Red dots in the pan base show energy conducting upwards. In the water and the room, particles are colored by temperature: red = warm, blue = cool. Temperatures are typical values for a kitchen, sped up.
A classic experiment. Five rods of the same size have one end in boiling water. Drawing pins are stuck on with wax every 3 cm; when the rod under a pin reaches 55 °C the wax melts and the pin drops. The rods are colored by temperature.
Model: one-dimensional heat flow along each rod (25 cm long, 6 mm thick) with energy lost to the 20 °C air from its surface. Real classroom rods are slower (the wax, the pins and imperfect contact soak up energy, so the first pin may take a minute or more), but the order is always the same.
Start with 200 g of water at 80 °C in a beaker in a 20 °C room, try different covers, and watch the cooling curves over 30 minutes. A flatter curve means a better insulator.
The curves follow Newton’s law of cooling: the water cools fastest when it is much hotter than the room. Heat-loss rates are typical classroom values, not measurements.
A Leslie cube is a metal box of hot water with differently finished sides. Turn each side to the infrared detector and compare the readings.
Good emitters are also good absorbers. That is why solar panels are black, and why survival blankets and the inside of a vacuum flask are shiny.
A vacuum flask (Thermos) is designed to stop all three kinds of heat transfer. Pick Basic (6 labels) or Advanced (9). Switch between the labeled diagram, a blank one, a label quiz and match the jobs. On a phone, swipe the diagram sideways.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the vacuum flask, sort 12 examples, read the conduction race results, multiple choice and explain questions, with an answer key.
Temperature tells you how fast the particles of something are moving or vibrating on average. When a hot thing touches a cold thing, the faster particles bump into the slower ones and share their energy. Energy never flows by itself from cold to hot: a cold drink in your hand warms up, it does not make your hand colder by “sending cold”.
In a solid the particles stay in place but vibrate. Heating one end makes those particles vibrate more, and they pass the vibration along. Metals conduct much better because they also have free electrons that move through the metal and carry energy quickly. Non-metals, liquids and especially gases are poor conductors, called insulators.
In liquids and gases the particles can move. The warmed part expands, becomes less dense and rises; cooler, denser fluid sinks to replace it. The loop is a convection current. It explains why a kettle heats all its water, why radiators warm whole rooms, and sea breezes. Trapping a gas so it cannot move (in fibers, foam or bubbles) stops convection, which is how most insulation works.
Every object gives out infrared radiation; hotter objects give out much more. It is an electromagnetic wave, so it can cross a vacuum: that is how the Sun warms the Earth. Matte black surfaces are the best emitters and absorbers; shiny surfaces are the worst and reflect infrared instead. A vacuum flask beats all three transfers at once: a vacuum (no conduction or convection), silvered walls (little radiation) and a stopper.
In the kitchen, switch between Conduction, Convection and Radiation and switch the heat off and on. Race the rods at ×20 and see why pans are metal but handles are wood. In the insulation lab compare “No cover” with “Vacuum flask”, then turn the shiny silver side of the Leslie cube to the detector.
| Material | Thermal conductivity k (W/m·K) | Good conductor or insulator? |
|---|---|---|
| Copper | 401 | Excellent conductor (pans, heat sinks) |
| Aluminum | 237 | Very good conductor |
| Steel | about 50 (stainless steel about 16) | Fair conductor |
| Glass | about 1 | Poor conductor |
| Water | 0.6 | Poor conductor (but convects) |
| Brick | about 0.7 | Poor conductor |
| Wood | 0.1–0.2 | Insulator (handles) |
| Foam, wool, fiberglass | 0.03–0.04 | Very good insulators (trapped air) |
| Still air | 0.026 | Very good insulator if it cannot move |
Loft insulation (fibers trap air: less conduction and convection), cavity wall insulation (foam stops convection in the gap), double glazing (a thin layer of gas between panes), draught excluders (stop convection through gaps) and shiny foil behind radiators (reflects infrared back into the room).
References: conductivity values from standard tables (e.g. Engineering ToolBox; Incropera & DeWitt, Fundamentals of Heat and Mass Transfer); emissivities: matte black paint ≈ 0.95, white paint ≈ 0.9, polished aluminum ≈ 0.05.
For AP Physics, A-level and first-year university. The rate of heat flow \(\dot Q\) (in watts) for each mechanism:
Thermal resistance works like electrical resistance: temperature difference plays the role of voltage and heat flow the role of current, so layers in a wall add in series, \(R_{\text{total}} = \sum R_i\). A thin layer of foam (k ≈ 0.035 W/m·K) can out-insulate a thick brick wall. In radiation, temperatures must be in kelvin, \(\sigma = 5.67\times10^{-8}\ \text{W m}^{-2}\text{K}^{-4}\), and doubling the absolute temperature multiplies the emitted power by 16. For small temperature differences all three give Newton’s law of cooling, \(dT/dt = -(T - T_{\text{room}})/\tau\) with \(\tau = mc/(UA)\), where UA is the total heat-loss coefficient in W/K; that is what the insulation lab plots. The net radiation formula assumes a small object in large surroundings.
Worked example: a brick wall 0.2 m thick (k = 0.72 W/m·K), 10 m², with 20 K across it: R = 0.2 / (0.72 × 10) = 0.0278 K/W, so \(\dot Q\) = 20 / 0.0278 ≈ 720 W. Adding 5 cm of foam (R = 0.05 / (0.035 × 10) = 0.143 K/W) gives Rtotal = 0.171 K/W and \(\dot Q\) ≈ 117 W: about six times less.
Conduction (energy passed between particles that touch, mainly in solids), convection (energy carried by a moving liquid or gas) and radiation (infrared electromagnetic waves, which need no particles).
Key takeaway: Conduction, convection and radiation.Metals have free electrons that move through the metal and carry energy quickly from the hot end to the cold end, as well as vibrating particles passing energy along. Non-metals only have the vibrations, so they conduct much more slowly.
Key takeaway: Free electrons carry energy fast.Convection needs the particles themselves to move from place to place and carry energy with them. In a solid the particles are held in fixed positions and can only vibrate.
Key takeaway: Convection needs a fluid that can flow.The vacuum between its double walls stops conduction and convection, the silvered surfaces reduce radiation, and the stopper reduces conduction, convection and evaporation at the top. It works both ways, so it keeps cold drinks cold too.
Key takeaway: It blocks all three transfers.Radiation is an electromagnetic wave, like light, so it needs no particles and can cross empty space. Conduction and convection cannot.
Key takeaway: Only radiation crosses a vacuum.Matte black surfaces absorb most of the infrared that falls on them and are also the best emitters. Shiny surfaces reflect most infrared. Solar water panels are matte black; survival blankets and fire suits are shiny.
Key takeaway: Good absorbers are good emitters.Both are at room temperature, but the metal conducts energy away from your warm hand much faster, so your skin cools quickly and the spoon feels cold. Your skin senses the rate of energy flow, not the temperature of the object.
Key takeaway: Feeling cold = losing energy quickly.These materials trap lots of small pockets of air. Air is a poor conductor, and because it cannot move, convection is stopped too. Wet insulation works badly because water conducts about 20 times better than air.
Key takeaway: Trapped, still air is the insulator.Air warmed by the heater rises, spreads across the ceiling, cools and sinks on the far side, then flows back along the floor to the heater. This convection current carries warmth around the whole room. An air conditioner is best high up for the opposite reason.
Key takeaway: Hot air rises, cold air sinks.The power radiated by a surface is P = εσAT⁴, where T is in kelvin, σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴ and ε is the emissivity (1 for a perfect black body). The net power exchanged with surroundings at Ts is εσA(T⁴ − Ts⁴). Try it in the Advanced calculator.
Key takeaway: Radiated power grows with T⁴.