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Thermal radiation and insulation
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All objects give out infrared radiation. Surface colour and texture decide how well they emit and absorb it.
Thermal radiation
Thermal radiation is infrared electromagnetic radiation. It needs no particles, so it can travel through a vacuum (as from the Sun).
All objects emit and absorb it. Hotter objects emit more radiation.
All objects emit and absorb it. Hotter objects emit more radiation.
Surfaces
Reducing unwanted energy transfer
Loft and cavity wall insulation: trap air, a poor conductor, and stop convection.
Double glazing: a layer of trapped air or gas between panes.
Vacuum flask: the vacuum stops conduction and convection; silvered surfaces reduce radiation; the stopper reduces convection and evaporation.
Practical (Leslie cube): fill a cube with hot water; an infrared detector shows the matt black face emits most and the shiny face least.
Double glazing: a layer of trapped air or gas between panes.
Vacuum flask: the vacuum stops conduction and convection; silvered surfaces reduce radiation; the stopper reduces convection and evaporation.
Practical (Leslie cube): fill a cube with hot water; an infrared detector shows the matt black face emits most and the shiny face least.
Why are the walls of a vacuum flask silvered?
- Shiny surfaces are poor emitters of radiation.
- They also reflect radiation back.
- So less energy is transferred by radiation.
Answer: To reduce energy transfer by radiation.
Thermal radiation is infrared and needs no medium. Hotter objects emit more. Matt black surfaces are the best emitters and absorbers; shiny surfaces are the worst and reflect well. Insulation traps air; a vacuum flask stops conduction, convection and radiation.
The interactive lesson includes the diagrams for this topic.
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