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work equations

Posted: Wed Jan 29, 2020 10:10 am
by Savannah Mance 4G
For the work formula for expansion it is - P x change of volume but for the other formula it has P x a x d. I was just wondering why the second equation doesn't have a negative sign in it.

Re: work equations

Posted: Wed Jan 29, 2020 2:23 pm
by Chem_Mod
They both can if one is using both to define the work done by/on the system.

Re: work equations

Posted: Wed Jan 29, 2020 4:59 pm
by Tyler Angtuaco 1G
The second equation is likely referring to the work done on the system, which is why it is positive. The first one you mention is what will be used most for the problems in the thermodynamics section since our observations are typically made in reference to the system and the work it is doing.

Re: work equations

Posted: Wed Jan 29, 2020 5:03 pm
by Angela Patel 2J
What about when we are calculating the change in internal energy in a system under constant pressure? Work is involved, do we always use the negative sign in the expression for work?

Re: work equations

Posted: Mon Feb 03, 2020 10:20 am
by Abhi Vempati 2H
Angela Patel 2J wrote:What about when we are calculating the change in internal energy in a system under constant pressure? Work is involved, do we always use the negative sign in the expression for work?


We don't just put the negative sign because work is involved; rather, we add the negative sign because the system is doing work. If work was being done on the system, the sign would be positive, signifying that you need to look at whether the system is doing work or if work is being done on the system. Hope this helps!

Re: work equations

Posted: Mon Feb 03, 2020 12:50 pm
by Dina Marchenko 2J
Angela Patel 2J wrote:What about when we are calculating the change in internal energy in a system under constant pressure? Work is involved, do we always use the negative sign in the expression for work?


You don't necessarily need to worry about when to include the negative sign or not within -P delta V. If work is being done on the system then the system is gaining work and energy. Work being done on the system is compression which means the volume will decrease. That will make delta V negative which will cancel out the negative sign and make internal energy positive overall. So the negative sign is there to represent a system losing energy when it does work on the surroundings, but the values you input into that same equation with work being done on a system will rework the negative sign and give the positive value you'd expect.

So don't worry about the negative sign. It's just there to make sure the internal energy value properly represents what's occurring with the system.