If 100 kJ of heat is absorbed by the system and 40 kJ of work is done on the system, what is the change of internal energy?
Correct answer: D. +140 kJ
- A. -60 kJ
- B. +60 kJ
- C. -140 kJ
- D. +140 kJ
Explanation
By the first law, the change in internal energy is the heat added to the system plus the work done on it, so 100 plus 40 gives plus 140 kJ. Both inputs raise the internal energy, which is why they add rather than cancel. Had the system done 40 kJ of work on the surroundings instead, the answer would have been plus 60 kJ, which is the distractor offered.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About First Law of Thermodynamics
The first law states that energy is conserved, so the change in a system's internal energy equals heat supplied plus work done on the system, ΔU = q + w. Questions use sign conventions, expansion and compression work, state functions versus path functions, and the relation between internal energy and enthalpy.
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