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Homework Help: Pressure law (how they combined to get pv/t)

  1. Dec 29, 2004 #1


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    Well .. i guess the question is in the title.I basically dont understand how they combined :

    P/T = Constant


    P . V = Constant to make P . V / T = Constant

    i can guess

    P/T = P . V which means P = P . V. T .. ummm help please!

    Last edited: Dec 29, 2004
  2. jcsd
  3. Dec 29, 2004 #2
    there is one more law your missing.


    don't assume all the constants are equal either. Each one of those constants are different from the other.

    If we know that P varies Directly with T and that V varies directly with T than PV varies directly with T.

    So you get PV/T=constant. The constant in this equation absorbs the other three. Yeah I know it's a bad explanation. I understand it in my mind perfectly but I don't know how to explain it any better for now.
  4. Dec 29, 2004 #3


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    You need to consider the complete expression of the Ideal Gas Law to understand the constants.

    PV = nRT

    So when you say PV = Constant you are saying that nRT, is a constant. The only true constant here is R, n is the number of moles of material present. Clearly the statement PV = C is only true in very special conditions. That is when the amount of material (number of atoms) present is constant and the Temperature is constant.

    To say the [itex] \frac P T = C [/itex] is to say that [itex] \frac {nR} V =C[/itex].
  5. Dec 29, 2004 #4


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    Ah yes.Thank you for your answers.The questions i am attemtping are rather easy and so they dont consider the amount of moles and molar mass and all of that stuff. Next section :)

  6. Dec 29, 2004 #5

    Integral. What your doing is reverse engineering a problem. Charles' Law and Boyle's Law were found before the ideal gas law and he wants to know how they combined that to get [itex] \frac {PV} T = C [/itex].

    It's like where you know the force of gravity is directly proportional to the mass and inversely proportional to the distance squared.

    How do you combine that to get [tex] F_g = G \frac {m_1 m_2} {r^2} [/tex]?
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