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2.3.3Thermal Physics

Radiation — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 36 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.3.3 Radiation — IGCSE PHYSICS notes: Describe thermal radiation, how surface colour and texture affect it, thermal equilibrium and the greenhouse effect.Source — all objects give off thermal radiation and hotter objects emit moreNature — infrared radiation, part of the electrom…

  • No medium needed
  • Black uses
  • Shiny surfaces
  • Both directions
  • Without greenhouse gases
  • Radiation exchange

Part 1 of 6Thermal radiation

Source
  • Source all objects give off thermal radiation and hotter objects emit more
Nature
  • Nature infrared radiation, part of the electromagnetic spectrum
Vacuum
  • Vacuum the only way thermal energy can cross a vacuum, for example Sun to Earth
No medium needed
  • No medium needed unlike conduction and convection

Part 2 of 6Effect of surface

SurfaceAbsorbingEmittingReflecting
BlackGoodGoodPoor
WhitePoorPoorGood
DullReasonableReasonablePoor
ShinyVery poorVery poorGood
Black uses
  • Black uses black clothes feel hotter in sun and black radiators and chargers cool better
Shiny surfaces
  • Shiny surfaces reflect radiation, absorb little, but also emit little so cool slowly

Part 3 of 6Thermal equilibrium

Absorbing
  • Absorbing an object that absorbs radiation gets hotter and emits more
Equilibrium
  • Equilibrium when the rate of absorbing equals the rate of emitting the temperature is constant
Both directions
  • Both directions energy still transfers both ways but the rates are equal
Imbalance
  • Receiving faster than losing gives a rising temperature
  • Losing faster than receiving gives a falling temperature

Part 4 of 6Temperature of the Earth

Without greenhouse gases
  • Without greenhouse gases the average surface temperature would be about −18 °C
Radiation exchange
  • Radiation exchange the Earth receives short-wavelength radiation from the Sun and emits longer-wavelength infrared
Factors
  • Factors the temperature depends on the rate radiation is reflected, absorbed and emitted

Part 5 of 6The greenhouse effect

Greenhouse gases
  • Greenhouse gases water vapour, methane and carbon dioxide absorb infrared from the Earth
Re-emission
  • Re-emission they emit it in all directions, some back to the surface
Effect
  • Effect the Earth is warmer than it would be without these gases
More CO2
  • More CO2 more radiation is absorbed and re-emitted to the surface, so surface temperature rises

Part 6 of 6More from the specification

More from the specification
Thermal radiation
  • Infrared energy emitted by all objects, travelling as electromagnetic waves without needing a medium.
Emitters and absorbers
  • Dark matt surfaces are good absorbers and emitters;
    • shiny light surfaces are poor absorbers and emitters, tested using a Leslie cube experiment.
Conduction in metals
  • Free electrons in the lattice of metals move and collide with ions, transferring energy faster than in other solids.
Conduction in solids
  • In non-metal solids energy passes by vibrations of particles in the lattice,
  • so they conduct poorly; space shuttle tiles are used as insulators.
Heated liquids
  • A heated liquid becomes less dense and rises, forming convection currents.
Thermal radiation
  • Infrared energy emitted by all objects, travelling as electromagnetic waves without needing a medium.
Emitters and absorbers
  • Dark matt surfaces are good absorbers and emitters;
    • shiny light surfaces are poor absorbers and emitters, tested using a Leslie cube experiment.
Conduction in metals
  • Free electrons in the lattice of metals move and collide with ions, transferring energy faster than in other solids.
Conduction in solids
  • In non-metal solids energy passes by vibrations of particles in the lattice, so they conduct poorly;
    • space shuttle tiles are used as insulators.
Heated liquids
  • A heated liquid becomes less dense and rises, forming convection currents.
Potassium permanganate
  • A purple compound used to show convection currents in water.
Liquid
  • State with a fixed volume that takes the shape of its container.
Lattice
  • The regular arrangement of atoms in a solid.
Solid
  • State with a fixed shape and a fixed volume.
Vibration
  • A repeated backwards and forwards movement.
Lattice vibrations
  • Vibrations of atoms or ions about fixed positions that pass energy on
Convection current
  • Circulating flow of fluid caused by density differences

Key terms

TermMeaning
Thermal radiationInfrared radiation emitted by all objects
EmitterSurface that gives out radiation
Thermal equilibriumConstant temperature because energy is absorbed and lost at equal rates
Greenhouse gasGas that absorbs infrared from the Earth and re-emits it

Avoid these mistakes

1
Saying shiny surfaces are good emitters — they are poor emitters and poor absorbers.
2
Saying radiation needs a medium — it travels through a vacuum.
3
Naming only pollution for fossil fuels — name carbon dioxide or another greenhouse gas.
4
Saying equilibrium means no energy transfer — transfer continues at equal rates.

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