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Homework Help: Internal Energy of an Expanding Gas

  1. Feb 22, 2010 #1
    1. The problem statement, all variables and given/known data
    2 moles of a monatomic ideal gas expands from state A to state B. PA = PB = 600 N/m2, VA = 3.0 m3 and VB = 9.0 m3.

    i. Calculate the work done by the gas as it expands.
    ii. Calculate the change in internal energy of the gas as it expands.
    iii. Calculate the heat added to or removed from the gas during this expansion.

    2. Relevant equations
    ΔE = q-w
    w = PΔV
    PV = nRT
    R = 8.314 J/mol*K

    3. The attempt at a solution
    i. w = PΔV
    w = (600)(9-6) = 3600 J

    ii. 4.186 J of work = cal of heat ?
    3600/4.186 = 860 cals

    I'm having troubles with ii and iii, I'm not sure if the converting of the work into calories is correct. I've looked at all the possible formulas and I'm not sure how to go about finding internal energy with only pressure and volume. Help, please!
  2. jcsd
  3. Feb 23, 2010 #2
    Have you come across the formula:
    [tex]U = \frac{3}{2}nRT[/tex]​
    for an ideal gas? This should be found in any standard textbook.
    So you just have to find [tex]U_{2} - U_{1}[/tex] for the change in internal energy.

    No, mechanical work is not equal to heat! They are separate concepts. You have to use the first law of thermodynamics
    [tex]\Delta U = W + Q[/tex]​
    where [tex]\Delta U[/tex] = change in internal energy, W = work done on the gas, Q = heat supplied to the system.
    I see that you have already written a variant of the equation under "relevant equations". You just have to use that equation and substitute the relevant values.
  4. Feb 23, 2010 #3
    Ahhh, I haven't seen that formula!
    I've got it now, thank you.
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