By convention in physics,

Work done by the system is +ve and work done on the system is -ve.

 

These signs are taken opp in chem i.e. +ve for work done on the system and -ve for work done by the system.

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The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:

 

The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines.

It is typical for chemistry texts to write the first law as ?U=Q+W. It is the same law, of course - the thermodynamic expression of the conservation of energy principle. It is just that W is defined as the work done on the system instead of work done by the system. In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. In the context of chemical reactions and process, it may be more common to deal with situations where work is done on the system rather than by it.

 

Thus,

 

 Work done by the system = +ve, as it should decrease U of the system.

 

work done on the system = -ve, as it should increase the U of the system.