Answers · Conduction and convection

Conduction and convection — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. TR1 · Return

Model answer. (a) Supplement. In a gas, the particles are far apart and rarely collide, so still air is a very bad conductor. The fibres hold the air still, so very little energy is conducted out through the bag. (b) Wrap one can in the filling and the other in the cotton sheet, the same thickness of each. Fill both with the same volume of hot water at the same temperature. Measure the temperature of each after the same time, for example 10 minutes. The can whose temperature has fallen less is wrapped in the better insulator. (c) Supplement. In a solid metal the ions stay in their places in the lattice and only vibrate: energy is passed on by these vibrations. The particles that do move through the metal are the free electrons, which carry energy quickly. No atoms travel along the zip. (d) Supplement. The steel mug feels hottest, the pottery mug less hot and the foam cup coolest. Steel is a good conductor, so energy passes from the soup through its wall into your hand quickly. Pottery is one of the solids in between: it conducts better than an insulator but less well than a metal. Foam is a thermal insulator, so energy reaches your hand only slowly.

A full answer includes

(a) I said the air particles are far apart.
(a) I said they rarely collide, so energy is passed on slowly.
(b) I wrapped one can in each material.
(b) I used the same thickness of each material.
(b) I used the same volume of water in each can.
(b) I started both cans at the same temperature.
(b) I measured the temperature of each can after the same time.
(b) I said the can whose temperature falls less has the better insulator.
(c) I said the ions stay in their places in the lattice and only vibrate.
(c) I said the particles that move through the metal are the free electrons.
(d) I said the steel mug feels hottest.
(d) I said the foam cup feels coolest.
(d) I said pottery is in between: it conducts better than foam but less well than steel.
(d) I said the better the wall conducts, the faster energy passes from the soup into my hand.
I did not say that the can which cools faster has the better insulator.

Also accept

(a) particles far apart, rarely collide (b) one can in each material, same water; temperature after the same time; smaller fall = better insulator (c) ions stay in place and vibrate; only free electrons move (d) steel hottest, pottery in between, foam coolest

Watch for

“A sleeping bag makes heat, so it keeps you warm.” — A sleeping bag makes no energy; it slows down the energy leaving your body.
“The can that cools faster is wrapped in the better insulator.” — A good insulator slows the energy leaving the water, so that can stays hot longer.
“The atoms of a metal travel along it, carrying the energy.” — The ions of a solid stay in their places and vibrate. Only the free electrons move through the metal.
“The pottery mug feels as cool as the foam cup, because pottery is an insulator.” — Pottery conducts several times better than foam, so it goes in between: it feels hotter than the foam.

2. TR2 · Return

Model answer. (a) On top of the food, for the same reason that an air conditioner is fixed high on a wall. (b) Air cooled by the ice pack contracts, becomes denser and sinks down over the food. Warmer air rises to the ice pack to take its place and is cooled in turn, so a convection current keeps all the food cold. (c) Stand a lit candle in a box with a glass front, under one of its two chimneys, and hold smouldering paper over the other chimney. The smoke goes down the chimney without the candle, across the box and up out of the chimney over the candle: warm air rises over the candle, and the cooler air drawn in carries the smoke. (d) Convection needs a fluid that can flow. The wall is a solid: its particles stay in their places, so energy passes through it by conduction (slowly, because plastic is an insulator).

A full answer includes

(a) I said on top of the food.
(b) I said air cooled by the ice pack becomes denser.
(b) I said this air sinks down over the food.
(b) I said warmer air rises to the ice pack to be cooled in turn.
(c) I put a lit candle under one chimney.
(c) I held smouldering paper over the other chimney.
(c) I said the smoke goes down that chimney, across the box and up over the candle.
(d) I said convection happens only in liquids and gases, which can flow.
(d) I said energy passes through the solid wall by conduction.
I did not write that cold rises from the ice pack.

Also accept

(a) on top (b) cooled air is denser and sinks; warmer air rises to be cooled (c) candle under one chimney; smoke drawn down the other and up over the candle (d) a solid cannot flow: conduction

Watch for

“The ice pack should go underneath, because cold rises.” — Cold air is denser, so it sinks. An ice pack on top cools the air that then sinks over the food.
“Energy gets through the plastic wall by convection.” — Convection needs the material to flow. The wall is solid, so energy is conducted through it.