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Where does the (5/2) come from in calculating thermal energy of diatomic gas

  1. Mar 26, 2012 #1
    So I am doing a second year thermodynamics course and would like to know. Do we just have to remember (5/2)PV for a diatomic gas, why is it 5/2 and also what is it for a monatomic gas. Also would we have to remember more complex ratios for exams?

  2. jcsd
  3. Mar 27, 2012 #2


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    This is value known as specific heat capacity at constant volume, or CV. You obtain said quantity by counting degrees of freedom and dividing it by 2. For diatomic gas, there are 7 total, 3 translational, 2 vibrational, and 2 rotational, but the 2 vibrational degrees of freedom are typically "frozen out". So you only count the 5. That gives you the 5/2. For monatomic gas, there are only the translational, so you get 3/2.

    Realistically, the value will be off by a bit. There is an associated quantity γ=CP/CV known as heat capacity ratio. You can look it up for gas of interest, and use the fact that CP=CV+1 to compute the actual ratio.

    Note that all these values are in units of R. So the actual specific heat capacity of diatomic gas at constant volume will be 5/2R per mole of gas.
    Last edited: Mar 27, 2012
  4. Mar 27, 2012 #3
    Thanks, that makes sense now. Except, my book doesn't go through the derivation, it doesn't give the unit R just the ratio. What does it physically represent?
  5. Mar 27, 2012 #4


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    Same as any heat capacity. How much energy you need to change the temperature. With gases, however, you can either hold the cylinder closed, and then the volume remain constant, but pressure changes with temperature, or you can have a piston in the cylinder, which keeps pressure constant, but lets volume vary. Because moving piston takes work, you need more heat to increase temperature when you keep pressure constant. Hence, CP is higher than CV.

    So suppose you want to change the temperature by ΔT in a closed cylinder, id est, constant volume. The amount of heat will be ΔQ = nCVΔT, or for diatomic gas, ΔQ = 5/2 nRΔT

    Keep in mind that the symbol CV may be used for specific heat capacity, as I have been doing, or for total heat capacity.
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