Absorbing infrared — answer key

1. LQ12 · Practice

Model answer. Dull black surfaces are good absorbers and poor reflectors: they take in most of the radiation that falls on them. White and shiny surfaces are poor absorbers and good reflectors, so they send most of it back.

A full answer includes

• The matt black plate.

Also accept

• matt black

Watch for

• “The shiny plate absorbs most, because it collects the light.” — A shiny surface reflects most of the radiation, so it absorbs little.

2. LQ13 · Practice

Options as numbered on the pupil sheet: 1 — Reflects most; 2 — Absorbs most.

Model answer. Dull black and dark surfaces are good absorbers: a matt black solar panel and dark tarmac take in most of the sunlight that falls on them. White and shiny surfaces are good reflectors: a white wall reflects most of the sunlight, shiny foil behind a radiator reflects infrared back into the room, and a shiny emergency blanket reflects the body’s own infrared back to it.

A full answer includes

• The matt black solar panel and the dark tarmac: absorb most.
• The white wall, the shiny foil and the shiny blanket: reflect most.

Also accept

• solar panel, tarmac – absorb most; white wall, foil, blanket – reflect most

Watch for

• “A white wall reflects all of the radiation, so it does not warm up at all.” — White reflects most of the radiation but still absorbs some, so it warms up, only more slowly.

3. LQ14 · Practice

Model answer. (a) A shiny surface is a good reflector and a poor absorber of infrared. Most of the radiation from the fire is reflected, so little energy goes into the suit. (b) A matt black surface is a good absorber and a poor reflector. It would take in most of the radiation from the fire, so the suit, and the firefighter inside it, would heat up quickly.

A full answer includes

• (a) A shiny surface is a good reflector and a poor absorber, so it reflects most of the infrared from the fire and absorbs little.
• (b) Matt black is a good absorber, so it would absorb most of the radiation and the suit would get hot.

Also accept

• (a) reflects most of the infrared, absorbs little (b) black absorbs most of it and gets hot

Watch for

• “The shiny layer works because it is a good emitter.” — Shiny surfaces are poor emitters and poor absorbers; the suit works because it reflects the radiation.

4. LQ15 · Practice

Model answer. (a) The sooty snow. (b) Both patches get the same sunlight. Dark soot is a good absorber, so the sooty snow takes in most of the radiation falling on it. Clean white snow is a good reflector and sends most of the sunlight back. The sooty snow gains energy faster, so it reaches its melting point and melts first.

A full answer includes

• (a) The sooty snow.
• (b) Dark soot is a good absorber, and clean white snow reflects most of the sunlight; the sooty snow takes in energy faster, so it warms and melts first.

Also accept

• (a) the sooty snow (b) dark absorbs more, white reflects most, so the sooty snow gains energy faster

Watch for

• “The clean snow melts first, because white surfaces let more sunlight in.” — White surfaces reflect most of the radiation; dark ones absorb it.

5. LQ16 · Practice

Options as numbered on the pupil sheet: 1 — Switch on the heater and start a stopwatch.; 2 — Swap the two plates round and repeat the test.; 3 — Stick a coin to the back of each plate with the same amount of wax, and stand the heater exactly halfway between the plates.; 4 — Note which coin falls first..

Model answer. First set up a fair test: the same amount of wax on each coin, and the heater the same distance from both plates, so that both receive the same radiation. Then switch on and start timing. The coin behind the better absorber falls first: the one on the black plate. Swapping the plates and repeating checks that the heater was not stronger on one side.

A full answer includes

• The same wax on each plate and the heater halfway; switch on and start timing; note which coin falls first; swap the plates and repeat.

Also accept

• wax and heater halfway; switch on; note the first coin; swap and repeat

Watch for

• “The heater should stand nearer the shiny plate, to give it a chance.” — Both plates must receive the same radiation, so the heater must be the same distance from each.

6. LQ17 · Practice

Model answer. (a) Can P. (b) P’s temperature rises faster and levels off higher, so P absorbs radiation faster than Q. Matt black is a better absorber than a shiny surface, so P is the black can. (c) Both cans must be the same distance from the heater, hold the same volume of water, start at the same temperature and be the same size and metal, so that only the surface differs. (d) The hotter a can gets, the faster it transfers energy away to its surroundings. Each can’s temperature stops rising when it transfers energy away at the same rate as it receives it from the heater.

A full answer includes

• (a) Can P.
• (b) P warms faster and reaches a higher temperature, so it absorbs more radiation: matt black is the better absorber.
• (c) Any two: the distance from the heater; the volume of water; the starting temperature; the size and metal of the cans.
• (d) As a can warms, it transfers energy away faster; its temperature stops rising when it transfers energy away as fast as it receives it.

Also accept

• (a) P (b) P warms faster and higher: better absorber (c) distance; volume; starting temperature (d) energy out as fast as energy in

Watch for

• “The temperatures stop rising because the cans are full of energy.” — The cans still absorb energy at the same rate; they stop warming because they now transfer it away just as fast.

7. LQ18 · Practice

Model answer. The temperature is constant, so the soup transfers energy away at the same rate as it receives it: 900 J every second, which is 900 W.

Checked automatically

• Rate of energy transfer to the surroundings: 900 W (within ±1 W)
  • Common wrong answer 0 : Energy still flows out: a constant temperature means it leaves as fast as it arrives.

A full answer includes

• 900 W (900 J every second): the same rate as it receives energy.

Also accept

• 900 W
• 900 J/s

Watch for

• “The soup transfers no energy away, because its temperature is not changing.” — Energy is still flowing in and out; the temperature is constant because the two rates are equal.

8. LQ19 · Practice

Options as numbered on the pupil sheet: 1 — Its temperature falls; 2 — Its temperature rises; 3 — Its temperature stays constant.

Model answer. When the can receives energy faster than it transfers energy away, its temperature rises. When the two rates are equal, its temperature stays constant, although energy is still flowing in and out. When a cloud cuts the energy coming in, the can transfers energy away faster than it receives it, so its temperature falls.

A full answer includes

• Receives energy faster than it transfers it away: the temperature rises.
• Equal rates: the temperature stays constant.
• Receives energy more slowly than it transfers it away: the temperature falls.

Also accept

• faster in – rises; equal – constant; slower in – falls

Watch for

• “If the temperature is constant, no energy is flowing.” — Energy flows in and out at equal rates.

9. LQ20 · Practice

Model answer. (a) At 20 °C the can was at the same temperature as the room, so it lost almost no energy, while it received energy from the lamp. Energy came in much faster than it went out, so the temperature rose quickly. (b) As the can warmed, it transferred energy away to its surroundings faster and faster. At about 36 °C it transferred energy away at the same rate as it received it, so its temperature stayed constant. (c) Its temperature fell back towards room temperature. The can was still transferring energy away, but it was now receiving much less, so energy went out faster than it came in.

A full answer includes

• (a) At first it was at room temperature, so it transferred very little energy away; it received energy much faster than it lost it.
• (b) The hotter it got, the faster it transferred energy away; at about 36 °C it transferred energy away as fast as it received it.
• (c) It fell towards 20 °C, because it was still transferring energy away but receiving much less.

Also accept

• (a) little energy out at first (b) energy out rose until it equalled energy in (c) fell: out faster than in

Watch for

• “The can stopped warming because the lamp stopped heating it.” — The lamp kept giving the can energy at the same rate; the can stopped warming because it then lost energy just as fast.

10. LQ21 · Practice

Model answer. More carbon dioxide absorbs more of the infrared emitted by the surface and sends part of it back down, so less escapes into space and the Earth warms. Less ice and snow means less sunlight is reflected and more is absorbed, so the Earth warms. More cloud and ice reflect more sunlight, which would cool the Earth, and less greenhouse gas lets more infrared escape, which would also cool it.

A full answer includes

• More carbon dioxide, so less infrared escapes; less ice and snow, so less sunlight is reflected.

Also accept

• more carbon dioxide; less ice and snow

Watch for

• “Greenhouse gases warm the Earth by making the Sun’s radiation stronger.” — They absorb some of the infrared emitted by the surface and send part of it back, so less escapes into space.

11. LQ22 · Practice

Model answer. (a) The glowing coal and the flames emit a lot of infrared radiation, which crosses the room and warms the person when it is absorbed. (b) The hot gases above the fire are less dense than the cooler room air, so they rise up the chimney, carrying energy with them by convection. (c) The hot coal touches the metal grate, and energy is conducted into it and through it. (d) As the hot gases rise up the chimney, cooler, denser air from the room is drawn in at the bottom to take their place, which also keeps the fire supplied with fresh air.

A full answer includes

• (a) By radiation (infrared) from the glowing coal and flames, across the room.
• (b) By convection: the hot gases are less dense than the room air, so they rise up the chimney.
• (c) By conduction from the hot coal into the metal.
• (d) As the hot gases rise, cooler air is drawn in to take their place.

Also accept

• (a) radiation (b) convection: hot gases rise (c) conduction (d) cool air drawn in to replace the rising gases

Watch for

• “The person is warmed mainly by hot air coming out of the fire.” — The hot gases go up the chimney; the person in front of the fire is warmed by radiation.

12. LQ23 · Practice

Options as numbered on the pupil sheet: 1 — Emits little infrared, and reflects most of what is emitted back; 2 — An insulator that stops warm air leaving the top by convection; 3 — Few, narrow joins, so little energy is conducted to the case; 4 — Almost no particles, so it stops conduction and convection.

Model answer. The stopper is a thermal insulator and stops warm air leaving the top by convection. The silvery surfaces are poor emitters, so the hot inner wall radiates little infrared, and good reflectors, so most of what is emitted is reflected back. The vacuum has almost no particles, so almost no energy can cross it by conduction or convection. The inner bottle touches the case only at the narrow neck and the small support, so very little energy is conducted through these joins.

A full answer includes

• A, the stopper: an insulator that stops warm air leaving the top by convection.
• B, the silvery surfaces: emit little infrared and reflect most of it back.
• C, the vacuum: stops conduction and convection across the gap.
• D, the neck and support: few, narrow joins, so little energy is conducted.

Also accept

• A convection at the top; B radiation; C conduction and convection across the gap; D conduction through the joins

Watch for

• “The vacuum stops radiation too.” — Radiation needs no medium, so it can cross a vacuum; it is the silvery surfaces that cut down radiation.

13. LQ24 · Practice

Model answer. (a) The hot coolant touches the inside of the tubes. Aluminium is a good thermal conductor, so energy is conducted quickly through the thin tube walls and along the fins. (b) The many thin fins give the radiator a very large surface area. More air touches the hot metal at once, so more energy is transferred to the air each second, mostly by convection and a little by radiation. (c) When the car drives along, air is pushed through the radiator all the time and carries the warm air away. In a traffic jam with no fan, little air flows through it, so the air between the fins warms up and carries energy away only slowly. The engine gives energy to the coolant faster than the radiator transfers it away, so the coolant and the engine get hotter and hotter.

A full answer includes

• (a) By conduction through the metal walls of the tubes and into the fins: aluminium is a good thermal conductor.
• (b) The fins give a large surface area, so more air touches the hot metal and more energy is transferred each second (by convection, and a little by radiation).
• (c) Standing still with no fan, little air flows through the radiator, so the warm air next to the fins is not carried away; energy leaves the coolant more slowly than the engine gives it energy, so its temperature rises. Driving along, the moving car pushes air through the radiator.

Also accept

• (a) conduction through the metal (b) a large surface area, so more energy to the air each second (c) no air flow: energy out slower than energy in, so the temperature rises

Watch for

• “A car radiator works mainly by radiation, as its name says.” — Despite its name, a car radiator transfers most of its energy to the air flowing through it, by convection; radiation is only a small part.

14. LQ25 · Practice

Model answer. (a) The cloud reflects some of the sunlight back into space before it reaches the surface, so the surface absorbs energy from the Sun more slowly. (b) The Earth still emits infrared into space at the rate set by its temperature, but it now absorbs energy more slowly than that. Energy goes out faster than it comes in, so the Earth cools. As it cools it emits more slowly, until it emits energy only as fast as it absorbs it, at a lower average temperature. (c) When the cloud has gone, the surface absorbs more sunlight again. Energy comes in faster than it goes out, so the Earth warms until the rates are equal again, back near its old temperature.

A full answer includes

• (a) It falls: more sunlight is reflected back into space, so less reaches the surface and is absorbed.
• (b) It falls: the Earth now absorbs energy more slowly than it emits it, so it cools until it emits only as fast as it absorbs.
• (c) It rises again: more sunlight reaches the surface, so the Earth absorbs energy faster than it emits it, and it warms until the rates balance again.

Also accept

• (a) it falls: more sunlight reflected (b) it falls: out faster than in, until they balance again (c) it rises: in faster than out, until they balance

Watch for

• “The ash cloud traps heat like a greenhouse gas, so the Earth warms.” — This cloud mainly reflects sunlight back into space before it reaches the surface, so the Earth absorbs less energy and cools.

15. LT2 · Apply

Model answer. (a) White paint is a good reflector: it reflects most of the sunlight that falls on it and absorbs less, so less energy goes into the roof and the rooms below. (b) A roof’s temperature stops rising when it transfers energy away as fast as it receives it, and the hotter it is, the faster it transfers energy away. The dark roof absorbs energy faster, so it has to get hotter before the rates are equal. (c) With many white roofs, more of the sunlight falling on the city is reflected back towards space. The city absorbs energy more slowly, so its energy in and out balance at a slightly lower temperature, in the same way that ice and snow help to keep the Earth cooler.

A full answer includes

• (a) White paint reflects most of the sunlight and absorbs less, so less energy goes into the roof and the rooms.
• (b) Supplement: the dark roof absorbs energy faster, so it must get hotter before it transfers energy away as fast as it receives it.
• (c) Supplement: more sunlight is reflected back towards space, so the city absorbs less energy and balances at a lower temperature.

Also accept

• (a) reflects most sunlight, absorbs less (b) absorbs faster, so it must be hotter to lose energy as fast (c) more reflected, less absorbed, balance at a lower temperature

Watch for

• “White roofs are cooler because white paint emits infrared faster.” — Matt white and dark paints emit infrared about equally well; the white roof is cooler because it reflects more sunlight.