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The mean free path is the average distance traveled by a moving molecule between collisions.
Imagine gas leaking out of a pipe. It would take a while for the gas to diffuse and spread into the environment. This is because gas molecules collide with each other, causing them to change in speed and direction. Therefore, they can never move in a straight path without interruptions. Between every two consecutive collisions, a gas molecule travels a straight path. The average distance of all the paths of a molecule is the mean free path.
Imagine a ball traveling in a box (Figure 1) where the ball represents a moving molecule. Everytime it hits the wall, a collision occurs and the direction of the ball changes. In figure 1, the ball hits the wall five times, causing five collisions. Between every two consecutive collisions, the ball travels an individual path. It travels a total of four paths between the five collisions; each path has a specific distance, d. The mean free path of this ball is the average distance of all four paths.
Figure 1. This figure shows a ball traveling in a box.
Every time it hits the wall, a collision occurs and the ball changes direction. The ball travels an individual path between every two consecutive collisions. Each path traveled by the ball has a distance, d.
Free Mean Path () = [d(1) + d(2) + d(3) + d(4)] / 4
In reality, the mean free path ( ) cannot be calculated by taking the average of all the paths because it is impossible to know the distance of each path traveled by a molecule. However, we can calculate it from the average speed (<c>) of the molecule divided by the collision frequency (). The formula for this is:
= <c> /
Since is equal to 1 / (average time between collisions), the formula can also be:
= <c> / [1 / (average time between collisions)]
= <c> X (average time between collisions)
In addition, since is also equal to (pi) d^2 <c> (N/v), where d is the diameter of the molecule and (N/v) is the density, the formula can be further modified to:
= <c> / [ (pi) d^2 <c> (N/v)]
= 1 / [ (pi) d^2 (N/v)]
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