Thermodynamics

7 NEET-UG practice questions on Thermodynamics, part of Physics. The first 7 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
In which process does the internal energy of an ideal gas remain unchanged?
1. AIsothermal
2. BIsobaric
3. CIsochoric
4. DAdiabatic
Show answerHide answer
Answer: A · Isothermal
The internal energy of an ideal gas depends only on its temperature. In an isothermal process the temperature is fixed, so ΔU = 0 and Q = W.
- B: At constant pressure, a change of volume changes the temperature.
- C: At constant volume, heat added raises the temperature.
- D: The temperature changes in an adiabatic process, since ΔU = −W.
Remember this
For an ideal gas, ΔU depends only on ΔT, whatever the path.
Question 2 · Medium
An ideal gas expands at 2 × 10⁵ Pa from 1 L to 3 L, cools at constant volume to 1 × 10⁵ Pa, is compressed at this pressure back to 1 L, and is finally heated at constant volume to its initial state. What is the net heat absorbed per cycle?
1. AZero
2. B400 J
3. C600 J
4. D200 J
Show answerHide answer
Answer: D · 200 J
The cycle is a rectangle on the P-V diagram, run clockwise, so net work = ΔP × ΔV = (1 × 10⁵)(2 × 10⁻³) = 200 J. Check: 400 J done by the gas in expansion minus 200 J done on it in compression. Over a cycle ΔU = 0, so Q = W = 200 J.
- A: This confuses ΔU = 0 with zero net heat.
- B: This counts only the work in the expansion.
- C: This adds the two work terms instead of subtracting.
Remember this
Net work in a cycle equals the enclosed area; ΔU = 0 makes net heat equal net work.
Question 3 · Medium
A Carnot engine works between a source at 500 K and a sink at 300 K. By how much must the source temperature be raised, with the sink unchanged, to make the efficiency 50%?
1. A100 K
2. B50 K
3. C30 K
4. D125 K
Show answerHide answer
Answer: A · 100 K
Efficiency 0.5 = 1 − 300/T₁ gives T₁ = 600 K, a rise of 100 K. The present efficiency is 1 − 300/500 = 40%.
- B: This is the fall in sink temperature that would achieve the same result.
- C: This takes 10% of the sink temperature.
- D: This scales T₁ in proportion to the efficiency, 500 × 0.5/0.4.
Remember this
Carnot efficiency depends only on T₂/T₁, in kelvin, and is not proportional to T₁.
Question 4 · Medium
An ideal refrigerator keeps its cold chamber at −3 °C in a room at 27 °C, and removes 900 J of heat from the chamber each second. What minimum electric power does it need?
1. A100 W
2. B810 W
3. C1000 W
4. D90 W
Show answerHide answer
Answer: A · 100 W
T₂ = 270 K and T₁ = 300 K, so the coefficient of performance α = T₂/(T₁ − T₂) = 270/30 = 9. W per second = 900/9 = 100 W. Check: heat released to the room per second is 900 × 300/270 = 1000 J, and 1000 − 900 = 100 J.
- B: This multiplies the 900 J by T₂/T₁.
- C: This is the heat released to the room each second.
- D: This uses T₁/(T₁ − T₂) = 10 as the coefficient of performance.
Remember this
α = Q₂/W = T₂/(T₁ − T₂), which can exceed 1, unlike the efficiency of an engine.
Question 5 · Easy
An ideal gas goes round the cycle marked on the P-V diagram. What net work does the gas do in one cycle?
Figure: MedExams
1. A−400 J
2. B+400 J
3. C+900 J
4. D+600 J
Show answerHide answer
Answer: B · +400 J
Net work is the enclosed area: (2 × 10⁵ Pa)(2 × 10⁻³ m³) = 400 J. The clockwise sense means the gas does positive net work.
- A: Reverses the sign; the cycle runs clockwise.
- C: Multiplies the largest pressure by the largest volume.
- D: Takes only the area under the top line.
Remember this
Net work equals the area enclosed; clockwise on a P-V diagram is positive.
Question 6 · Hard
A monatomic ideal gas (Cv = 3R/2, Cp = 5R/2) runs through the cycle ABCD drawn. What is the efficiency of this cycle?
Figure: MedExams
1. A8/9
2. B2/9
3. C4/15
4. D1/3
Answer and explanation in LearnFree account
Question 7 · Medium
An ideal gas is compressed from state S to the same final volume by two separate routes: isothermal (iso, solid) and adiabatic (adi, dashed). Which graph shows the two routes correctly?
Figure: MedExams
1. AGraph A
2. BGraph C
3. CGraph B
4. DGraph D
Answer and explanation in LearnFree account

Thermodynamics

In which process does the internal energy of an ideal gas remain unchanged?

An ideal gas expands at 2 × 10⁵ Pa from 1 L to 3 L, cools at constant volume to 1 × 10⁵ Pa, is compressed at this pressure back to 1 L, and is finally heated at constant volume to its initial state. What is the net heat absorbed per cycle?

A Carnot engine works between a source at 500 K and a sink at 300 K. By how much must the source temperature be raised, with the sink unchanged, to make the efficiency 50%?

An ideal refrigerator keeps its cold chamber at −3 °C in a room at 27 °C, and removes 900 J of heat from the chamber each second. What minimum electric power does it need?

An ideal gas goes round the cycle marked on the P-V diagram. What net work does the gas do in one cycle?

A monatomic ideal gas (Cv = 3R/2, Cp = 5R/2) runs through the cycle ABCD drawn. What is the efficiency of this cycle?

An ideal gas is compressed from state S to the same final volume by two separate routes: isothermal (iso, solid) and adiabatic (adi, dashed). Which graph shows the two routes correctly?