Radiation (Cambridge O Level Physics)

• All objects give off thermal radiation
  • The hotter an object is, the more thermal radiation it emits
  • Thermal radiation is the part of the electromagnetic spectrum called infrared
• 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
• The colour of an object affects how good it is at emitting and absorbing thermal radiation:

Thermal Equilibrium

• As an object absorbs thermal radiation it will become hotter
• As it gets hotter it will also emit more thermal radiation
  • The temperature of a body increases when the body absorbs radiation faster than it emits radiation

• Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it is emitting radiation
  • At this point, the object will be in thermal equilibrium

Radiation in Equilibrium

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

• If the rate at which an object receives energy is less than the rate at which it transfers energy away then the object will cool down

• If the rate at which an object transfers energy away is less than the rate at which it receives energy then the object will heat up

• The process will always move towards thermal equilibrium

• The amount of thermal radiation emitted by an object depends on a number of factors:
  • The surface colour of the object (black = more radiation)
  • The texture of the surface (shiny surfaces = more radiation)
  • The surface area of the object (greater surface area = more area for radiation to be emitted from)

Absorbing and Emitting Powers of Different Coloured Objects

Colour	Absorbing	Emitting
Black	Good absorber	Good emitter
Dull / dark	Reasonable absorber	Reasonable emitter
White	Poor absorber	Poor emitter
Shiny	Very poor absorber (reflects radiation)	Very poor emitter

• 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 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

 

Leslie Cube Emission

An image of a hot object taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface