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Free Thermodynamics MCQs with Answers

714 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.

Thermal equilibrium and heat explain how temperature and energy transfer are related, while molar specific heats describe the heat required by a gas at constant volume or constant pressure. The first law connects heat supplied, work done and change in internal energy, with sign conventions kept distinct from temperature changes.

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Thermal Equilibrium and HeatMolar Specific Heat of a GasFirst Law of Thermodynamics
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714 questions · page 7 of 36

121. The bicycle pump works based on:

  • A. 1st Law of thermodynamics
  • B. 2nd Law of thermodynamics
  • C. Law of conservation of energy
  • D. Law of entropy

Explanation: An adiabatic process is one where no heat enters or leaves a system. Here, we compress a gas adiabatically inside a bicycle pump.

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Correct answer: 1st Law of thermodynamics

122. According to the first law of thermodynamics, ΔU = Q + W, where ΔU Is the increase in internal energy of the system, Q is the heat transferred to the system and W is the external work done by the system.Which of the following is NOT a correct expression?

  • A. At constant temperature: Q = -W
  • B. When no work is done: ΔU = Q
  • C. In gaseous system: ΔU = Q + P Δ V
  • D. When work is done by the system: ΔU = Q - W

Explanation: There are two sign conventions:1. ∆U= Q+W: Here, work done BYthe gas/system is taken POSITIVE. 2.

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Correct answer: When work is done by the system: ΔU = Q - W

123. The efficiency of the Carnot's Engine working between 150°C and 50°C is:

  • A. 22.3%
  • B. 20.0%
  • C. 23.6%
  • D. 30.6%
  • E. 33.6%

Explanation: The efficiency of a Carnot engine is calculated using the formula η% = (1 - T2/T1) × 100%, where T1 is the temperature of the hot…

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Correct answer: 23.6%

124. A system absorbs 1000 Joules of heat and delivers 600 Joules of work, while losing 100 Joules of heat by conduction to the atmosphere, the change in the internal energy of the system will be:

  • A. 600 Joules
  • B. 900 Joules
  • C. 300 Joules
  • D. 400 Joules
  • E. 1600 Joules

Explanation: Change in internal energy of the system is given by:Change in internal energy = Heat absorbed - Work done - Heat lostChange in internal…

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Correct answer: 300 Joules

125. The equation ΔU = ΔQ refers to:

  • A. Isothermal process
  • B. Adiabatic process
  • C. Isochoric process
  • D. Isobaric process
  • E. Carnot cycle

Explanation: For equation ΔU = ΔQ + W to be ΔU = ΔQ, the work done should be equal to 0.

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Correct answer: Isochoric process

126. The amount of heat at constant volume is called as:

  • A. Internal energy
  • B. Enthalpy
  • C. Entropy
  • D. Temperature
  • E. Pressure

Explanation: At constant volume, the heat of the reaction is equal to the change in the internal energy of the system.

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Correct answer: Internal energy

127. A gas is enclosed in a container fitted with a piston of cross-sectional area of 0.10 m2. The pressure of the gas is maintained at 8000 N/m2. When heat is slowly transferred, the piston is pushed up through a distance of 4.0 cm. If 42 J heat is transferred to the system during the expansion, what is the change in the internal energy of the system?

  • A. 5 J
  • B. 10 J
  • C. 20 J
  • D. 30 J
  • E. 40 J

Explanation: To determine the change in internal energy, apply the first law of thermodynamics: ΔU = Q - W, where ΔU is the change in internal energy…

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Correct answer: 10 J

128. A fixed mass of gas in a thermally insulated container is compressed. After compression, the temperature of the gas will have:

  • A. Fallen, since more molecules bombarded the container and so they must be moving slower
  • B. Fallen, since more molecules collide more frequently with one another and so their average speed is lower
  • C. Remains constant if the compression is very slow
  • D. Risen, since doing work on the gas increases the kinetic energy of the molecules
  • E. Risen, since there are more intermolecular collisions and so more heat is produced by them

Explanation: If an ideal gas is compressed adiabatically, its temperature rises because the heat produced cannot be lost to the surroundings.

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Correct answer: Risen, since doing work on the gas increases the kinetic energy of the molecules

129. With the usual notation, the first law of thermodynamics applied to one mole of an ideal gas can be written in the following form:CVΔT=ΔQ - PΔVIn a change for which Boyle's law is obeyed, which of the following would necessarily be zero?

  • A. ΔQ
  • B. CV
  • C. ΔT
  • D. P
  • E. ΔV

Explanation: Boyle's Law states that when the temperature of a given mass of confined gas is constant, the product of its pressure and volume is also…

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Correct answer: ΔT

130. The rapid escape of air from a burst tire is an example of:

  • A. Isothermal
  • B. Adiabatic
  • C. Isobaric
  • D. Isochoric

Explanation: The rapid escape of air from a burst tyre is an example of an adiabatic process.

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Correct answer: Adiabatic

131. A frictionless heat engine can be 100% efficient if:

  • A. The temperature of the sink is 0°C.
  • B. The temperature of the sink is 0 K.
  • C. The temperature of the source is 0.
  • D. The temperature of the sink is equal to the temperature of the source.

Explanation: A frictionless heat engine(Carnot engine) has the following formula for efficiency: Maximum efficiency n = 1 Tc/Th where Th is the…

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Correct answer: The temperature of the sink is 0 K.

132. The internal energy of the system decreases in an adiabatic process. Which of the following must be true regarding this process?

  • A. Heat flows out of the system
  • B. Work is done by the system
  • C. Work is done by the surroundings
  • D. The potential energy of the system is changing

Explanation: According to the first law of thermodynamics, Q = ΔU + W, where Q is the heat added to the system, ΔU is the change in internal energy…

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Correct answer: Work is done by the system

133. When an ideal gas (γ=5/3) is heated under constant pressure, what percentage of given heat energy will be utilized in doing external work?

  • A. 0.4
  • B. 0.5
  • C. 0.3
  • D. 0.2

Explanation: The percentage of heat utilized is 40% or 0.4.

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Correct answer: 0.4

134. Heat exchange between the system and surroundings at constant volume is shown by the relation:

  • A. ΔH= qv+ w
  • B. ΔE=(q)v
  • C. ΔH=(q)v
  • D. ΔE=(q)v + PV

Explanation: Since we have a constant volume, it is safe to assume that no work is being done on the system, or by the system.

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Correct answer: ΔE=(q)v

135. The first law of thermodynamics can be stated as?

  • A. Q = ΔU + W
  • B. Q + ΔU = W
  • C. Q = ΔU x W
  • D. None of these

Explanation: The first law of thermodynamics states that the change in internal energy of a system equals the net heat transfer into the system minus…

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Correct answer: Q = ΔU + W

136. If one mole of an ideal gas is heated at constant pressure, then the first law of thermodynamics can be written as:

  • A. CpΔT = CvΔT + PΔV
  • B. CvΔT = CpΔT + PΔV
  • C. CpΔT = CvΔT + VΔP
  • D. ΔCpT = ΔCvT + PΔV

Explanation: Heat supplied to the gas at constant pressure isΔQ=nCPΔTChange in internal energy,ΔU=nCVΔTChange in work done,ΔW = PΔVAccording to the…

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Correct answer: CpΔT = CvΔT + PΔV

137. The first law of thermodynamics is a special case of :

  • A. Newton's law
  • B. Charles's law
  • C. Conservation of energy
  • D. Conservation of entropy

Explanation: First law of thermodynamics states that change in internal energy of the system is equal to heat transferred to the system and work done…

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Correct answer: Conservation of energy

138. For an adiabatic process, the first law of thermodynamics can be written as:

  • A. Work done by the system = decrease in internal energy of the system
  • B. Work done by 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 an adiabatic process, we have no heat exchange thus the internal energy would be depending, solely, upon the work done, either on the…

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Correct answer: Work done by the system = decrease in internal energy of the system

139. A thermally insulated rigid container contains an ideal gas. It is heated through a resistance coil of 100Ω by passing a current of 1 A for five minutes, then the change in internal energy of the gas is:

  • A. 0 kJ
  • B. 10 kJ
  • C. 20 kJ
  • D. 30 kJ

Explanation: Heat energy supplied to the Ideal gas = I2RtHeat energy supplied to the Ideal gas = 12x100x5x60Heat energy supplied to the Ideal gas =…

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Correct answer: 30 kJ

140. What will be the change in internal energy when work done by the system is 210 J and heat absorbed is 20 J?

  • A. -190
  • B. 190
  • C. 90
  • D. -90

Explanation: In thermodynamics, heat entering the system is taking as positive, and heat leaving the system is taken as negative.Similarly, work done…

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Correct answer: -190
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