This note covers the three ways thermal energy is transferred: conduction, convection and radiation. It also looks at what affects the rate of transfer and how everyday objects such as vacuum flasks, radiators and solar panels use these ideas.
Key ideas
Thermal energy always flows from a hotter place to a colder place. It can travel in three ways.
Conduction. Conduction is the transfer of thermal energy through a material without the material itself moving. It happens mainly in solids.
- In all solids: particles at the hot end vibrate more strongly. They pass this extra vibration to neighbouring particles, which pass it on again along the solid.
- In metals (extended): metals also contain free electrons that move through the structure. Hot electrons move quickly, collide with particles and with other electrons, and carry energy rapidly along the metal. This makes metals much better conductors than non-metals.
Materials that conduct well are called thermal conductors (metals). Materials that conduct badly are thermal insulators (wood, plastic, wool, and air and other gases). Gases are poor conductors because their particles are far apart and collide only occasionally. Liquids (other than molten metals) are poor conductors because their particles are not held in fixed positions to pass vibrations on.
Convection. Convection is the transfer of thermal energy by the movement of a liquid or gas. It cannot happen in a solid.
- The fluid is heated, its particles move faster and spread further apart, so the fluid becomes less dense.
- The warmer, less dense fluid rises.
- Cooler, denser fluid sinks to take its place.
- This circulation is a convection current.
Convection currents explain why a room heats up best when the radiator is low down, why sea breezes occur, and why the heating element in a kettle is at the bottom.
Radiation. Thermal radiation is the transfer of energy by infrared electromagnetic waves. It needs no particles, so it can cross a vacuum. This is how energy reaches the Earth from the Sun. All objects emit infrared radiation, and they also absorb it.
The amount of radiation emitted or absorbed depends on:
- Surface colour and texture: dull black surfaces are the best emitters and the best absorbers. Shiny, white or silvered surfaces are the worst emitters and absorbers, and they reflect radiation well.
- Surface temperature: hotter objects emit more radiation each second.
- Surface area: a larger surface area emits and absorbs more radiation.
Rate of energy transfer. Energy is lost faster when the temperature difference between the object and its surroundings is larger. In an experiment comparing two identical containers of hot water, the one with the dull black surface cools faster than the one with the shiny surface.
Applications.
- Vacuum flask: the vacuum between the two walls stops conduction and convection. The silvered walls reduce radiation. The plastic stopper is an insulator and prevents convection from the top.
- Cooking pans: metal bases conduct energy to the food, and plastic or wooden handles stay cool.
- Home insulation: trapped air in loft insulation and in the gap of double glazing reduces conduction and convection. Shiny foil behind a radiator reflects radiation back into the room.
- Solar water heaters: dull black pipes absorb as much radiation as possible.
- Car radiators and heat sinks: have a large surface area and fins to lose energy faster.
Definitions and formulae
| Term or formula | Meaning / when to use |
|---|---|
| Conduction | Energy passes through a material by particle vibrations (and by free electrons in metals) with no overall movement of the material. |
| Convection | Energy carried by the movement of heated fluid in a convection current. Only in liquids and gases. |
| Radiation | Energy transferred by infrared waves. Can travel through a vacuum. |
| Thermal conductor / insulator | A material that allows energy to pass easily / poorly through it. |
| Good emitter and absorber | A dull black surface. A shiny surface is poor at both but is a good reflector. |
| density = mass ÷ volume | Explains why heated fluid rises: it is less dense than the cooler fluid around it. |
| E = Pt, Q = mcΔT | Used to turn a temperature drop into energy lost and an average power. |
Worked examples
Example 1 Two identical cans each hold 0.30 kg of water at 80 °C. One can is painted dull black and the other is polished silver. The water in the black can cools to 60 °C in 6.0 minutes. The water in the silver can takes 9.5 minutes. The specific heat capacity of water is 4200 J/(kg °C). Find the average rate of energy loss for each can.
Temperature drop = 80 − 60 = 20 °C for both cans.
Energy lost: Q = mcΔT = 0.30 × 4200 × 20 = 25 200 J.
Black can: time = 6.0 × 60 = 360 s, so power = 25 200 ÷ 360 = 70.0 W.
Silver can: time = 9.5 × 60 = 570 s, so power = 25 200 ÷ 570 = 44.2 W.
Answer: black can 70.0 W; silver can 44.2 W. The dull black surface is the better emitter, so it loses energy faster.
Example 2 A metal spoon and a plastic spoon are placed in hot soup. After a minute the metal handle is warm but the plastic handle is cool. Explain this.
The soup heats the ends of both spoons. The particles at the hot end vibrate faster and pass energy to their neighbours.
In the metal, free electrons also gain energy, move quickly along the spoon and collide with other particles, transferring energy much faster along the handle.
Plastic has no free electrons, so energy passes along only by particle vibrations, which is slow. Plastic is a thermal insulator.
Answer: metal conducts energy quickly because of its free electrons; plastic has none, so it conducts poorly.
Example 3 A heater is placed on the floor of a room. Explain how it warms the whole room, and why it would not warm the room as well if it were fixed near the ceiling.
The air next to the heater warms up. Its particles move faster and spread out, so the air becomes less dense.
This warm, less dense air rises. Cooler, denser air from the sides moves in to take its place and is then heated. A convection current spreads thermal energy through the room.
A heater near the ceiling would warm only the air above it. Warm air is less dense and stays at the top, so no circulating current forms.
Answer: a low heater creates a convection current; a ceiling heater does not.
Common errors
- Common error: saying "heat rises". It is the warm fluid that rises, because it is less dense than the cooler fluid around it.
- Common error: saying cold travels from a cold object to a warm one. Energy always travels from hot to cold.
- Common error: explaining convection in a solid. Convection needs a fluid that can flow, so it happens only in liquids and gases.
- Common error: saying a shiny surface "does not absorb" any radiation or that black surfaces "attract" heat. A shiny surface is a poor absorber and a good reflector, and a dull black surface is a good absorber and a good emitter.
- Common error: saying radiation needs air. Infrared waves can travel through a vacuum.
Exam tip: When a question asks for an explanation of a vacuum flask, go through conduction, convection and radiation one at a time and name the feature that reduces each one.
Check yourself
- Name the way thermal energy reaches the Earth from the Sun and explain how you know it does not need a medium.
Answer
Radiation (infrared and other electromagnetic waves). Space is a vacuum, and energy still arrives, so no particles are needed. - Why are metals better thermal conductors than wood?
Answer
Metals contain free electrons that move through the material and carry energy quickly, as well as particle vibrations. Wood has no free electrons, so energy passes by vibrations only. - A black cup of tea at 90 °C and a white cup of tea at 90 °C are left in the same room. Which cools faster by radiation and why?
Answer
The black cup. A dull black surface is a better emitter of infrared radiation than a white surface. - A 0.50 kg sample of water cools from 60 °C to 40 °C in 420 s (c = 4200 J/(kg °C)). Find the average power lost.
Answer
Q = 0.50 × 4200 × 20 = 42 000 J. P = Q ÷ t = 42 000 ÷ 420 = 100 W.