Emitting infrared — answer key

1. LQ1 · Practice

Model answer. Thermal radiation is infrared radiation. Every object emits it, whatever its temperature: a cup of tea, your hand, the walls and even an ice cube. A hotter object emits more infrared each second than a cooler one, but a cold object still emits some. Infrared is invisible, so an object does not have to glow to emit it.

A full answer includes

• Thermal radiation is infrared radiation, and all objects emit it; hotter objects emit more each second.

Also accept

• all objects emit infrared; hotter objects emit more each second

Watch for

• “Only hot objects give out infrared.” — Every object emits infrared. A cold object emits less than a hot one, but it still emits.
• “An object gives out infrared only when it glows.” — Infrared is invisible. Your hand emits infrared all the time without glowing.

2. LQ2 · Practice

Options as numbered on the pupil sheet: 1 — Convection; 2 — Radiation; 3 — Conduction.

Model answer. Space has almost no particles, so only radiation can carry the Sun’s energy across it. Energy passes along a metal spoon by conduction, from particle to particle through the metal. Warm air rising from a heater is convection: the warmed air is less dense and rises. The hot air from a campfire goes upwards, not sideways, so your face is warmed by the infrared radiation the fire emits.

A full answer includes

• Sun to Earth across space: radiation.
• Along the spoon: conduction.
• Warm air rising: convection.
• Your face by the campfire: radiation.

Also accept

• Sun – radiation; spoon – conduction; warm air – convection; campfire – radiation

Watch for

• “The campfire warms your face by convection.” — Convection carries the hot air and smoke upwards. Energy reaches your face sideways, by radiation.
• “Energy from the Sun reaches us by convection through space.” — Convection needs a liquid or gas that can flow, and space has almost no particles.

3. LQ3 · Practice

Model answer. (a) Air warmed by the mug becomes less dense and rises straight up, so it does not flow sideways to a hand held beside the mug. (b) The mug emits infrared radiation, which travels across the gap to your hand and warms it when it is absorbed. (c) Yes. All objects emit infrared, including your hand. The mug is hotter, so it emits more infrared towards your hand than your hand emits towards the mug, and your hand gains energy overall.

A full answer includes

• (a) Warm air rises from the mug, so it goes upwards, not sideways to your hand.
• (b) By infrared (thermal) radiation from the mug.
• (c) Yes: all objects emit infrared; the hotter mug sends more to your hand than your hand sends back.

Also accept

• (a) warm air rises, not sideways (b) infrared radiation (c) yes, all objects emit infrared

Watch for

• “Your hand does not emit infrared because it is cooler than the mug.” — Every object emits infrared. Your hand emits less than the hot mug, so it gains energy overall, but it still emits.

4. LQ4 · Practice

Model answer. (a) The Sun emits radiation, including infrared, which travels through empty space and is absorbed by the suit. (b) Conduction passes energy from particle to particle, and convection needs a liquid or gas that can flow. Space has almost no particles, so neither can carry energy there: thermal radiation is the only transfer that does not need a medium. (c) Yes. Every object emits infrared, whatever its temperature, so the suit still emits it in the shadow; it emits less as it cools.

A full answer includes

• (a) By radiation from the Sun, including infrared, which crosses empty space.
• (b) Conduction and convection need a medium (particles), and space has almost no particles.
• (c) Yes: all objects emit infrared, even cool ones.

Also accept

• (a) radiation (infrared) across space (b) they need particles, a medium; space has almost none (c) yes

Watch for

• “Radiation needs air to travel through, like sound.” — Thermal radiation needs no medium: it crosses empty space, which is how sunlight reaches the Earth.
• “In the cold shadow the suit stops emitting infrared.” — All objects emit infrared. A cooler suit emits less, but it still emits.

5. LQ5 · Practice

Model answer. All four cans are at the same temperature, so only the surface differs. Dull (matt) surfaces are good emitters and shiny surfaces are poor ones, and black emits better than white. Matt black is the best emitter of all. Matt white comes close behind, and the polished aluminium can emits least.

A full answer includes

• The can painted matt black.

Also accept

• matt black

Watch for

• “The shiny can emits most, because it looks bright.” — Looking bright in light does not mean emitting infrared. Shiny surfaces are poor emitters of infrared.
• “Matt white hardly emits any infrared.” — Matt white paint emits almost as well as matt black; it is shiny metal that emits very little.

6. LQ6 · Practice

Model answer. Dull surfaces are good emitters of infrared and shiny surfaces are poor emitters. Black surfaces emit better than white ones, so matt black emits slightly more than matt white at the same temperature. But texture matters much more than colour: matt white emits almost as well as matt black, while polished metal emits very little.

A full answer includes

• (1) good
• (2) poor (bad)
• (3) more (slightly more)
• (4) bigger (larger)

Also accept

• good; poor; more; bigger

Watch for

• “Colour matters more than texture: white surfaces hardly emit.” — Matt white emits almost as well as matt black. Shiny surfaces are the poor emitters.

7. LQ7 · Practice

Model answer. (a) A dull black surface is the best emitter of infrared, so the warm pipes radiate energy to the room quickly. (b) Shiny surfaces are poor emitters of infrared, so polished pipes would radiate energy more slowly. The liquid inside would not cool as well, and the fridge would work less well.

A full answer includes

• (a) A dull black surface is a good emitter of infrared, so the pipes radiate energy away quickly.
• (b) They would give out energy by radiation more slowly, because shiny surfaces are poor emitters.

Also accept

• (a) dull black is a good emitter (b) slower: shiny surfaces are poor emitters

Watch for

• “Black is chosen because it absorbs heat from inside the pipes.” — The aim is to give energy out to the room. Dull black is chosen because it is a good emitter.

8. LQ8 · Practice

Model answer. In a fair test only the surface changes, because that is what is being tested. Hot water fills the cube, so every side is at the same temperature. The detector must be the same distance from each side, because a nearer detector catches more radiation, and it must face the same area of each side. The colour and texture are what change from side to side, and the colour of the detector’s stand makes no difference.

A full answer includes

• The temperature of the water, the distance from the side to the detector and the area of the side the detector faces.

Also accept

• water temperature; distance to the detector; area the detector faces

Watch for

• “The colour and texture of the sides must be kept the same.” — The surface is what is being tested, so it is the one thing that changes.

9. LQ9 · Practice

Options as numbered on the pupil sheet: 1 — Dull aluminium; 2 — Matt black; 3 — Polished aluminium; 4 — Matt white.

Model answer. All four sides are at the same temperature and the same distance from the detector, so the readings compare the surfaces. Matt black is the best emitter, so it gives the highest reading, B (110 W/m²). Matt white is close behind, D (104 W/m²). The metal sides emit far less: dull aluminium, A (23 W/m²), is ahead of polished aluminium, C (6 W/m²), because a shiny surface is the poorest emitter.

A full answer includes

• A: dull aluminium.
• B: matt black.
• C: polished aluminium.
• D: matt white.

Also accept

• A dull aluminium; B matt black; C polished aluminium; D matt white

Watch for

• “Side D must be dull aluminium, because white paint hardly emits.” — Matt white emits almost as well as matt black, so it gives the second-highest reading.
• “The polished side gives the highest reading because it is shiny and bright.” — Shiny surfaces are poor emitters of infrared, so polished aluminium gives the lowest reading.

10. LQ10 · Practice

Model answer. The higher the temperature of a surface, the faster it emits infrared, so every side gives a higher reading: about twice as high in the experiment. The surfaces have not changed, so matt black is still the highest and polished aluminium still the lowest.

A full answer includes

• Every side gives a higher reading, and the order of the sides stays the same.

Also accept

• every side higher; same order

Watch for

• “Only the water changes, so the readings stay the same.” — A hotter surface emits infrared faster, so every reading rises.

11. LQ11 · Practice

Model answer. Both panels have the same surface and temperature, so each square metre radiates at the same rate. The second panel has 1.60 ÷ 0.80 = 2 times the surface area, so it radiates 2 × 240 W = 480 W.

Checked automatically

• Power radiated by the second panel: 480 W (within ±5 W)
  • Common wrong answer 240 : That is the first panel’s power: the second panel has twice the area.
  • Common wrong answer 120 : A bigger area radiates more, not less.

A full answer includes

• 1.60 m² is twice 0.80 m², so the power doubles: 2 × 240 W = 480 W.

Also accept

• 480 W

Watch for

• “The power stays 240 W, because the temperature is the same.” — Each square metre radiates the same, and the second panel has twice as many square metres.
• “A bigger panel radiates less, because the energy is spread out.” — More surface means more of the panel emitting at once, so more energy is radiated each second.

12. LT1 · Apply

Model answer. (a) Conduction and convection both need particles, a medium, to carry energy, and space has almost none. Thermal radiation needs no medium, so it is the only way to lose energy into space. (b) A matt black finish, because a dull black surface is the best emitter of infrared and so radiates energy away fastest. Matt white would be nearly as good; a shiny finish would be a poor emitter. (c) Make the surface area larger, for example with a bigger panel or one with fins, and run the panel at a higher surface temperature: the rate of emission depends on both.

A full answer includes

• (a) Space has almost no particles; conduction and convection need a medium, and radiation does not.
• (b) Matt black or matt white, because both are good emitters of infrared (matt black is the best; a shiny finish would be poor).
• (c) Supplement: a larger surface area (a bigger panel, or fins); a higher surface temperature.

Also accept

• (a) no particles in space; only radiation needs no medium (b) matt black (or matt white): a good emitter (c) bigger area; hotter surface

Watch for

• “A shiny panel is best, because shiny things are used in space.” — Shiny surfaces are poor emitters; to get rid of energy the panel needs a good emitter.
• “The panel can lose energy by convection into space.” — Convection needs a liquid or gas that can flow, and space has almost no particles.