Answers · Thermal radiation

Thermal radiation — final check key

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

1. LK1 · Final check

Model answer. (a) Thermal radiation is infrared radiation. (b) Conduction and convection need a medium: particles to pass energy along, or a liquid or gas to flow. Space has almost no particles, but radiation needs no medium, so it crosses space. (c) Dull, matt surfaces are good emitters of infrared and shiny surfaces are poor emitters. Black surfaces emit better than white ones, but texture matters more: matt black is the best emitter and polished metal the worst. (d) Fill a Leslie cube with hot water so that all its sides are at the same temperature. Stand a radiation detector a fixed distance in front of one side, facing its middle. Turn the cube so that each side faces the detector in turn, at the same distance, and note each reading once it has settled. The side with the highest reading is the best emitter. (e) A hotter surface emits faster, and a larger surface area emits more each second.

A full answer includes

(a) Infrared radiation.
(b) Space has almost no particles; conduction and convection need a medium, and radiation does not.
(c) Dull (matt) surfaces are good emitters and shiny surfaces poor ones; black emits better than white; matt black is the best.
(d) Supplement: fill the cube with hot water; the detector at the same distance from each side, facing the same area; turn the cube and note the reading for each side; the higher the reading, the better the emitter.
(e) Supplement: the surface temperature and the surface area.

Also accept

(a) infrared (b) no particles in space; radiation needs no medium (c) dull good, shiny poor; black better than white (d) hot water; same distance and area; read each side (e) temperature and area

Watch for

“White surfaces hardly emit infrared.” — Matt white emits almost as well as matt black; shiny surfaces are the poor emitters.

2. LK2 · Final check

Model answer. (a) The dull black surface: dull black surfaces are good absorbers, white surfaces poor ones. (b) A shiny surface reflects most of the infrared that falls on it. (c) Stand two identical cans, one painted dull black and one shiny or white, the same distance from a lamp or heater. Put the same volume of water at the same starting temperature in each, with a thermometer. Switch on and read both thermometers every few minutes; the can whose temperature rises faster has the better absorbing surface. Swap the cans and repeat to check. (d) It must transfer energy away at the same rate as it receives energy. (e) Carbon dioxide absorbs some of the infrared emitted by the Earth’s surface and sends part of it back down, so less escapes into space. The Earth then transfers energy away more slowly than it receives it, so it warms until the two rates are equal again.

A full answer includes

(a) The dull black surface.
(b) It reflects it.
(c) Supplement: two identical cans (or plates), one dull black and one shiny or white, the same distance from a lamp or heater, with the same volume of water at the same starting temperature; measure each temperature over time (or time how long wax takes to melt); the faster rise shows the better absorber.
(d) Supplement: it must transfer energy away at the same rate as it receives energy.
(e) Supplement: carbon dioxide absorbs some of the infrared emitted by the surface and sends part back, so less escapes; the Earth warms until it emits as fast as it absorbs again.

Also accept

(a) dull black (b) reflects it (c) two cans the same distance from a lamp; compare temperature rises (d) energy out at the same rate as energy in (e) less infrared escapes, so the Earth warms

Watch for

“A white surface absorbs no radiation at all.” — White absorbs some of the radiation; it reflects most of it.