Question

The incorrect formula for entropy for a calorically perfect gas is____

(a) ds=0

(b) s=s(p,T)

(c) s2-s1=cvln\(\frac {T_2}{T_1}\)+Rln\(\frac {v_2}{v_1}\)

(d) s2-s1=cpln\(\frac {T_2}{T_1}\)-ln\(\frac {p_2}{p_1}\)

I got this question in semester exam.

My enquiry is from A Brief Review of Thermodynamics topic in portion Compressible Flow – Preliminary Aspects of Aerodynamics

Answer 1

Correct answer is (a) ds=0

Easy explanation: The change in entropy is zero only for an adiabatic process of a gas. Calorically perfect gas has constant specific heats, which makes the integration between the initial and final state easier. The final form of the equation comes out as s2-s1=cvln\(\frac {T_2}{T_1}\)+Rln\(\frac {v_2}{v_1}\) and s2-s1=cpln\(\frac {T_2}{T_1}\)-ln\(\frac {p_2}{p_1}\). As visible from these equations s=s(p, T).