A system delivers 1275 J of heat while the surroundings perform 854 J of work on it. What is ΔU of the system?
A) -2129 J
B) -421 J
C) +421 J
D) +2129 J
E) -1275 J

Question

Quizplus · Accepted Answer

The change in internal energy (ΔU) of a system can be calculated using the first law of thermodynamics, which is ΔU = Q - W, where Q is the heat added to the system and W is the work done by the system. In this case, the system absorbs 1275 J of heat (Q = +1275 J) and the surroundings do work on the system, meaning work is done on the system rather than by it, so W = -854 J. Plugging these values into the formula gives ΔU = 1275 J - (-854 J) = 1275 J + 854 J = 2129 J. However, since work is done on the system, the correct interpretation should actually lead to ΔU = Q - W = +1275 J - 854 J = +421 J, indicating an increase in the system's internal energy.

A System Delivers 1275 J of Heat While the Surroundings