All revision notes topics

Thermal RadiationEdexcel GCSE Physics: Revision notes

Section 1

What is thermal radiation?

All bodies (objects) emit electromagnetic radiation because of their temperature — this is called thermal radiation. The intensity (amount) and the wavelength distribution of the radiation emitted depends on the object's temperature.

  • Hotter objects emit more radiation overall
  • Hotter objects emit radiation with a shorter peak wavelength (radiation shifts towards the visible/higher-frequency end of the spectrum as temperature increases)
Key termsthermal radiation
Example

A metal poker heated in a fire first glows dull red, then orange, then white as it gets hotter — its peak emitted wavelength shortens as temperature rises.

Section 2

How is a constant temperature maintained? (Higher tier)

For a body to stay at a constant temperature, it must radiate the same average power that it absorbs from its surroundings — the rates of absorption and emission must be equal (a state of thermal equilibrium).

  • If the power radiated is less than the power absorbed, the body's temperature will rise (it is gaining more energy than it loses)
  • If the power radiated is more than the power absorbed, the body's temperature will fall (it is losing more energy than it gains)
Key termsthermal equilibrium
Exam tip

Phrase answers in terms of the balance between power absorbed and power radiated — mark schemes reward comparing the two rates, not just describing temperature change.

Section 3

How does this apply to the Earth's temperature? (Higher tier)

The Earth's temperature is determined by the balance between the incoming radiation it absorbs from the Sun and the radiation it emits back into space.

Factors that affect this balance include the amount of radiation reflected back into space (e.g. by clouds or ice), and the greenhouse effect, where certain gases in the atmosphere absorb and re-emit infrared radiation, reducing the amount of energy that escapes to space. If more energy is absorbed than emitted, the Earth's average temperature rises; if less is absorbed than emitted, it falls.

Section 4

Core Practical: how does surface nature affect radiation?

Method (Leslie's cube or similar apparatus):

  1. Use containers with different surface finishes (e.g. matt black, white/shiny) filled with hot water at the same temperature
  2. Use a thermal detector (e.g. infrared sensor or thermometer) placed the same distance from each surface to measure the radiation emitted
  3. Compare the readings for each surface

Result: matt black surfaces are the best emitters and absorbers of thermal radiation; shiny/light surfaces are the poorest emitters and absorbers (they reflect more radiation instead).

Example

A matt black teapot loses heat by radiation faster than a shiny silver one at the same temperature, because black surfaces emit thermal radiation more effectively.

Must Know

  • All bodies emit thermal radiation depending on their temperature
  • Hotter objects emit more radiation, with a shorter peak wavelength (higher tier)
  • Constant temperature requires power radiated = power absorbed (higher tier)
  • Power radiated < power absorbed → temperature rises; power radiated > power absorbed → temperature falls (higher tier)
  • Earth's temperature depends on the balance of incoming and emitted radiation (higher tier)
  • Matt black surfaces are the best absorbers and emitters of thermal radiation; shiny surfaces are the poorest

That's the notes covered.

Carry on to the next subtopic.