Question
Fri December 07, 2012 By:

why sign conventions for work done on the system and by the system is different in physics and chemistry?

Expert Reply
Sat December 08, 2012
It's sometimes easier to find the change in energy first (whether +ve or -ve) and decide whether it is positive or negative afterwards.

energy lost ===> ?E is negative ==> work done by system
energy gained => ?E is positive ==> work done on system 

A good example is the first law of thermodynamics:

change in internal energy = heat added to system - work done by system; OR, equivalently:
change in internal energy = heat added to system + work done on system

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