Mean Free Path
Mean free Path
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- On the basis of kinetic theory of gases, it is assumed that the molecules of a gas are continuously colliding against each other.
- Molecules move in straight line with constant speeds between two successive collisions.
- Thus path of a single molecule is a series of zig-zag paths of different lengths as shown in fig -.
- These paths of different lengths are called free paths of the molecule
- Mean Free Path is the average distance traversed by molecule between two successive collisions.
- If s is the Total path travelled in Ncoll collisions, then mean free path
Expression for mean free path :
- Consider a gas containing n molecules per unit volume.
- We assume that only one molecule which is under consideration is in motion while all others are at rest.
- If σ is the diameter of each molecule then the moving molecule will collide with all these molecules where centers lie within a distance from its centre as shown in fig
- If v is the velocity of the moving molecule then in one second it will collide with all molecules with in a distance σ between the centres.
- In one second it sweeps a volume πσ2v where any other molecule will collide with it.
- If n is the total number of molecules per unit volume, then nπσ2v is number of collisions a molecule suffers in one second.
- If v is the distance traversed by molecule in one second then mean free path is given by
λ = total distance traversed in one second /no. of collision suffered by the molecules
- This expression was derived with the assumption that all the molecules are at rest except the one which is colliding with the others.
- However this assumption does not represent actual state of affair.
- More exact statement can be derived considering that all molecules are moving with all possible velocities in all possible directions.
- More exact relation found using distribution law of molecular speeds is
its derivation is beyond our scope.
- Revision Notes