Conduction — answer key

1. TQ1 · Practice

Options as numbered on the pupil sheet: 1 — Thermal insulator; 2 — Good thermal conductor.

Model answer. A baking tray, a radiator panel and the base of a kettle must pass energy on quickly: from the oven to the food, from the hot water to the room, and from the element to the water. They need a good thermal conductor, such as a metal. An oven glove, the walls of a cool box and the lining of a winter boot must let energy through only slowly: from the hot tray to your hand, from the warm air to the food, and from your foot to the snow. They need a thermal insulator.

A full answer includes

• I matched the baking tray to good thermal conductor.
• I matched the radiator panel to good thermal conductor.
• I matched the base of the kettle to good thermal conductor.
• I matched the oven glove to thermal insulator.
• I matched the walls of the cool box to thermal insulator.
• I matched the boot lining to thermal insulator.
• I did not match the radiator panel to thermal insulator.

Also accept

• baking tray, radiator panel, kettle base – good conductor; oven glove, cool box walls, boot lining – insulator

Watch for

• “The radiator panel should be an insulator, so that it keeps its heat in.” — A radiator is there to pass energy from the hot water inside it to the room, so it must let energy through quickly: a good conductor.
• “The walls of the cool box should be a good conductor, to let the cold reach the food.” — Nothing called “cold” flows. The walls must slow down the energy that flows in from the warm air outside, so they need an insulator.
• “The oven glove should be a good conductor, so that it takes the heat away from the tray.” — If the glove conducted well, energy from the hot tray would reach your hand quickly. An insulator lets it through only slowly.

2. TQ2 · Practice

Model answer. Both slides are at 45 °C, warmer than your skin, so energy flows from the slide into your skin. Steel is a good thermal conductor, so energy reaches your skin from a large part of the slide quickly and your skin heats up fast: it feels very hot. Plastic is a thermal insulator, so energy reaches your skin only slowly and it feels much less hot. Touch tells you how fast energy flows into or out of your skin, not the temperature of what you touch.

A full answer includes

• I chose: steel conducts well, so it passes energy into my skin faster.
• I did not choose that steel stores more heat.

Also accept

• steel conducts energy into the skin faster

Watch for

• “Plastic reflects more sunlight, so less energy reaches your hand.” — The sunlight has already warmed both slides to the same 45 °C. Your hand gets its energy from the slide you touch, and steel passes it on much faster.
• “Steel stores more heat than plastic, so it feels hotter.” — What you feel is the rate at which energy flows into your skin. Steel conducts well, so energy reaches your skin quickly.
• “Energy flows from your hand into the plastic, so it feels cool.” — The plastic is at 45 °C and your skin at about 33 °C, so energy flows into your hand from both slides: only slowly from the plastic.

3. TQ3 · Practice

Model answer. Both rods started at 21 °C and stood in the same hot water. In 6 minutes the far end of the brass rod warmed by 22 °C (21 °C to 43 °C), but the far end of the steel rod by only 7 °C (21 °C to 28 °C). Energy was conducted along the brass much faster, so brass is the better thermal conductor. The test is fair, because the rods are the same size and in the same water.

A full answer includes

• I chose: brass is the better conductor: its far end warmed up faster.
• I did not choose steel for keeping its energy in.

Also accept

• brass is the better conductor: its far end warmed faster

Watch for

• “Steel is the better conductor: it kept its energy in for longer.” — A good conductor lets energy through quickly. The steel let less energy reach its far end, so it is the worse conductor.
• “They conduct equally well: both rods started at 21 °C.” — The same start is what makes the test fair. What you compare is how fast each far end warms after that.
• “Neither result counts: the far ends never reached the water temperature.” — You do not need to wait that long. In the same time, the rod whose far end warms more conducts better.

4. TQ4 · Practice

Options as numbered on the pupil sheet: 1 — Iron; 2 — Glass; 3 — Aluminium; 4 — Brass.

Model answer. The sooner a pin drops, the sooner enough energy reached it, so the better the rod conducts. Aluminium (1 min 50 s) is the best conductor, then brass (5 min 15 s), then iron (7 min 40 s). The glass pin had not dropped after 30 minutes, so glass is by far the worst conductor of the four, although it still conducts a little.

A full answer includes

• I put aluminium first: its pin dropped soonest, at 1 min 50 s.
• I put brass second.
• I put iron third.
• I put glass last: its pin was still hanging at 30 min.
• I did not put the slowest rod first.

Also accept

• aluminium, brass, iron, glass

Watch for

• “The rod whose pin takes longest conducts best, because it holds on to the energy.” — A good conductor lets energy through quickly, so its pin drops first.
• “Glass does not conduct at all.” — Glass is a bad conductor, not a perfect insulator: energy does get along the glass, only very slowly.

5. TQ5 · Practice

Model answer. (a) Take an aluminium block and a plastic block of the same size that have both been in the room for an hour, so that both are at room temperature. Stand them side by side and put an ice cube of the same size on top of each at the same moment. (b) Keep the same: the size of the blocks, their starting temperature, the size of the ice cubes and the room they stand in. Only the material changes. (c) Measure the time from putting each ice cube down until it has completely melted. (d) The ice on the aluminium melts much sooner. Energy flows from each block into its ice cube, and aluminium conducts energy from the whole block to the ice much faster, which shows that aluminium is a good thermal conductor and plastic is a thermal insulator.

A full answer includes

• (a) I used an aluminium block and a plastic block of the same size.
• (a) I left both blocks in the room, so that both are at room temperature.
• (a) I put an ice cube on top of each block at the same moment.
• (b) I named three of: the size of the blocks, their starting temperature, the size of the ice cubes, the room.
• (c) I measured the time until each ice cube has melted.
• (d) I predicted that the ice on the aluminium melts first.
• (d) I stated that this shows aluminium conducts thermal energy faster than plastic.
• I did not predict that the ice on the plastic melts first because the plastic feels warmer.

Also accept

• (a) same-size aluminium and plastic blocks at room temperature, an ice cube on each (b) three of: block size, starting temperature, ice cube size, room (c) time for each ice cube to melt (d) the ice on the aluminium melts first: aluminium conducts better

Watch for

• “The ice on the plastic melts first, because the plastic feels warmer to touch.” — Both blocks are at room temperature. The plastic only feels warmer because it takes energy from your hand slowly; in the same way it passes energy to the ice slowly.
• “Touch the two blocks and see which feels colder.” — Touch is not a measurement. The time for each ice cube to melt is a number you can read and compare.

6. TQ6 · Practice

Model answer. Two things changed at once: the material and the distance to the pin. Energy reaches a pin 6 cm away much sooner than one 12 cm away, so the aluminium pin could drop first even if aluminium were no better. In a fair test both pins are the same distance from the water. (Aluminium is in fact the better conductor, but this test cannot show it.)

A full answer includes

• I chose: the aluminium pin was nearer the water, so it could drop first for that reason.
• I did not choose that the result must be wrong.

Also accept

• the pins were at different distances, so the test was not fair

Watch for

• “Aluminium is a worse conductor than steel, so the result must be wrong.” — Aluminium does conduct better, so the result is not wrong. The problem is that the test cannot show it.
• “The test should have used thicker rods, so that the pins would drop faster.” — Faster pins would not make the test fair. What matters is that only the material changes, and here the distance changed too.
• “Steel needs longer to warm up, so the test did not run for long enough.” — Running longer would not help: the aluminium pin would still have started much nearer the water.

7. TQ24 · Practice

Model answer. (a) Runs 1, 2 and 4 are within 5 s of each other, but run 3 (254 s) is more than 70 s longer than any of them: it is the anomalous result. Something went wrong in that run, for example the water had cooled or the blob of wax was bigger. (b) Mean = (176 s + 181 s + 179 s) ÷ 3 = 536 s ÷ 3 = 178.7 s = 179 s (3 s.f.).

Checked automatically

• (a) Run number of the anomalous result: 3 (exact value)
  • Common wrong answer 1 : 176 s is the shortest, but within a few seconds of runs 2 and 4; the anomalous result is far from the rest.
• (b) Mean time: 179 s (3 significant figures)
  • Common wrong answer 198 : You included run 3. Leave it out before you add.
  • Common wrong answer 134 : You left run 3 out but divided by 4. Divide by the 3 results you added.

A full answer includes

• (a) I wrote run 3.
• (b) I added the other three times: 176 s + 181 s + 179 s = 536 s.
• (b) I divided by 3.
• (b) My answer is 179 s.
• I did not include run 3 in the mean.

Also accept

• (a) run 3 (b) 179 s

Watch for

• “Run 1 is the anomalous result, because it is the shortest time.” — 176 s is the shortest, but it is within a few seconds of runs 2 and 4. The anomalous result is the one far from all the rest.
• “Every result must go into the mean.” — An anomalous result comes from something going wrong in that run. Leave it out, then find the mean of the rest.

8. TQ7 · Practice

Model answer. In every solid, the particles are held in fixed positions and vibrate. At the hot end they vibrate more strongly and push against their neighbours, which then vibrate more strongly too, so energy is passed along from particle to particle by lattice vibrations: this is thermal conduction. Stone is not a metal and has no free electrons, so this is its only way, which is why it conducts slowly. The particles stay in their places: only energy travels.

A full answer includes

• I chose: particles at the hot end vibrate more and pass the vibration to their neighbours.
• I did not choose that hot particles travel along the rod.

Also accept

• lattice vibrations, passed from particle to particle

Watch for

• “Free electrons pick up energy at the hot end and carry it along the rod.” — Free electrons are found in metals. Stone is not a metal, so it has none.
• “Hot particles of stone travel along the rod and take the energy with them.” — In a solid, the particles stay in their places and only vibrate. What travels is energy.
• “The particles at the hot end grow bigger and push the energy along.” — Particles do not grow when they are heated. They vibrate more strongly, and pass that vibration on.

9. TQ8 · Practice

Model answer. Every solid conducts by lattice vibrations: the particles at the hot end vibrate more strongly and pass the vibration on to their neighbours. So both the copper and the rubber have “yes” for lattice vibrations. Copper is a metal, so it also has free (delocalised) electrons, which gain kinetic energy at the hot end and carry it quickly through the metal. Rubber is not a metal and has no free electrons, so it conducts only by the slower vibrations. Row D.

A full answer includes

• I chose row D.
• I did not choose a row where the rubber has no lattice vibrations.

Also accept

• Row D

Watch for

• “Row A.” — Rubber is a thermal insulator, but it still conducts a little: its particles vibrate and pass energy on, as in every solid.
• “Row B.” — Copper is a solid, so its ions vibrate and pass energy on too. The free electrons are an extra way, not the only way.
• “Row C.” — Copper is a metal, so it has free electrons, which carry energy much faster than the vibrations. That is why it conducts so well.

10. TQ9 · Practice

Model answer. (a) In both skewers, the particles at the hot end vibrate more strongly and pass the vibration on to their neighbours: lattice vibrations. (b) Steel is a metal, so it also has free electrons, also called delocalised electrons, which move about between the ions. At the hot end the free electrons gain kinetic energy, move quickly and travel a long way through the steel before they hand the energy on to ions further along. That is much faster than handing the energy on from one particle to the next, so the steel conducts far faster than the wood, which has only the vibrations.

A full answer includes

• (a) I wrote lattice vibrations.
• (a) I said the vibrations are passed from particle to particle.
• (b) I said steel, a metal, also has free (delocalised) electrons.
• (b) I said the free electrons gain kinetic energy at the hot end.
• (b) I said they move quickly through the steel.
• (b) I said they travel a long way before they pass the energy on.
• (b) I said the wood has only the slower lattice vibrations.
• I did not write that the particles of wood do not vibrate.

Also accept

• (a) lattice vibrations (b) steel also has free electrons, which carry energy fast over long distances; wood has only vibrations

Watch for

• “Steel conducts better because its particles vibrate and the particles of wood do not.” — The particles of wood vibrate too. The difference is that steel also has free electrons.

11. TQ10 · Practice

Model answer. In a gas the particles are far apart, with a lot of empty space between them. A fast particle passes on energy only when it collides with another one, and that does not happen often, so energy spreads through still air very slowly. The bubbles keep the air still, so the energy from the warm air outside reaches the milk only slowly.

A full answer includes

• I chose: the particles are far apart, so they rarely collide to pass on energy.
• I did not choose that gas particles cannot move.

Also accept

• particles far apart, few collisions, energy passed on slowly

Watch for

• “Its particles are far apart, so they cannot vibrate or move at all.” — Gas particles move fast in all directions. They are so far apart that they rarely meet to hand on energy.
• “The air in the bubbles is cold, so it passes cold into the milk.” — Nothing called “cold” flows. The air slows down the energy flowing in from the warm air outside.
• “Its particles are light, so each one is too small to carry energy.” — Each gas particle does carry energy as it moves. It can pass that energy on only when it hits another particle.

12. TQ11 · Practice

Model answer. (a) The particles of a liquid are close together but are not held in a fixed lattice: they slide past each other, so vibrations are not passed on well, and water has no free electrons. Energy is passed down only slowly, by collisions between neighbouring particles. (The warm water also stays at the top, so no convection current carries energy down.) (b) The reading still rises only slowly: oil is a liquid with no free electrons, so, like most liquids, it conducts badly. (c) The reading would rise much faster. Mercury is a liquid metal, so it has free electrons, which carry energy quickly down through the liquid, as in a solid metal.

A full answer includes

• (a) I said the particles of water are not held in a fixed lattice.
• (a) I said water has no free electrons.
• (a) I said energy is passed down only slowly, by collisions between neighbouring particles.
• (b) I predicted the reading still rises only slowly.
• (b) I said oil, like water, has no free electrons.
• (c) I predicted the reading would rise much faster.
• (c) I said mercury is a metal, so it has free electrons.
• (c) I said the free electrons carry energy quickly down through the liquid.
• I did not say that the particles of water are far apart.

Also accept

• (a) no fixed lattice, no free electrons: energy passed on slowly (b) still slow: oil has no free electrons (c) much faster: mercury has free electrons

Watch for

• “The thermometer stays cool because the cold sinks and keeps it cold.” — Nothing called “cold” moves. So little energy is conducted down through the water.
• “Water conducts badly because its particles are far apart.” — That is true of a gas. In water the particles are close together, but they are not held in a fixed lattice and there are no free electrons.

13. TQ12 · Practice

Options as numbered on the pupil sheet: 1 — Thermal insulator; 2 — Good thermal conductor; 3 — In between.

Model answer. Brass and steel are metals, with numbers of 50 or more: good thermal conductors. Granite and porcelain conduct far less well than the metals (steel conducts about 18 times better than granite), but still about 9 to 70 times better than rubber and cork, so they are in between, with solids such as glass, brick and concrete. Rubber and cork have the smallest numbers: thermal insulators.

A full answer includes

• I matched brass (110) to good thermal conductor.
• I matched steel (50) to good thermal conductor.
• I matched granite (2.8) to in between.
• I matched porcelain (1.5) to in between.
• I matched rubber (0.16) to thermal insulator.
• I matched cork (0.04) to thermal insulator.
• I did not put granite or porcelain with the insulators.

Also accept

• brass, steel – good conductor; granite, porcelain – in between; rubber, cork – insulator

Watch for

• “Steel goes in between, because 50 is far below 110.” — Steel is a metal, and 50 is still about 18 times more than granite. All the metals are good conductors.
• “Granite and porcelain are thermal insulators.” — They conduct about 9 to 70 times better than rubber or cork, so with three groups they go in between.
• “Granite goes with the good conductors.” — Steel conducts about 18 times better than granite (2.8).
• “Rubber goes in between.” — Porcelain conducts about 9 times better than rubber (0.16), so rubber is an insulator.

14. TT1 · Apply

Model answer. (a) Aluminium is a very good thermal conductor, so energy from the element passes through the plate quickly and reaches the cloth. (b) Plastic is a thermal insulator: energy is conducted along the handle only slowly, so the part you hold stays much cooler. (c) Supplement. The particles of the plastic near the hot iron vibrate more strongly and pass the vibration on to their neighbours: lattice vibrations. Plastic has no free electrons, so this is slow, but some energy does get through. (d) Supplement. The particles of a gas are far apart and rarely collide, so still air conducts even worse than plastic. A hollow handle replaces some of the plastic with trapped air, so even less energy is conducted to the outside, where you hold it.

A full answer includes

• (a) I said aluminium is a good thermal conductor.
• (a) I said energy passes quickly from the element through the plate to the cloth.
• (b) I said plastic is a thermal insulator.
• (b) I said energy reaches my hand only slowly.
• (c) I said the particles of the plastic vibrate and pass the vibration on to their neighbours.
• (c) I said plastic has no free electrons, so this is slow.
• (d) I said still air conducts even worse than plastic.
• (d) I said the particles of air are far apart and rarely collide.
• I did not write that plastic stops thermal energy completely.

Also accept

• (a) aluminium conducts well (b) plastic insulates (c) lattice vibrations only, no free electrons (d) trapped air conducts even worse than plastic

Watch for

• “The plastic handle stops thermal energy completely.” — An insulator slows energy down; it does not stop it. Left switched on for long enough, the handle warms too.
• “Air is a good conductor, so the hollow handle should get hotter.” — The particles of air are far apart and rarely collide, so still air is one of the worst conductors.