MCQs / Physics / Heat Engines, Second Law and Entropy

Heat Engines, Second Law and Entropy MCQs — Physics (page 2)

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.

26. Which is true of all natural processes?

  1. they are perfectly reversible
  2. they are irreversible and degrade energy
  3. they decrease entropy
  4. they create energy

Answer: B) they are irreversible and degrade energy

Explanation: Second law. SLO P-11-C-20 · Understanding

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27. The efficiency of real machines is always less than 100% because:

  1. energy is destroyed
  2. energy is created
  3. some energy is always degraded (e.g. by friction)
  4. machines don't use energy

Answer: C) some energy is always degraded (e.g. by friction)

Explanation: Second law. SLO P-11-C-20 · Understanding

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28. A shuffled pack of cards is unlikely to return to its original order because:

  1. cards repel
  2. the ordered arrangement is just one of an enormous number of possibilities
  3. shuffling destroys cards
  4. energy is conserved

Answer: B) the ordered arrangement is just one of an enormous number of possibilities

Explanation: Analogy for entropy. SLO P-11-C-21 · Understanding

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29. Leaving a refrigerator door open in a closed room will:

  1. warm the room slightly
  2. have no effect
  3. cool the room
  4. freeze the room

Answer: A) warm the room slightly

Explanation: Heat given out = heat removed + work. SLO P-11-C-17 · Understanding

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30. During the adiabatic stages of a Carnot cycle:

  1. the volume is constant
  2. no heat is exchanged and the temperature changes
  3. the pressure is constant
  4. heat is absorbed at constant temperature

Answer: B) no heat is exchanged and the temperature changes

Explanation: Q = 0. SLO P-11-C-16 · Understanding

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31. Entropy is highest for a system that is:

  1. coldest
  2. most disordered
  3. most ordered
  4. smallest

Answer: B) most disordered

Explanation: Definition. SLO P-11-C-22 · Understanding

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32. In a Carnot cycle, QCQH =

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

Answer: B) TCTH

Explanation: Basis of the efficiency formula. SLO P-11-C-16 · Understanding

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33. A heat engine takes in 2400 J from the hot reservoir and rejects 1800 J to the cold reservoir. Its efficiency is:

  1. 25%
  2. 12.5%
  3. 75%
  4. 57.1%

Answer: A) 25%

Explanation: W = QH - QC = 600 J; eta = WQH = 25%. SLO P-11-C-13 · Knowledge

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34. When a gas is compressed at constant temperature, its entropy:

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

Answer: C) decreases

Explanation: The molecules have less space (fewer arrangements). SLO P-11-C-22 · Understanding

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35. Which device needs an external energy input to operate, according to the second law?

  1. a cup of tea cooling
  2. a refrigerator
  3. a hot iron cooling
  4. an ice cube melting in a warm room

Answer: B) a refrigerator

Explanation: Pumping heat uphill. SLO P-11-C-15 · Understanding

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36. Ultimately, most of the energy we use ends up as:

  1. low-temperature heat in the environment
  2. fuel
  3. gravitational potential energy
  4. electricity

Answer: A) low-temperature heat in the environment

Explanation: Degraded energy. SLO P-11-C-20 · Understanding

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37. The working substance in a steam engine is:

  1. steam (water vapour)
  2. the piston
  3. the boiler
  4. coal

Answer: A) steam (water vapour)

Explanation: It absorbs and rejects heat. SLO P-11-C-13 · Knowledge

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38. Low-grade energy is:

  1. thermal energy at a temperature close to the surroundings
  2. gravitational PE of a dam
  3. electrical energy
  4. chemical energy in fuel

Answer: A) thermal energy at a temperature close to the surroundings

Explanation: Hard to use for work. SLO P-11-C-19 · Understanding

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39. Two gases mixed together do not separate by themselves because:

  1. energy is lost
  2. they become one gas
  3. they react
  4. the mixed state has far more possible arrangements

Answer: D) the mixed state has far more possible arrangements

Explanation: Separation would lower entropy. SLO P-11-C-21 · Understanding

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40. A refrigerator removes 400 J from inside using 100 J of work. Its coefficient of performance is:

  1. 0.25
  2. 5
  3. 4
  4. 8

Answer: C) 4

Explanation: K = QCW = 400100 = 4. SLO P-11-C-17 · Understanding

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41. The Clausius statement of the second law says:

  1. energy is created in engines
  2. temperature is constant
  3. all heat can be converted into work
  4. heat cannot flow by itself from a colder body to a hotter body

Answer: D) heat cannot flow by itself from a colder body to a hotter body

Explanation: A refrigerator needs work. SLO P-11-C-15 · Understanding

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42. Carnot's engine is reversible, so if run backwards it acts as:

  1. a generator of energy
  2. a refrigerator (or heat pump)
  3. a stronger engine
  4. a thermometer

Answer: B) a refrigerator (or heat pump)

Explanation: Heat is pumped from cold to hot. SLO P-11-C-16 · Understanding

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43. The entropy of an isolated system tends to:

  1. oscillate
  2. remain zero
  3. decrease
  4. increase

Answer: D) increase

Explanation: Second law. SLO P-11-C-19 · Understanding

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44. A tidy room tends to become untidy unless work is done because:

  1. there are many more disordered arrangements than ordered ones
  2. energy is destroyed
  3. order is forbidden
  4. gravity pulls things apart

Answer: A) there are many more disordered arrangements than ordered ones

Explanation: Statistical tendency to disorder. SLO P-11-C-21 · Understanding

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45. Which change increases disorder?

  1. cooling a gas
  2. freezing water
  3. evaporating a liquid
  4. condensing steam

Answer: C) evaporating a liquid

Explanation: Liquid → gas. SLO P-11-C-18 · Understanding

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46. Entropy is a measure of:

  1. mass
  2. the total energy
  3. how dispersed (unavailable) the energy of a system is
  4. temperature

Answer: C) how dispersed (unavailable) the energy of a system is

Explanation: Higher entropy, lower quality. SLO P-11-C-19 · Understanding

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47. The coils at the back of a refrigerator are warm because:

  1. heat flows in from the room
  2. they are electric heaters
  3. the motor is broken
  4. the refrigerant condenses there and releases heat

Answer: D) the refrigerant condenses there and releases heat

Explanation: Condenser. SLO P-11-C-17 · Understanding

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48. A ball bouncing to lower and lower heights is an example of:

  1. a reversible process
  2. an irreversible process
  3. perpetual motion
  4. a Carnot cycle

Answer: B) an irreversible process

Explanation: Energy is dissipated. SLO P-11-C-14 · Understanding

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49. Which has the highest entropy?

  1. 1 kg of water at 0 °C
  2. 1 kg of ice at 0 °C
  3. 1 kg of water at 100 °C
  4. 1 kg of steam at 100 °C

Answer: D) 1 kg of steam at 100 °C

Explanation: Gas is most disordered. SLO P-11-C-22 · Understanding

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50. 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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