# Mean Free Paths

1. May 5, 2006

### lando45

My teacher has set me this question:

Which of the following statements about the mean free path is correct?

1. The mean free path decreases as the size of the molecules increases.
2. The mean free path increases with the size of the molecules.
3. The mean free path decreases as the density of molecules increases.
4. The mean free path increases with the density of molecules.
5. The mean free path depends on the temperature of the gas.
6. The mean free path is the average distance travelled by a molecule between collisions.
7. The mean free path is the longest distance a particle can travel without colliding.

It doesn't specify how many of the statements are correct, so I tried selecting 1, 5 and 6, but that's incorrect.

I believe the definition of "mean free path" to the be the average distance a particle travels in a container before it collides with another particle.

So I chose 1 because increasing the size of the particles would make collisions more frequent, due to less free space in the container, and therefore a lower mean free path.

I chose 5 because if the temperature of the gas increases, the particles will have more energy, and will therefore move more eratically, causing more collisions, and therefore a low mean free path.

And finally, I chose 6 because this is basically a definition of the term.

Now why didn't I get this question right? Are the 3 options I've chosen correct? Should I be selecting any additional options? Or are the ones I've selected incorrect?

2. May 5, 2006

### daveb

Have you seen the formula for mean free path? From this can you tell what cause it to increase and decrease.