Temperature Balances (AQA GCSE Physics)

Absorption & Emission of Radiation

• 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 equilibrium

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

• If, however, the object starts to absorb radiation again at a higher rate than it radiates it, then the object will heat up again

  • Likewise, if it loses radiation at a greater rate than it absorbs it, then the object will cool down again

• The temperature of a body can be regulated by balancing how much incoming radiation is absorbed and emitted (or reflected)

• This is used for emergency blankets, to keep a trauma victim warm:

  • Rescue teams use light-coloured, shiny emergency blankets to keep accident survivors warm

  • A light, shiny outer surface emits a lot less radiation than a dark, matt (non-glossy) surface

  • This keeps the patient warm, as less infrared radiation is emitted than if an ordinary blanket had been used

Emergency blankets keep a patient warm by emitting less of their body heat through it

Temperature of the Earth

• If the Earth had no atmosphere, the temperature on the surface would drop to about −180 °C at night, the same as the Moon’s surface at night

  • This would happen because the surface would be emitting all the radiation from the Sun into space

• The Earth receives the majority of its heat in the form of thermal radiation from the Sun

  • At the same time, the Earth emits its own thermal radiation, with a slightly longer wavelength than the thermal radiation it receives (the surface temperature of the Earth is significantly smaller than the surface temperature of the Sun)

• Some gases in the atmosphere, such as water vapour, methane, and carbon dioxide (greenhouse gases) absorb and reflect back longer-wavelength infrared radiation from the Earth and prevent it from escaping into space

  • These gases absorb the radiation and then emit it back to the surface

• This process makes the Earth warmer than it would be if these gases were not in its atmosphere

The Earth receives thermal radiation from the Sun but emits its own thermal radiation at the same time

• The temperature of the Earth, therefore, depends on several factors, such as the rate that light and infrared radiation from the Sun are:

  • Reflected back into space

  • Absorbed by the Earth’s atmosphere or by the Earth’s surface

  • Emitted from the Earth’s surface and from the Earth’s atmosphere into space

The Greenhouse Effect

• The rate of absorption and emission of radiation on Earth contributes to the Greenhouse Effect

  • This is the natural process that warms the Earth's surface from the Sun

• The Sun's thermal radiation reaches the Earth's atmosphere where:

  • Some radiation is reflected back to space

  • Any radiation not reflected is absorbed and re-radiated by greenhouse gases

• The absorbed radiation then warms the atmosphere and the surface of the Earth

  • This is similar to what happens in a greenhouse to keep a humid, and warm temperature to grow plants