KTEST 6 2027 Practice Test

In thermodynamics, the first law states ΔU = Q - W. If a system absorbs 3 J of heat and does 5 J of work, what is ΔU?

2 J -2 J -8 J 8 J

Correct Answer:

-2 J

Explanation:

The main idea is the first law of thermodynamics, which says ΔU = Q − W. Here, heat added to the system is positive 3 J, and the system does 5 J of work on the surroundings, which is also taken as positive in this convention. Plugging in gives ΔU = 3 − 5 = −2 J. A negative change means the system’s internal energy decreases by 2 joules because the energy leaving as work is larger than the energy entering as heat.

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