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The first law of thermodynamic is sometimes written as

[itex]\Delta[/itex]U = [itex]\Delta[/itex]Q-P [itex]\Delta[/itex]V and sometimes as [itex]\Delta[/itex]U = Q+W, and sometimes [itex]\Delta[/itex]U = Q - W. I am confused about all these.

I also know that W is positive when the system in question does work to its surrounding and negative otherwise. Looking at [itex]\Delta[/itex]U = [itex]\Delta[/itex]Q-P [itex]\Delta[/itex]V, i want to conclude that every expansion entails positive work and every compression negative work. I'm I correct to do so?