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....
Thanks for your informative response. I perceive the scaling argument
to be elegant and powerful. At present, however, I don't follow it in
this case. I assume the equation to which you refer when you write "you
can't have an extensive variable on one side of the equation and not the
other" is:
(1) dU/dV (const T) = 3P + 3V dP/dV (const T)
On the left is the ratio of a change in an extensive variable to a
change in an extensive variable. That ratio is an intensive variable.
The first term on the right is clearly an intensive variable. The
second term on the right is an extensive variable times the ratio of a
change in an intensive variable to a change in an extensive variable.
The ratio is a reciprocal extensive variable. The product of the
extensive variable 3V times that reciprocal extensive variable is thus
an intensive variable. As such, every term in equation (1) scales like
the zeroth power of the size of the system and we can't use the scaling
argument to establish the fact that the second term on the right must
vanish.