PHYS 317 - Reading Assignment #3

Read sections 1.6, and answer the following questions by email (, hardcopy, or web form before class Wednesday, Sep 3.

  1. Name a few systems that have a constant CV (at least constant over some temperature range), and at least one system for which CV shouldn't be constant.

    When all the energy is contained in quadratic degrees of freedom, then U = N f kT/2, and CV = N f k/2 = constant. This would apply to a monatomic ideal gas, or a diatomic ideal gas in the range where f=5 or f=7 (but not in between). Also a solid at high T.

    An example where CV isn't constant is a diatomic ideal gas in the temperature range where kT is just barely large enough to excite the vibrational mode. In the region we are crossing from f=5 to f=7, so the specific heat is changing with temperature.

  2. Which quantity, the heat capacity or the specific heat capacity, is independent of N?

    The specific heat capacity (or specific heat) is independent of N.

  3. Explain briefly why "enthalpy capacity" is a better name for the heat capacity at constant pressure, CP.

    One can increase the enthalpy of a system at constant pressure either by heat flow or by other kinds of work (like a microwave) without heat flow. Either way, CP = (dH/dT) at constant P works fine, so it is more general than just a ``heat'' capacity.

  4. Was there anything you had particular difficulty with in this reading? Is so, describe briefly.

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