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An Introduction to Thermal Physics
Found in: Page 159
An Introduction to Thermal Physics

An Introduction to Thermal Physics

Book edition 1st
Author(s) Daniel V. Schroeder
Pages 356 pages
ISBN 9780201380279

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Short Answer

The formula for CP-CV derived in the previous problem can also be derived starting with the definitions of these quantities in terms of U and H. Do so. Most of the derivation is very similar, but at one point you need to use the relation P=-(F/V)T.

CP-CV=TSVTVTP

See the step by step solution

Step by Step Solution

Given

CP-CV

Explanation

The specific heat of a substance can be of two types:(i) specific heat at constant pressure CP(ii) specific heat at constant volume CV

They are given by

CV=UTVCP=HTP

Where, U = internal energy, H = enthalpy, V = volume and P = pressure.

Lets consider U=U(V, T), and differentiate above expression with respect to T, we get
dU=UVTdV+UTVdT

Similarly write expression for enthalpy H=U+P V.

Write the expression for d H at constant pressure d H=d U+P d V

Substitute dU=UVTdV+UTVdT

We get,

dH=UVTdV+UTVdT+pdV=UVT+PdV+UTVdT

Simplify, divide both sides of the above expression by d T,

HTP=UVT+PVTP+UTV...........................(1)

Substitute CP for HTP and CV for UTV

CP=UVT+PVTP+CV.............................(2)

Substitute -FVTfor P in equation (2)

CP=(U-F)VTVTP+CV

Now substitute TS for (U-F)

CP=TSVTVTP+CV

So, the value of CP-CV=TSVTVTP

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