Thermal conduction, convection and radiation

A spoon handle warms up in tea, air moves above a radiator, and sunlight travels through nearly empty space. How does energy follow these three different routes?

Leave one end of a metal spoon in hot tea. After a while, the handle warms too, although metal has not flowed from the cup to your fingers. Particles transfer energy as they interact with their neighbours; mobile electrons also make an important contribution in metals. This transfer of energy without bulk flow of matter is called conduction. Cooler regions receive energy from hotter ones, gradually reducing the temperature difference.

Why does a wooden handle protect your fingers better than a metal handle of the same shape? Energy travels through different materials at different rates. Metals are usually good conductors, whereas still air conducts heat poorly. Pores in insulation trap air and restrict its bulk motion. Imagine wrapping ice in insulation: it would melt more slowly. This shows that insulation slows transfer in both directions; it does not produce heat.

Now follow a coloured stream of water in a heated container. It moves with the warmed water: energy travels with that portion of matter. Transfer of internal energy by flowing liquid or gas is called convection. This differs from transfer along a stationary spoon. In an ordinary solid, particles cannot form such a freely moving flow. Heat can still spread through the solid, but by another mechanism.

Why does air above a radiator usually rise? Heating makes it expand, so the same mass occupies more volume: its density falls. The denser surroundings in a gravitational field push it up, much as water pushes up a less dense object. Cooler air moves into the space it leaves. This is natural convection. Remove gravity and this particular upward flow disappears. The explanation also requires heating to reduce density.

What if we want warm air to move downwards? A fan can drive it against the direction of natural buoyant motion. A pump does the same for water in a heating system. This is forced convection: an external device makes the fluid move. In both natural and forced convection, energy moves with the matter; what differs is the cause of the flow. Air movement is therefore not always caused by the air being warmer than its surroundings.

There is no continuous material between the Sun and Earth to pass energy from neighbour to neighbour, so a third mechanism is needed. Thermal radiation carries energy in electromagnetic waves and can travel through a vacuum. An object both emits and absorbs radiation; at equilibrium, these transfers balance. For now, “electromagnetic waves” names this way of transferring energy. We will study how it works later.

Place otherwise identical light and dark surfaces in the Sun. If the dark surface absorbs more of the incident light, with everything else equal, it warms more. Reflected energy does not enter the object. Shiny surfaces often reflect well. But visible colour concerns only visible light: it does not automatically tell us how a material absorbs invisible thermal radiation. We must relate a coating's properties to the radiation being compared.

How could we fairly test two types of insulation? Use identical containers and equal amounts of water, with the same starting temperatures and surroundings. Change only the insulation, and compare cooling over the same time t in seconds. A smaller temperature drop means that the same amount of water has lost less energy. In a real container, conduction, convection and radiation can all occur together, so one effective layer may not block every route.

Why does a vacuum flask benefit from a vacuum gap, reflective walls and a tight stopper?

First list the energy routes: through gas between the walls, radiation between surfaces, and transfer through the neck and solid connections.

The vacuum gap contains very few particles, so conduction through the gas and convection are greatly reduced. Radiation can cross a vacuum, so reflective walls are needed to reduce it.

A tight stopper limits transfer through the neck. Solid connections still conduct energy, so the contents eventually change temperature.

Different parts of the flask reduce different routes of heat transfer. It therefore keeps things hot or cold, but cannot insulate them forever.

In conduction, energy passes between particles; in convection, the liquid or gas itself moves.

The natural rise of a warmed fluid depends on density and gravity; a pump or fan drives a flow in another way.

Radiation also crosses a vacuum, so removing air is not enough for complete thermal insulation.

Next we will calculate the energy needed for a given temperature rise.