Question

The fundamental thermodynamic relation for a rubber band is given by 𝒅𝑼 = 𝑻𝒅𝑺 + 𝝉𝒅𝑳, where 𝑻 is the absolute temperature, 𝑺 is the entropy, 𝝉 is the tension in the rubber band, and 𝑳 is the length of the rubber band. Which one of the following relations is CORRECT:

Options :

  1. ( βˆ‚ T βˆ‚ S ) L = ( βˆ‚ Ο„ βˆ‚ L ) S

  2. Ο„ = ( βˆ‚ U βˆ‚ S ) L

  3. T = ( βˆ‚ U βˆ‚ S ) Ο„

  4. ( βˆ‚ T βˆ‚ L ) S = ( βˆ‚ Ο„ βˆ‚ S ) L

Show Answer

Answer :

( βˆ‚ T βˆ‚ L ) S = ( βˆ‚ Ο„ βˆ‚ S ) L

Solution :

If there is a relation between x, y & z, then z may be expressed as a function of x & y.

If an equation of type dZ = Mdx + Ndy is an exact differential, then it can be written as  ( βˆ‚ M βˆ‚ y ) x = ( βˆ‚ N βˆ‚ x ) y

Now,

Comparing this thermodynamic relation dU = TdS + Ο„dL with  ( βˆ‚ M βˆ‚ y ) x = ( βˆ‚ N βˆ‚ x ) y we get,

Z = U, M = T, X = S, N = Ο„, y = L

The thermodynamic relation can be written as,  ( βˆ‚ T βˆ‚ L ) S = ( βˆ‚ Ο„ βˆ‚ S ) L

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