Closed Systen Thermodynamics

[Anastasia_3B]

Hello,

I'm having slight difficulty understanding why the work done in a system is negative. I know Professor Lavelle had a pretty good explanation in lecture, but i'm wondering if anyone has a better explanation.

Thank you!!

[205995790]

The negative sign associated with PV work done indicates that the system loses energy. If the volume increases at constant pressure (ΔV > 0), the work done by the system is negative, indicating that a system has lost energy by performing work on its surroundings.

[606025647]

The sign of work depends on compression or expansion. If a gas expands, delta V is positive so -PdeltaV will be negative, which makes sense because as the gas expands in does work on the surroundings. If a gas compresses, delta V is negative so -PdeltaV will be positive, because the surrounds are doing work to compress the gas.

[Matthew_Park_2K]

Hi, another way to think about the concept is what way energy is flowing. Say a piston in a gas chamber expands, this work done is negative as energy is leaving the system (gas) to do work (expand the piston). Similarly, if the piston compressed, energy is entering the system to condense the gas, so the net energy is positive and thus work is also positive!

[Sophia Jin 3B]

If the work is negative, this means that the system is losing energy, hence the negative sign. If the system is doing work, it is losing energy. But if work is being done on the system, work would be positive, as the system gains energy.