Black Body Radiation

• What do all objects do?

  • Emit and absorb infrared radiation
  • no matter what temperature
• Another term for infrared radiation

  • thermal radiation
• The hotter the body (object) :

  • the more infrared radiation it radiates in a given time
  • greater amount of shorter-wavelength radiation released ( e.g x-rays, gamma)
• A perfect black body (2 marks)

  • An object that absorbs all the radiation incident (exposed) to it.
  • does not transmit or reflect radiation
• What does a black body NOT do?

  • Reflect or transmit any radiation
• A good absorber is also a..

  • Good emitter
• A perfect black body can be

  • The best possible emitter
• What does the amount + type of radiation emitted depend on?

  • object's temperature
• Intensity of wavelength emitted

  • how much energy is transferred to a given area in a certain amount of time
• Intensity equation

  • power per unit area
  • P/A
• How does higher temperatures affect intensity?

  • As the temperature of an object increases, the intensity of every emitted wavelength increases.
• Distribution of wavelength emitted

  • Distribution of wavelengths emitted by an object depends on the object's temperature
• Peak wavelength distribution

  • As the temperature of an object increases, the peak wavelength increases.

  
• Hotter objects

  • emit a greater intensity of every wavelength
• Describe how the intensity and distribution of wavelengths emitted by an object changes as the object's temperature increases

  • intensity of every wavelength increases
  • peak wavelength increases
• Temperature of an object depends on

  • amount of heat energy transferred to it
• Heat energy increases

  • when energy is transferred to the object
• Heat energy decreases

  • when the object emits radiation
• A body at constant temperature

  • absorbing radiation at the same rate it is emitting radiation
• An object emitting more radiation..

  ..than absorbing radiation

  will DECREASE in temperature
• An object absorbing more radiation..

  ...than emitting more radiation

  will increase in temperature
• Temperature of the Earth

  • rate of absorption of radiation
  • rate of emission of radiation
  • amount of radiation reflected back into space
• What may radiation be absorbed or reflected by?

  • the atmosphere (gases)
  • clouds
  • surface of the Earth
• What causes the internal energy of Earth to increase?

  • solar, light and infrared radiation absorbed by the earth
  • the surface of the earth increases in temperature
• How may emitted infrared radiation travel?

  • travel through the atmosphere and back into space
  • absorbed and reflected by greenhouse gases in the earth's atmosphere

  
• Peak of wavelength of a hotter object

  • further to the left
• An metal ball is heated by a flame at 4 J/s. It emits radiation at a rate of 2 J/s. Explain what happens to the temperature of the metal ball.

  • metal is gaining more heat than it is radiating
  • overall heat energy of the ball increases
  • metal ball gets hotter
• The best emitters of infrared radiation have

  • dark matt surfaces
• The worst emitters of infrared radiation have

  • white shiny surfaces
• Thermal imaging cameras work better at night than during the day. Explain why.

  • cooler in the night
  • so surroundings emit less infrared radiation than in daytime
• Light shiny surfaces

  • poor emitters
  • poor absorbers
  • of infrared radiation
• Shiny Teapot - Light Shiny surface

  • Poor emitter of infrared radiation
  • Teapot stays hotter for longer
• Light shiny surfaces

  • good reflectors of infrared radiation
• When an object has reached room temperature

  • emit and absorb infrared radiation at the same rate
• If a lot of energy is being emitted to shiny light surfaces..

  • will reflect most of the infrared radiation
  • will take a long time for the shiny light surface to heat up
• Surfaces

  • colours
  • texture
• Why are cooling fins in the back of a fridge black?

  • good emitters of infrared radiation
  • Maximise energy transfer to the surroundings
• Why does at night the air at point A cools faster than the air at point B?

  • at night, more radiation is emitted from the Earth than absorbed from space.
  • cloud reflects radiation towards the Earth which allow solar radiation for radiation at A
  • there is no cloud cover so a larger proportion of radiation will be emitted into space
• Why need different models?

  • Different models appropriate for different situations
• Why may some theories be replaced?

  • New evidence
  • evidence cannot be explained using an existing model
  • New model explains new evidence