MCQs / Physics / Heat Engines, Second Law and Entropy

Heat Engines, Second Law and Entropy MCQs — Physics (page 7)

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.

151. 'Degradation of energy' means that energy:

  1. becomes spread out and less available to do useful work
  2. becomes more concentrated
  3. is destroyed
  4. is created

Answer: A) becomes spread out and less available to do useful work

Explanation: Total energy is conserved, but quality falls. SLO P-11-C-19 · Understanding

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152. Natural processes go in the direction of:

  1. concentrating energy
  2. decreasing disorder
  3. increasing disorder and energy spreading out
  4. creating energy

Answer: C) increasing disorder and energy spreading out

Explanation: Spontaneous direction. SLO P-11-C-20 · Understanding

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153. A Carnot engine works between 373 K and 273 K. Its efficiency is:

  1. 26.8%
  2. 36.6%
  3. 86.6%
  4. 73.2%

Answer: A) 26.8%

Explanation: eta = 1 - TCTH = 1 - 273373 = 26.8%. SLO P-11-C-16 · Understanding

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154. Burning a piece of paper is:

  1. a Carnot cycle
  2. irreversible
  3. reversible
  4. reversible if done slowly

Answer: B) irreversible

Explanation: Chemical and thermal changes can't be retraced. SLO P-11-C-14 · Understanding

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155. When we say energy is 'used up', we really mean it is:

  1. converted into less useful forms (degraded)
  2. destroyed
  3. stored forever
  4. turned into mass

Answer: A) converted into less useful forms (degraded)

Explanation: Energy is conserved. SLO P-11-C-19 · Understanding

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156. Entropy is a state function, which means its change depends on:

  1. the time taken
  2. only the initial and final states
  3. the path taken
  4. the shape of the container

Answer: B) only the initial and final states

Explanation: Like internal energy. SLO P-11-C-22 · Understanding

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157. An isolated system left to itself moves towards:

  1. greater order
  2. lower temperature always
  3. lower entropy
  4. a state of greater disorder (higher entropy)

Answer: D) a state of greater disorder (higher entropy)

Explanation: Second law. SLO P-11-C-21 · Understanding

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158. The 'heat death' of the universe refers to a state where:

  1. entropy is maximum and no energy is available for work
  2. temperature is infinite
  3. entropy is zero
  4. all energy is destroyed

Answer: A) entropy is maximum and no energy is available for work

Explanation: Uniform temperature. SLO P-11-C-19 · Understanding

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159. According to the second law, the entropy of an isolated system:

  1. stays constant in all processes
  2. is always zero
  3. never decreases
  4. always decreases

Answer: C) never decreases

Explanation: It increases in real processes. SLO P-11-C-15 · Understanding

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160. Order can be created locally (e.g. in a freezer making ice) only if:

  1. energy is destroyed
  2. the second law fails
  3. no work is done
  4. the entropy of the surroundings increases by more

Answer: D) the entropy of the surroundings increases by more

Explanation: Total entropy still increases. SLO P-11-C-21 · Understanding

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161. Electrical energy transmitted over cables is partly degraded as:

  1. light only
  2. extra current
  3. heat in the wires
  4. magnetic monopoles

Answer: C) heat in the wires

Explanation: I^2R loss. SLO P-11-C-20 · Understanding

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162. If 500 J of heat leaves a body at 250 K reversibly, its entropy change is:

  1. -0.5 J/K
  2. +2 J/K
  3. -2 J/K
  4. +125,000 J/K

Answer: C) -2 J/K

Explanation: Delta S = Q/T = -500/250. SLO P-11-C-22 · Understanding

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163. A Carnot engine works between 600 K and 300 K. Its efficiency is:

  1. 75%
  2. 50%
  3. 100%
  4. 52%

Answer: B) 50%

Explanation: eta = 1 - TCTH = 1 - 300600 = 50%. SLO P-11-C-16 · Understanding

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164. The second law sets a limit on:

  1. the charge on an electron
  2. the speed of light
  3. the mass of a gas
  4. the efficiency of heat engines

Answer: D) the efficiency of heat engines

Explanation: Max = Carnot efficiency. SLO P-11-C-15 · Understanding

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165. Which is consistent with the second law?

  1. an engine with 100% efficiency
  2. a hot drink cooling to room temperature
  3. a cold drink spontaneously warming above room temperature
  4. heat flowing from ice to a hot stove

Answer: B) a hot drink cooling to room temperature

Explanation: Natural direction of heat flow. SLO P-11-C-15 · Understanding

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