Free First Law of Thermodynamics MCQs with Answers
297 First Law of Thermodynamics MCQs from Physics, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
The first law relates heat supplied, work done and the change in internal energy through energy conservation. Problems use sign conventions and apply the law to isothermal, adiabatic, isobaric and isochoric processes. Internal energy is a state function, while heat and work depend on the path followed.
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297 questions · page 3 of 15
- A. W = ΔU = Q
- B. Q = -W
- C. Q = W
- D. W = -ΔU
Explanation: The first law of thermodynamics states that the ΔU = Q - W.Adiabatic Process: An adiabatic process is defined as a thermodynamic process…
Correct answer: W = -ΔU- A. 150 J
- B. 300 J
- C. 250 J
- D. 500 J
Explanation: According to the first law of thermodynamics, ΔU = Q - W. Heat is supplied, so Q = +400 J, and work is done by the system, so W = +150 J.
Correct answer: 250 J- A. Isochoric
- B. Isobaric
- C. Isothermal
- D. Isentropic
Explanation: For an ideal gas, internal energy is a function of temperature. In an isothermal process, the temperature is constant (ΔT = 0), so the…
Correct answer: Isothermal44. In which process the entire heat supplied to the gas is converted to the internal energy of the gas?
- A. Isochoric process
- B. Isobaric process
- C. Isothermal process
- D. Adiabatic process
Explanation: For all heat supplied (Q) to be converted into internal energy (ΔU), the work done (W) must be zero.
Correct answer: Isochoric process- A. Decreases
- B. Increases
- C. Becomes zero
- D. Remains constant
Explanation: For an ideal gas, internal energy depends only on its temperature. Since an isothermal process occurs at a constant temperature by…
Correct answer: Remains constant- A. Positive in each case
- B. Negative for I and positive for II
- C. Positive for I and negative for II
- D. Zero in I and positive for II
Explanation: An adiabatic process, has no heat transfer (Q=0). For an isothermal expansion, the internal energy does not change (ΔU=0), but the gas…
Correct answer: Zero in I and positive for II47. The heat rejected by a Carnot engine operating between 600 K and 300 K with 2000 J of heat input is:
- A. 1000 J
- B. 1500 J
- C. 600 J
- D. 1200 J
Explanation: Efficiency of a Carnot engine is η = 1 - (Tcold /Thot ) = 1 - (300/600) = 0.5. Work done is W = η × Qin = 0.5 × 2000 = 1000 J.
Correct answer: 1000 J- A. 4.00
- B. 1.00
- C. 0.25
- D. Impossible to determine
Explanation: Thermal efficiency (η) is the ratio of useful work output to the total heat input (η = Work / Heatin).
Correct answer: 0.2549. The efficiency of a heat engine in terms of the temperature of the reservoir and sink is defined as:
- A. T₁/T₂
- B. 1+T₁/T₂
- C. 1-T₂/T₁
- D. T₂/T₁
Explanation: The maximum possible efficiency of a heat engine operating between a hot reservoir at temperature T₁ and a cold reservoir at temperature…
Correct answer: 1-T₂/T₁- A. PV = constant
- B. PVᵞ = constant
- C. P/V=RT/n
- D. PVy=nRT
Explanation: This is the mathematical relationship that describes an adiabatic process for an ideal gas.
Correct answer: PVᵞ = constant- A. Isothermal process
- B. Adiabatic process
- C. Neither of these options is correct.
- D. Equal in both cases
Explanation: For an expansion from the same initial state to the same final volume, the pressure in an isothermal process remains higher than in an…
Correct answer: Isothermal process- A. 20%
- B. 25%
- C. 4%
- D. 50%
Explanation: From the conservation of energy, the heat input (Q₁) must equal the work done (W) plus the heat rejected (Q₂).
Correct answer: 20%- A. Slow compression of an elastic spring
- B. Slow evaporation of a substance in an isolated vessel
- C. Slow compression of a gas
- D. A chemical explosion
Explanation: An irreversible process is one that cannot return to its initial state along the same path.
Correct answer: A chemical explosion- A. Change in pressure
- B. Change in temperature
- C. Change in volume
- D. All of these are correct
Explanation: In an adiabatic process, there is no heat exchange (Q=0), so first law of thermodynamics becomes ΔU = -W.
Correct answer: Change in temperature- A. 80°C, 37°C
- B. 99°C, 30°C
- C. 99°C, 25°C
- D. 99°C, 37°C
Explanation: The initial efficiency is η = 1/6 = 1 - Tsink /Tsource . When the sink temperature becomes Tsink - 62, the new efficiency is 2(1/6) = 1/3.
Correct answer: 99°C, 37°C- A. Work done by the system = increase in internal energy of the system
- B. Work done on the system = increase in internal energy of the system
- C. Work done on the system = decrease in internal energy of the system
- D. Work done on the system = decrease in internal energy of the system + heat released
Explanation: In adiabatic process, Q=0, so ΔU = -W. This means that if work is done on the system (W is negative), internal energy increases (ΔU is…
Correct answer: Work done on the system = decrease in internal energy of the system- A. Pressure
- B. Volume
- C. Temperature
- D. All of these
Explanation: For an ideal gas, there are no intermolecular forces, so its internal energy consists only of the kinetic energy of its molecules.
Correct answer: Temperature- A. 8200 J
- B. 5600 J
- C. 7900 J
- D. 6400 J
Explanation: First, convert the heat absorbed to Joules: Q = 2 kcal × 4200 J/kcal = 8400 J (using 1 cal ≈ 4.2 J).
Correct answer: 7900 J- A. Temperature increases
- B. Mechanical energy comes into the system
- C. ΔQ = Zero
- D. All of these
Explanation: During an adiabatic contraction, work is done on the system, which is an input of mechanical energy.
Correct answer: All of these- A. Pressure is constant
- B. Volume is constant
- C. Temperature is constant
- D. Entropy is constant
Explanation: In isochoric process, volume is constant (ΔV = 0), work done (W=PΔV) is zero, regardless change in pressure or temp.
Correct answer: Volume is constant