Thermodynamics of Gas: Work Done by vs. Work Done On

[hoyy1kolko]

I read from my book work done by gas is positive sign and work done on gas is negative sign.

I read through the information on internet,it states that work done by gas is negative and work done on gas is positive.I am confusing now.Please explain to me.Thank you.

 

[Mapes]

It's a matter of convention whether incoming energy is taken as positive or negative. That is, you can choose either definition and get the same answer as long as you're consistent. (However, it does make it confusing if the definition isn't made clear at the start of a solution.)

 

[kaymant]

Indeed, its a matter of convention. Usually in physics we take the work done by the gaseous system as positive. The motivation behind this convention comes the definition of work. When the system expands, its exerting some force in the outward direction and the displacement of the boundary is also outward. Hence, the work done by the gas is positive. On the other hand, when the gas contracts, it is exerting the force in the outward direction, yet the displacement of the boundary is inwards. Hence, the work done on the system is negative work from the standpoint of the system.
Usually in chemistry we take the work done on the system as positive. The reference taken in this case is the energy of the system. If something increases the energy, it is assigned positive value.

 

[Studiot]

See post#4 of this thread

https://www.physicsforums.com/showthread.php?t=479853

 

[Q_Goest]

I read from my book work done by gas is positive sign and work done on gas is negative sign.

I read through the information on internet,it states that work done by gas is negative and work done on gas is positive.I am confusing now.Please explain to me.Thank you.

I'll stick to the standard convention and take a guess at the context you read this in. Let's say we have two bodies or systems, A and B. A might be a bucket of water or a cylinder with a piston in it. B might be the environment.

When A does work on, transfers heat to or otherwise transfers energy to B, the energy in A decreases and the energy in B increases an equal amount.

- conversely -

When A has work done on it, has heat transferred to it or otherwise has energy transferred from B, the energy in A increases and the energy in B decreases an equal amount.

So when you read that the "work done by gas is positive" (or an increase) that probably was to say that there was a transfer of energy to something and that something had an increase in energy.

When you read that "work done by gas is negative", that probably was to say that the total energy of the gas decreased when it did work.