# Thermodynamics Questions and Answers – Types of Equilibrium

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This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Types of Equilibrium”.

1. For an isolated system,
a) dS<0
b) dS>0
c) dS=0
d) none of the mentioned

Explanation: We have dU+pdV-TdS<0 and for isolated system, U and V are constant.

2. The entropy of an isolated system always ____ and reaches ____ when equilibrium is reached.
a) remains constant, maximum
b) decreases, minimum
c) increases, maximum
d) none of the mentioned

Explanation: This is because dS>0 for an isolated system.

3. Which constraints must be imposed on system to make the Helmholtz function decrease?
a) constant T and p
b) constant U and T
c) constant U and V
d) constant T and V

Explanation: In these constraints, the Helmholtz function decreases and becomes minimum at final equilibrium state.

4. If the constraints are constant p and T, then the Gibbs function of a system decreases.
a) true
b) false

Explanation: The Gibbs function becomes minimum at the final equilibrium state.

5. Which of the following statement is true?
a) a system is in equilibrium state if, when it is disturbed, it comes back to its original state
b) if there is a spontaneous change in the state, the system is not in equilibrium
c) during a spontaneous change, the entropy of system increases
d) all of the mentioned

Explanation: During a spontaneous change, the entropy of system increases and reaches a maximum when equilibrium is reached.

6. A system is said to be in a state of unstable equilibrium when
a) dG<0, dF<0, dS<0
b) dG<0, dF<0, dS>0
c) dG>0, dF>0, dS>0
d) dG>0, dF>0, dS<0

Explanation: These conditions refer to a spontaneous change which makes unstable equilibrium.

7. What is the criterion of stability?
a) dG<0, dF<0, dS<0
b) dG<0, dF<0, dS>0
c) dG>0, dF>0, dS>0
d) dG>0, dF>0, dS<0

Explanation: For these conditions, a system is said to be in a state of stable equilibrium.

8. For a system to be in a state of neutral equilibrium,
a) dS=dG=dF=0
b) dS=0, dG=dF<0
c) dS=0, dG=dF>0
d) none of the mentioned

Explanation: The thermodynamic criterion of equilibrium thus remains at constant value for all possible variations.

9. If a system is stable to small but not large disturbances, it is said to be in metastable equilibrium.
a) true
b) false

Explanation: This is the definition of metastable equilibrium and an example can be mixture of oxygen and hygrogen.

10. Which of the following is true for a system at equilibrium?
a) S=Smax and ∂S=0
b) F=Fmin and ∂F=0
c) G=Gmin and ∂G=0
d) all of the mentioned

Explanation: These are necessary but not the sufficient conditions for equilibrium.

11. For thermal stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) both of the mentioned
d) none of the mentioned

Explanation: Since T>0K , Cv>0 for thermal stability.

12. For mechanical stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) (∂p/∂V)<0, at constant temperature
d) all of the mentioned

Explanation: This is the condition for mechanical stability.

13. For total stability,
a) Cv>0
b) (∂p/∂V)<0, at constant entropy
c) (∂p/∂V)<0, at constant temperature
d) all of the mentioned

Explanation: All these three conditions are required for stability.

14. The third law is a fundamental law of nature and cannot be proved.
a) true
b) false

Explanation: The third law is not derivable from second law and cannot be proved as is the case with zeroth, first and second laws.

15. For magnetic cooling, ____ salt is used.
a) diamagnetic
b) paramagnetic
c) both of the mentioned
d) none of the mentioned

Explanation: A paramagnetic salt like gadolinium sulphate is used for magnetic cooling.

Sanfoundry Global Education & Learning Series – Thermodynamics.
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