Work (negative or positive)

[eriko vong 3D]

Hi! I was really confused on why the system loses energy (work is negative) when it expands and was hoping to get more clarification on how to know when work is negative or positive.

[Irene Oh 2A]

Hey! It's because it is the system (the expanding gas when the piston moves out) that does work on the surroundings. Volume increases as the system loses energy. If the system does work on the surroundings, work is negative. However, when the work is done on the system (ex: gas being compressed), then work is positive because energy is being added when the piston is compressed

[SanthoshNandakumar3L]

The work equation is negative because the sign cancels out with the change in volume value (delta V). For example, if the volume is expanding then the the final volume (larger) - initial volume (smaller) results in a positive value times the negative gives a negative value, which means the system is doing work on its surroundings. On the other hand, if the volume is compressed then the the final volume (smaller) - initial volume (larger) results in a negative value times the negative gives a positive value, which means the surroundings is doing work on the system. Hope this helps.

[Nikta Seyedin 3D]

Hello,
For the answer to your question and to clarify the concept in the simple terms, it is helpful to think about gas expansion as work being done by the system. This is because when the gas has expanded, it can apply a force on the container and then the work will be done by the system. In this situation for the gas expansion, the change in volume will be positive, and since the equation is w=-pdeltav, thus work will be negative. However, during gas compression, one need to perform work on the system. In this situation for gas compression then, the change in volume will be negative, and since the equation is w=-pdeltav, the work will be positive. I hope this helps!

[Andrew Dinh 2L]

When the system gains volume (positive delta V), it has to push air molecules out of the way, so it does work on the surroundings. Therefore, w would be negative. Conversely, if the system loses volume (negative delta V), work is being done on the system, so w would be positive. If it's easier, you can remember that the signs for delta V and work will be opposite of each other.

[Shubhreet Bhullar 3C]

Work is positive if it is done on the system and work is negative if it is done by the system. This simply denotes that the external pressure was positive and the system did work so the work is negative (w=-Pex delta V). Now, if the external pressure was negative, this means that work was done on the system to change the internal energy so the Pex would be negative and the work would be positive. hope this helps!

[Rylee Kubo 2K]

Delta V is related to change in volume (so when final is greater than intial, delta V is positive and when intial is greater than final, delta V is negative). When the system expands, the delta V is positive, so when multiplied by -P, work is negative and does work on its surroundings.

[Nikki Ma 3B]

if the system has work being done on it --> +work
if the system is the one doing work ---> - work