MCQs / Physics / Heat Engines, Second Law and Entropy

Heat Engines, Second Law and Entropy MCQs — Physics (page 5)

165 multiple-choice questions on Heat Engines, Second Law and Entropy from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

101. For an ideal (Carnot) refrigerator, K =

  1. 1 - TCTH
  2. THTC
  3. TCTH - TC
  4. TH - TCTC

Answer: C) TCTH - TC

Explanation: From QC/QH = TC/TH. SLO P-11-C-17 · Understanding

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102. A power station's waste heat in cooling water is an example of:

  1. created energy
  2. degraded (low-grade) energy
  3. destroyed energy
  4. high-grade energy

Answer: B) degraded (low-grade) energy

Explanation: Too cool to drive another engine efficiently. SLO P-11-C-19 · Understanding

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103. Which natural process degrades energy?

  1. an ideal Carnot cycle
  2. a perfectly elastic collision in a vacuum
  3. a ball rolling to rest on grass
  4. a frictionless pendulum

Answer: C) a ball rolling to rest on grass

Explanation: Friction converts KE to heat. SLO P-11-C-20 · Understanding

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104. Why is the Carnot engine an ideal (not real) engine?

  1. it has friction
  2. it uses only reversible processes
  3. it has 100% efficiency
  4. it uses irreversible processes

Answer: B) it uses only reversible processes

Explanation: Real engines have losses. SLO P-11-C-14 · Understanding

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105. A refrigerator does not violate the Clausius statement because:

  1. it has no working substance
  2. it is 100% efficient
  3. heat flows spontaneously from cold to hot
  4. external work is done on the refrigerant

Answer: D) external work is done on the refrigerant

Explanation: Heat is pumped, not flowing by itself. SLO P-11-C-17 · Understanding

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106. A Carnot cycle consists of:

  1. two isobaric and two isochoric processes
  2. four adiabatic processes
  3. two isothermal and two adiabatic reversible processes
  4. four isothermal processes

Answer: C) two isothermal and two adiabatic reversible processes

Explanation: Isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression. SLO P-11-C-16 · Understanding

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107. In a light bulb, much of the electrical energy is degraded to:

  1. kinetic energy of the bulb
  2. heat
  3. chemical energy
  4. nuclear energy

Answer: B) heat

Explanation: Only some becomes light, which is also eventually absorbed as heat. SLO P-11-C-20 · Understanding

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108. A Carnot engine works between 500 K and 300 K. Its efficiency is:

  1. 66.7%
  2. 80%
  3. 60%
  4. 40%

Answer: D) 40%

Explanation: eta = 1 - TCTH = 1 - 300500 = 40%. SLO P-11-C-16 · Understanding

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109. Which has the lowest entropy?

  1. a perfect crystal near 0 K
  2. steam
  3. liquid water
  4. air

Answer: A) a perfect crystal near 0 K

Explanation: Most ordered. SLO P-11-C-22 · Understanding

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110. Over time, a building left unattended:

  1. deteriorates (becomes more disordered)
  2. stays perfect
  3. becomes more ordered
  4. gains energy

Answer: A) deteriorates (becomes more disordered)

Explanation: Natural tendency. SLO P-11-C-21 · Understanding

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111. The Carnot limit applies to:

  1. only petrol engines
  2. only steam engines
  3. only refrigerators
  4. all heat engines working between the given temperatures

Answer: D) all heat engines working between the given temperatures

Explanation: Universal limit. SLO P-11-C-23 · Knowledge

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112. As the entropy of the universe increases, the energy available to do work:

  1. becomes infinite
  2. increases
  3. stays the same
  4. decreases

Answer: D) decreases

Explanation: Energy becomes more dispersed. SLO P-11-C-19 · Understanding

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113. No heat engine can be 100% efficient because:

  1. work cannot be done
  2. energy is destroyed
  3. friction is infinite
  4. some heat must always be rejected to the cold reservoir

Answer: D) some heat must always be rejected to the cold reservoir

Explanation: Second law (Kelvin statement). SLO P-11-C-13 · Knowledge

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114. Which state of a substance has the greatest disorder?

  1. liquid
  2. solid
  3. gas
  4. crystal at 0 K

Answer: C) gas

Explanation: Gas particles move freely in all directions. SLO P-11-C-18 · Understanding

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115. High-grade (useful) energy includes:

  1. heat spread in the environment
  2. electrical and mechanical energy
  3. heat at room temperature
  4. waste heat in rivers

Answer: B) electrical and mechanical energy

Explanation: Can be converted easily. SLO P-11-C-19 · Understanding

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116. Temperature and disorder are linked because temperature measures:

  1. the pressure only
  2. the total mass
  3. the average random kinetic energy of the particles
  4. the order of the particles

Answer: C) the average random kinetic energy of the particles

Explanation: More random motion = more disorder. SLO P-11-C-18 · Understanding

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117. Dissolving sugar in hot tea increases disorder because:

  1. molecules stop moving
  2. sugar forms crystals
  3. the tea cools
  4. sugar molecules spread randomly through the liquid

Answer: D) sugar molecules spread randomly through the liquid

Explanation: More arrangements. SLO P-11-C-18 · Understanding

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118. Entropy increases when:

  1. a gas condenses
  2. a liquid freezes
  3. a solid dissolves in a liquid
  4. a gas is compressed slowly at constant T

Answer: C) a solid dissolves in a liquid

Explanation: Particles spread out. SLO P-11-C-22 · Understanding

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119. When 3340 J melts 10 g of ice at 273 K, the entropy of the ice increases by about:

  1. 3340 J/K
  2. 12.2 J/K
  3. 0.082 J/K
  4. 9.1 10^5 J/K

Answer: B) 12.2 J/K

Explanation: Delta S = Q/T = 3340/273. SLO P-11-C-18 · Understanding

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120. If TH = TC, the Carnot efficiency is:

  1. 100%
  2. infinite
  3. 50%
  4. zero

Answer: D) zero

Explanation: No temperature difference, no work. SLO P-11-C-23 · Knowledge

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121. A Carnot engine works between 750 K and 300 K. Its efficiency is:

  1. 60%
  2. 70%
  3. 40%
  4. 150%

Answer: A) 60%

Explanation: eta = 1 - TCTH = 1 - 300750 = 60%. SLO P-11-C-16 · Understanding

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122. The Carnot cycle on a p–V diagram encloses an area equal to:

  1. the internal energy
  2. the net work done per cycle
  3. the heat rejected
  4. zero

Answer: B) the net work done per cycle

Explanation: Area of a closed loop. SLO P-11-C-16 · Understanding

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123. When water vapour condenses to liquid, the disorder of the water:

  1. becomes infinite
  2. stays the same
  3. decreases
  4. increases

Answer: C) decreases

Explanation: Molecules become more ordered (but heat released increases the surroundings' disorder). SLO P-11-C-18 · Understanding

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124. Entropy, S, is a thermodynamic quantity that measures:

  1. the temperature
  2. the degree of disorder of the particles in a system
  3. the total energy
  4. the pressure

Answer: B) the degree of disorder of the particles in a system

Explanation: More disorder, more entropy. SLO P-11-C-22 · Understanding

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125. Sand castles get washed away but never build themselves because:

  1. the first law
  2. water creates energy
  3. sand is heavy
  4. disordered sand arrangements are vastly more probable

Answer: D) disordered sand arrangements are vastly more probable

Explanation: Entropy. SLO P-11-C-21 · Understanding

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