Transfer of Thermal Energy

Thermal energy travels from hot to cold by three mechanisms, and nearly every question in this topic is answered by naming the right one and explaining how it works.

Conduction passes energy through a material without the material itself moving. Convection carries energy by moving the material itself, so it happens only in fluids — liquids and gases. Radiation transfers energy as infrared electromagnetic waves and needs no material at all, which is how the Sun's energy crosses the vacuum of space to reach us.

Identifying which one is at work is usually straightforward. If a solid is involved, think conduction. If a fluid is free to move, think convection. If there is a gap, or a vacuum, or the energy is arriving from something hot but not touching, think radiation.

Heat one end of a metal bar and the particles there vibrate more strongly. They collide with their neighbours and pass the vibration along, so energy travels through the bar while the bar itself stays put.

That mechanism works in every solid, but it is slow. Metals conduct far better than anything else because they have a second, much faster route: free electrons. These drift throughout the metal, pick up kinetic energy at the hot end and carry it directly to the cold end, colliding with ions along the way.

This is why metals feel cold to the touch and wood does not, even in the same room at the same temperature. The metal conducts energy away from your hand quickly; the wood does not. Your hand is sensing the rate of energy loss, not the temperature.

Liquids and gases are poor conductors, and a trapped layer of air is one of the best insulators available. Wool, fur, fibreglass, double glazing and a duvet all work the same way: they trap air and stop it moving.

Heat a fluid from below and the part nearest the heat expands. Expanding makes it less dense, so it rises. Cooler, denser fluid sinks to take its place, is heated in turn, and rises as well. The circulating flow that results is a convection current.

The full chain — heated, expands, less dense, rises, cooler fluid sinks to replace it — is what earns the marks. Answers that say only "hot air rises" leave out the density step that explains why.

This is why a heating element sits at the bottom of a kettle and a freezer compartment at the top of a refrigerator. Put the element at the top and only the surface layer would heat, because there would be nothing to drive the circulation.

Convection is also behind sea breezes. During the day the land warms faster than the sea, air above it rises, and cooler air flows in from the sea to replace it.

Energy going in raises the temperature — except during a change of state, where it goes into breaking bonds instead. All three transfer mechanisms are ways of getting that energy into or out of a substance.

Every object emits infrared radiation, and the hotter it is the more it emits. Radiation needs no medium, travels at the speed of light and is the only mechanism that works across a vacuum.

What a surface does with radiation depends on its colour and texture, and the rule is neatly symmetrical. A matt black surface is the best absorber and also the best emitter. A shiny silver or white surface is the best reflector and the worst absorber — and correspondingly the worst emitter.

That symmetry explains a great deal. Houses in hot countries are painted white to reflect solar radiation and stay cool. The cooling fins on the back of a refrigerator are painted black to radiate energy away efficiently. Emergency blankets are shiny to reflect body heat back to the casualty.

A vacuum flask defeats all three mechanisms at once, which makes it the standard exam question on this topic. The vacuum between its double walls stops conduction and convection, since neither can cross empty space. The silvered surfaces reflect radiation back. The stopper prevents convection through the top, and the plastic supports conduct poorly.