Transfer of Thermal Energy

Thermal energy moves from hotter to colder places in three ways. Conduction passes vibrations between particles (and free electrons in metals). Convection carries energy as warm, less dense fluid rises. Radiation is infrared waves, which need no material at all. Dull black surfaces are the best emitters and absorbers; shiny surfaces are the best reflectors.

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Section 1 of 14

Learning objectives

Explain conduction, including why metals are good conductors.
Explain convection currents in terms of density.
Describe infrared radiation and the effect of surface colour and texture.
Apply all three to everyday examples such as heaters, insulation and the vacuum flask.

Pause and think

On a cold morning, a metal bench and a wooden bench are at the same temperature. Why does the metal one feel much colder to sit on?

Section 2 of 14

Core idea

Start hereWhy a metal spoon feels colder than wood

A metal spoon and a wooden spoon in the same drawer are at the same temperature, yet the metal feels colder. It isn't colder; it carries heat away from your hand faster. Heat moves in three ways, and each needs different conditions.

The big idea

Conduction: vibrations and free electrons in solids. Convection: warm fluid rises because it's less dense. Radiation: infrared waves, needs no medium.

1. Conduction

Why metals conduct best

$\text{hot end: particles vibrate more}$
Pass energy to neighbours: slow.
$\text{metals: free electrons}$
Move quickly through the lattice, carrying energy far: fast.

Air and plastics have no free electrons and widely spaced particles: insulators.

2. Convection

A convection current in a pan of water

Water above the flame expands, becomes less dense and rises; cooler, denser water sinks to take its place.

A convection current

Drawing diagram…

Only in fluids (liquids and gases): particles must be free to move.

3. Radiation

Cooling of two identical cans of hot water

The black can radiates energy faster, so it cools more quickly.

Dull black: best absorber and emitter.
Shiny silver: worst (best reflector).
Travels through a vacuum (sunlight).

Worked exampleA vacuum flask

Explain each feature.

Goal: Link features to transfers.
Strategy: One transfer at a time.

Vacuum: no particles, so no conduction or convection across the gap.
Silvered walls: reflect infrared, reducing radiation.
Stopper: reduces convection and evaporation at the top.

AnswerAll three transfers are reduced.

✓ CheckKeeps cold drinks cold too ✓.

Common misconception"Heat rises" Hot fluid rises; heat itself spreads in all directions by conduction and radiation.

Pause and think

Why is the freezer at the top of old fridges?

Summary

Conduction, convection, radiation.
Black emits/absorbs best.

Infrared: Thermal radiation.
Insulator: Poor conductor.

HistoryHerschel finds infrared. In

1800

William Herschel split sunlight with a prism and put thermometers in each colour. The thermometer just beyond the red end, where there was no visible light at all, got hottest. He had discovered infrared radiation.

Fun factPenguins huddle by physics. Emperor penguins survive Antarctic winters at

-40

°C by huddling in groups of thousands. Huddling reduces the surface area each penguin exposes to the cold, cutting energy loss by up to half.

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