The jittering of smoke specks is evidence for the particle model
Definition
Brownian motion
The continuous, random, jerky movement of microscopic particles suspended in a liquid or a gas, caused by collisions with the moving particles of that liquid or gas.
- Tiny specks suspended in a fluid never settle into stillness, and instead keep wandering in short jerks that change direction unpredictably.
- The standard demonstration uses a smoke cell: a small glass cell of smoky air is lit brightly from the side and viewed from above through a microscope.
- The smoke specks show up as bright points of light because they scatter the light sideways into the microscope, while the air around them stays dark.
- Each bright point is seen to move along a haphazard, zig-zag path made of short straight steps in unrelated directions.
- The same effect appears in a liquid, and that is where it was first noticed, when pollen grains suspended in water were watched under a microscope.
- The movement never stops and never settles into a pattern, however long the cell is left undisturbed.
- That behaviour is evidence for the kinetic particle model, because something invisible must exist and must be moving in order to keep knocking the specks about.
- It shows more than that the particles exist: because the specks are pushed in every direction rather than drifting one way, the invisible particles must be moving randomly.
Uneven collisions explain the zig-zag path
- The air in the smoke cell is made of molecules that are far too small to see, moving fast and in random directions.
- A single smoke speck is being struck by these molecules on all sides at once, many thousands of times in the blink of an eye.
- At any instant, purely by chance, the hits on one side happen to be more numerous or harder than the hits on the opposite side.
- That imbalance leaves a small resultant force on the speck, and the speck moves off in the direction of that force.
- A moment later chance favours a different side, so the resultant force points somewhere new and the speck jerks off in that new direction.
- Repeating that over and over is exactly what produces the random, jerky, unpredictable path seen through the microscope.
- Warming the air makes the molecules faster, so the specks jiggle more violently, which points straight back at the motion of the molecules as the cause.
Molecules and microscopic particles are not the same thing
- Use atoms or molecules for the invisible particles of the gas or liquid, and microscopic particles for the visible specks of smoke or pollen.
- A smoke speck is microscopic but still enormous compared with an air molecule, and is itself built from very many molecules.
- The air molecules are much lighter than the speck but much faster, so a collision still delivers a real nudge.
- One molecule on its own would barely shift the speck, so it is the uneven total of very many collisions that produces each visible jump.
- Smaller suspended specks are seen to jiggle more than larger ones, because the same imbalance of collisions moves a smaller mass further.
Example
- Dust caught in a shaft of sunlight through a front-room window drifts about instead of falling steadily to the carpet.
- Much of that slow drifting is air currents, but the small restless jitter on top of it is the same effect as the smoke cell shows.
- The dust motes are large enough to see, while the air molecules knocking them about are not.
- Each mote is visible for the same reason a smoke speck is: it scatters the light that falls on it.
Common Mistake
- Do not claim you can see the air molecules, because what you see is the smoke specks and the molecules stay invisible.
- Do not draw or describe the path as smooth, circular or wavy, because it is a series of straight jerks in random directions.
- Do not put the movement down to a draught or a convection current, because the cause is random collisions and a draught would push every speck the same way.
- Do not write "the particles hit the particles", because a marker cannot tell which particles you mean.
Exam technique
- A full explanation needs three linked steps: fast randomly moving molecules, unequal collisions on opposite sides, and a resultant force in a constantly changing direction.
- Name the two kinds of particle separately in every sentence, calling the invisible ones molecules and the visible ones microscopic particles; this is where the mark is most often lost.
- If asked what the observation shows, say the fluid is made of particles that are moving and moving randomly.
- A description of the apparatus scores for the light shone in from the side and the microscope, so include both.
Self review
- What is observed when the smoke in a smoke cell is viewed through a microscope?
- Why do the smoke specks appear as bright points of light?
- Explain why a smoke speck keeps changing direction.
- In the smoke cell, which particles are the molecules and which are the microscopic particles?
- What does this observation tell you about the motion of the particles of a gas?
