Heat Energy: Conduction, Convection and Radiation

Heat and temperature

Temperature tells you how hot or cold something is. It is measured in degrees Celsius (°C) or kelvin (K).

Heat (thermal energy) is energy that flows from a hotter object to a colder one. It is measured in joules (J).

Heat always flows from hot to cold, never the other way by itself. It keeps flowing until both objects reach the same temperature. This is called thermal equilibrium.

1. Conduction

Conduction is heat passing through a material by particles bumping into their neighbours. The particles themselves stay in place. They vibrate and pass the energy along.

• Happens best in solids, because the particles are packed closely together.
• Metals are the best conductors. They have free electrons that carry energy quickly.
• Poor conductors are called insulators: wood, plastic, wool, air, polystyrene.

That is why Ruby's metal spoon got hot and the wooden one did not.

2. Convection

Convection is heat carried by the movement of a liquid or gas (a fluid). It cannot happen in solids.

1. The fluid near the heat source warms up.
2. Its particles spread apart, so it becomes less dense.
3. The warm, less dense fluid rises.
4. Cooler, denser fluid sinks to take its place.
5. This sets up a loop called a convection current.

Examples: smoke rising from a campfire, a heater warming a whole room, sea breezes, boiling water in a pot, hot air balloons.

3. Radiation

Radiation is heat travelling as infrared waves. It needs no particles at all, so it can travel through empty space. This is how the Sun's energy reaches Earth across 150 million km of vacuum.

All objects give out infrared radiation. Hotter objects give out more.

So houses in hot places are often painted white, and emergency blankets are shiny silver.

Reducing heat transfer

A vacuum flask (thermos) stops all three:

• Vacuum between the two walls: stops conduction and convection (no particles).
• Silvered walls: reflect radiation back.
• Plastic stopper: an insulator that stops conduction and convection out of the top.

Home insulation works in a similar way: ceiling batts trap air (a poor conductor) so it cannot move to form convection currents.

Quick summary

• Heat flows from hot to cold until temperatures are equal.
• Conduction: particles pass on vibrations. Best in solids, especially metals.
• Convection: hot fluid rises, cold fluid sinks, forming currents. Liquids and gases only.
• Radiation: infrared waves, which can cross a vacuum.
• Dark dull surfaces absorb and emit radiation best. Shiny surfaces reflect it.
• Insulators like trapped air reduce heat transfer.

Quiz