2 Brownian motion

• If we look through a microscope at the smoke rising from a burning candle, we see tiny carbon particles moving here and there in random zig-zag motion.
• This motion is called Brownian motion after the Scottish botanist Robert Brown, who discovered in 1827 that pollen grains suspended in water move constantly and randomly.

Explanation.

• The molecules of air (or any gas) move randomly in all directions with different speeds and collide with each other and with the smoke particles.
• At any instant the number (or force) of the collisions on one side of a carbon particle may be larger than on the opposite side, so the particle is pushed a short distance in one direction.
• A moment later the unbalance is in another direction.
• The motion of the molecules is caused by heat and is never-ending.

Conclusion.

• Gas molecules are in continuous random motion; they collide with each other and with the walls of the container.
• (The collisions with the walls are the cause of gas pressure.)