Radiation (Cambridge (CIE) IGCSE Physics)

Thermal radiation

What is thermal radiation?

• All objects give off thermal radiation

  • The hotter an object is, the more thermal radiation it emits

  • Thermal radiation is infrared radiation (part of the electromagnetic spectrum)

• Thermal radiation is the only way in which heat can travel through a vacuum

  • It is the way in which heat reaches us from the Sun through the vacuum of space

  • Thermal radiation does not need a medium to travel, unlike convection and conduction

Effects of different surfaces

What two factors affect the rate of thermal energy transfer?

• The amount of thermal radiation emitted by an object depends on a number of factors:

  • The surface colour of the object (black = more radiation emitted and absorbed)

  • The texture of the surface (shiny surfaces = less radiation emitted and absorbed)

Table of the effect of coloured surfaces on absorbing and emitting ability

• Black objects are very good at absorbing thermal radiation, for example black clothes make you feel hotter in sunny weather

  • Black objects are also very good at emitting thermal radiation, which is the reason that some chargers for laptops, and radiators in cars are coloured black - it helps them to cool down

• Shiny objects reflect thermal radiation and so absorb very little

  • They also emit very little, though, and so take longer to cool down

The infrared emitted from a hot object can be detected using a special camera. The dull black side of the cube (left) is seen to glow brighter with infrared radiation that the shiny light side of the cube (right)

Thermal equilibrium

Extended tier only

• As an object absorbs thermal radiation it will become hotter

  • As it gets hotter it will also emit more thermal radiation

• Eventually, an object will reach a point where it is absorbing radiation at the same rate as it is emitting radiation

  • At this point, the object will be in thermal equilibrium

  • At thermal equilibrium, an object has constant temperature

An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat

Heating & cooling by energy transfer

Extended tier only

• If the rate at which an object receives energy is greater than the rate at which it transfers energy away:

  • then the object's temperature will increase

• If the rate at which an object transfers energy away is greater than the rate at which it receives energy:

  • then the object's temperature will decrease

• The process will always move towards thermal equilibrium

  • As temperature increases, the object emits more radiation until the rates are equal

  • As temperature decreases, the object transfers less energy until the rates are equal