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Theme B — The Particulate Nature of Matter

B.1.12 Thermal Radiation — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.1.12 Thermal Radiation — IB PHYSICS SL notes: Describe thermal radiation and black-body radiation, compare emitters and absorbers and relate them to the temperature of an object.Thermal radiation = heat transfer by electromagnetic radiation, normally in the infrared region.All …

  • All objects
  • Hotter objects
  • No medium needed
  • Surface colour
  • Surface area
  • Leslie cube

Part 1 of 8Thermal radiation

Thermal radiation
Thermal radiation heat transfer by electromagnetic radiation, normally in the infrared region.
All objects
  • All objects every object above absolute zero emits a spectrum of thermal radiation, whatever its temperature.
Cause
  • Cause electric charges within atoms vibrate, and the thermal motion rises with temperature.
Hotter objects
  • Hotter objects they radiate more infrared in a given time.
No medium needed
  • No medium needed it is the only method of transfer that works through a vacuum, which is how the Sun's energy reaches Earth.
Speed
  • Speed it travels at the speed of light, c = 3.00 × 108 m s−1, and can be reflected, absorbed or transmitted.

Part 2 of 8Factors affecting emission

Surface colour
  • Surface colour dark, dull objects are better emitters and absorbers.
    • (black emits more)
Texture
  • Texture light, shiny surfaces are worse emitters and absorbers and are good reflectors.
Surface area
  • Surface area a greater area emits more radiation.
Temperature
  • Temperature a higher temperature emits more.
Leslie cube
  • Leslie cube the black face emits more infrared than the shiny face of the same hot cube.
Applications
  • Applications a black car roof gets hot, emergency blankets are shiny and a flask has silvered walls.
Solar heaters
  • Solar heaters black panels absorb as much as possible.

Part 3 of 8Emission and absorption together

Net effect
  • Net effect an object absorbs more than it emits and warms, emits more and cools, or has equal rates and stays at constant temperature.
Equilibrium in sunlight
  • Equilibrium in sunlight an object warms until the power radiated equals the power absorbed.
Matt black roof
  • Matt black roof it reaches a higher temperature than a white one but also cools quickly at night.
Shiny cylinder
  • Shiny cylinder a shiny cylinder of hot water loses less by radiation than a black one.
Meteorite on the Moon
  • Meteorite on the Moon it loses energy by conduction and radiation only, because the Moon has no atmosphere for convection.

Part 4 of 8Black bodies

Perfect black body
Perfect black body an object that absorbs all the radiation incident on it and does not reflect or transmit any.
Best emitter
  • Best emitter a good absorber is also a good emitter, so a black body is the best possible emitter.
Approximation
  • Approximation a small hole in a hollow blackened cavity, the Sun and stars.
Colour
  • Colour it appears black because all colours are absorbed.
Emission spectrum
  • Emission spectrum a continuous curve of intensity against wavelength whose shape depends only on the temperature.

Part 5 of 8Black-body curves

Black-body curves
Higher temperature
  • Higher temperature the peak moves to a shorter wavelength and a higher intensity, and the intensity rises at every wavelength.
Peak position by temperature
  • Peak position by temperature around 300 K the peak is in the infrared, around 1000 °C the object glows red, around 6000 °C the peak is in the visible,
    • and still hotter objects emit ultraviolet or X-rays.
Shorter wavelength
  • Shorter wavelength waves of smaller wavelength carry more energy per photon.

Part 6 of 8Comparison of the three mechanisms

MechanismMedium neededHow it works
ConductionYes (mostly solids)Particle vibrations and free electrons
ConvectionYes (fluids)Bulk movement of fluid
RadiationNoElectromagnetic waves
  • Question cues
    • colour or a shiny surface suggests radiation; a vacuum means radiation only.

Part 7 of 8Link to the greenhouse effect

Sun's radiation
  • Sun's radiation mostly visible and short wavelength, passing through the atmosphere.
Earth's surface
  • Earth's surface it absorbs it and re-emits longer-wavelength infrared.
Greenhouse gases
  • Greenhouse gases they absorb this infrared and re-emit it in random directions, some returning to the surface.
Without the effect
  • Without the effect the Earth would average about 255 K (−18 °C) instead of about 288 K.
Distinction
  • Distinction this effect is not the same as ozone depletion.

Part 8 of 8More from the specification

1
Specific Latent Heat of Fusion and Vaporisation
  • Specific latent heat of vaporisation is the thermal energy needed per unit mass to convert a liquid into a gas at constant temperature;
    • supplied energy breaks intermolecular bonds between molecules,
  • increasing potential energy without increasing molecular kinetic energy.
2
Phase Changes and Temperature Plateaus
  • When a substance undergoes cooling or heating phase changes, a temperature-time graph exhibits flat plateaus where temperature remains constant while latent heat is released or absorbed.
3
Calorimetric Heat Transfer
  • When a hot solid (such as copper) is immersed in a liquid, thermal energy flows from the metal to the liquid according to conservation of energy: mc·cc·ΔTc = mliq·cliq·ΔTliq + mvap·Lv .
    • (if vaporisation occurs)
4
Density
  • Mass per unit volume.
5
Fusion
  • Joining of light nuclei into a heavier one.
6
Kinetic energy
  • K = ½mv2, energy of motion.
7
Apparent brightness
  • The power received per unit area at the observer
8
Phase change
  • Change of matter from one state to another
9
Thermal conductivity
  • Material constant relating the rate of conduction to the temperature gradient and area
10
Internal energy
  • Total kinetic and potential energy of the particles in a system
11
Potential energy
  • Energy stored by the configuration of a system.
12
Liquid
  • State with a fixed volume that takes the shape of its container.
13
Copper
  • Metal widely used for electrical wires.
14
Molecule
  • Tiny particle of the fluid, too small to be seen.
15
Luminosity
  • The total power of radiation emitted by a star.
16
Capacity
  • Charge a cell can supply, in A h or C.

Key terms

TermMeaning
Thermal radiationHeat transfer by electromagnetic waves, normally infrared
Black bodyObject that absorbs all radiation falling on it and emits the maximum possible
EmitterSurface that radiates electromagnetic waves
AbsorberSurface that takes in radiation falling on it
Leslie cubeHot cube with differently finished faces used to compare thermal emission
InfraredElectromagnetic radiation of wavelength longer than visible light

Avoid these mistakes

1
Saying radiation needs a medium — it travels through a vacuum.
2
Saying shiny surfaces are good emitters — dark, matt surfaces emit best.
3
Saying cold objects emit no radiation — any object above 0 K emits.
4
Confusing a good absorber with a good reflector — they are opposites.
5
Saying a black body appears black at all temperatures — it glows when hot.

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