PV Work

[Hemichordate]

So I know that for PV work, if the volume is constant, work = 0. But can the system still be doing non PV work at constant volume?

I guess I'm still a bit confused about the concept of PV work.

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[Hemichordate]

bump

 

[G1SG2]

So I know that for PV work, if the volume is constant, work = 0. But can the system still be doing non PV work at constant volume?

I guess I'm still a bit confused about the concept of PV work.

Hmm, what type of non PV work do you have in mind? PV work is done when a gas pushes up on its surroundings by expanding, thus doing work on its surroundings. Work is also force*distance, so I guess it can do work but the distance part implies some type of volume change, which is not possible at constant volume so I'm not too sure...

 

[Hemichordate]

If you lift a box 5 feet into the air, would that be considered non-PV work? In that case, wouldn't the volume not change?

 

[DamienPro]

If you lift a box 5 feet into the air, would that be considered non-PV work? In that case, wouldn't the volume not change?

pdeltaV is work pertaining to a gas in a container, whereas work associated with lifting a box is F*d. Don't mix them up.

 

[crevmed]

W = P deltaV is for work done by or on a gas in a system. W= F deltax is not related to gas and is a different system doing work, in that case the person lifting the box is doing the work (applying a lifting force for acertain distance).

 

[G1SG2]

If you lift a box 5 feet into the air, would that be considered non-PV work? In that case, wouldn't the volume not change?

That's not work done by a gas.

 

[matth87]

This question confuses me...

 

[matth87]

If you lift a box 5 feet into the air, would that be considered non-PV work? In that case, wouldn't the volume not change?

If the volume of your system increases while the pressure stays the same (isobaric) then the work your system does will be negative.

Your system is doing work so it is losing energy.

If the volume of your system decrease while the pressure stays the same (isobaric) then the work on your system will be positive.

Your system is being worked on so it is gaining energy.

This can be seen from the following equation
w = -Pressure(external) x delta(Volume)
increasing volume = -w
decreasing volume = +w
* Pressure = constant

 

[crunchiee]

I am also confused with the PV work concept.

So from my understanding PV work only applies to the work done by gas right?

Then how is its relationship with enthalpy and gibbs free energy explained? also only when gas is involved? because the enthalpy equation is H=U+PV and i get so confused as to when it is constant pressure and temperature or not

 

[JaySmoove]

On the MCAT, PV work questions will always be at constant pressure. So...we dont have to worry about pressure. If the volume of the system is constant (doesnt change), then there is no pressure volume work done. Its all about the change in voume. Thats all you need to know. As far as a system being capable of non-pressure volume work at a constant volume, No.