Pupil sheet · Conduction and convection
Conduction and convection — final check
Name ______________________ Date ____________
Work alone, with the lesson closed. Answer every question; there are no hints.
1. identify / state / justify / explain
A school is choosing door handles for a cold corridor: brass, glass or plastic. (a) A pupil plans a test. She fits a brass rod 8 mm thick and a plastic rod 4 mm thick into the side of a tank of hot water. On each rod she sticks a drawing pin 6 cm from the tank: the pin on the brass rod with butter, the pin on the plastic rod with candle wax. Identify the two faults in her plan, and state how to put each one right. (b) Supplement. In a second test she lays a tile of each material, all the same size and 1 cm thick, on the same hot plate at 75 °C, and times how long the top of each tile takes to reach 40 °C. The table shows her results. Identify the group each material belongs to: good thermal conductor, in between, or thermal insulator. Use the data to justify the group you give to glass. (c) Supplement. A pupil writes: “Brass conducts better than glass because the particles of brass vibrate faster.” Explain why this is incorrect. (d) Supplement. A pupil says: “Water conducts thermal energy well, because its particles are close together, like the particles of a solid.” Explain why this is incorrect.
| Tile | Time for the top to reach 40 °C / s |
|---|---|
| brass | 4 |
| glass | 70 |
| plastic | 360 |
(a) Two faults
(b) Supplement: the three tiles
(c) Supplement: brass and glass
(d) Supplement: water
2. predict / explain / suggest
A convector heater stands on the floor against one wall of a cold room. It draws in cool air at the bottom, warms it, and lets it out through a grille at the top. Someone pushes a sofa against the heater, so that the back of the sofa covers the grille. (a) Predict how warm the far side of the room now gets, compared with before, and explain your prediction in terms of density. (b) A pupil says: “It does not matter: the energy will still reach the far side by conduction through the air.” Explain why this is incorrect. (c) In a separate test, a coloured ice cube floats at one end of a tank of water at room temperature. Predict the path the cold coloured water takes as the ice melts, and explain it in terms of density. (d) Thick porridge heated in a pan burns at the bottom unless it is stirred, but thin soup heated in the same way does not. Suggest why.
(a) The covered grille
(b) Not conduction
(c) The ice cube
(d) Porridge and soup