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Chapter 1 — Chapter 1 — Thermal Physics
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📘Easy summary
Thermal physics deals with the relationship between heat, work, temperature and other forms of energy. A large system containing many atoms or molecules is a macroscopic system (properties: temperature, pressure); a system of a single atom or molecule is a microscopic system (properties: kinetic energy, momentum). Brownian motion is the random and irregular motion of molecules in a gas. Robert Brown, a botanist (the book prints 1927; it was 1827), observed under a microscope that tiny pollen particles moved randomly in a liquid; later it was proved to be an effect of molecular motion. Collisions transfer momentum and energy, so a molecule follows a zig-zag path. Causes: lighter (smaller) particles obtain greater speeds from collisions; speed is inversely proportional to the viscosity of the fluid — low viscosity gives faster Brownian movement. Einstein explained it in 1905 and Perrin verified it in 1908 (Nobel Prize 1926).
⭐Key points
- Macroscopic: T, P; microscopic: K.E., momentum.
- Brownian motion = random zig-zag motion.
- Lighter particles and low viscosity → faster motion.
🔬Diagram
🌿Real-life example
A drop of ink spreads through still water because water molecules keep hitting the ink particles randomly.
💡Exam tip
- Learn the causes of Brownian motion: mass of particle and viscosity of fluid.
✅Quick check
1. Brownian motion is faster in a fluid of:
2. Temperature is a property of a:
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