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If work is done on the system by the surroundings, then work is positive. If work is done by the system onto the surroundings, then work is negative. This is because when work is done by the system, the system loses energy.
you can think about it the same way as when enthalpy is positive or negative—> it’s negative if the system is giving off heat, just like if the system is doing work (or “giving off” work so to speak), and like with enthalpy, it’s positive when the system is gaining heat, or when heat is being given to/absorbed by the system, just like when work is being done on the system.
alex_4l wrote:W is positive when work is done BY the system
W is negative when work is done ON the system
That being said, this is only for the equation of delta U = Q + W implying that it is a closed system with no other factors involved