Sign convention for Q and W in the first law of thermodynamics

[FermiParadox]

. I know this is a really stupid question, but how the heck did the Q - W get to the other side of the equation? Shouldn't it be W - Q? I'm going to feel really stupid when I hear the answer, I'm sure, and I'm already done with the class so it's ultimately irrelevant, but I'd love to know.

 

[Bandit127]

Since you don't have an expert answer yet, I will try for an easy one. I am sure the experts will shoot me down if my thinking is wrong...

Q(dot) - W_s(dot) is still on the left in the final equation.

Substitute each term for a letter. Let us use u, v and w. Put a pair of brackets round Q(dot) - W_s(dot) and that is u.

Then you start with u + v - w = 0

Adding w gives u + v = w

Subtracting v gives u = w - v

u still equals Q(dot) - W_s(dot)

(The term on the right hand side has been simplified from w - v for your final equation).

 

[FermiParadox]

Since you don't have an expert answer yet, I will try for an easy one. I am sure the experts will shoot me down if my thinking is wrong...

Q(dot) - W_s(dot) is still on the left in the final equation.

Substitute each term for a letter. Let us use u, v and w. Put a pair of brackets round Q(dot) - W_s(dot) and that is u.

Then you start with u + v - w = 0

Adding w gives u + v = w

Subtracting v gives u = w - v

u still equals Q(dot) - W_s(dot)

(The term on the right hand side has been simplified from w - v for your final equation).

Dude, that makes total sense. It simultaneously explains where the "change in" signs on enthalpy and kinetic and potential came from. Thanks a ton.