1. The problem statement, all variables and given/known data One mole of an ideal gas at 300K expands isothermally from a pressure of 2x10^6 Nm^-2 to a pressure of 2x10^5 Nm^-2. Calculate the work done ON the gas. 2. Relevant equations dW = PdV 3. The attempt at a solution Following through the integral, it becomes clear that work is either plus or minus 5.74kJ. My teacher has said he uses the sign convention that work done ON a system is always negative. I'm trying to understand what this means. Imagine a piston, if you push the piston you will do work, but if you pull the piston are you not also doing work too? According to the convention does this mean you will do negative work on the gas if you pull OR push? The reason I'm getting so confused is that in the mark scheme, the answer is given as -5.74kJ. This means the work done ON the gas in an expansion is negative. But later on, I'm told, using the same sign convention, compressing a gas means negative work has been done ON the gas. Surely that is a contradiction? Has my teacher confused himself??