- #106

Hak

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TSny said:It is a matter of convention as to whether the symbol ##W## in the first law is positive when the system does work on the surroundings or is positive when the surroundings does work on the system.

Thank you so much. I understand now.TSny said:For the first convention where ##W## represents the work donebythe system, the first law is $$\Delta U = Q - W.$$ For the second convention where ##W## represents the work doneonthe system, the first law is $$\Delta U = Q+W.$$ ##W## will differ in sign for the two conventions. You are free to choose either convention for this problem. If you use the first convention and calculate ##W## for, say, the first stroke of the small pump, then you would get a negative value for ##W##. This means that the air in the pump did negative work on the piston. Therefore, the piston did positive work on the air in the pump. (This follow's from Newton's third law of motion.)

If you use the second convention, then you would find that ##W## has a positive value. Therefore, the air did positive work on the air in the pump. This agrees with the first convention.