【Daily Question】Theoretical Foundation 2020.05.13
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Multiple-choice question: A certain amount of ideal gas goes from state a to state b through a certain process; it absorbs 335 J of heat, and the system does 126 J of work on its surroundings； If the system moves from state a to state b through another process, and the work done by the system on its surroundings is 42 J, then the amount of heat transferred into the system during this process is: ( ) A. 530 J B. 167 J C. 251 J D. 335 J Answer: C Hint: An additional reward will be given to those who can explain the solution process; the answer can be found in the response!
C. The increment ΔE depends only on the initial and final states of the system, and not on the process undergone by the system; Qab=335=ΔEab+126, ΔEab=209 J, and Q′ab=ΔEab+42=251 J.
C. When an ideal gas moves from state a to state b, the increase in internal energy ΔE remains the same. From the first law of thermodynamics, Q = (E2 – E1) + W = ΔE + W, we have: Q1 – W1 = Q2 – W2. Therefore, 335 – 126 = Q2 – 42, which gives Q2 = 251 J
Analysis: Heat is absorbed to go from a to b: 335 J; work done on the surroundings is 126 J, so the total is 209 J ； If 42 J of work is done on the system, then heat must be absorbed: 209 J + 42 J = 251 J