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### Equation Explanation

Posted: Wed Feb 01, 2017 11:28 pm
Does the equation delta S=q/T only apply to ideal gases? Or does it apply to any isothermal, reversible process?

Also, are there any differences between the equations delta S=q/T and delta S=q(rev)/T?

Thanks!

### Re: Equation Explanation

Posted: Wed Feb 01, 2017 11:33 pm
Hi Ariana,

The equation $\Delta S = \frac{q}{T}$ assumes that temperature is constant - a reversible, isothermal expansion/compression. As such, you can only use this equation, and $\Delta S = nRT ln (\frac{V2}{V1})$, for an isothermal expansion/compression.

Note that, even if you don't have an isothermal expansion (say both temperature and volume increases), because entropy is a state function, you can turn the expansion into a two-step process (an isothermal expansion and isovolumetric expansion) so that you can use your formulas.

Hope this helps!