 Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2014/paper-2/section-c/c11/solution

With

F = U - TS

d F = - S d T + γ d A .

Equality of mixed derivatives of

F (T, A)

(\frac{\partial S}{\partial A})_{T} = - (\frac{\partial γ}{\partial T})_{A} .

Use the area convention already specified by

γ d A

; do not insert an additional factor for two film faces.

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