Matter and the particle model · Model
Brownian motion
Some specks in a drop of water refuse to settle. They jiggle, hour after hour, with nothing touching them and nothing alive inside them. Explaining that jiggle is how the world was talked into believing in atoms.
Start here
The specks have been jiggling for an hour and they will not stop.
A sealed cell of still air with a wisp of smoke in it, lit from the side and viewed under a microscope. The bright specks never settle. They jerk a short way, stop, jerk somewhere else — never in the same direction twice, and never coming to rest.
What is moving them?
Invisible particles, hitting them from every side. At any instant a smoke speck is being struck by billions of air molecules; the strikes very nearly cancel, and the small leftover imbalance shoves it a short way in a random direction. A moment later the imbalance points somewhere else. Nothing is alive, nothing is blowing and nothing has a plan — the jiggle is direct visible evidence that air is made of separate moving particles.
Brownian motion is the constant random jiggling of small specks suspended in a fluid. It happens in air and in liquids, it never stops, and it has nothing to do with currents, with life or with the observer.
The explanation is entirely about size. A fluid is made of molecules in constant motion, and a suspended speck is being struck by enormous numbers of them at once — from the left, the right, above and below, all at the same instant. Those strikes almost cancel. What is left over is a small unbalanced push in a direction that nobody chose, and a moment later the leftover points somewhere else.
This is why the speck has to be the right size. Something as small as a molecule would be knocked clean across the cell; something as large as a grain of sand is struck so evenly that the imbalance is nothing against its weight. A smoke speck sits in the narrow band where the imbalance is big enough to shift it and the speck is big enough to see.
At the bench · a smoke cell under a microscope
You can see the specks. You cannot see what is hitting them.
Change a control to begin
Choose what is under the microscope and how warm it is. Only the second bar is anything you could actually watch — the other two are what the model says is going on underneath.
Commit first. You warm the cell from 20 °C to 80 °C. What happens to the jiggling?
What is under the microscope
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What you can see
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What you cannot see
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Temperature
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Strikes each second
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Key fact
Brownian motion is the random jiggling of a visible speck caused by unbalanced strikes from the invisible molecules of the fluid around it. It never stops, it gets more violent as the temperature rises, and it is direct evidence that fluids are made of separate moving particles.
Think again
“The smoke specks are moving under their own power.”
They have no power. A smoke speck is a fragment of burnt material with no store of energy it can use and no mechanism to use one; it moves only because it is pushed. Brown himself worried about exactly this, and settled it by finding the same jiggling in dust from a piece of window glass and in a fragment of the Sphinx — neither of which had ever been alive.
“You are watching the air molecules hit the specks.”
You are watching one side of the collision only. A molecule is a few tenths of a nanometre across, thousands of times below what any light microscope can resolve, and no lens will ever show one. What the microscope shows is the effect: a speck you can see, moving in a way that only makes sense if something you cannot see is hitting it. That is the whole force of the argument, and it is why the experiment mattered so much.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the model
A student watches smoke specks jiggling and says the specks are being pushed by other smoke specks. What is wrong with that?
Rung 2 · The one that catches people
Why do we watch smoke specks rather than watching the air molecules themselves?
Rung 3 · Explain
Describe what you would see through the microscope in a smoke cell, and explain the movement in terms of particles.
Rung 4 · Take it somewhere new
Robert Brown first saw this in 1827 with pollen in water and could not explain it; Einstein explained it in 1905 and it was taken as proof that atoms are real. Explain why a jiggling speck counts as evidence for atoms.
Key note
Brownian motion is the random jiggling of small visible specks suspended in a fluid, caused by unbalanced collisions from the fluid’s own molecules. The strikes come from every direction and nearly cancel; the tiny leftover imbalance moves the speck, and changes direction constantly. Warming the fluid speeds the molecules up and makes the jiggling wilder. Nothing alive and no current is involved, and the molecules themselves are never seen — only their effect.
Going further
Einstein’s 1905 paper did not simply say that molecules were doing it. It predicted how far a speck of a given size should wander in a given time, at a given temperature, in a fluid of a given thickness — a number that could be measured. Jean Perrin spent four years measuring it, found the prediction held, and used it to work out how many molecules there are in a mole. Both men have Nobel prizes partly for this, and the atomic hypothesis stopped being a hypothesis.
The same mathematics turns up wherever something takes a large number of small random steps. It is used to model how a pollutant spreads through groundwater, how a share price wanders, and how proteins find their way about inside a cell. The name for the general case is a random walk, and the smoke cell is the cheapest place to see one.
Before this lesson
Connects to
- Diffusion
The same random molecular motion, followed over a room rather than under a microscope.
- Solids, liquids and gases
Where the constant motion of the particles in a fluid is established.
- Heating and thermal equilibrium
Why warming the cell speeds every molecule up.
At GCSE this becomes
- The kinetic theory of gases, diffusion rates and Graham’s law, and Brownian motion as evidence for the particle model.
Where to next
Ask Mr Badmus AI
Not sure why a speck you can see proves something you cannot?
Teacher demonstration. The smoke cell uses a lit taper and a small glass cell — watch, don't handle.
The bench is a teaching model. Typical molecular speeds are quoted for the gas or liquid named, scaled with the square root of absolute temperature from about 500 m/s for air and about 590 m/s for water at 20 °C; real distributions are broad and no molecule holds one speed for long. The jiggle figure is an illustrative displacement rate, not a measurement. It is scaled with the same square root, and it falls as the speck grows: the bigger the speck, the more completely the strikes on it cancel, so a smoke speck jiggles further than a pollen grain. Size ratios are order-of-magnitude comparisons between a typical speck and a typical molecule.
Lesson content © MrBadmusAI.