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Entropy and Potential Temperature Questions and Answers

101.

In an isentropic process there is no change in :

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

In an isentropic process there is no change in heat, that is, no heat is exchanged.

An isentropic process is one at constant entropy. Since \(dS = dQ_{rev}/T\), constant entropy in a reversible process means dQ = 0: the process is reversible and adiabatic. The dry adiabatic ascent of an air parcel is the standard example, and it is why potential temperature, which is tied to entropy, is conserved.

Temperature, volume and pressure all change in such a process: a rising parcel expands and cools as its pressure falls. Isothermal, isochoric and isobaric processes are the ones that keep those quantities fixed.

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102.

In a thermodynamic system entropy φ is defined as ............. where dh is heat added, p and T are pressure and temperature.

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

Entropy is defined as dh/T, the heat added divided by the absolute temperature.

In thermodynamics the change in entropy for a reversible process is

\[ d\phi = \frac{dh}{T} \]

where dh is the heat added and T the absolute temperature (in kelvin) at which it is added. The same amount of heat produces a larger entropy change when added at a low temperature than at a high one. In meteorology this definition underlies potential temperature and the analysis of dry adiabatic processes, which are isentropic.

Dividing by pressure (dh/p or dh/pT) or by the square of temperature (dh/T2) does not give entropy.

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