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?

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?

• A. 2 J
• B. -2 J
• C. -8 J
• D. 8 J

Answer: (B)

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.

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.