Question 1

Which of the following statements best describes a heat engine?

Accepted Answer

A device that transforms heat energy into useful work by operating in a cyclical process between two reservoirs.

Question 2

The thermal efficiency $$\eta of a $$heat engine is defined as:

Accepted Answer

$$\eta = \dfrac{W_{out}}{Q_H}$$

Question 3

A refrigerator is fundamentally different from a heat engine because:

Accepted Answer

It uses external work to transfer heat from a cold reservoir to a hot reservoir.

Question 4

Which equation correctly expresses the first law of thermodynamics for a cyclic process in a heat engine?

Accepted Answer

$$Q_H = Q_C + W_{out}$$

Question 5

The maximum possible efficiency of a heat engine operating between a hot reservoir at temperature $$T_H$$ and a cold reservoir at $$T_C is $$given by:

Accepted Answer

$$\eta_{max} = 1 - \dfrac{T_C}{T_H}$$

Question 6

A Carnot engine operates between $$T_H = 500\,\mathrm{K}$$ and $$T_C = 300\,\mathrm{K}. $$What is its maximum possible efficiency?

Accepted Answer

$$0.60$$

Question 7

Which of the following is a correct statement of the second law of thermodynamics as it applies to heat engines?

Accepted Answer

No heat engine operating between two reservoirs can be more efficient than a Carnot engine operating between the same reservoirs.

Question 8

The coefficient of performance $$K$$ for a refrigerator is defined as:

Accepted Answer

$$K = \dfrac{Q_C}{W_{in}}$$

Question 9

A power plant operates between $$T_H = 673\,\mathrm{K}$$ and $$T_C = 298\,\mathrm{K}. $$What is the maximum possible efficiency for converting heat to electricity?

Accepted Answer

$$0.56$$

Question 10

Which process in the Carnot cycle involves heat transfer at constant temperature?

Accepted Answer

Isothermal expansion

Question 11

Why can’t a real engine achieve the Carnot efficiency?

Accepted Answer

Because real engines always have some irreversibilities and friction.

Question 12

In a Brayton cycle, which of the following steps is responsible for the work output?

Accepted Answer

Adiabatic expansion

Question 13

If a refrigerator operates between $$T_H = 350\,\mathrm{K}$$ and $$T_C = 250\,\mathrm{K}$$, what is the maximum possible coefficient of performance $$K_{max}$$?

Accepted Answer

$$K_{max} = \dfrac{250}{350 - 250} = 2.5$$

Question 14

Which of the following best describes an energy reservoir?

Accepted Answer

A part of the environment so large that its temperature does not change when heat is transferred.

Question 15

A heat engine absorbs $$Q_H = 1000\,\mathrm{J}$$ from the hot reservoir and exhausts $$Q_C = 600\,\mathrm{J} to $$the cold reservoir per cycle. What is the work output per cycle?

Accepted Answer

$$W_{out} = 400\,\mathrm{J}$$

Question 16

Which of the following is NOT a step in the Carnot cycle?

Accepted Answer

Isobaric heating

Question 17

If a refrigerator removes $$Q_C = 500\,\mathrm{J} of $$heat from the cold reservoir and requires $$W_{in} = 200\,\mathrm{J} of $$work input, what is its coefficient of performance?

Accepted Answer

$$K = 2.5$$

Question 18

Which of the following statements about the second law of thermodynamics is correct?

Accepted Answer

It forbids the construction of a perfect heat engine with $$\eta = 1.$$

Question 19

In a real-world power plant, why is the actual thermal efficiency much lower than the Carnot efficiency?

Accepted Answer

Because of irreversibilities, friction, and practical engineering limitations.

Heat Engines and Refrigerators: Principles, Efficiency, and Thermodynamic Limits

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