Answers · Thermal radiation

Thermal radiation — return questions key

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

1. LR1 · Return

Model answer. (a) Yes. Every object emits infrared, whatever its temperature, so the walls do too; they emit less than the hot tank. (b) Shiny. A shiny surface is a poor emitter of infrared, so the tank loses energy by radiation more slowly and the water stays hot for longer. A dull black surface is the best emitter and would lose energy fastest. (c) The rate of emission depends on the surface area as well as the temperature. The larger tank has more surface at the same temperature, so it emits more energy each second.

A full answer includes

(a) Yes: all objects emit infrared, including the walls at room temperature.
(b) Shiny, because a shiny surface is a poor emitter, so the tank loses less energy by radiation.
(c) Supplement: it increases, because a larger surface area emits more radiation each second at the same temperature.

Also accept

(a) yes (b) shiny: a poor emitter (c) it increases: a larger surface area

Watch for

“Walls at room temperature do not emit infrared.” — All objects emit infrared, whatever their temperature.
“Dull black keeps the water hot because black absorbs heat.” — The tank is losing energy, not gaining it. A shiny surface is a poor emitter, so it loses less by radiation.

2. LR2 · Return

Model answer. (a) The black car. A dull black surface is a good absorber, so it takes in most of the sunlight; white paint reflects most of it. (b) As the roof warms, it transfers energy away faster. By the afternoon it transfers energy away at the same rate as it receives it, so its temperature stays constant. (c) Energy is conducted from the coolant through the metal walls of the tubes and into the fins, and the air flowing between the fins carries it away by convection. The radiator also emits a little infrared.

A full answer includes

(a) The black car: a dark surface is a good absorber, and white reflects most of the sunlight.
(b) Supplement: it transfers energy away at the same rate as it receives it.
(c) Supplement: conduction through the metal tubes and fins, and convection by the moving air (also a little radiation).

Also accept

(a) black: a good absorber (b) energy out as fast as energy in (c) conduction and convection

Watch for

“The white car reflects all the sunlight, so it does not warm up at all.” — White reflects most of the sunlight but absorbs some, so it warms up, only less.