Lesson B · 4.6–4.10
By the end you should explain conduction, convection and radiation in exam sentences, describe the 4.9 practicals, and say how a house or a flask reduces wasted energy.
How I mark thermal questions
“Heat rises” scores zero. “Hot air expands, becomes less dense than the surrounding air, and rises” scores the marks. Every explain answer needs a mechanism: particles, density, or infrared — not the name of the process alone.
4.6 · Conduction
Conduction is the transfer of thermal energy through a material without the material itself flowing. It is the main method in solids.
When you heat one end of a rod, the particles there vibrate with a larger amplitude. They collide with their neighbours and pass on kinetic energy. That is the lattice-vibration story, and it works in every solid.
Metals are the best conductors because they also have free (delocalised) electrons. Those electrons move through the metal and transfer energy quickly from the hot end to the cold end. Insulators (wood, plastic, air, glass wool) have no free electrons and weakly coupled particles, so energy creeps along slowly.
Convection does not happen in solids
Particles in a solid are fixed in a lattice. They vibrate; they do not circulate. If you write “convection in the metal rod,” that mark is gone.
4.6–4.7 · Convection
Convection is the transfer of thermal energy by the bulk movement of a fluid (a liquid or a gas). Solids cannot convect.
The exam sentence I want every time:
The fluid is heated, so it expands and becomes less dense than the surrounding cooler fluid [1]. The warmer, less dense fluid rises [1]. Cooler, denser fluid sinks to take its place, and a convection current is set up [1].
Room heater / radiator
Air next to the heater warms, expands, rises. Cooler air from the floor is drawn in. The room fills from the top down. That is why a heater is placed low on the wall, not on the ceiling.
Coastal breeze
By day the land heats faster than the sea. Warm air over the land rises; cooler sea air is drawn onshore. At night the land cools faster and the breeze can reverse.
Fridge freezer at the top
Cold air is denser, so it sinks from the freezer compartment and cools the fridge below. If the freezer were at the bottom, the cold air would stay there.
Boiling pan / kettle
Water at the bottom is heated, rises, cooler water sinks. You see the current if you drop a crystal of potassium permanganate at the side of a beaker.
4.6, 4.8 · Radiation
Thermal radiation is infrared electromagnetic radiation. It does not need particles, so it is the only way energy reaches us from the Sun through space. Hotter surfaces emit more radiation (and a greater fraction at shorter wavelengths).
| Surface | Emitter | Absorber | Reflector |
|---|---|---|---|
| Dull / matt black | best | best | worst |
| Shiny silver / white | worst | worst | best |
A black car in the sun heats up faster than a silver one (better absorber). A shiny emergency blanket keeps you warm because it is a poor emitter and a good reflector of infrared. The silvering on a vacuum flask reflects radiation back into the drink (or keeps radiation out).
Black is not “hotter because it is black”
Say “dull black is the best absorber of infrared” or “the best emitter.” Colour without infrared / radiation is not enough for the mark.
4.9 · Core practical
You can be asked for method, variables, errors and safety on any one of these three classic set-ups. Learn the verbs: measure, time, control, repeat.
Equal lengths of different metals (copper, aluminium, iron, brass) are heated at one end, often with a Bunsen. A drawing pin is stuck to the far end of each rod with a dab of wax or petroleum jelly. The pin that falls first belongs to the best conductor.
A large beaker of cold water. Drop a crystal of potassium permanganate at one side of the bottom. Heat gently under that crystal with a small flame. Purple streaks rise above the heat and loop around the beaker — a convection current you can see.
A Leslie’s cube is a hollow metal cube with four faces: dull black, shiny black, dull white/silver, shiny silver. Fill it with very hot water. Hold an infrared detector (or a thermometer in a blackened cork, or your cheek at a safe distance) the same distance from each face. The dull black face gives the largest reading — best emitter.
Boiling-tube version: identical tubes, one painted dull black, one wrapped in foil, same volume of hot water, lids, thermometers. Record temperature against time. The black tube cools faster (best emitter). To test absorption, put the same tubes in front of a heater and see which warms faster.
Method marks want a sequence
“Fill the cube with hot water. Place the detector 5 cm from the dull black face and record the reading. Repeat at the same distance for each face. Compare.” That is four marks of method. “Use a Leslie cube” is one word and no marks.
4.10 · Reducing unwanted transfer
Unwanted transfer from a house is usually conduction through walls and roof, convection through draughts, and radiation from hot surfaces. Each fix targets a pathway.
| Measure | What it reduces | Why (exam sentence) |
|---|---|---|
| Loft insulation (glass wool / fibre) | conduction (and convection in the loft) | Trapped air is a poor conductor; fibres stop air circulating |
| Cavity-wall foam | conduction and convection in the cavity | Foam traps air; air cannot form a convection current across the gap |
| Double glazing | conduction (and convection between panes) | Trapped air (or argon) between the panes is an insulator |
| Draught excluders | convection | Stop warm air leaving and cold air entering through gaps |
| Shiny foil behind radiators | radiation | Reflects infrared back into the room instead of into the wall |
| Carpets, curtains | conduction / draughts | Trapped air; reduce flow at windows |
A flask keeps a drink hot (or cold) by attacking all three pathways:
“The vacuum stops heat” is not enough
Name the processes: vacuum prevents conduction and convection; silvering reduces radiation; stopper reduces convection through the opening. One feature, one process, every time.
Watch
Also useful
1.
Why are metals better thermal conductors than plastics?
2.
Explain how a convection current is set up above a room heater.
3.
State the job of the vacuum and of the silvering in a vacuum flask.