Transfer of thermal energy
Three ways heat travels
- A metal spoon left in hot soup soon burns your hand.
- Yet the Sun warms you across millions of kilometres of empty space.
- Thermal energy always flows from hot to cold — by three routes: conduction, convection and radiation.
Conduction
- Conduction carries energy through a material without the material moving.
- Heated particles vibrate more and pass the energy to their neighbours.
- Metals are the best conductors: their free (delocalised) electrons carry energy quickly.
- Insulators (air, wood, plastic) conduct badly — which is why we use them to keep things warm.
Why are metals such good conductors of thermal energy?
Metals have free (delocalised) electrons that move through the metal and carry energy rapidly from the hot end to the cold end.
Convection
- Convection happens in liquids and gases.
- Heated fluid expands, becomes less dense, and rises; cooler, denser fluid sinks to take its place.
- This circulation is a convection current — it carries energy around the fluid.
- It cannot happen in a solid, because the particles cannot move from place to place.
In convection, heated fluid rises because it has become:
Heating makes the fluid expand, so it is less dense and floats upward; cooler, denser fluid sinks to replace it.
Convection can happen in a solid.
Convection needs particles to move from place to place, so it only happens in liquids and gases — not solids.
Radiation
- Thermal radiation is energy carried by infrared waves.
- All objects emit it, and it needs no material — it can cross empty space (this is how the Sun's energy reaches us).
- A dull, black surface is a good emitter and absorber of infrared.
- A shiny, white surface is a poor emitter and a good reflector — which is why hot-water tanks and spacecraft are often shiny.
Which method of heat transfer can travel through empty space (a vacuum)?
Radiation is carried by infrared waves, which need no material — so the Sun's energy reaches us across empty space.
Match each method of heat transfer to how it works.
Conduction works through a material; convection moves the fluid itself in a current; radiation is infrared waves that need no material.
Which surface is the best emitter of thermal radiation?
Dull black surfaces are the best emitters (and absorbers) of infrared; shiny, light surfaces are poor emitters and good reflectors.
You've got it
- thermal energy always flows from hot to cold
- conduction: through a material, particle to particle; metals best (free electrons)
- convection: warm fluid rises, cool sinks — a current; never in solids
- radiation: infrared, needs no material; dull black = best emitter/absorber