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Posted: Mon Feb 04, 2019 1:16 am
How can we determine whether the work is performed by or on the system? What makes work positive or negative in terms of the system?
Posted: Mon Feb 04, 2019 1:31 am
Work is positive (done on the system) if it adds energy to the system. For example, if you increase the pressure of a gas using a piston, you are doing work on the gas because you are adding energy to it. If you heat a gas, you are adding energy to it, therefore you are doing work on it.
Posted: Mon Feb 04, 2019 1:38 am
So does that mean work can't be negative?
Posted: Mon Feb 04, 2019 2:24 am
Work can be negative if it is done by the system, opposite of doing work on the system in that heat is released in contrast to adding heat, or energy.
Posted: Mon Feb 04, 2019 9:08 am
If work is done on the system it is positive. If work is done by the system it is negative. This is kind of like temperature changes in a system being endothermic and exothermic. If heat is being given into a system it’s positive and if heat is leaving the system it’s negative.
Posted: Mon Feb 04, 2019 9:43 am
I think of work like exothermic and endothermic. Doing work on a system is positive because you are adding energy. Work by a system is negative because energy is released.
Posted: Mon Feb 04, 2019 10:00 am
If work is positive, then it is adds energy into the system. If it is negative, energy is released!