# Thermodynamics Questions and Answers – Second Law Efficiency-1

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This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Second Law Efficiency-1”.

1. The first law efficiency is defined as the ratio of the output energy to the input energy.
a) true
b) false

Explanation: First law efficiency = output energy / input energy.

2. Which of the following statement is true about the first law?
a) it is concerned only with the quantities of energy
b) it disregards the form in which the energy exists
c) it does not discriminate between the energies available at different temperatures
d) all of the mentioned

Explanation: It is the second law which provides a means of assigning a quality index to energy.

3. With the concept of exergy available, which of the following is possible?
a) to analyse means of minimizing the consumption of available energy to perform a given process
b) to ensure most efficient possible conversion of energy
c) both of the mentioned
d) none of the mentioned

Explanation: These statements tell us why the concept of exergy is so important.

4. Second law efficiency is defined as
a) actual exergy intake / minimum exergy intake
b) minimum exergy intake / actual exergy intake
c) actual exergy intake / maximum exergy intake
d) maximum exergy intake / minimum exergy intake

Explanation: It is the ratio of minimum exergy which must be consumed to do a task divided by the actual amount of exergy consumed in performing the task.

5. For a power plant, second law efficiency can be given by desired output(W) / available energy(Wmax).
a) true
b) false

Explanation: Here, A=Wmax and Amin=W, hence second law efficiency = Amin/A = W/Wmax.

6. Second law efficiency can also be given as
a) 1 / ( first law efficiency * Carnot efficiency)
b) Carnot efficiency * first law efficiency
c) Carnot efficiency / first law efficiency
d) first law efficiency / Carnot efficiency

Explanation: First law efficiency = W/Q1 = (W/Wmax)*(Wmax/Q1) = second law efficiency * Carnot efficiency.

7. If work is involved, Amin= ____ and if heat is involved, Amin= ____
a) w, Q(1+To/T)
b) W, Q(1-To/T)
c) Q(1+To/T), W
d) Q(1-To/T), W

Explanation: This is because, Wmax=Q1(1-To/T).

8. If solar energy Qr is available at a reservoir storage temperature of Tr and if quantity of heat Qa is transferred by the solar collector at temperature Ta, then which of the following is true?
a) first law efficiency = Qa/Qr
b) second law efficiency = exergy output / exergy input
c) second law efficiency = (first law efficiency)*(1-To/Ta)/(1-To/Tr)
d) all of the mentioned

Explanation: Second law efficiency = first law efficiency / Carnot efficiency.

9. In case of a heat pump, second law efficiency is given as
a) (first law efficiency)*(1-Ta/To)
b) (first law efficiency)*(1+To/Ta)
c) (first law efficiency)*(1-To/Ta)
d) none of the mentioned

Explanation: First law efficiency = Qa/Wi and second law efficiency = Qa*(1-To/Ta)/Wi.

10. Both first law efficiency and second law efficiency indicate how effectively the input has been converted into the product.
a) true
b) false

Explanation: First law of efficiency does this on energy basis and second law efficiency does it on exergy basis.

11. For proper utilization of exergy, it is desirable to make first law efficiency ____ and the source and use temperatures should ____
a) as close to unity, be different
b) as close to unity, match
c) as close to zero, match
d) as close to zero, be different

Explanation: If first law efficiency is close to unity, the all the energy carried in by heat transfer is used and no heat is lost to the surroundings.

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