Physics Practice Test: Thermodynamics
- Thermodynamics
- 10 questions
- 15 minutes
These 10 questions cover heat, temperature and the laws of thermodynamics. Use c = 4186 J/(kg·°C) for water and always convert to kelvin before using gas laws or Carnot efficiency.
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Question 1
How much heat is needed to warm 2.0 kg of water from 20°C to 50°C? (c = 4186 J/(kg·°C))
Correct answer: A
Q = mcΔT = 2.0 × 4186 × 30 ≈ 2.5 × 105 J. 4.2 × 105 J uses the final temperature of 50°C instead of the 30°C change.
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Question 2
How much heat is needed to melt 0.50 kg of ice that is already at 0°C? (latent heat of fusion = 3.34 × 105 J/kg)
Correct answer: C
Q = mL = 0.50 × 3.34 × 105 ≈ 1.7 × 105 J, and the temperature stays at 0°C while the ice melts. 6.7 × 105 J divides by the mass instead of multiplying.
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Question 3
A room is at 300 K. What is this temperature in degrees Celsius?
Correct answer: D
T(°C) = T(K) − 273 = 300 − 273 = 27°C. 573 adds 273 instead of subtracting it.
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Question 4
A gas absorbs 500 J of heat and does 200 J of work on its surroundings. What is the change in its internal energy?
Correct answer: B
First law: ΔU = Q − W = 500 − 200 = +300 J. 700 J adds the work, which would be right only if work were done on the gas.
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Question 5
A sealed rigid container of gas is heated from 27°C to 327°C. By what factor does the pressure change?
Correct answer: C
At constant volume, P is proportional to the kelvin temperature: 300 K to 600 K, a factor of 2. Using Celsius (327/27 ≈ 12) is the classic mistake.
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Question 6
A heat engine takes in 1000 J of heat and releases 600 J to the cold reservoir in each cycle. What is its efficiency?
Correct answer: B
The work output is 1000 − 600 = 400 J, so the efficiency is 400/1000 = 40%. 60% is the fraction of heat wasted, not the useful part.
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Question 7
What is the maximum possible efficiency of a heat engine working between a hot reservoir at 600 K and a cold reservoir at 300 K?
Correct answer: D
The Carnot efficiency is 1 − TC/TH = 1 − 300/600 = 50%. No real engine between these temperatures can do better, and 100% would break the second law.
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Question 8
How does energy from the Sun reach Earth?
Correct answer: A
Space between the Sun and Earth is almost empty, and conduction and convection both need matter. Electromagnetic radiation can travel through a vacuum.
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Question 9
0.20 kg of water at 80°C is mixed with 0.30 kg of water at 20°C in an insulated cup. What is the final temperature?
Correct answer: A
Heat lost equals heat gained: 0.20(80 − T) = 0.30(T − 20), so T = 44°C. 50°C is the simple average, which would be right only for equal masses.
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Question 10
Which statement agrees with the second law of thermodynamics?
Correct answer: C
The second law says heat flows spontaneously from hot to cold and the entropy of an isolated system never decreases. A refrigerator moves heat the other way only because work is done on it.
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