<p>In AP Physics, internal energy (delta U) = Q + W. But I saw in some sat ii tests, it said Q - W... where work is positive when done by the system, and negative when done on the system. Which is correct????</p>
<p>They both are, just the interpretation is different. In the AP book, they assume that w = - (PV). In the some sat books, w = pv. Both are correct but the AP book sees work done ON the system as positive work (positive work would be a decrease in volume of the system-> negatives cancel out) while the sat looks at work by the system (positive work would be an increase in volume-> no negatives and so the equation remains positive). However, when using the law of thermodynamics equation U = Q + W (AP) and U = Q - W, they are both basically the same thing and incorporate work done on the system through the system of equations: w= -PV → U = Q - PV and w= PV → U = Q - PV. Hopefully that sort of made sense… IF anything, read the explanations in the AP book and the other sources and you’ll find out they are basically asking the same thing.</p>
<p>I KNOW they are the same thing, but they need to state whether W is positive if the work is done on the gas or by the gas… I got one question wrong becuase it did not state, and I solved it in AP way…</p>
<p>Lol the real test won’t be that ambiguous I’m sure.</p>