Integrating Maxwell-Boltzmann speed distribution

1. Dec 6, 2010

zebala

1. The problem statement, all variables and given/known data

Let P(v) represent the Maxwell-Boltzmann speed distribution. Basically what it comes down to is that I have to find the definite integral (0,inf) of P(v)*v^2 and get vrms from this.

2. Relevant equations

We are given the definite integral from 0 to inf for the function (x^4)*exp(-x^2), which is (3/8)*Sqrt(Pi).

3. The attempt at a solution

I first move all the constants in front of the integral and then the integral simplifies to (v^4)*exp(-mv^2/kT). However, I have no idea what to do with the constants stuck in the exponent. How would I continue from here?

Thank you beforehand.

2. Dec 6, 2010

phyzguy

You have to make a substitution. Set:
x=v*sqrt(m/(kT)). (Except I think it's really x=v*sqrt(m/(2kT)) and you forgot the 2. E= 1/2 mv^2).

3. Dec 6, 2010

zebala

Thanks, that seems to be a working solution! Do you know what the answer should be? 3kT/m? For some reason I get Sqrt(9kT/2m).

EDIT: Forgot to replace dv with the corresponding operator for x, dx*[(m/2kT)^(-1/2)]. Got it now!

Last edited: Dec 6, 2010