Thermal Radiation (Edexcel GCSE Physics): Revision Note

Thermal Radiation

• All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves

  • These electromagnetic waves usually lie in the infrared region of the spectrum but could be emitted in the form of visible light or other wavelengths, depending on the temperature

• The hotter object, the more infrared radiation it radiates in a given time

The infrared radiation emitted from a hot object can be detected using a special camera

• The intensity and wavelength distribution of any emitted waves depends on the temperature of the body

• This can be represented on a thermal radiation curve

  • As the temperature increases, the peak of the curve moves

  • This moves to a lower wavelength and a higher intensity

The peak of a thermal radiation curve moves to the left with increasing temperature

• From the electromagnetic spectrum, waves with a smaller wavelength have higher energy (e.g. UV rays, X-rays)

• When an object gets hotter, the amount of thermal radiation it emits increases

• This increases the energy emitted and therefore the wavelength of the emitted radiation decreases

  • At room temperature objects emit thermal radiation in the infrared region of the spectrum

  • At around 1000 °C an object will emit a significant amount of red light

  • At 6000 °C an object will mainly emit white or blue light (and some ultraviolet)

  • At even higher temperatures objects will emit ultraviolet or even X-rays

Thermal Equilibrium

Higher Tier Only

• 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

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

Radiation & Temperature

Higher Tier Only

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

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

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

• This is how an emergency blanket works, 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

The reflective nature of a emergency blanket helps to keep a trauma patient warm