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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 8 of 15

141. For a heat engine 'A' the ratio of Q2 to Q1 is 2/3 while for that of heat engine 'B', the ratio of Q2 to Q1 is 1/3. What is the value ηA: ηB?

  • A. 1:3
  • B. 1:2
  • C. 2:3
  • D. 2:1

Explanation: To find the value of the efficiency ratio (ηA:ηB) between two heat engines A and B, we need to compare the ratios of their heat outputs…

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Correct answer: 1:2

142. During adiabatic compression work done is equal to 200 J. What will be the change in internal energy?

  • A. -200 J
  • B. +200 J
  • C. 100 J
  • D. 0 J

Explanation: In an adiabatic system no heat exchange between the system and surrounding.

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Correct answer: +200 J

143. If heat energy of 100 J is taken out but 400 J work is done on the system, the change in internal energy is:

  • A. -300 J
  • B. 300 J
  • C. -100 J
  • D. 0 J

Explanation: ΔQ=ΔW+ΔU100 J =400 J+ΔU100 J - 400 J= ΔUΔU=300 J

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Correct answer: 300 J

144. Heat and work are equivalent. This means

  • A. When we supply heat to a body we do work on it
  • B. When we do work on a body we supply heat to it
  • C. The temperature of a body can be increased by doing work on it
  • D. Heat and work are not inter convertible

Explanation: From the given statement it can be derived Q= W and hence Internal energy change must me zero.

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Correct answer: The temperature of a body can be increased by doing work on it

145. A system can be taken from the initial state P1V1 to the final state P2V2 by two different methods. Let Q and W represent the heat given to the system and the work done by the system.Which of the following must be the same in both the methods?

  • A. Q
  • B. W
  • C. Q + W
  • D. Q - W

Explanation: Internal energy would be the same irrespective of the path.U (internal energy) = Q - W

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Correct answer: Q - W

146. When 20J of work was done on a gas, 40J of heat energy was released If the initial internal energy of the gas was 70J. what is the final internal energy?

  • A. 50J
  • B. 60J
  • C. 90J
  • D. 110J

Explanation: The change in internal energy is 20-40 = -20 J so the final internal energy is x where x-70= -20X=50

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Correct answer: 50J

147. A carnot engine working between 200k and 400k has a work output of 600 J per cycle. How much heat energy is supplied to the engine from the source of each cycle.

  • A. 1400 J
  • B. 1200 J
  • C. 1700 J
  • D. 1300 J

Explanation: T₁ =400kT₂-200kW=800JWe know efficiency=1-T2/T1-WW/Q=1-200/400=600/0.5=1200JAs it is numerical, it can have only one answer.

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Correct answer: 1200 J

148. During each cycle, a heat engine with an efficiency of 25% takes in 800J of energy. How much waste heat energy is expelled in each cycle?

  • A. 100J
  • B. 200J
  • C. 400J
  • D. 600J

Explanation: If the efficiency of the engine is 25%, then the energy that is output for useful work is 25% of the input energy, which is 25% of 800 J=…

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Correct answer: 600J

149. The sum of which of the following forms of energies, possessed by the atoms and molecules of a system, describes its internal energy?I. RotationalII. VibrationalIII. Translational

  • A. I, II and III
  • B. I and II only
  • C. I and III only
  • D. II and III only

Explanation: The internal energy of a system is the total energy contained within, including kinetic and potential energies associated with the motion…

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Correct answer: I, II and III

150. Which of the following diagrams show(s) adiabatic compression(s)?

  • A. I only
  • B. I and II
  • C. III only
  • D. II and III

Explanation: In an adiabatic process, there is no heat exchange with the surroundings Adiabatic compression is usually depicted as a curve on a PV…

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Correct answer: III only

151. If a system absorbs 2090 J of heat and does 400 J of work, then the change in internal energy of the system will be

  • A. -1690 J
  • B. 0 J
  • C. 1690 J
  • D. 2490 J

Explanation: The First Law of Thermodynamics states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q)…

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Correct answer: 1690 J

152. The maximum possible efficiency of a heat engine operating between the temperature limits of 373 K and 673 K as input and output respectively is

  • A. 25%
  • B. 44.6%
  • C. 55.4%
  • D. 75%

Explanation: The maximum possible efficiency of a heat engine operating between temperature limits is given by the Carnot efficiency formula:Efficiency…

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Correct answer: 44.6%

153. For adiabatic process, the first law of thermodynamics is:

  • A. w = ∆u = Q
  • B. Q = -w
  • C. Q = w
  • D. w = -∆u

Explanation: An adiabatic process is one in which no heat is gained or lost by the system.

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Correct answer: w = -∆u

154. Conversion of heat energy completely into work violates:

  • A. Zeroth law of thermodynamics
  • B. First law of thermodynamics
  • C. Second law of thermodynamics
  • D. Both, the first and second laws of thermodynamics

Explanation: Conversion of heat energy completely into work implies no loss of energy.

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Correct answer: Second law of thermodynamics

155. The property of the system that does not change during adiabatic change is:

  • A. Temperature
  • B. Volume
  • C. Pressure
  • D. Amount of heat

Explanation: In adiabatic process, heat remains constant. Q =constant ΔQ = O Hence, option D is correct.

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Correct answer: Amount of heat

156. The equation of 1st Law of thermodynamics is:

  • A. Q= ∆U+W
  • B. ΔU= Q-W
  • C. W= Q+ ∆U
  • D. Q= W
  • E. B and A

Explanation: The first law of thermodynamics is given as ΔU = Q - W or Q = ΔU + W, where ΔU is the change in internal energy of a system, Q is the net…

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Correct answer: B and A

157. First law of thermodynamics concerns with the conservation of:

  • A. Heat
  • B. Work
  • C. Momentum
  • D. Energy

Explanation: The correct statement is D. Energy. The first law of thermodynamics concerns the conservation of energy, stating that the total energy of…

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Correct answer: Energy

158. In a certain process, 200J of heat energy is supplied to a system and at the same time 50J of work is done by the system. The increase in the internal energy of the system is:

  • A. 25 J
  • B. 100 J
  • C. 150 J
  • D. 250 J

Explanation: The first law of thermodynamics states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (Q)…

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Correct answer: 150 J

159. A Fixed mass of an ideal gas absorbs 1000 J of heat and expands under a constant pressure of 20 kPa from a volume of 25 x 10^-3 m3 to a volume of 50 x 10^-3 m3 . What is the change in internal energy of the gas?

  • A. +1000 J
  • B. +5000 J
  • C. 0J
  • D. +500 J

Explanation: W = PΔVΔV = V2 - V1ΔV = 50 * 10-3 - 25 * 10-3 ΔV = 25 * 10-3 W = 20,000 * 25 * 10-3 W = 500 JCalculate the change in internal energy.

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Correct answer: +500 J

160. According to first law of thermodynamics, what is the relationship between the change in the internal energy of a closed system ΔU, the head added to the system (Q) and the work done by the system W.

  • A. ΔU=Q+W
  • B. ΔU=Q-W
  • C. ΔU=Q*W
  • D. ΔU=Q/W

Explanation: The first law of thermodynamics, also known as the law of energy conservation, states that the change in internal energy (ΔU) of a closed…

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Correct answer: ΔU=Q-W
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