1.3.2 Brownian Motion

Brownian Motion You might well have studied Brownian Motion before. It is unlikely that you fully appreciated or understood it - it took Albert Einstein to really get to the bottom of it back in 1905. One of the first pieces of evidence for how gases move was discovered by Robert Brown a botanist who noticed that pollen grains in water moved about randomly. Robert Brown noticed that small grains of pollen (clarkia) suspended in water, moved in a haphazard or random fashion.

It was only visible under a microscope (he used fairly low magnification). Being a botanist he logically thought that the grains of pollen were alive. He then tried various other small particles such as carbon (in ink) and realised that the motion was not truely due to life. Einstein used it to measure Avogadro’s number. At the time there were those who were not convinced of Kinetic Theory. This was the first definitive evidence that atoms or molecules existed.

The experimental setup for viewing Brownian motion

A plot of the smoke particles position over a period of time might look like this: Note that · There is no appreciable displacement of the particle · The particle travels more or less in straight lines · The motion is RANDOM At any point in time the forces on the smoke particle are uneven causing a net force and therefore acceleration in that direction. Resultant force Their light mass means they change direction quickly and easily.

Observations Explanations Small bright dots are seen The dark smoke particles scatter the bright light in all directions. They consequently look brighter than the surroundings. The microscope can only focus on a small plane within the smoke cell. If a smoke particle rises or falls far from the plane, it goes out of focus and seems to disappear. Some disappear all of a sudden

Observations Explanations They move Because they are quite small, they can be hit by uneven numbers of air molecules on opposing sides. This causes a net force on them and they move. at random The tiny air molecules are in random motion (energy distributed randomly) so this process will occur without any pattern. Over small distances The molecules are small and light compared to the heavier, big smoke particles which are ‘quite’ close together.

Conclusions The visible motion could adequately be explained by Kinetic Theory. Air consists of millions of molecules moving randomly at high speeds Visible smoke particles are battered by invisible air molecules. Although smoke particles are small, air molecules are even smaller and so must have very high velocities in order to have enough momentum to cause Brownian motion of the smoke particles.

But (i) he wasn't the first to record the observation, and (ii) he did NOT observe the motion of actual pollen grains.