Work done by the system, positive or negative?

[Mahmoud Mahmoud]

My chemistry course says that work done by the system is negative, but physics course says that work done by the system is positive, I'm sure I'm missing something and I really need to understand it. Thanks.

 

[DrClaude]

It is a question of convention. Both approaches are valid, so long as one uses the right sign in from of W.

For a particular calculation, once you've decided on a convention, just be careful to be consistent.

 

[Chestermiller]

To add to what @DrClaude said, if the first law is written as $\Delta U=Q-W$, then W is the work done by the system on the surroundings. If it is written as $\Delta U=Q+W$, then W is the work done by the surroundings on the system.