MCQs / Physics / Heat Engines, Second Law and Entropy

Heat Engines, Second Law and Entropy MCQs — Physics

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.

1. A refrigerator is:

  1. a perpetual motion machine
  2. a device that destroys heat
  3. a heat engine that makes cold
  4. a heat engine working in reverse, moving heat from cold to hot using work

Answer: D) a heat engine working in reverse, moving heat from cold to hot using work

Explanation: Work input is required. SLO P-11-C-17 · Understanding

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2. When a car brakes, its kinetic energy is degraded into:

  1. chemical energy
  2. low-temperature heat spread through the brakes and air
  3. high-grade electrical energy
  4. nuclear energy

Answer: B) low-temperature heat spread through the brakes and air

Explanation: Less useful form. SLO P-11-C-19 · Understanding

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3. An engine does 400 J of work with 25% efficiency. The heat taken in is:

  1. 1200 J
  2. 1600 J
  3. 400 J
  4. 100 J

Answer: B) 1600 J

Explanation: QH = W/eta. SLO P-11-C-13 · Knowledge

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4. The second law is often described as the law of:

  1. conservation of energy
  2. constant pressure
  3. thermal equilibrium
  4. increasing entropy

Answer: D) increasing entropy

Explanation: Entropy of the universe increases. SLO P-11-C-15 · Understanding

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5. Adding heat to a system generally increases its:

  1. order
  2. potential to be reversed
  3. mass
  4. entropy

Answer: D) entropy

Explanation: Delta S = Q/T for reversible heating. SLO P-11-C-18 · Understanding

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6. The maximum possible efficiency of an engine working between 227 °C and 27 °C is:

  1. 60%
  2. 80%
  3. 40%
  4. 88.1%

Answer: C) 40%

Explanation: Convert to kelvin: TH = 500 K, TC = 300 K. etamax = 1 - 300500 = 40%. SLO P-11-C-23 · Knowledge

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7. The efficiency of a Carnot engine depends only on:

  1. the working substance
  2. the speed of the engine
  3. the temperatures of the hot and cold reservoirs
  4. the size of the engine

Answer: C) the temperatures of the hot and cold reservoirs

Explanation: eta = 1 - TC/TH. SLO P-11-C-16 · Understanding

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8. An ideal refrigerator works between 273 K and 300 K. Its coefficient of performance is about:

  1. 27
  2. 1.1
  3. 10.1
  4. 0.09

Answer: C) 10.1

Explanation: K = 27327 approx 10.1. SLO P-11-C-17 · Understanding

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9. The efficiency of a heat engine is:

  1. WQH
  2. QCQH
  3. WQC
  4. QHW

Answer: A) WQH

Explanation: Useful output ÷ input. SLO P-11-C-13 · Knowledge

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10. A refrigerator removes 600 J from inside using 200 J of work. Its coefficient of performance is:

  1. 3
  2. 4
  3. 6
  4. 0.333

Answer: A) 3

Explanation: K = QCW = 600200 = 3. SLO P-11-C-17 · Understanding

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11. A car petrol engine is an example of:

  1. a heat engine
  2. a solar cell
  3. a heat pump
  4. a refrigerator

Answer: A) a heat engine

Explanation: It converts heat from burning fuel into work. SLO P-11-C-13 · Knowledge

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12. Which process degrades energy the most?

  1. lifting a weight
  2. friction turning kinetic energy into heat
  3. storing energy in a spring ideally
  4. charging a battery perfectly

Answer: B) friction turning kinetic energy into heat

Explanation: Dissipation. SLO P-11-C-19 · Understanding

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13. Heat flows from a body at 300 K to one at 400 K only if:

  1. the 300 K body is larger
  2. work is done (e.g. by a heat pump)
  3. never, even with work
  4. the bodies touch

Answer: B) work is done (e.g. by a heat pump)

Explanation: Clausius statement. SLO P-11-C-15 · Understanding

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14. A refrigerator removes 900 J from inside using 300 J of work. Its coefficient of performance is:

  1. 3
  2. 6
  3. 0.333
  4. 4

Answer: A) 3

Explanation: K = QCW = 900300 = 3. SLO P-11-C-17 · Understanding

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15. The first law says energy is conserved; the second law tells us:

  1. work equals heat
  2. energy is created
  3. the direction in which processes occur naturally
  4. temperature is absolute

Answer: C) the direction in which processes occur naturally

Explanation: e.g. hot to cold. SLO P-11-C-15 · Understanding

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16. A refrigerator removes 400 J from its contents using 100 J of electrical work. The heat given out to the kitchen is:

  1. 100 J
  2. 500 J
  3. 400 J
  4. 300 J

Answer: B) 500 J

Explanation: QH = QC + W = 500 J. SLO P-11-C-17 · Understanding

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17. In a reversible process, the total entropy of system + surroundings:

  1. becomes zero
  2. increases
  3. decreases
  4. stays constant

Answer: D) stays constant

Explanation: Irreversible processes increase it. SLO P-11-C-14 · Understanding

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18. Perpetual motion of the second kind refers to:

  1. an engine that creates energy
  2. a frictionless wheel
  3. an engine that converts heat entirely into work from a single reservoir
  4. a refrigerator

Answer: C) an engine that converts heat entirely into work from a single reservoir

Explanation: Forbidden by the second law. SLO P-11-C-15 · Understanding

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19. A reversible process is one that:

  1. cannot be undone
  2. involves friction
  3. happens quickly
  4. can be retraced exactly so that both system and surroundings return to their original states

Answer: D) can be retraced exactly so that both system and surroundings return to their original states

Explanation: An idealisation. SLO P-11-C-14 · Understanding

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20. Why are real engines less efficient than the Carnot limit?

  1. friction, heat losses and other irreversibilities
  2. the first law fails
  3. the Carnot limit is too low
  4. they use more fuel

Answer: A) friction, heat losses and other irreversibilities

Explanation: Losses. SLO P-11-C-23 · Knowledge

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21. To increase the maximum efficiency of power stations, engineers:

  1. use smaller turbines
  2. cool the boiler
  3. use lower steam temperatures
  4. use higher steam temperatures

Answer: D) use higher steam temperatures

Explanation: Larger TH. SLO P-11-C-23 · Knowledge

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22. When a moving car stops, its kinetic energy:

  1. is stored in the brakes as KE
  2. is degraded to internal energy in the brakes, tyres and air
  3. becomes potential energy
  4. is destroyed

Answer: B) is degraded to internal energy in the brakes, tyres and air

Explanation: It spreads out as heat. SLO P-11-C-20 · Understanding

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23. An engine with 30% efficiency takes in 2000 J. The work done is:

  1. 600 J
  2. 1400 J
  3. 300 J
  4. 2000 J

Answer: A) 600 J

Explanation: W = 0.3 2000. SLO P-11-C-13 · Knowledge

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24. In a refrigerator, heat is taken from:

  1. the compressor only
  2. the room to the inside
  3. the inside (cold reservoir) and given to the room (hot reservoir)
  4. nowhere

Answer: C) the inside (cold reservoir) and given to the room (hot reservoir)

Explanation: Heat pumped 'uphill'. SLO P-11-C-17 · Understanding

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25. A heat engine:

  1. converts all heat into work
  2. creates energy
  3. takes heat from a hot reservoir, converts part into work and rejects the rest to a cold reservoir
  4. moves heat from cold to hot

Answer: C) takes heat from a hot reservoir, converts part into work and rejects the rest to a cold reservoir

Explanation: Basic principle. SLO P-11-C-13 · Knowledge

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