51. When a substance is heated, the disorder of its particles:
- stays the same
- increases
- becomes zero
- decreases
Answer: B) increases
Explanation: Particles move more randomly. SLO P-11-C-18 · Understanding
MCQs / Physics / Heat Engines, Second Law and Entropy
Heat Engines, Second Law and Entropy MCQs — Physics (page 3)
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.
52. A real engine is less efficient than a Carnot engine because:
- it uses a different gas
- it needs no reservoirs
- it is larger
- real processes are irreversible (friction, heat losses)
Answer: D) real processes are irreversible (friction, heat losses)
Explanation: Losses. SLO P-11-C-16 · Understanding
53. Perfume released in one corner of a room soon fills it because:
- air pushes it in one direction
- molecules spread randomly into the most probable (disordered) state
- it is lighter than air
- it is attracted to walls
Answer: B) molecules spread randomly into the most probable (disordered) state
Explanation: Diffusion increases entropy. SLO P-11-C-21 · Understanding
54. To reduce the disorder of a system, we must:
- isolate it
- do work on it, increasing disorder elsewhere
- wait
- cool the universe
Answer: B) do work on it, increasing disorder elsewhere
Explanation: e.g. tidying a room uses your energy. SLO P-11-C-21 · Understanding
55. The maximum efficiency of a heat engine is given by:
- eta = 1 - TCTH
- eta = TH - TC
- eta = TCTH
- eta = 1 - THTC
Answer: A) eta = 1 - TCTH
Explanation: Carnot limit, temperatures in kelvin. SLO P-11-C-23 · Knowledge
56. Doubling TH (in kelvin) with TC fixed:
- does not change the efficiency
- halves the efficiency
- increases the maximum efficiency
- makes it 100%
Answer: C) increases the maximum efficiency
Explanation: TC/TH gets smaller. SLO P-11-C-23 · Knowledge
57. 1000 J of heat is added reversibly at 250 K. The entropy change is:
- 250,000 J/K
- 0.25 J/K
- 1250 J/K
- 4 J/K
Answer: D) 4 J/K
Explanation: Delta S = 1000/250. SLO P-11-C-18 · Understanding
58. Why can't the heat from a cup of tea that has cooled be used to reheat it?
- the first law forbids it
- the energy was destroyed
- heat has no energy
- the energy is spread out at low temperature (degraded)
Answer: D) the energy is spread out at low temperature (degraded)
Explanation: It has become unavailable. SLO P-11-C-20 · Understanding
59. A diesel engine ignites fuel by:
- a spark plug
- electricity only
- the heat of highly compressed air
- a flame
Answer: C) the heat of highly compressed air
Explanation: Compression ignition. SLO P-11-C-13 · Knowledge
60. The increase in entropy when heat Q is added reversibly at temperature T is:
- Delta S = QT
- Delta S = QT
- Delta S = Q - T
- Delta S = TQ
Answer: B) Delta S = QT
Explanation: Unit J/K. SLO P-11-C-18 · Understanding
61. A heat engine could be 100% efficient only if:
- both reservoirs were at the same temperature
- the cold reservoir were at absolute zero
- the hot reservoir were at 0 K
- there were no friction
Answer: B) the cold reservoir were at absolute zero
Explanation: Impossible in practice. SLO P-11-C-23 · Knowledge
62. The Kelvin–Planck statement of the second law says:
- no engine working in a cycle can convert all the heat it takes in into work
- energy is conserved
- entropy decreases
- heat flows from cold to hot
Answer: A) no engine working in a cycle can convert all the heat it takes in into work
Explanation: Some heat must be rejected. SLO P-11-C-15 · Understanding
63. Slowly compressing a gas in a frictionless cylinder in contact with a heat bath is:
- close to reversible
- strongly irreversible
- impossible
- explosive
Answer: A) close to reversible
Explanation: Quasi-static isothermal change. SLO P-11-C-14 · Understanding
64. A temperature difference between two bodies is useful because:
- it makes mass
- it allows a heat engine to do work
- it destroys energy
- it increases entropy
Answer: B) it allows a heat engine to do work
Explanation: Once equalised, no work can be extracted. SLO P-11-C-19 · Understanding
65. A refrigerator does not violate the second law because:
- heat flows naturally from cold to hot
- it has 100% efficiency
- work is done on it to move heat from cold to hot
- it creates cold
Answer: C) work is done on it to move heat from cold to hot
Explanation: External work is supplied. SLO P-11-C-15 · Understanding
66. Which of these is irreversible?
- an ideal Carnot cycle
- heat flowing from a hot body to a cold one
- a very slow, frictionless compression of a gas
- a quasi-static isothermal expansion
Answer: B) heat flowing from a hot body to a cold one
Explanation: Heat never flows back by itself. SLO P-11-C-14 · Understanding
67. Heat Q flows from a hot body at TH to a cold body at TC. The total entropy change is:
- Q(TH - TC)
- 0
- QTC - QTH 0
- QTH - QTC 0
Answer: C) QTC - QTH 0
Explanation: Cold body gains more entropy than the hot body loses. SLO P-11-C-22 · Understanding
68. Gas released from a cylinder into a vacuum chamber:
- becomes liquid
- spreads to fill it and does not return
- returns to the cylinder
- stays in one corner
Answer: B) spreads to fill it and does not return
Explanation: Free expansion increases disorder. SLO P-11-C-21 · Understanding
69. A falling raindrop at terminal velocity converts its gravitational potential energy into:
- kinetic energy
- chemical energy
- heat in the air (degraded energy)
- nuclear energy
Answer: C) heat in the air (degraded energy)
Explanation: Its KE is constant. SLO P-11-C-20 · Understanding
70. The part of a refrigerator where the refrigerant evaporates and absorbs heat is the:
- evaporator (inside the fridge)
- condenser (back coils)
- thermostat
- compressor
Answer: A) evaporator (inside the fridge)
Explanation: Evaporation absorbs latent heat. SLO P-11-C-17 · Understanding
71. When ice melts to water, the disorder of the molecules:
- stays the same
- is destroyed
- decreases
- increases
Answer: D) increases
Explanation: The regular lattice breaks down. SLO P-11-C-18 · Understanding
72. Ocean water has a huge internal energy, but a ship can't run on it alone because:
- the energy is too small
- the first law forbids it
- water doesn't contain energy
- there is no colder reservoir to reject heat to
Answer: D) there is no colder reservoir to reject heat to
Explanation: Second law. SLO P-11-C-15 · Understanding
73. Heat spreading through a metal rod until its temperature is uniform is an example of:
- work being created
- a reversible process
- a system becoming less orderly (entropy increasing)
- entropy decreasing
Answer: C) a system becoming less orderly (entropy increasing)
Explanation: Energy spreads out. SLO P-11-C-21 · Understanding
74. When hot and cold water mix, the total energy is unchanged but:
- energy is destroyed
- entropy decreases
- entropy increases and the ability to do work is lost
- the water heats up more
Answer: C) entropy increases and the ability to do work is lost
Explanation: The temperature difference is gone. SLO P-11-C-19 · Understanding
75. A heat engine:
- converts all heat into work
- creates energy
- takes heat from a hot reservoir, converts part into work and rejects the rest to a cold reservoir
- 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
Practise Heat Engines, Second Law and Entropy MCQs
More Physics chapters
- Physical Quantities and Measurement MCQs
- Motion MCQs
- Vectors MCQs
- Electricity MCQs
- Kinematics and Projectile Motion MCQs
- Circular and Rotational Motion MCQs
- Momentum and Collisions MCQs
- Deformation of Solids MCQs
- Work and Energy MCQs
- Fluid Statics and Dynamics MCQs
- Kinetic Theory and First Law of Thermodynamics MCQs
- Wave Intensity and the Doppler Effect MCQs