# Homework Help: Change of entropy

1. Sep 17, 2009

### Petar Mali

I would like to find change of entropy of composite of two ideal gases which have sane tenperature and preasure and different volumes $$V_1$$ and $$V_2$$.

I think about this and I have idea but I'm not sure am I right?

Equation of state of ideal gas is

$$PV=RT$$

And we have

$$Tds=C_VdT+PdV$$

$$P=\frac{RT}{V}$$

$$dS=C_V\frac{dT}{T}+RT\frac{dV}{V}$$

First term is equal to zero!

$$dS=RT\frac{dV}{V}$$

I think now that change of entropy is

$$S=\int^V_{V_1}RT\frac{dV}{V}+\int^V_{V_2}RT\frac{dV}{V}$$

This is change of entropy! Am I right?

Can I use that

$$V=V_1+V_2$$ ?

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