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October 31, 2014

October 31, 2014

Posted by **sTELLA** on Sunday, March 2, 2008 at 7:23pm.

I posted this question few hours ago, and someone gave me such hints to do it. However, I don't quite get it, can anyone please explain more about it?THANKS A LOT!

p V = n R T is state equation before and after

Now work done at constant temp

dW = -p dV

but p = (n R T)/V and here n R T is constant given

so

dW = -(nRT) dV/V

so

work done = (nRT) ln(V1/V2)

by the way, that is also the heat out since internal energy depends only on T which is constant.

- PHYSICS -
**Damon**, Sunday, March 2, 2008 at 7:40pmStella,

I assumed that you have had calculus.

If you have not, then your book must say something like:

For an ideal gas:

With compression at constant temperature,

work in = (n R T) ln (V1/V2)

or

With compression at constant temperature

heat out = (n R T) ln (V1/V2)

Since change in internal energy = heat in - work out

and internal energy depends only on temperature

your work in is the same as the heat out

You are given the work in, so you have :

work in

n, number of moles

R gas constant

V1 and V2

so you can calculate T

- PHYSICS -
**sTELLA**, Sunday, March 2, 2008 at 7:48pmwork in = (n R T) ln (V1/V2)

what is the "1n" standing for?

- PHYSICS -
**Damon**, Sunday, March 2, 2008 at 7:50pmln is natural log. Log to the base e. It is on your calculator, probably on the same key as e^x

- PHYSICS -
**Damon**, Sunday, March 2, 2008 at 7:55pmThis must be in your physics book. Look in the index for isothermal compression or expansion or simply compression or expansion. It may be in a paragraph on "work done during volume changes".

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