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Basic question on Specific Heat Ratio of gases

  1. Aug 14, 2004 #1
    I have been trying to understand the concept of specific heat ratios. From what I gather, the ratio is defined as the quotient of the Specific heat at constant pressure and the Specific heat ratio at constant volume.

    Could anybody give me an insight into what leads to gases having different specific heats when under constant pressure or constant volume. I'm guessing it's to do with the difference in particle interactions when at constant volume and at constant pressure, but how exactly?

    And does the ratio of specific heat have any physical meaning attached to it? If so, how would you define the ratio other than in a mathematical way.
  2. jcsd
  3. Aug 14, 2004 #2


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    Basically, what deterines the specific heat of a gas is its structure - is it monoatomic, diatomic, triatomic, or possibly even larger.

    For a monoatomic ideal gas, there are three degrees of freedom corresponding to translation.

    For a diatomic gas, one has more degrees of freedom. The molecule can rotate, and vibrate. Quantum mechanics also enters here, the different degrees of freedom "activate" depending on the temperature.

    Triatomic gasses have even more degrees of freedom than diatomic- there are more vibrational and rotational modes.

    To make life even more interesting, forces between atoms affect the heat capacity as well, causing some departure away from the ideal gas equation.

    To get all the details of what degrees of freedom are active when, I'd suggest consulting a good physics book. Haliday & Resnick has some discussion of this IIRC, at least in the older editions.
  4. Aug 16, 2004 #3
    Thanks for the explanation pervect. However, I am still not clear about what leads to a gas having some heat capacity at constant pressure and why that heat capacity has to be different at constant volume. I understand that different gases have different specific heat capacities (shc) because of the different degrees of freedom they may have, from what you explained. But why does the same gas (say a monoatomic gas) have a different shc at constant volume than it has at constant pressure.
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