THE FIRST LAW OF THERMODYNAMICS, AND
THERMAL PROCESSES!
ΔU = Q + W
The change in internal energy of a system equals
the heat added to the system plus the work done on
the system. (Beware of sign conventions in other books!) Note
that for an ideal gas, U = (3/2)NkT, so that if the
temperature does not change the internal energy cannot change.
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An expanding gas is doing work, a gas being
compressed is having work done on it. The sign convention
adopted in our text makes W < 0 if the gas is doing work, and
W > 0 if work is being done on the gas. We can say in general
that W = - ∫ViVf
p dV.
ISOTHERMAL PROCESS: The product pV
must remain constant.
ISOVOLUMETRIC PROCESS: ΔU = Q.
ISOBARIC PROCESSES... note the existence
of the important molar specific heats, Cp and Cv,
vital particularly in understanding thermal processes in
gases.
ADIABATIC PROCESSES... Q = 0.
Processes that cycle... ΔU = 0.
The equipartition theorem assigns
(1/2)kT to each degree of freedom possessed by an
atom or molecule. This causes CV
to increase by R/2 for each new degree of freedom.
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