GCSE · Physics · Edexcel · Spec 1PH0

All bodies emit radiation: temperature dependence

Right now you are giving out infrared radiation. So is your desk, your phone and the ice in your drink. Nothing is switched off.

Turn up the temperature

Temperature setting (1 = coolest, 5 = hottest) 1

Temperature setting (1 = coolest, 5 = hottest): 1. Visible light given out (curve height at the far left): 0.0

Slide the temperature setting from 1 to 5. Every setting is a body giving out radiation. The curve shows how intense each wavelength is. Drawn for shape, not to scale.

Watch out: Setting 1 is not "off". Its curve is low, but it is still a body giving out infrared. Only the amount and the wavelengths change.

Physics · Radiation

Which of these sounds like you?

A glass of ice water sits on a table in a warm room.

Be honest. Which idea is closest to what you thought before this lesson?

Only hot things give out radiation. Something cold, like the ice, gives out nothing.Cold things give out "cold", and that is what you feel when you stand near them.Every object gives out infrared radiation all the time, and absorbs it all the time too, whatever its temperature.An object stops giving out radiation once it is the same temperature as the room around it.

How sure are you?

I'm sureI think soI'm guessing

Only hot things give out radiation. Something cold, like the ice, gives out nothing.

This is a common misconception

Easy idea to hold, because we notice radiation from hot things. But any object with a temperature is constantly emitting electromagnetic waves into its surroundings. The ice has a temperature, so it is emitting infrared right now. Cold changes how much, not whether.

Cold things give out "cold", and that is what you feel when you stand near them.

This is a common misconception

There is no "cold" being given out. What every body emits is infrared radiation: electromagnetic waves. A cooler body emits less of it every second than a hotter one. Nothing flows out that is the opposite of heat.

Every object gives out infrared radiation all the time, and absorbs it all the time too, whatever its temperature.

This matches the evidence

That is it. All bodies, whatever their temperature, emit and absorb infrared radiation all the time. Temperature changes how much is emitted and which wavelengths, never whether emission happens.

An object stops giving out radiation once it is the same temperature as the room around it.

This is a common misconception

Emission never switches off. Bodies emit and absorb infrared all the time, whatever their temperature, so there is no temperature at which an object goes quiet.

Emitting is always on. Temperature only changes how much, and which wavelengths.

Physics · Radiation

Put the peaks in order

Rank four bodies by the wavelength of their most intense radiation, shortest first.

1 · Shortest wavelength of most intense radiation4 · Longest wavelength of most intense radiation

Body X
Warmer than Body W, cooler than Body Y.
Body Z
So hot that it glows.
Body W
The coolest of the four. Still emitting.
Body Y
Hotter than Body X, but not hot enough to glow.

Physics · Radiation

Where is the glow line?

What is each body emitting? Decide, then check your reason.

Still to sort

Emits mainly infrared (0)

At lower temperatures the wavelengths emitted are mainly infrared.

Where the line is: Still emitting plenty. It just has no visible glow.

Glowing: visible as well as infrared (0)

Hot enough to glow.

Where the line is: Visible wavelengths join the infrared. The infrared does not stop.

6 of 6 still to sort.

Physics · Radiation

Now say it in words

Describe how the radiation emitted by a body changes as its temperature rises, up to the point where it glows. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

It emits more radiation every second. (1 mark)
The intensity of every wavelength it emits increases. (1 mark)
The range of wavelengths gets wider, including shorter wavelengths that were not emitted at the lower temperature. (1 mark)
The wavelength of the most intense radiation (the peak) decreases: it moves to shorter wavelengths. (1 mark)
When it is hot enough it glows: it emits visible wavelengths as well as infrared. (1 mark)

Model answer

As the temperature of the body rises it emits more radiation every second. The intensity of every wavelength it emits increases, and the range of wavelengths gets wider to include shorter wavelengths that were not emitted at the lower temperature. The wavelength of the most intense radiation gets shorter, so the peak moves towards shorter wavelengths. When the body is hot enough it glows, emitting visible wavelengths as well as infrared.

Five changes, one story: more, brighter everywhere, wider, shifted shorter, and finally a glow.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Even you. Turn up the temperature and watch what changes.

What you need to know

Every object gives out infrared radiation all the time, whatever its temperature. Yes, even ice.
Objects absorb infrared all the time too. Neither emitting nor absorbing ever switches off.
Have a goA classmate says: "That ice cube is freezing, so it is giving out zero infrared." Right or wrong?
Wrong. The ice is emitting infrared right now, like every object.
Emitting never switches off at any temperature, so cold cannot mean "gives out nothing".
Infrared is electromagnetic waves with frequencies just lower than red light.
Hotter bodies emit more infrared every second.
Have a goYou switch on an electric hob. As the ring heats up from cold, what happens to the infrared it gives out each second?
It increases.
The hotter a body gets, the more infrared it emits in every second.
A body emits a whole range of wavelengths, not just one, and its temperature decides how intense each one is.
Raise the temperature and every wavelength it emits gets more intense.
The range widens too, adding shorter wavelengths that weren't emitted at the lower temperature.
The peak, the wavelength of the most intense radiation, moves to shorter wavelengths.
Have a goBody A has its peak at 3 units on the wavelength scale. It is heated. Which is the better guess for its new peak: 2 units or 5 units?
2 units.
Heating moves the peak towards shorter wavelengths, and 2 is shorter than 3.
At lower temperatures, what a body emits is mainly infrared.
Get it hot enough and it glows: visible wavelengths join the infrared.

The big picture

Every body emits and absorbs infrared radiation all the time, whatever its temperature. Raise the temperature and the body emits more every second, every wavelength gets more intense, the range stretches to include shorter wavelengths, and the peak moves to shorter wavelengths. Get it hot enough and it glows: visible wavelengths as well as infrared.

Key points

1All bodies emit and absorb infrared radiation all the time, whatever their temperature.

2Infrared is electromagnetic waves with frequencies just lower than red light.

3Raising a body's temperature: more radiation emitted every second, and more intensity at every wavelength.

4Raising the temperature also widens the range to shorter wavelengths and moves the peak to shorter wavelengths.

5At lower temperatures the radiation is mainly infrared. When a body is hot enough it glows, emitting visible wavelengths as well as infrared.

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Worked example

Problem

Two bodies are drawn on the same axes. The most intense radiation from Body P has a wavelength of 4 units. The most intense radiation from Body Q has a wavelength of 2 units. Which body is hotter, and which emits more radiation every second?

⚠ Watch out

Thinking only hot objects give out radiation, so a cold object gives out nothing (or gives out "cold"). Every body emits infrared all the time. Temperature changes how much and which wavelengths, never whether.

🧠

Memory hook

Everything glows in infrared all the time. Heating it turns the whole curve up, stretches it to the left and slides its peak left, until the left edge reaches visible light and you see the glow.

✓

Check yourself

Close your eyes (or cover the screen). Name the five things that change when a body gets hotter, then say what a cold object is still doing.

Flashcards

(11)

Does a cold object give out infrared radiation?

Yes. All bodies, whatever their temperature, emit and absorb infrared radiation all the time.

What is infrared radiation?

Electromagnetic waves with frequencies just lower than those of red light.

A body gets hotter. What happens to the radiation it emits every second?

It increases. Hotter bodies emit more infrared in a set time.

Does a body emit just one wavelength?

No. It emits a range of wavelengths, and the range and their intensities depend on its temperature.

A body gets hotter. What happens to the intensity at each wavelength it emits?

The intensity of every wavelength emitted increases.

A body gets hotter. What happens to the range of wavelengths it emits?

It gets wider, to include shorter wavelengths that were not emitted at the lower temperature.

What is the "peak" of a body's emission?

The wavelength of the most intense radiation it emits.

A body gets hotter. Which way does the peak move?

To shorter wavelengths: the wavelength of the most intense radiation decreases.

What does a body emit at lower temperatures?

Wavelengths that are mainly infrared.

When does a body glow?

When it is hot enough to emit visible wavelengths as well as infrared.

Body Q is hotter than Body R. Which has its peak at the shorter wavelength?

Q. Hotter means a shorter peak wavelength.

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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Turn up the temperature

Which of these sounds like you?

Put the peaks in order

Where is the glow line?

Now say it in words

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