Contents
1. Kinetic Particle Model
All matter is made of particles (atoms or molecules) in constant motion. Temperature is a measure of the average kinetic energy of the particles.
| State | Arrangement | Motion | Forces |
|---|---|---|---|
| Solid | Regular lattice, closely packed | Vibrate about fixed positions | Strong |
| Liquid | Irregular, close together | Move randomly, slide past each other | Moderate |
| Gas | Widely spaced, random | Move rapidly in all directions | Very weak |
When a substance is heated, particles gain kinetic energy and move faster. During melting or boiling, energy input is used to overcome attractive forces — particle speed does not increase, so temperature stays constant.
Students write "the particles move faster". Wrong — during melting, the energy input breaks intermolecular bonds, not increase speed. Kinetic energy stays constant; potential energy increases. Temperature only rises again after all the solid has melted.
2. Heat Transfer Methods
Conduction
Transfer of thermal energy through a material without the material itself moving. Occurs mainly in solids. In metals, free electrons transfer energy quickly — metals are good conductors. Non-metals and gases are poor conductors (insulators).
Metals conduct heat well because free (delocalised) electrons can transfer kinetic energy rapidly from hot regions to cold regions. Wood, plastic and air are poor conductors because they have no free electrons.
Convection
Transfer of thermal energy by the movement of a fluid (liquid or gas). Hot fluid becomes less dense, rises; cooler fluid sinks to replace it, forming a convection current. Convection cannot occur in solids.
A radiator heats the air next to it. The warm air expands, becomes less dense, and rises. Cooler air from the rest of the room flows in at the bottom to replace it. This creates a circulation that warms the whole room.
Radiation
Transfer of thermal energy as infrared electromagnetic waves — requires no medium (works in a vacuum). All objects emit and absorb radiation. The rate depends on:
- Temperature — hotter objects emit more radiation.
- Surface type — dull black surfaces are good emitters and good absorbers; shiny silver surfaces are poor emitters and poor absorbers (good reflectors).
- Surface area — larger area emits and absorbs more.
3. Specific Heat Capacity
The energy needed to raise the temperature of 1 kg of a substance by 1 °C (or 1 K).
How much energy is needed to heat 0.5 kg of water (c = 4200 J/kg°C) from 20°C to 80°C?
ΔT = 80 − 20 = 60°C
E = mcΔT = 0.5 × 4200 × 60 = 126 000 J = 126 kJ
Water has a very high specific heat capacity (4200 J/kg°C) — it takes a lot of energy to heat up and a lot of time to cool down. This makes it useful as a coolant in car engines and as a heat store.
4. Latent Heat
The energy needed to change the state of 1 kg of a substance at constant temperature.
- Specific latent heat of fusion (l_f): energy to melt or freeze 1 kg.
- Specific latent heat of vaporisation (l_v): energy to boil or condense 1 kg.
Find the energy needed to melt 0.2 kg of ice. (l_f of water = 334 000 J/kg)
E = ml = 0.2 × 334 000 = 66 800 J = 66.8 kJ
- Q = mcDeltaT: energy needed to heat/cool. c of water = 4200 J/(kg degC) - very high, good coolant/store.
- Q = mL: energy for change of state (melting/boiling). Temperature does NOT change during state change.
- Conduction: solids (especially metals - free electrons). Gases are poor conductors.
- Convection: fluids only. Hot fluid less dense, rises; cool fluid denser, sinks. Forms convection current.
- Radiation: no medium needed. Dull black surfaces best emitter and absorber. Shiny white: poor emitter/absorber.
- Evaporation causes cooling: faster molecules escape, leaving lower average KE (lower temperature) behind.
5. Common Exam Traps
Conduction occurs in solids; convection requires a fluid that can flow. "Heat travels through the metal rod by convection" is wrong — it is conduction.
Radiation can travel through a vacuum (that is how the Sun's heat reaches Earth). Conduction and convection both require a medium.
A black surface is the best absorber of radiation AND the best emitter. A silver surface is the worst absorber AND worst emitter. Students often get the emitter part wrong.
Key Terms — Flashcard Review
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🎯 Practice Quiz — Test Yourself
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.