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If a system is doing work on its surroundings (such as expanding volume against a piston), then work should be considered negative. If a system is having work done on it, then work is positive for the equation of internal energy (delta U).
If work is done ON the system, then work will be positive and internal energy will increase as a result. On the other hand, if work is done by a system (so expanding outward if you think of a piston), then work is negative and the internal energy of the system will decrease as a result. (Based on the equation deltaU = q + w)
If work is done on the system then the surrounding is putting energy into the system. The system would be gaining energy and the surroundings would be losing energy. If the system is doing work then energy is being put into the surroundings. The system would be losing energy and the surroundings would be gaining energy.
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