11, 2007 #3
    I forgot to tell that the longitude of the spring in equilibrium is 3L (which means that x=2L, I think?) So, what I did is:

    P= F/A


    And volume is one, because its an ideal gas, so



    And thats all I know
  5. Apr 11, 2007 #4
    Forget it! I did it :rofl: :rolleyes: :tongue2:


    Thanks anyways!! :)
  6. Apr 11, 2007 #5
    Thanks, I was wondering where the 6 came from :confused:

    Thats getting real close,

    the importat links are being made:

    There is nothing that says ideal gas has volume of 1,
    in fact it is 22.4 L for n=1.

    what we can say P=nRT/(A*L) where L here is the Length at equilibrium under final condition.

    we know that force is P*A=nRT/L

    we also know that F=K*L where L is above.

    then k=nRT/L^2 now relate the L I used vs that you were given at equilibrium as I had some trouble understanding problem description.
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