# Maxwell speed distribution

1. Apr 22, 2011

### Je m'appelle

1. The problem statement, all variables and given/known data
Show that the number $$N(0,v)$$, of molecules of an ideal gas with speeds between 0 and v is given by

$$N(0,v) = N \left[ erf(\xi) - \frac{2}{\sqrt{\pi}} \xi e^{-\xi^2} \right]$$

Where,

$$erf(\xi) = \frac{2}{\sqrt{\pi}} \int_{0}^{\xi} e^{-x^2} dx$$

And,

$$\xi^2 = \left(\frac{mv^2}{2kT} \right)$$

2. Relevant equations

$$\frac{dN_v}{N} = \sqrt{\frac{2}{\pi}} \left( \frac{m}{kT}\right)^{\frac{3}{2}} v^2 e^{\frac{-mv^2}{2kT}} \ dv$$

3. The attempt at a solution

Alright, so I managed to get to the following

$$\frac{dN_v}{N} = \frac{4}{\sqrt{\pi}} x^2 e^{-x^2} dx$$

Where,

$$\alpha = \frac{m}{2kT}, \ x = \sqrt{\alpha}v$$

=========================\\===========================

So far so good, but now when checking the solution on the textbook it claims the following algebraic manipulation which I can't follow

$$\frac{2N}{\sqrt{\pi}}\int_{0}^{\xi} x (2xe^{-x^2} dx) = \frac{2N}{\sqrt{\pi}} x e^{-x^2} |_{\xi}^{0} \ + \ N \frac{2}{\sqrt{\pi}} \int_{0}^{\xi} e^{-x^2} dx$$

What was done on the integral above, how can you "split" it like that?

2. Apr 22, 2011

### L-x

Integration by parts

3. Apr 22, 2011

### Dick

Re: I need a hand understanding this integral manipulation

It's integration by parts. udv=d(uv)-vdu with u=x and dv=(2x)e^(-x^2)dx.

4. Apr 22, 2011

### Je m'appelle

But of course! Agh, how didn't I see that, I feel very stupid right now.

Thanks for the highlight, L-x!

5. Apr 22, 2011

### L-x

No worries. It's not immidiately obvious, although the integral is written in a way which gives you a clue: x(2x) instead of 2x^2

6. Apr 22, 2011