6.11 Black body radiation and the temperature of the Earth
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Which of these objects emits infrared radiation?
Choose one answer.
Correct Not quite: the answer is All objects
Mark scheme
C: All objects
Model answer
All objects (C).
State what happens to the peak wavelength of the radiation emitted by an object as its temperature increases.
Mark scheme
it gets shorter / decreases
Model answer
The peak wavelength gets shorter.
Describe how the total radiation emitted by an object changes as the temperature of the object increases.
Mark scheme
it increases
Model answer
The total radiation emitted increases.
Explain what is meant by a perfect black body.
Mark scheme
it absorbs all the radiation that falls on it (1); it is the best possible emitter of radiation at any temperature (1)
Model answer
A perfect black body absorbs all of the radiation that hits it, and emits the maximum possible radiation for its temperature.
An object is at a constant temperature. State and explain how the radiation it absorbs compares with the radiation it emits.
Mark scheme
they are the same / equal (1); if they were not equal the temperature would rise or fall (1)
Model answer
The rate of absorption equals the rate of emission, otherwise the temperature would change.
A stone in the Sun absorbs more radiation than it emits. Describe what happens to its temperature and why it stops changing.
Mark scheme
the temperature rises (1); as it gets hotter it emits more radiation (1); until the rate of emission equals the rate of absorption and the temperature stays constant (1)
Model answer
The stone warms up. As it gets hotter it emits more radiation, until the rate of emission equals the rate of absorption and the temperature becomes constant.
Explain why the Sun emits mostly visible light but the Earth emits mostly infrared radiation.
Mark scheme
the Sun is much hotter than the Earth (1); the hotter an object, the shorter the wavelength of its peak radiation / hotter objects emit shorter wavelengths (1)
Model answer
The Sun is much hotter than the Earth. The hotter an object is, the shorter the wavelength at which it emits most strongly.
Explain how an increase in the amount of carbon dioxide in the atmosphere affects the temperature of the Earth.
Mark scheme
carbon dioxide is a greenhouse gas that absorbs infrared radiation (1); it re-emits the radiation in all directions, including back towards the Earth (1); so less infrared escapes to space (1); the surface temperature rises until the radiation emitted balances the radiation absorbed (1)
Model answer
Carbon dioxide absorbs infrared radiation from the Earth's surface and re-emits it in all directions, including back to the surface. Less radiation escapes to space, so the Earth warms up until the radiation going out balances the radiation coming in.
Describe two factors, other than greenhouse gases, that can affect the temperature of the Earth, and explain how each does so.
Mark scheme
reflectivity of the surface / ice / clouds (1); more reflection means less is absorbed and the Earth cools (1); volcanic ash / dust (1); reflects or absorbs sunlight so reducing the radiation reaching the surface (1)
Model answer
More ice and cloud reflects more sunlight into space, so less is absorbed and the Earth cools. Volcanic ash reflects sunlight, so less reaches the surface, and the temperature falls for a time.
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