According to the first law, if a system absorbs 10 J of heat and does 2 J of work, what is ΔU?

According to the first law, if a system absorbs 10 J of heat and does 2 J of work, what is ΔU?

• A. 12 J
• B. 2 J
• C. -8 J
• D. 8 J

Answer: (D)

The first law of thermodynamics says that the change in internal energy equals the heat added to the system minus the work done by the system: ΔU = Q − W. Here, the system gains heat of 10 J (Q = +10 J) and does 2 J of work on the surroundings (W = +2 J). So ΔU = 10 − 2 = 8 J. The internal energy increases by 8 joules because part of the added heat goes into doing work, leaving a net rise in internal energy smaller than the heat input. If no work were done, ΔU would be 10 J; doing work reduces that net increase, which is exactly what happens here.

The first law of thermodynamics says that the change in internal energy equals the heat added to the system minus the work done by the system: ΔU = Q − W. Here, the system gains heat of 10 J (Q = +10 J) and does 2 J of work on the surroundings (W = +2 J). So ΔU = 10 − 2 = 8 J. The internal energy increases by 8 joules because part of the added heat goes into doing work, leaving a net rise in internal energy smaller than the heat input. If no work were done, ΔU would be 10 J; doing work reduces that net increase, which is exactly what happens here.