MCQs / Physics / Heat Engines, Second Law and Entropy

Heat Engines, Second Law and Entropy MCQs — Physics (page 6)

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.

126. All real (natural) processes are:

  1. impossible
  2. neither
  3. reversible
  4. irreversible

Answer: D) irreversible

Explanation: Friction, heat loss, turbulence. SLO P-11-C-14 · Understanding

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127. Rusting of iron is a natural process in which:

  1. the total entropy of the universe increases
  2. entropy decreases
  3. energy is created
  4. no energy changes

Answer: A) the total entropy of the universe increases

Explanation: All spontaneous processes increase total entropy. SLO P-11-C-20 · Understanding

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128. Compared with the energy input, the useful energy output of any real process is:

  1. unrelated
  2. more
  3. less
  4. equal

Answer: C) less

Explanation: Some is degraded. SLO P-11-C-20 · Understanding

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129. Living organisms maintain order by:

  1. creating energy
  2. breaking the second law
  3. decreasing the universe's entropy
  4. using energy and increasing the entropy of their surroundings

Answer: D) using energy and increasing the entropy of their surroundings

Explanation: They release heat and waste. SLO P-11-C-21 · Understanding

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130. Which change would increase a Carnot engine's efficiency the most?

  1. lowering the temperature of the cold reservoir
  2. using a bigger cylinder
  3. running it faster
  4. changing the gas

Answer: A) lowering the temperature of the cold reservoir

Explanation: eta depends on TC/TH. SLO P-11-C-16 · Understanding

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131. Mixing of two different gases is:

  1. isothermal and reversible
  2. reversible
  3. irreversible
  4. quasi-static

Answer: C) irreversible

Explanation: They don't unmix by themselves. SLO P-11-C-14 · Understanding

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132. A perfect crystal at absolute zero has:

  1. infinite entropy
  2. maximum disorder
  3. the same disorder as a gas
  4. minimum disorder (zero entropy)

Answer: D) minimum disorder (zero entropy)

Explanation: Third law of thermodynamics. SLO P-11-C-18 · Understanding

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133. A heat engine takes in 500 J from the hot reservoir and rejects 350 J to the cold reservoir. Its efficiency is:

  1. 70%
  2. 15%
  3. 30%
  4. 58.8%

Answer: C) 30%

Explanation: W = QH - QC = 150 J; eta = WQH = 30%. SLO P-11-C-13 · Knowledge

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134. Both statements of the second law (Kelvin and Clausius) are:

  1. only true for gases
  2. unrelated
  3. equivalent — violating one would violate the other
  4. contradictory

Answer: C) equivalent — violating one would violate the other

Explanation: They can be shown to imply each other. SLO P-11-C-15 · Understanding

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135. A Carnot engine has efficiency 40% and its cold reservoir is at 300 K. Its hot reservoir is at:

  1. 375 K
  2. 500 K
  3. 420 K
  4. 750 K

Answer: B) 500 K

Explanation: 1 - TCTH = 0.4 Rightarrow TH = 3000.6 = 500 K. SLO P-11-C-23 · Knowledge

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136. The second law of thermodynamics implies that:

  1. work is impossible
  2. some energy always becomes less useful in real processes
  3. energy decreases
  4. all energy can be used

Answer: B) some energy always becomes less useful in real processes

Explanation: Degradation of energy. SLO P-11-C-15 · Understanding

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137. A change of state from liquid to gas involves:

  1. no change in entropy
  2. a decrease in entropy
  3. negative heat
  4. a large increase in entropy

Answer: D) a large increase in entropy

Explanation: Much more disorder. SLO P-11-C-22 · Understanding

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138. A Carnot engine absorbs 1000 J at 500 K and rejects heat at 300 K. The work it does is:

  1. 1000 J
  2. 200 J
  3. 600 J
  4. 400 J

Answer: D) 400 J

Explanation: eta = 0.4. SLO P-11-C-16 · Understanding

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139. In the Carnot efficiency formula, temperatures must be in:

  1. any unit
  2. degrees Celsius
  3. kelvin
  4. degrees Fahrenheit

Answer: C) kelvin

Explanation: Absolute temperatures. SLO P-11-C-23 · Knowledge

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140. A bouncing ball eventually stops because:

  1. energy is destroyed
  2. its energy is degraded to heat and sound
  3. its mass decreases
  4. gravity disappears

Answer: B) its energy is degraded to heat and sound

Explanation: Each bounce dissipates energy. SLO P-11-C-20 · Understanding

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141. The coefficient of performance of a refrigerator is:

  1. K = 1 - TCTH
  2. K = WQC
  3. K = QHQC
  4. K = QCW

Answer: D) K = QCW

Explanation: Heat removed per joule of work. SLO P-11-C-17 · Understanding

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142. In a four-stroke petrol engine, the stroke in which work is done by the gas is:

  1. the exhaust stroke
  2. the power stroke
  3. the compression stroke
  4. the intake stroke

Answer: B) the power stroke

Explanation: Burning fuel expands. SLO P-11-C-13 · Knowledge

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143. During the isothermal expansion of a Carnot cycle, the gas:

  1. absorbs heat from the hot reservoir
  2. rejects heat to the cold reservoir
  3. is compressed
  4. has no heat exchange

Answer: A) absorbs heat from the hot reservoir

Explanation: At TH. SLO P-11-C-16 · Understanding

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144. Boltzmann related entropy to the number of microscopic arrangements W by:

  1. S = kln W
  2. S = W^k
  3. S = Wk
  4. S = kW

Answer: A) S = kln W

Explanation: k = Boltzmann constant. SLO P-11-C-22 · Understanding

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145. The maximum possible efficiency of an engine working between 427 °C and 27 °C is:

  1. 93.7%
  2. 42.9%
  3. 57.1%
  4. 99%

Answer: C) 57.1%

Explanation: Convert to kelvin: TH = 700 K, TC = 300 K. etamax = 1 - 300700 = 57.1%. SLO P-11-C-23 · Knowledge

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146. A refrigerator removes 600 J from its contents using 200 J of electrical work. The heat given out to the kitchen is:

  1. 400 J
  2. 800 J
  3. 200 J
  4. 600 J

Answer: B) 800 J

Explanation: QH = QC + W = 800 J. SLO P-11-C-17 · Understanding

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147. An ink drop spreading through water is:

  1. irreversible
  2. a heat engine
  3. quasi-static
  4. reversible

Answer: A) irreversible

Explanation: It never re-gathers itself. SLO P-11-C-14 · Understanding

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148. At higher temperature, the particles of a gas have:

  1. a fixed arrangement
  2. no energy
  3. fewer arrangements
  4. more possible arrangements of positions and energies

Answer: D) more possible arrangements of positions and energies

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

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149. A Carnot engine has efficiency 25% and its cold reservoir is at 270 K. Its hot reservoir is at:

  1. 338 K
  2. 360 K
  3. 270 K
  4. 540 K

Answer: B) 360 K

Explanation: 1 - TCTH = 0.25 Rightarrow TH = 2700.75 = 360 K. SLO P-11-C-23 · Knowledge

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150. A reversible process must be carried out:

  1. with large friction
  2. as fast as possible
  3. with sudden explosions
  4. infinitely slowly (quasi-statically) without friction

Answer: D) infinitely slowly (quasi-statically) without friction

Explanation: Always near equilibrium. SLO P-11-C-14 · Understanding

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