W = P/\V question and Expansion, confused

[onedirection]

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So we have W = P/\V

If the object is expanding Vf > Vo, therefore W is positive

+ Work --> + E ---> Increased tempearture right?


But an increase in volume means that it is doing work on the surroundings, so E is negative?

 

[sakabato93]

So we have W = P/\V

If the object is expanding Vf > Vo, therefore W is positive

+ Work --> + E ---> Increased tempearture right?


But an increase in volume means that it is doing work on the surroundings, so E is negative?

W= -PdeltaV where P is external pressure.

If work is done BY the system on the surroundings, the sign of the W is negative. If work is done UPON the system by the surroundings, the sign of W is positive. Then if there is no heat transfer between the surroundings and the system (adiabatic), then deltaE=W or deltaE=-PdeltaV

 

[jtran7]

So we have W = P/\V

If the object is expanding Vf > Vo, therefore W is positive

+ Work --> + E ---> Increased tempearture right?


But an increase in volume means that it is doing work on the surroundings, so E is negative?

you just need to remember the equation deltaU = Q - deltaW, it can also be written as deltaU = Q + W depending on how you view the situation ie you say work done on system is +ve.

Lets just stick with deltaU = Q - deltaW (work done on system is -ve, work done by system on surrounding is +ve)

if there is a gas that expands it is doing Work on its surrounding so if P is constant an increase in V will result in positive work being done. Consequently, you would have a decrease in delta U because you would have a positive deltaW and assuming no heat is added or removed to the system, deltaU = (0) - (+W)

But lets say you have the previous situation but there is loss of heat to the surround then Q would be negative so then the change in deltaU would be even more negative because deltaU = (-Q) - (+W)

 

[onedirection]

So we have W = P/\V

If the object is expanding Vf > Vo, by expanding it's doing work on the surroundings, therefore W is negative

- Work --> - E ---> decreased tempearture

Okay so this is what it actually is ^^ right htere?

 

[sakabato93]

So we have W = P/\V

If the object is expanding Vf > Vo, by expanding it's doing work on the surroundings, therefore W is negative

- Work --> - E ---> decreased tempearture

Okay so this is what it actually is ^^ right htere?

Yeah. Remember, under adiabatic conditions, expanding gases will DECREASE in temperature while compressed gases will INCREASE in temperature.

Also your sign on the equation for work is wrong. W= (negative)P(external)deltaV(system)

 

[sakabato93]

To be honest, the signs for the equation aren't that big of a deal, as long as you remember to assign signs properly and intuitively as you solve problems. Anything that is done TO the system is positive, any thing that is done BY the system is negative.

 

[onedirection]

Now I"m confused again, I did an EK 1001 question

And it's saying to get a gas to a higher pressure and volume, heat must flow into the gas system


oh actually nvm, I realized that this was not an adiabatic condition

 

[frochocinco]

sakabato93...how do you reason that expanding gases causes a decrease in temperature?

from the ideal gas law, if pressure is constant and the volume is increasing, wouldnt temperature have to increase to maintain equilibrium (V1/T1 = V2/T2)

please correct me if im wrong, i'm just not following your logic