Course: Combined Science + Separate Physics | Tier: Foundation + Higher | See also: Reducing Unwanted Energy Transfers, Black Body Radiation, RP 2: Thermal Insulation
Note: AQA doesn’t ask you to define conduction and convection, but using these ideas makes your answers on insulation, thermal conductivity and infrared much clearer.
📌 KEY DEFINITIONS – learn them word for word
High thermal conductivity: the material transfers energy by conduction quickly. Insulators have a low thermal conductivity.
Infrared radiation: all objects emit and absorb infrared. The hotter the object, the more infrared it emits in a given time. Matt black surfaces are the best emitters and absorbers; shiny surfaces are the worst.
Dissipated energy: energy that spreads out to the surroundings and is wasted, usually by heating.
Notes
Conduction
Conduction happens mainly in solids. When one end of a solid is heated, its particles vibrate more and pass energy on to neighbouring particles. Metals are the best conductors because they also have free electrons that move through the metal and transfer energy quickly. Non-metals, liquids and especially gases are poor conductors, so they make good insulators.
Convection
Convection happens in liquids and gases (fluids). The heated fluid expands and becomes less dense, so it rises. Cooler, denser fluid sinks to take its place. This circulating flow is a convection current. Convection can’t happen in solids, because the particles can’t move around. Trapping air in small pockets (in foam, fibres or feathers) stops convection currents.
Radiation (infrared)
All objects emit and absorb infrared radiation. It is an electromagnetic wave, so it needs no particles and can travel through a vacuum – which is how energy from the Sun reaches the Earth. Matt black surfaces are the best absorbers and emitters. Shiny silver surfaces are the worst absorbers and emitters, because they reflect infrared.
✏️ Copy into your book
| How energy is transferred | Where it happens | How to reduce it | |
|---|---|---|---|
| Conduction | Particles vibrate and pass energy to neighbours; free electrons in metals | Mainly solids | Use materials with a low thermal conductivity (e.g. plastic, trapped air) |
| Convection | Hot fluid becomes less dense and rises; cooler fluid sinks | Liquids and gases | Trap air in small pockets; put a lid on |
| Radiation | Infrared electromagnetic waves | Anywhere, even a vacuum | Use shiny, silvered surfaces |
The vacuum flask
A vacuum flask keeps hot drinks hot and cold drinks cold by reducing every way energy can be transferred.
✏️ Copy into your book: the vacuum flask
- Vacuum between the double walls: there are no particles, so there is no conduction or convection across the gap.
- Silvered (shiny) walls: reflect infrared, reducing energy transfer by radiation.
- Plastic or cork stopper: a poor conductor, and it stops convection currents and evaporation from the top.
- Plastic supports and outer case: poor conductors, reducing conduction to the outside.
- For a cold drink, the flask works the same way – it stops energy getting in.
Lubrication
When surfaces rub together, friction does work and energy is dissipated by heating the parts and the surroundings. Lubricants such as oil and grease form a thin layer between moving parts.
✏️ Copy into your book: lubrication
Lubrication reduces friction between moving parts. Less work is done against friction, so less energy is dissipated by heating. More of the input energy is transferred usefully, so the machine is more efficient. It also reduces wear and noise.
Fill the gaps
Copy the passage into your book and fill in the numbered gaps using the word bank.
Conduction happens mainly in (1)______. Metals are good conductors because they have free (2)______. Gases are poor conductors, so trapped air is a good (3)______. In convection, a heated fluid becomes less (4)______ and rises, and cooler fluid (5)______. Infrared radiation can travel through a (6)______ because it doesn’t need particles. (7)______ black surfaces are the best emitters and absorbers. In a vacuum flask, the shiny walls (8)______ infrared. Lubrication reduces (9)______, so less energy is (10)______.
Word bank: dense · dissipated · electrons · friction · insulator · matt · reflect · sinks · solids · vacuum
Check your answers
1 solids · 2 electrons · 3 insulator · 4 dense · 5 sinks · 6 vacuum · 7 matt · 8 reflect · 9 friction · 10 dissipated
Questions
Q1 (F) Why are metals good thermal conductors?
Show answer
They have free electrons that move through the metal and transfer energy quickly, as well as vibrating particles passing energy to their neighbours.
Q2 (F) Why can’t convection happen in a solid?
Show answer
The particles in a solid can only vibrate about fixed positions. They can’t move from place to place, so no convection current can form.
Q3 (F) How does energy from the Sun reach the Earth through space?
Show answer
By radiation (electromagnetic waves, including infrared). Space is a vacuum, so conduction and convection can’t happen – they need particles.
Q4 (F) Explain why the heater in a room is usually placed low down near the floor.
Show answer
Air next to the heater warms up, becomes less dense and rises. Cooler, denser air sinks to replace it and is heated in turn. This sets up a convection current that circulates warm air around the whole room.
Q5 (F/H) Explain how each part of a vacuum flask reduces energy transfer from a hot drink.
Show answer
The vacuum has no particles, so it stops conduction and convection between the walls. The silvered walls reflect infrared, reducing radiation. The plastic stopper is a poor conductor and stops convection currents and evaporation from the top. The plastic supports are poor conductors.
Q6 (F/H) Why does a winter coat filled with feathers keep you warm?
Show answer
The feathers trap air. Air has a very low thermal conductivity, so it reduces energy transfer by conduction, and because the air is trapped it can’t move to transfer energy by convection.
Q7 (F/H) Why are the pipes on the back of a fridge painted matt black?
Show answer
Matt black is the best emitter of infrared radiation, so the pipes transfer energy to the surroundings as quickly as possible.
Q8 (F/H) A bike chain is oiled. Explain how this improves the efficiency of the bike.
Show answer
The oil reduces friction between the moving parts. Less work is done against friction, so less energy is dissipated by heating. More of the energy the rider puts in is transferred usefully to the kinetic store of the bike, so the efficiency increases.
Q9 (H) Two identical cans of hot water are left to cool – one painted matt black, one shiny silver. Which cools faster? Explain.
Show answer
The matt black can. Matt black surfaces are better emitters of infrared radiation than shiny silver surfaces, so it transfers energy to the surroundings at a higher rate. (For a fair test, the cans must start at the same temperature with the same volume of water.)
