Change in the energy content of an isobaric process

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Homework Statement:
A PV diagram shows an isobaric expansion, I'm asked to know the signs of: work done on, heat added to, and change in energy content of the system.
Relevant Equations:
W= integral (P dV), dU = Q + W
Homework Statement: A PV diagram shows an isobaric expansion, I'm asked to know the signs of: work done on, heat added to, and change in energy content of the system.
Homework Equations: W= integral (P dV), dU = Q + W

Since this is an expansion, the system does work on the surrounding and therefore the work done on the system is -ve.
Also, to preserve a constant pressure, heat has to be added, therefore heat added is +ve.

The energy content(dU) is the sum of those too, and since the process is isobaric not adiabatic, dU has a value.
How can I know whether it is negative or positive?
 

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  • #2
kuruman
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What expression do you know for dU that involves the temperature change dT?
 
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vela
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The energy content(dU) is the sum of those too, and since the process is isobaric not adiabatic, dU has a value.
You should really write either ##dU = dQ + dW##, if you're dealing with infinitesimal quantities, or ##\Delta U = Q + W##, if not. I'll assume you really meant ##\Delta U##, not ##dU##.

##\Delta U## is always going to have some value which depends only on where you start and where you end up, not the process, since ##U## is state variable.

Did you mean ##\Delta U## won't be 0? That claim would be true for both isobaric and adiabatic processes, so your logic doesn't make sense.
 

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