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2.1.4Thermal Physics

Brownian Motion — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 10 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.1.4 Brownian Motion — IGCSE PHYSICS notes: Describe Brownian motion, explain its cause and state what it shows about the particle model.Brownian motion = the random, zigzag motion of small visible particles suspended in a fluid, caused by collisions with the much smaller, fast-…

  • Drawing the path
  • Smoke cell
  • Resultant force
  • Random walk
  • p1V1 = p2V2
  • Robert Brown first saw tiny pollen grains jiggling constantly in water through a microscope in 1827

Part 1 of 6What Brownian motion is

Brownian motion
Brownian motion the random, zigzag motion of small visible particles suspended in a fluid, caused by collisions with the much smaller, fast-moving molecules of the fluid.
Robert Brown first saw tiny pollen grains jiggling constantly in water through a microscope in 1827.
It is also seen with specks of smoke in air and fat droplets in milk.
It never stops and has no pattern.

Part 2 of 6The explanation

1
The smoke particle is bombarded by fast, randomly moving air molecules from all directions.
2
At any instant the number and strength of collisions are not equal on all sides.
3
A tiny unbalanced resultant force acts on the particle.
4
The particle is pushed in one direction for a moment, then the imbalance changes and so does the motion.
5
Many collisions per second make the direction change rapidly, giving a random zigzag path.
6
The presence of the molecules is inferred from the motion of the larger particles.

Part 3 of 6What affects the motion

1
Smaller particles move more noticeably because the unbalanced force gives them a larger acceleration.
2
A higher temperature makes molecules move faster, so collisions are stronger and the motion more vigorous.
3
The motion does not die down because the molecules do not run out of energy.
4
Air currents are not the cause, because currents would carry all specks in the same direction.
5
Each particle moves independently and randomly; a steady drift of all specks shows a draught.

Part 4 of 6What it shows

1
Brownian motion is direct evidence for the kinetic particle model.
2
It shows that matter is made of tiny particles in constant random motion.
3
It supports the idea that particles in a gas have a range of speeds and transfer momentum to objects in their path.
4
The same kind of collisions give gas pressure on container walls.

Part 5 of 6Scale and observation

A smoke particle is thousands of times larger than an air molecule, so very many collisions are needed to move it noticeably.
The motion is a random walk, not a drift, with the particle wandering without going far in a straight line.
Smoke particles are small clumps of soot or ash, visible because they reflect light.
Use a dim room, a strong lamp and a sealed cell.
Drawing the path
  • Drawing the path use short straight lines of different lengths joined by sudden changes of direction.

Part 6 of 6More from the specification

More from the specification
States of matter properties
  • States of matter properties solids possess fixed shape and volume, liquids have fixed volume but adapt shape, while gases fill containers completely.
Absolute zero in kelvin
  • Absolute zero in kelvin zero kelvin represents the absolute lowest temperature where thermal kinetic energy of molecules reaches a theoretical minimum.
Thermal expansion of fluids
  • Thermal expansion of fluids heating a liquid or gas substance increases particle kinetic energy, causing the volume to expand under constant pressure.
States of Matter and Particle Arrangement
  • A solid has a fixed shape and fixed volume due to strong intermolecular forces holding particles in a lattice;
    • a liquid has a fixed volume but assumes the shape of its container,
  • while gases expand to fill any volume.
Brownian Motion in Fluids
  • Microscopic smoke particles in air or pollen grains in liquid water exhibit continuous, erratic zigzag motion caused by bombardment from smaller, invisible molecules moving at high speeds.
Absolute Temperature and Gas Pressure
  • The absolute thermodynamic temperature scale is measured in kelvin;
    • (T / K = θ / °C + 273)
  • as thermal energy increases, the average kinetic energy of gas particles increases,
  • resulting in more frequent, forceful collisions with container walls.
Solid
  • State with a fixed shape and a fixed volume.
Inversely proportional
  • One quantity falls as the other rises in the same ratio.
Average speed
  • Total distance divided by total time.
Volume
  • The amount of space an object occupies.
Liquid
  • State with a fixed volume that takes the shape of its container.
Kelvin
  • SI unit of temperature on the absolute scale.
p1V1
p1V1 p2V2
What the symbols mean: p1, V1 = pressure and volume before · p2, V2 = after; only for a fixed mass of gas at constant temperature

Key terms

TermMeaning
Brownian motionRandom motion of microscopic particles suspended in a fluid.
Smoke cellGlass cell used to view smoke particles under a microscope.
Resultant forceNet unbalanced force from collisions on one side.
Random walkPath of many short steps in random directions.
MoleculeTiny particle of the fluid, too small to be seen.

Avoid these mistakes

1
Saying smoke particles are moved in one direction by air molecules — the pushes are unbalanced and random.
2
Thinking you can see the air molecules — only the larger smoke particles are visible.
3
Saying Brownian motion is caused by convection currents.
4
Thinking Brownian motion stops after a while — it is continuous.
5
Calling the motion diffusion — the correct name is Brownian motion.

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