Transfer of thermal energy

Key points

2.3.1 Conduction

• Thermal conduction is the transfer of thermal energy through a material from particle to particle, without the material itself moving.
• A good thermal conductor lets thermal energy pass through it quickly. Metals are good thermal conductors.
• A bad thermal conductor is called a thermal insulator. Examples: plastic, wood, polystyrene, wool and air.
• Many insulators, such as wool and polystyrene foam, work well because they trap air. Air is a very bad conductor.
• Experiment: comparing conductors with rods
  1. Take rods of different materials (for example copper, aluminium, steel and glass). They must have the same length and thickness.
  2. Fix an identical drawing pin to the far end of each rod. Use the same amount of the same wax on each rod.
  3. Heat the near ends of all the rods equally, for example by putting them in the same hot water, or in the same flame at the same time.
  4. Time how long each pin takes to fall. The pin falls when the wax melts.
  5. The pin on the best conductor falls first, because thermal energy reaches its far end fastest.
• Experiment: paper on wood and metal: wrap one sheet of paper tightly around a rod that is half wood and half metal. Hold it briefly over a flame. The paper over the wood scorches (turns brown). The paper over the metal does not. The metal conducts the thermal energy away from the paper. The wood does not.
• Experiment: water is a bad conductor: hold a piece of ice at the bottom of a test tube of water with a small weight (for example metal gauze). Heat the top of the tube. The water at the top boils, but the ice at the bottom does not melt for some time. Very little thermal energy is conducted down through the water.
• In all solids, conduction happens by lattice vibrations. The particles at the hot end gain energy and vibrate more. They push on their neighbours, which then vibrate more too. The vibrations pass along the solid from particle to particle. Extended
• Metals also contain free (delocalised) electrons. These electrons are not attached to any one atom and can move through the whole metal. Extended
• At the hot end, the free electrons gain kinetic energy. They move quickly through the metal and collide with particles in cooler parts. Each collision transfers energy. This is much faster than lattice vibrations alone. So metals are good thermal conductors. Extended
• Gases are bad conductors. The particles are far apart. A particle can only pass on energy when it collides with another particle. In a gas, this happens much less often than in a solid. So energy passes along very slowly. Extended
• Most liquids are bad conductors. Their particles are not held in a fixed lattice, so vibrations are not passed on well. They also have no free electrons. Extended
• Extra detail: liquid metals, such as mercury, do conduct well, because they have free electrons.
• Conduction is not “good” or “bad” only. Many solids are in between: they conduct thermal energy better than thermal insulators, but much less well than good thermal conductors such as metals. Examples: glass and concrete. Extended
• Example: a tiled or concrete floor feels colder to bare feet than a carpet. The tiles conduct thermal energy away from your feet faster than the carpet does. Extended

2.3.2 Convection

• A fluid is a liquid or a gas.
• Convection is the transfer of thermal energy by the movement of a fluid. The warm fluid itself moves and carries the energy with it.
• Convection is an important way that thermal energy moves through liquids and gases. It cannot happen in a solid, because the particles of a solid cannot move from place to place.
• How convection works, step by step:
  1. The fluid near the heat source is heated.
  2. The warm fluid expands. Its mass stays the same, so its density decreases.
  3. The warm, less dense fluid rises.
  4. Cooler, denser fluid sinks to take its place.
  5. The cooler fluid is heated in turn, and rises. This continuous loop of moving fluid is a convection current.
• When a fluid is cooled from above (for example by an air conditioner), the cool fluid becomes denser and sinks. This also sets up a convection current.
• Experiment: convection in water: put one small crystal of potassium permanganate (a purple dye) at the bottom of a beaker of water, near one side. Heat the beaker gently just under the crystal. Purple streams rise above the heat, move across the top, and sink down the other side. They show the path of the convection current.
• Experiment: convection in air: use a box with a candle under one chimney and a second chimney over the cool side. Hold smoking paper over the cool chimney. The smoke is drawn down this chimney, across the box and up the chimney above the candle. The air above the candle is heated, becomes less dense and rises. Cooler air is drawn in to replace it.

2.3.3 Radiation

• Thermal radiation is infrared radiation. It is part of the electromagnetic spectrum (see 3.3).
• All objects emit infrared radiation, whatever their temperature.
• Thermal radiation does not need a medium (a material to travel through). It can travel through a vacuum.
• This is how thermal energy from the Sun reaches the Earth through space. Conduction and convection cannot happen in a vacuum, because they need particles.
• The colour and texture of a surface affect how well it emits, absorbs and reflects infrared radiation:
  • black surfaces are better emitters and better absorbers than white surfaces.
  • dull (matt) surfaces are better emitters and better absorbers than shiny surfaces.
  • white and shiny surfaces are good reflectors of infrared radiation.

• Example: a white shirt reflects much of the Sun’s infrared radiation and stays cooler. A black bag absorbs more and soon feels hot.
• An object at a constant temperature must transfer energy away at the same rate as it receives energy. Extended
• If an object receives energy faster than it transfers energy away, its temperature rises. Extended
• If an object receives energy more slowly than it transfers energy away, its temperature falls. Extended
• The rate of emission of radiation is greater when the surface temperature is higher. Extended
• The rate of emission is also greater when the surface area is larger. Extended
• So a warming object emits more and more radiation as its temperature rises. It stops warming when the total rate at which it transfers energy away (by radiation, and also by conduction and convection) equals the rate at which it receives energy. Extended
• The Earth’s temperature depends on a balance. The Earth and its atmosphere absorb some of the radiation from the Sun. They also emit infrared radiation, and some of it escapes into space. If energy is absorbed from the Sun at the same rate as it escapes into space, the average temperature stays constant. Extended
• Factors that change this balance: Extended
  • how much radiation from the Sun reaches the Earth.
  • how much of it is reflected back into space. Clouds, ice and snow reflect a lot of it.
  • greenhouse gases in the atmosphere, such as carbon dioxide, methane and water vapour. They absorb some of the infrared radiation emitted from the Earth’s surface. They then emit infrared radiation in all directions, so some of it goes back towards the surface. So less energy escapes into space.
• More greenhouse gases mean that energy escapes into space more slowly. The Earth then absorbs energy faster than it loses it, so its temperature rises until a new balance is reached. Extended
• Experiment: good and bad emitters (a Leslie cube) Extended
  1. A Leslie cube is a hollow metal cube. Its four sides have different surfaces, for example dull black, shiny black, dull white and shiny silver.
  2. Fill it with very hot water. All four sides are now at the same temperature.
  3. Place an infrared detector (or a thermometer with a blackened bulb) at the same distance from each side in turn.
  4. The reading is highest for the dull black side and lowest for the shiny silver side. So dull black is the best emitter.
• Another way: two identical cans, one dull black and one shiny, each with the same volume of hot water at the same starting temperature, with lids. Measure the temperature of each every minute. The water in the dull black can cools faster, so dull black is the better emitter. Extended
• Experiment: good and bad absorbers Extended
  1. Take two identical metal plates. Paint one dull black. Make the other shiny.
  2. Stick a drawing pin to the back of each plate with the same amount of wax.
  3. Place a heater midway between them, so each plate is the same distance from the heater.
  4. The pin on the dull black plate falls first. This plate absorbs infrared radiation faster, so its wax melts first.
• Another way: two identical cans, one dull black and one shiny, each with the same volume of cold water at the same starting temperature, placed the same distance from a heater. The water in the dull black can warms up faster. Extended

2.3.4 Consequences of thermal energy transfer

• A kitchen pan:
  • The base is made of metal, a good conductor. Thermal energy is conducted quickly from the hot plate or flame through the base to the food.
  • The handle is made of plastic or wood, a thermal insulator. Very little thermal energy is conducted along it, so it stays cool enough to hold.
• Heating a room by convection:
  • A heater is placed low down, near the floor. The air next to it is heated, expands, becomes less dense and rises.
  • Cooler, denser air sinks and moves towards the heater to take its place. A convection current carries warm air all around the room.
• Cooling a room: an air conditioner is placed high up on the wall. It cools the air near the ceiling. The cool air is denser, so it sinks. Warmer air rises towards the air conditioner to be cooled.
• Sea breeze: on a sunny day, the land warms up faster than the sea. The air above the land is heated, expands, becomes less dense and rises. Cooler, denser air from above the sea moves in to take its place. This is a convection current.
• In some situations, more than one type of thermal energy transfer is important. Extended
• A fire burning wood or coal Extended
  • Radiation: the burning fuel emits infrared radiation in all directions. This is how a person in front of the fire is warmed. The radiation does not need to heat the air on the way.
  • Convection: the hot gases above the fire are less dense, so they rise up and away (up the chimney). Cooler air is drawn in at the bottom to replace them. This brings fresh air, with oxygen, to the fire. Convection carries energy upwards, so it does little to warm a person sitting in front of the fire.
  • Conduction: air is a bad conductor, so very little energy reaches the person by conduction through the air. Energy is conducted through the metal grate and the walls of the fireplace.
• A radiator in a car Extended
  • Hot water (the coolant) from the engine flows through the thin metal tubes of the radiator.
  • Conduction: thermal energy is conducted through the metal walls of the tubes and into the metal fins joined to them. Metal is a good conductor.
  • Convection: air flowing past the fins is heated, and it carries the energy away.
  • Radiation: the hot fins also emit some infrared radiation.
  • The many thin fins give a large surface area. This increases the rate of energy transfer to the air, by convection and by radiation.
  • Despite its name, a car radiator transfers most of its energy to the air by convection, not by radiation.
• A vacuum flask (keeps hot drinks hot and cold drinks cold) Extended
  • The vacuum between the two walls stops conduction and convection, because these need particles.
  • The shiny silvered walls are poor emitters and good reflectors. They reduce thermal energy transfer by radiation.
  • The stopper is made of an insulator, so it reduces conduction. It also stops warm air rising out of the flask (convection) and stops evaporation from the top.

Common mistakes

• Students write “heat rises”. / The mark scheme wants: the warm fluid expands, becomes less dense and rises; cooler, denser fluid sinks.
• Students write that convection happens in solids. / The mark scheme wants: convection happens only in fluids (liquids and gases), because the fluid must move.
• Students write that thermal radiation needs air to travel through. / The mark scheme wants: thermal radiation is infrared, and it needs no medium; it can travel through a vacuum.
• Students write that shiny surfaces are good emitters. / The mark scheme wants: dull black surfaces are the best emitters and absorbers; shiny (and white) surfaces are the best reflectors.
• Students write that metals conduct well because their particles vibrate more. / The mark scheme wants: metals have free (delocalised) electrons that move through the metal and carry energy quickly. Extended
• Students write that an object at a constant temperature receives no energy. / The mark scheme wants: it receives energy and transfers energy away at the same rate. Extended

Exam tips

• Name the method of transfer every time: conduction, convection or radiation. Then give the reason.
• Describe a conduction experiment with a fair test: same length and thickness of rod, identical pins with the same amount of wax, same heat source, same distance from the heat. Say what you measure (time for the pin to fall) and how you decide (shortest time = best conductor).
• Explain convection as a chain: heated → expands → less dense → rises; cooler, denser fluid sinks to replace it → convection current.
• Explain a design choice (pan, room heater, air conditioner) by linking the material or position to the transfer: “metal base because it is a good conductor”; “heater low down because warm air rises”.
• Explain conduction in metals using free electrons as well as lattice vibrations. Extended
• Describe emitter and absorber experiments with the control variables: same starting temperature (or same heater), same distance, same size of surface. Only the surface type changes. Extended
• Explain complex cases (fire, car radiator, vacuum flask) by going through all three methods in turn and saying how important each one is. Extended
• A typical 1-mark answer: “Thermal radiation is infrared radiation.”
• A typical 2-mark answer to “Why is a pan handle made of plastic?”: “Plastic is a thermal insulator (1). So very little thermal energy is conducted to your hand, and the handle stays cool enough to hold (1).”
• A typical 3-mark answer to “Explain how a heater warms the whole room”: “The air near the heater is heated and expands (1). It becomes less dense and rises (1). Cooler, denser air sinks to take its place, so a convection current circulates the warm air (1).”