# Homework Help: Probability that a molecule will travel a distance at least equal to the mean free pa

1. Aug 17, 2010

### Oojee

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

a) Find the probability that a molecule will travel a distance at least equal to the mean free path before its next collision.
b)After what distance of travel since the last collision is the probability of having suffered the next collision equal to 1/2?

2. Relevant equations
Exponential probability distribution
f(r) = Ae-r/$$\lambda$$

where A = a constant, $$\lambda$$ = mean free path

3. The attempt at a solution

P = Integral (limits $$\lambda$$ to $$\infty$$)f(r) dr / Integral (limits 0 to $$\infty$$) f(r) dr
I will do the calculations of both parts but not sure about the probability. I have found probability by number of molecules with distance between collision greater than lambda / total number of molecules. Is this probability correct?

2. Aug 17, 2010

### vela

Staff Emeritus
Re: probability that a molecule will travel a distance at least equal to the mean fre

What exactly is f(r) supposed to represent? You say it's a probability distribution but of what exactly?

3. Aug 17, 2010

### Oojee

Re: probability that a molecule will travel a distance at least equal to the mean fre

f(r) is a continuous function that gives the probability for a molecule to travel a distance r before having a collision.

4. Aug 17, 2010

### vela

Staff Emeritus
Re: probability that a molecule will travel a distance at least equal to the mean fre

OK, your calculation is correct. Typically, though, you normalize f(r). In other words, you set the constant A, which is a normalization constant, so that

$$\int_0^\infty f(r)\,dr = 1$$

You're effectively doing this in your calculation by dividing by the integral from 0 to infinity.

5. Aug 18, 2010

### Oojee

Re: probability that a molecule will travel a distance at least equal to the mean fre

@ vela
thank you.