13.1 Brownian Motion

13.1 Brownian Motion

Brownian motion is the random and irregular motion of molecules in a gas. In 1927, Robert Brown, a botanist observed under microscope that tiny particles of pollens plant were moving around in the liquid randomly. These particles were identified as dust particles. Later, it was proved to be one of the effects of molecular motion.

A molecule in a gas changes its path after collision with another molecule. When it keeps on colliding with other molecule, the colliding molecule follows a random or zig-zag motion. In fact, collision transfers or exchanges the momentum and energy between the molecules (Fig. 13.1).

Brownian motion describes randomness and chaos, therefore, it represents one of the simple models of randomness. There are various reasons and causes of this motion which are given as under:

1. The size of the molecules is inversely proportional to the speed of the motion. We know that the transfer of momentum is inversely proportional to the mass of the particles. Lighter particles obtain greater speeds from collisions
2. The speed of the particles is inversely proportional to the viscosity of the fluid. Low viscosity of the fluid results in the faster Brownian movement.
3. Viscosity describes the magnitude of the internal friction in a fluid. It represents the resistance to flow of the fluid.
4. Brownian movement causes the molecules/particles in a fluid to be in constant motion.

(Fig. 13.1: Particles are in random motion)

For your information

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Albert Einstein explained the pollen movement in a liquid assisted by the molecules in 1905. In 1908, a French physicist J Perrin experimentally verified Einstein's explanation which earned him the 1926 Nobel Prize in physics.