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Homework Help: Specific Heat

  1. Jan 17, 2012 #1
    1. What is the value of Q in this problem? Is it a value that's stated in the question or does it need calculating?

    A sample consisting of 5 moles of an ideal gas at a temperature of 300 K and a
    pressure of 1.00 × 105 Pa is heated to a temperature of 500 K at constant pressure.
    The amount of heat transferred to the gas is 29.1 kJ.

    2. Relevant equations
    No equations needed really, I just need to make sure I'm using the right values!

    3. The attempt at a solution
    Is Q = 29.1 kJ = 29100 J (or would that be U?)
    Is Q the same value for constant volume and constant pressure while ΔU changes because ΔU = Q + W and there is no work in a constant volume situation?
  2. jcsd
  3. Jan 17, 2012 #2
    In this example the gas is at constant pressure which means that it will expand and therefore external work is done during the expansion.
    All of the heat energy does not go to raise the internal energy (raise the temperature) of the gas
    If the gas is heated at constant volume no external work is done ands all of the heat energy goes to internal energy and raise the temperature.
  4. Jan 17, 2012 #3
    I got that bit, I just wasn't sure whether we were actually given Q or U but I've worked it out:

    ∆U = Q = 29100 J
    ∆U = C_v (T_2-T_1)
    etc. Using Q as that gave me a sensible ratio of C_p to C_v so I'm happy!
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