Question

During an adiabatic expansion of an ideal gas, which stays constant?

a) U

b) H

c) S

Internal Energy stays constant?

Which one of the following is true?

a) The entropy of an isolated system can only stay constant or increase during a process

b) We can construct a heat engine that extracts heat from a hot reservoir and delivers an equal amount of work

c) We can construct a refrigerator whose sole effect is to transfer heat from a cooler object to a hotter one

Entropy of isolated system stays constant?

Thank you!

Answer 1

(1).​​​​​​Correct answer = S (entropy)

In Adiabatic Expansion system isolated from surrounding which means there is not exchange of heat from surrounding that means dq=0.

• U ​​​​​​= Internal Energy, according to first law of thermodynamics, ∆U = q + w ​​​​​​, here q= change in heat which is zero so work done during expansion is equal to change in internal energy. So internal energy of system doesn't stay constant during expansion.
• H= enthalpy of the system, ​​​

  The Enthalpy H is defined as H=U+PV. Therefore,ΔH=ΔU+PΔV+VΔP

  For an adiabatic process, q=0. therefore from the first law of thermodynamics,
  ΔU=q+w=q−PΔV
  ΔU=w=−PΔV
  Substituting this in the first equation you get,
  ΔH=VΔP so during expansion H does not stay constant.

• S= Entropy, entropy is a function of heat and Temperature, dS =dq/T, here change in heat is zero so During expansion Entropy of the system stays constant.

(2).correct answer is a).

• Explanation for a) :- For entropy (explains in first question)
• Explanation for b) :-That means thermal efficiency of process is 100 %, which is impossible according to second law of thermodynamics.
• Explanation for c) :- According to the second law of thermodynamics, it is impossible for any process to have heat transfer from a cooler to a hotter object as its sole result.