GCSE · Physics · AQA · Spec 8463

Perfect black bodies and Earth's temperature (physics only)

Even an ice cube gives out radiation. So if everything is giving radiation out all the time, why doesn't it all just keep cooling down? The answer is a balance.

Heat it up. Watch what it gives out.

Temperature (1 = coolest) 1

Temperature (1 = coolest): 1

Drag the dot up to make the object hotter. The temperature runs from 1 (coolest) to 6 (hottest), and the numbers on both axes are relative, not real measurements.

Exam line: Hotter object: more radiation emitted at every wavelength, and the peak moves to shorter wavelengths.

Watch out: Look at setting 1. The curve is low, but it never lies flat on the axis. A cool object still emits radiation: just less of it, and more of it at longer wavelengths.

Perfect black bodies

Reason it through

Why would a perfect black body be the best possible emitter of radiation?

Link 1 of 4

First link · your turn

Start with the definition. What does a perfect black body do with the radiation that falls on it?

Locked — reveal the link above first

Higher

A steady temperature

The mug that stopped cooling

A mug of tea was left on the kitchen table hours ago. It has cooled down, and now its temperature isn't changing at all.

What is happening to radiation at the mug right now? Pick the idea closest to what you think.

Nothing. Its temperature has stopped changing, so it has stopped absorbing and emitting radiation.It's still emitting radiation, but it has stopped absorbing any.It's absorbing radiation from the room, but it's too cool now to emit any.It's absorbing and emitting radiation at the same rate.

How sure are you?

I'm sureI think soI'm guessing

Nothing. Its temperature has stopped changing, so it has stopped absorbing and emitting radiation.

This is a common misconception

Tempting, but no. The mug never stops emitting: every object emits radiation, whatever its temperature, as you saw on the curve. It keeps absorbing radiation from the room too. Its temperature is steady because it absorbs radiation at the same rate as it emits it.

It's still emitting radiation, but it has stopped absorbing any.

This is a common misconception

Half right. It is still emitting, like every object. But it hasn't stopped absorbing: radiation from the room still reaches it and is absorbed. A steady temperature tells you the two are happening at the same rate.

It's absorbing radiation from the room, but it's too cool now to emit any.

This is a common misconception

This is the idea the curve was built to catch. There's no 'too cool to emit': all bodies emit radiation. A cooler mug emits less, but it still emits. Right now it absorbs radiation at the same rate as it emits it.

It's absorbing and emitting radiation at the same rate.

This matches the evidence

Exactly. Both processes are still going on, all the time. Because the mug absorbs radiation at the same rate as it emits radiation, it gains energy as fast as it loses it, so its temperature stays constant.

A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. Steady means balanced, not stopped.

Higher

Radiation at the Earth

Follow the radiation

View: Both together. Showing 3 layers: Space, atmosphere and surface, Radiation arriving, Radiation emitted

View

Layers

Explore

Tap an arrow to find out what it shows.

Radiation arriving and radiation leaving, at the same time. The Earth's temperature depends on how these compare.

Switch between the views, then tap each part of the picture to see what it shows.

Higher

What sets the Earth's temperature?

Build the web

Add the arrows you think belong here: each one means 'this affects that'. Then compare your web with the lesson's.

Radiation reflected into space
Rate the Earth absorbs radiation
Rate the Earth emits radiation
The Earth's temperature (outcome)

Add a link

This factor……made this happen or worse

No links yet.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Perfect black bodies and the temperature of the Earth

What you need to know

All bodies (objects) emit radiation, whatever their temperature.
The intensity and wavelength distribution of the radiation a body emits depend on its temperature.
A perfect black body absorbs all the radiation incident on it, so it would be the best possible emitter.
Higher: a body's temperature depends on the balance between the radiation it absorbs and the radiation it emits.
Higher: the Earth's temperature depends on the rates of absorption and emission of radiation and on reflection of radiation into space.

The big picture

All bodies emit radiation, whatever their temperature. Temperature sets the intensity and wavelength distribution: a hotter body emits more at every wavelength, with its peak at a shorter wavelength. A perfect black body absorbs all the radiation that falls on it, and since a good absorber is also a good emitter, it would be the best possible emitter. Higher: a temperature stays constant when a body absorbs and emits radiation at the same rate, and rises when it absorbs faster. The Earth's temperature depends on those rates and on how much radiation is reflected into space.

Key points

1Every body emits radiation all the time. Cold objects still emit; they just emit less.

2As a body gets hotter, it emits more radiation at every wavelength, and the peak of its emission moves to shorter wavelengths.

3A perfect black body absorbs all the radiation incident on it. A good absorber is also a good emitter, so it would be the best possible emitter at its temperature.

4Higher: a body at constant temperature absorbs radiation at the same rate as it emits it. Its temperature rises when it absorbs faster than it emits, and falls when it emits faster than it absorbs.

5Higher: a hotter body emits more, so a warming body's rate of emission rises until it matches its rate of absorption, and the temperature levels off.

6Higher: radiation reaching the Earth is partly reflected into space and partly absorbed by the atmosphere and surface; the surface and atmosphere both emit radiation, some of which leaves into space.

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Worked example

Problem

A small satellite moves out of the Earth's shadow into sunlight. Its temperature rises for a while, then stops rising and stays constant at a higher value. Assume radiation is the only way the satellite gains or loses energy. Explain these changes in terms of the radiation it absorbs and emits.

⚠ Watch out

Thinking that an object at a steady temperature has stopped absorbing and emitting radiation, or that a cold object gives out no radiation at all. Every object emits radiation, and a steady temperature means it is absorbing and emitting at the same rate.

🧠

Memory hook

Hotter means taller and shorter: the curve grows taller, and its peak slides to shorter wavelengths. Steady means in equals out.

✓

Check yourself

Cover the page. Name the two things about an object's radiation that change as it heats up, and say why a perfect black body would be the best emitter. Higher: what keeps a temperature steady?

Flashcards

(13)

Which bodies emit radiation?

All of them, whatever their temperature. Even cold objects emit radiation.

Which two features of the radiation a body emits depend on its temperature?

Its intensity and its wavelength distribution.

A body gets hotter. What happens to the intensity of the radiation it emits?

It increases: the body emits more radiation at every wavelength.

A body gets hotter. What happens to the peak of its emission curve?

It moves to a shorter wavelength (and gets higher).

What is a perfect black body?

An object that absorbs all of the radiation incident on it.

What does 'incident' radiation mean?

Radiation that arrives at, or falls on, the object.

What links a good absorber to a good emitter?

A good absorber is also a good emitter, so the best possible absorber would be the best possible emitter.

Higher: what is true of a body whose temperature is constant?

It is absorbing radiation at the same rate as it is emitting radiation.

Higher: a body takes in radiation faster than it gives radiation out. What happens to its temperature?

It goes up.

Higher: why does a warming body's temperature eventually level off?

Hotter bodies emit more, so its rate of emission rises until it equals its rate of absorption.

Higher: name the factors from this lesson that the Earth's temperature depends on.

The rates of absorption and emission of radiation, and reflection of radiation into space.

Higher: what happens to the Sun's radiation when it reaches the Earth?

Some is reflected back into space; the rest is absorbed by the atmosphere and the surface.

Higher: how does the Earth lose energy by radiation?

Its surface and atmosphere emit radiation, and some of it leaves into space.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Perfect black bodies and Earth's temperature (physics only)

Heat it up. Watch what it gives out.

The mug that stopped cooling

Build the web

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