Transfer of Thermal Energy

Conduction: thermal energy through a solid

Conduction is the transfer of thermal energy through a material from a hotter region to a colder region, without the material itself moving. In a solid the particles are held in a fixed lattice and vibrate about fixed positions. The particles at the hot end gain energy and vibrate more strongly; they collide with their neighbours and pass the energy on along the chain, particle to particle, so the vibrations spread from hot to cold. The particles themselves stay where they are, so nothing flows.

Metals conduct far better than non-metals because they contain free (delocalised) electrons. These electrons are not tied to any particular ion, so they can move through the lattice. An electron near the hot end gains kinetic energy and travels quickly towards the cold end, where it collides with the ions and passes that energy on. Because the electrons drift through the metal so much faster than the lattice vibrations creep along, metals are the best thermal conductors there are.

Conduction is poor in gases and most liquids because their particles are far apart and the forces between them are weak. In a gas the particles are so widely spaced that collisions between them are rare, so energy is handed on very slowly. Liquids conduct a little better than gases because their particles are closer together, but most liquids are still poor conductors. That is why water can be boiling at the top of a pan while the bottom is still cool, and why air trapped in a foam or a woolly jumper makes such a good insulator.

Between the two extremes sit many ordinary solids. Glass, brick, concrete and wood conduct thermal energy better than a true thermal insulator such as air or foam, but far less well than a good conductor such as copper or aluminium. So they are neither useless nor ideal for a job that needs fast or slow transfer - you would not make the base of a saucepan from glass, and you would not make an oven door from copper.

Experiments: good and bad thermal conductors

The standard experiment uses rods of different materials, all the same length and the same thickness, heated equally at one end. Small pins are stuck to each rod with wax at equal distances along it. As thermal energy travels along a rod, the wax melts and the pins drop off one after another. The rod whose pins fall off first is the best conductor; the rod whose pins fall off last, or not at all, is the best insulator. Keeping the rods the same size and heating them equally is what makes it a fair test, because then the only difference is the material.

A second test uses the feel of the material. Hold a metal rod and a wooden rod of the same size that have both been standing in the same room. The metal feels colder, but this does not mean it is at a lower temperature - both are at room temperature. The metal simply conducts thermal energy away from your skin faster, and your skin detects that faster loss as cold. Getting this reason right is worth a mark in almost every conduction question.

You can also compare surfaces by touch only if you are careful, so the pin-and-wax method is preferred because it gives a visible, timed result. Whichever method you use, the conclusion is the same: metals are good thermal conductors, and materials such as wood, plastic, foam and air are good thermal insulators.

Convection: thermal energy transfer in liquids and gases

Convection is the transfer of thermal energy in a liquid or a gas by the movement of the fluid itself. When a fluid is heated it expands, so the same mass now fills a larger volume and its density falls. The warmer, less dense fluid rises, and cooler, denser fluid flows in to take its place. That cooler fluid is then heated and rises in turn, so a continuous convection current is set up that carries thermal energy through the fluid.

Convection cannot happen in a solid, because the particles are locked into a lattice and cannot move from place to place. Convection needs the particles to be free to flow, which is why it happens only in liquids and gases. In a solid the only method of transfer available is conduction.

Experiment with a liquid. Fill a beaker with cold water and, using a pipette, let a crystal of potassium permanganate (a purple dye) sink to the bottom near one side. Heat the water gently at that point. A purple streak rises from the heat source, spreads across the surface and then sinks down the far side, tracing out the convection current. If you heat the water at the top instead, almost no current forms, because the hot water is already at the top and has nowhere to rise to.

Experiment with a gas. A convection box is a rectangular box with a glass front and two chimneys. Put a lighted candle under one chimney and hold a smouldering piece of paper near the other. Smoke is drawn into the box and rises out of the candle chimney, showing that air heated by the flame expands, becomes less dense and rises, while cooler air is drawn in to replace it.

• Heating a fluid makes it expand, so its density falls.
• The less dense fluid rises; cooler, denser fluid sinks to replace it.
• This circulation is a convection current, and it can only happen in a liquid or a gas.
• Heating a fluid from the top produces almost no current, because the warm fluid is already at the top.

Radiation: infrared and the effect of surfaces

Thermal radiation is infrared radiation, part of the electromagnetic spectrum. Every object emits it, whatever its temperature - a block of ice emits infrared just as a fire does, though a hotter object emits far more. The radiation comes from the vibrations of the particles and the movement of the electrons inside the object.

Radiation does not need a medium. It can travel through a vacuum, which is how energy from the Sun reaches the Earth across empty space. This is the key difference from conduction and convection, both of which need particles to carry the energy. If you were asked how a fire could warm you in the vacuum of space, radiation is the only one of the three methods that would work.

The rate at which an object emits radiation depends on its surface temperature and its surface area. A hotter surface emits much more radiation every second, and a larger surface area emits more because there is more surface to radiate from. This is why a car radiator is built with many thin fins: the fins give a large surface area, so the radiator loses energy to the air quickly.

The colour and texture of a surface control how much infrared it emits, absorbs and reflects. A black, dull (matt) surface is the best emitter and the best absorber of infrared, and it reflects very little. A white or shiny (silvered) surface is a poor emitter and a poor absorber, and it is a good reflector of infrared. The rule to remember is that a good absorber is always a good emitter - the same surface that soaks up infrared well also gives it out well.

Experiments: good and bad emitters and absorbers

Emitters. A Leslie cube is a metal cube with its four vertical faces finished differently - for example black matt, white matt, shiny silver and a second dull face. Fill it with hot water and point an infrared detector, or a thermometer with a blackened bulb, at each face in turn, always holding it the same distance away. The black matt face gives the highest reading, so it is the best emitter, and the shiny face gives the lowest, so it is the worst emitter.

Absorbers. Take two identical metal cans, one painted black and matt and one white or shiny. Put a thermometer in each and stand them the same distance from a heater or a lamp. The black can warms up faster, so a black surface is the better absorber of infrared radiation. If you then switch off the heater and let both cool, the black can cools fastest, confirming that the best absorber is also the best emitter.

A quick version of the same test uses two thermometers side by side, one with its bulb painted black and one left shiny, placed the same distance from a hot object. The blackened bulb's reading rises faster, so black surfaces absorb infrared best. In every one of these experiments you must keep the surfaces the same distance from the source and the same size, so that the only thing you are changing is the surface itself.

Constant temperature and the balance of radiation

An object is at a constant temperature when it transfers energy away from itself at exactly the same rate as it receives energy. The energy leaving equals the energy arriving, so its internal store does not change and the temperature stays the same. This is a dynamic balance, not a state in which nothing is happening: energy is still flowing in and out, just at equal rates.

If the object receives energy faster than it transfers energy away, its internal store fills up and its temperature rises. If it transfers energy away faster than it receives it, its internal store empties and its temperature falls. The temperature settles only when the two rates become equal again, which is why a hot drink eventually reaches room temperature and then stays there.

This balance controls the temperature of the Earth. The Earth receives radiation from the Sun and emits its own infrared radiation into space, and its average surface temperature is set by where those two rates balance. Greenhouse gases such as carbon dioxide and methane in the atmosphere absorb some of the infrared radiation emitted by the Earth's surface and re-emit part of it back down. Adding more of these gases makes the Earth lose energy to space more slowly, so the balance moves to a higher temperature and the Earth warms. Anything that reduces the incoming radiation - such as dust from a large volcanic eruption reflecting sunlight away - would tip the balance the other way and cool the Earth.

Consequences: the vacuum flask and everyday applications

The vacuum flask. A vacuum flask (thermos) is designed to slow down all three methods of thermal energy transfer at once. It has a double-walled glass bottle with the air removed from between the walls: the vacuum stops conduction and convection, because there are no particles to carry energy across the gap. Both glass walls are silvered so that they reflect infrared radiation back into the flask, reducing radiation. The stopper is made of plastic or cork, which are poor conductors, so little energy escapes through the top, and the bottle rests on insulating pads. The flask does not make a drink hot or keep it hot for ever - it only slows the transfer of thermal energy, so the drink stays near its starting temperature for much longer.

Kitchen pans and heating a room. The base and body of a saucepan are made of a good conductor such as aluminium or copper so that thermal energy from the hob passes quickly by conduction into the food, while the handle is made of wood or plastic, poor conductors, so that only a little energy reaches your hand and you can hold it safely. A radiator heats a room mainly by convection: it warms the air next to it, that air expands, becomes less dense and rises, and cooler, denser air flows in to replace it, so a convection current carries warm air around the room. A radiator painted matt black also transfers a useful amount of energy by radiation.

A fire burning wood or coal. A fire uses all three methods. Thermal energy from the flames reaches you mainly by radiation, which is why you feel warm even when sitting to one side of the fire, and why a fire would still warm you across a vacuum. Convection carries hot gases and smoke up the chimney, which draws fresh air in to feed the flames and circulates warm air around the room. Conduction carries thermal energy along the metal grate or poker, which is why the metal end of a poker left in the fire becomes too hot to hold while its wooden handle does not.

A car radiator. A car engine becomes very hot and must be cooled. Water, mixed with antifreeze, is pumped around the engine, where it gains thermal energy by conduction from the hot metal. The hot water is then pumped to the radiator at the front of the car. There, the many thin fins give a large surface area, so the water loses thermal energy to the air partly by conduction through the metal fins, partly by convection as moving air carries the energy away, and partly by radiation from the hot fins.