the sign (negative or positive)of work being done on the system or on the surroundings
work done on the system --------------------> positive
work done on the surroundings ---------------------> negative
explanation :
mathematical expression for first law of thermodynamics
∆E = q + w
When a system does work (w) on the surroundings, its internal energy decreases. In this case, work is taken as negative (-w).
Now, if 'q' is the amount of heat absorbed by the system and 'w' is the work done by the system, then change in internal energy becomes
∆E = q + (-w) = q - w
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