How to remove confusion for sign in work done ..sometimes it comes + and sometime negative?

It all has to do with the basic definition of work, W = F·?x? Work is positive if F and ?x are in the same direction, and negative if they are in opposite directions. Imagine you are pushing on a piston containing a gas. You are exerting a force on it (the "system"), and it is exerting an equal-but-opposite force back on you (the "surroundings") If the gas is allowed to expand as you push on it, the force on the system (i.e. the gas) is opposite to the displacement, and so there is negative work done on the system. Also, the force on the surroundings (i.e. you) is in the same direction as displacement, so positive work is done on the surroundings. In other words, the system does work on the surroundings, or positive work is done by the system rather than on the system. If a gas compresses, things are the other way around. In thermodynamics, one adopts a definition of "work" and specifies whether it is positive for work done by the system or on the system. As far as I'm aware, it's common to say work done by the system is positive, or in other words: dW = P dV? Hope that helps. read less