Numford

Heat and thermodynamicsClass 11, Kinetic Theory

Dimensional formula of Mean free path

[L]

Symbolλ SI unitmetre (m)

The average distance a molecule travels between collisions. It is a length.

Derivation

How to get the dimensional formula of mean free path

  1. Start from the definition: mean free path = 1 / (number density × area).
  2. volume = length3, so volume is [L]3 = [L3].
  3. number density = 1 / volume, so number density is 1 / [L3] = [L-3].
  4. area = length × length, so area is [L] × [L] = [L2].
  5. Put in the dimension of each part: 1 / ([L-3] × [L2]).
  6. Add the powers of each base quantity: [L].
Same dimensions

Quantities with the dimensions [L]

Equal dimensions do not make them the same quantity. Compare mean free path with the others in the full grid.

Practice

Three questions on mean free path

1Which of these has the same dimensions as mean free path?

2What is the SI unit of mean free path?

3Which one has different dimensions from the others?

Questions

Mean free path: quick answers

What is the dimensional formula of mean free path?

The dimensional formula of mean free path is [L]. It comes from mean free path = 1 / (number density × area).

What is the SI unit of mean free path?

The SI unit of mean free path is the metre (m).

Which quantities have the same dimensions as mean free path?

Mean free path shares [L] with length, displacement, radius of gyration, wavelength, amplitude, focal length, fringe width, de Broglie wavelength. Equal dimensions do not make them the same quantity.

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