Heat Always Moves…
Heat flows from hotter to cooler objects, never the other way round unless energy is supplied.
Method 1: Conduction
Conduction happens in solids (and stationary fluids). Vibrating particles pass kinetic energy to neighbours.
Experiment: Hold one end of a metal rod. Heat the other end. The heat gradually conducts toward your hand.
| Good conductors | Poor conductors (insulators) |
|---|---|
| Copper, iron, steel | Wood, plastic, rubber |
| Aluminium | Glass, air, wool |
Why do metal objects feel colder than wooden ones at room temperature?
Both are at the same temperature. Metal conducts heat away from your hand quickly → feels cold. Wood conducts slowly → feels less cold.
Method 2: Convection
Convection happens in liquids and gases. Heating creates currents as warm (less dense) fluid rises and cool (denser) fluid sinks.
Sea breeze: During the day, land heats faster than sea. Warm air over land rises → cool sea air flows in as a breeze.
Method 3: Radiation
All objects emit infrared radiation (a form of electromagnetic wave). Hotter objects emit more.
No material is needed — radiation works through a vacuum.
Surface colour matters:
- Dark/dull surfaces: best absorbers and emitters
- Light/shiny surfaces: best reflectors
Thermos flasks reduce all three: a vacuum stops conduction/convection; silvered walls reflect radiation.
Real world: double-glazed windows
A double-glazed window has two glass panes separated by a layer of argon gas. The gas is a poor conductor (insulator), greatly reducing heat loss by conduction. The surfaces also slow radiation. A well-fitted double-glazed window can cut heat loss by up to 50% compared to single glazing.
Real world: thermos flask (vacuum flask)
A thermos flask keeps coffee hot by minimising all three transfer methods: a vacuum between two walls eliminates conduction and convection (no medium for particles to transfer energy through); silvered inner walls reflect infrared radiation back into the liquid. The result is the coffee stays hot for several hours.
Real world: emergency foil blanket
A shiny metallic emergency blanket reflects about 90% of infrared radiation emitted by a person's body back towards them. The shiny surface is a poor emitter (and good reflector) of radiation, dramatically slowing heat loss for someone in hypothermia risk until medical help arrives.
Practice Problems
Name two applications of good thermal insulators in everyday life.
Show answer
Why are solar panels black?