Isothermal Expansion/Compression

[t0ny]

I have a slight hiccup in understanding this. If you are expanding isothermally, it means that any heat lost to do the expansion will be compensated by addition of heat, to my understanding.

So q=P.delta(V)

So where I get stuck is for the value of P that we use, since P will decrease as V increases, since they are inversely proportional. Do we use Initial Pressure? Or Final pressure once expansion is done. I'm not exactly sure why we don't use delta P.

My second hiccup arises with difference between change in Enthalpy (delta(H)) and change in internal energy (delta(U)). Not quite sure what the difference is and when the two are the same.

Any clarification with this would he greatly appreciated. Thanks!

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[t0ny]

Anyone? ;(

 

[Cawolf]

To solve for the work without constant pressure you need to integrate PdV over the change in volume which solves to find that W = -nRTln(Vb/Va).

In this equation the enthalpy is the same as heat.