Heat Transfer

The change in internal energy (ΔU) of a system can be determined using the first law of thermodynamics, which states that the change in internal energy is equal to the heat (Q) added to the system minus the work (W) done by the system:
ΔU = Q - W
In this case, the system gains xJ of heat (Q = xJ), and yJ of work is done on it (W = yJ).
Substituting these values into the equation, we have:
ΔU = xJ - yJ
Simplifying further, we get:
ΔU = (x - y)J
Therefore, the correct option is (A) x - y.