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Physics · Waves

Black body radiation

Explain that all objects emit infrared radiation, describe perfect black bodies, explain how temperature affects emission, and (Higher tier) explain the balance of radiation that decides the temperature of a body or the Earth (Physics only; Required Practical 10).

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    What is infrared?

    Show answerHide answer

    Electromagnetic radiation felt as heat

  2. 2

    What makes an object hotter?

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    Absorbing energy faster than it emits it

  3. 3

    What is absorption?

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    Taking in the energy of radiation

  4. 4

    What is reflection?

    Show answerHide answer

    A wave bouncing off a surface

  5. 5

    What colour surface absorbs best?

    Show answerHide answer

    Matt black

Learning Objectives

  1. 1State that all bodies emit and absorb infrared radiation.
  2. 2Describe a perfect black body and why it is the best emitter.
  3. 3Explain that the intensity and wavelength distribution of emission depend on temperature.
  4. 4Explain how the temperature of a body or of the Earth is related to absorption and emission (Higher tier).

BLACK BODY RADIATION

All bodies, whatever their temperature, emit and absorb infrared radiation. The hotter the body, the more infrared it radiates in a given time.

A perfect black body absorbs all the radiation that falls on it and is also the best possible emitter.

Required Practical 10: Method

Compare how surfaces radiate infrared.

  1. 1 Fill the cube

    Fill a Leslie cube with hot water and put in the lid.
  2. 2 Detector

    Hold the infrared detector at the same distance from each face.
  3. 3 Record

    Note the detector reading for each surface.
  4. 4 Control

    Keep the distance, water temperature and time the same.
  5. 5 Result

    The matt black surface emits most infrared; the shiny silver surface emits least.

A Body at Constant Temperature (Higher)

Explain why a body at a constant temperature is absorbing and emitting radiation at the same rate.

Show the solutionHide the solution
  1. 1 Absorbing more than emitting The temperature rises
  2. 2 Emitting more than absorbing The temperature falls
  3. 3 Constant temperature Absorption equals emission

AnswerIf the rates are equal the temperature does not change.

The Earth's Temperature (Higher)

Explain what happens to the temperature of the Earth's surface if the atmosphere absorbs more of the infrared radiation emitted by the surface.

Show the solutionHide the solution
  1. 1 More is absorbed And re-radiated back to the surface
  2. 2 Emission to space Decreases
  3. 3 Absorption exceeds emission So the surface temperature rises until a new balance

AnswerThe temperature of the surface increases.

Key Points

Learn these.

  • All bodies

    Emit infrared, whatever their temperature.

  • Hotter

    More radiation in a given time and a shorter peak wavelength.

  • Good absorber

    Also a good emitter (matt black surfaces).

  • Good reflector

    A poor absorber and a poor emitter (shiny silver).

Hot or Cold?

Two cans of hot water, one matt black and one shiny silver, cool for 10 minutes. Which cools faster? Explain.

1. Compare emission.

2. Use the black body idea.

A good answer shows: The matt black can, because it is a better emitter of infrared radiation, so it loses energy faster.

Can I...?

  1. 1State that all bodies emit infrared.
  2. 2Define a perfect black body.
  3. 3Say a good absorber is a good emitter.
  4. 4Describe Required Practical 10.
  5. 5Explain the effect of temperature.
  6. 6Describe the curves.
  7. 7Explain constant temperature.
  8. 8Explain the Earth's energy balance.

Summary & Exam Focus

  • All bodies emit and absorb infrared.
  • A perfect black body absorbs all radiation and is the best emitter.
  • Hotter bodies emit more with shorter wavelengths.
  • Temperature is steady when absorption equals emission.

Exam focus

Explain why a matt black surface is a better emitter of infrared radiation than a shiny silver surface. (2 marks) (2 marks)

Good absorbers are good emitters.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Black body
An object that absorbs all the radiation that hits it.
Emit
Give out radiation.
Absorb
Take in radiation.
Intensity
The amount of radiation per second per unit area.
Infrared
Electromagnetic radiation emitted by all objects.
Equilibrium
When absorption equals emission.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 State 2 marks

    State whether a matt black surface or a shiny silver surface is a better emitter of infrared radiation, and give the reason.

    Show answerHide answer

    Model answer

    A matt black surface, because a good absorber is also a good emitter.

    Mark scheme

    • Matt black — 1 mark
    • Good absorber is a good emitter — 1 mark
  2. Question 2 Describe 4 marks

    A student investigates how the surface of a Leslie cube affects the infrared radiation emitted. Describe how the investigation is carried out and how to keep it a fair test.

    Show answerHide answer

    Model answer

    Fill a Leslie cube with hot water. Use an infrared detector to measure the radiation from each of the different faces. Keep the detector at the same distance from each face and keep the water temperature the same. Record the readings; the matt black face should give the greatest reading.

    Mark scheme

    • Hot water in a Leslie cube — 1 mark
    • Detector at each face — 1 mark
    • Control distance and temperature — 1 mark
    • Compare readings — 1 mark
  3. Question 3 Use the graph 3 marks

    The graph shows how the intensity of radiation emitted by a hot object depends on the wavelength at two different temperatures, A and B. (a) Which curve is for the hotter object? (b) Give two reasons for your answer.

    Two emission curves, A lower with a longer peak wavelength and B higher with a shorter peak wavelength.
    Show answerHide answer

    Model answer

    (a) Curve B. (b) It has a higher intensity (more radiation emitted), and its peak is at a shorter wavelength.

    Mark scheme

    • Curve B — 1 mark
    • Higher intensity — 1 mark
    • Peak at shorter wavelength — 1 mark
  4. Question 4 Explain 3 marks

    The temperature of a cup of tea stays constant at 50 °C when it is kept warm by a heater. Explain this in terms of radiation.

    Show answerHide answer

    Model answer

    The tea absorbs radiation and energy from the heater at the same rate as it emits radiation to the surroundings, so its temperature stays constant.

    Mark scheme

    • Absorbs energy at some rate — 1 mark
    • Emits at the same rate — 1 mark
    • So constant temperature — 1 mark
  5. Question 5 Explain 4 marks

    Explain how the Earth's temperature is related to the radiation it absorbs and emits. Explain what happens if the atmosphere contains more greenhouse gases.

    Show answerHide answer

    Model answer

    The Earth's temperature is constant when it absorbs radiation from the Sun at the same rate as it emits infrared to space. More greenhouse gases absorb more of the emitted infrared and send some back, so the surface absorbs more than it emits and its temperature rises.

    Mark scheme

    • Temperature depends on absorption and emission — 1 mark
    • Constant when equal — 1 mark
    • Greenhouse gases absorb emitted infrared — 1 mark
    • So the temperature increases — 1 mark

Quick check

  1. All objects emit...

    1. Ainfrared radiation
    2. Bonly light
    3. Conly sound
    4. Dno radiation
    Show answerHide answer

    A: infrared radiation

    At any temperature.

  2. A perfect black body is a perfect...

    1. Areflector
    2. Babsorber and emitter
    3. Cinsulator
    4. Dtransmitter
    Show answerHide answer

    B: absorber and emitter

    It absorbs all radiation.

  3. A hotter object emits...

    1. Aless radiation
    2. Bno radiation
    3. Cmore radiation
    4. Dsound only
    Show answerHide answer

    C: more radiation

    Greater intensity.

  4. Which surface emits infrared best?

    1. AShiny silver
    2. BShiny white
    3. CMirror
    4. DMatt black
    Show answerHide answer

    D: Matt black

    Good absorber, good emitter.

  5. A body at constant temperature absorbs radiation...

    1. Aat the same rate as it emits
    2. Bfaster than it emits
    3. Cslower than it emits
    4. Dnot at all
    Show answerHide answer

    A: at the same rate as it emits

    Balance.

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