Brownian Motion
5 Minutes
Brownian motion and matter for kids: explore how tiny invisible particles collide to make pollen and dust move. An English science lesson.
- Aristotle and Democritus debating how matter is divided
- Robert Brown observing moving pollen grains in water
- How colliding particles create Brownian motion in dust
What this lesson teaches
Brownian motion is the irregular, zigzag movement of tiny visible objects, such as dust specks or pollen grains, caused by constant collisions with invisible particles in a liquid or gas.
The debate about matter
- Aristotle proposed that all matter was continuous and made up of water, earth, fire, air, and space, believing any substance could be cut into smaller pieces without end.
- Democritus disagreed and stated that matter is discontinuous and cannot be divided endlessly.
- Democritus argued that matter reaches a final point beyond which it cannot be cut any further, calling this basic building block the atom.
Robert Brown's experiment
In the nineteenth century, Robert Brown watched flower pollen grains floating in water through a microscope. The pollen grains moved continuously in an irregular way, making people wonder if they were alive. Brown tested the same setup with lifeless dust particles and found that they moved in the exact same manner. He concluded that the particles were not living organisms, but he passed away before he could discover what pushed them.
How the motion happens
Albert Einstein explained that all matter is made of vast numbers of tiny moving particles. In solids, neighboring particles are held tightly in place, but in liquids and gases, particles have more freedom and dart around haphazardly. In the air, around one hundred trillion invisible air particles collide with a single floating dust particle every second. These endless, uneven collisions knock the dust around, producing the dancing movement seen in a beam of sunlight through a window.
The tiny scale of atoms
- Matter has mass, which is the amount of matter in an object, and volume, which is the space it occupies.
- A single drop of water holds one trillion water particles, which is a thousand times more than the total human population of the world.
- A particle is so small that the tiny ball inside a ballpoint pen would appear as tall as two Eiffel Towers combined to it.
- Scientists later confirmed that these colliding particles are atoms by photographing them, including molybdenum atoms, using scanning electron microscopes and transmission electron microscopes.