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Insulators

Heat and matter · level 3, lesson 3 of 6

A question to start

A vacuum flask keeps soup hot all day, and keeps juice cold too. How does one flask do both?

The idea

An insulator slows heat down. It does not stop heat completely, but it makes it leak far more slowly.

The best insulators trap lots of still air in tiny pockets: wool, feathers, foam, fluffy fibres. Air conducts heat badly, and when it is trapped it cannot move around and carry heat away.

A vacuum flask is even better. It has two walls with nothing, a vacuum, between them:

vacuum gap:no conduction,no convection shiny walls:reflect radiationstopperhot soup
  • With no particles in the gap, heat cannot conduct across it, or be carried by moving air.
  • The walls are shiny, so they reflect heat that tries to cross as radiation.
  • The stopper stops warm air escaping from the top.

It works both ways: heat cannot easily get out of hot soup, or into cold juice.

Worked examples

1. A woolly hat. Wool traps air between its fibres. Your head's heat leaks out slowly.

2. Loft insulation. Thick fluffy layers in a roof trap air, so less of the house's heat escapes.

3. A spacesuit. It has many thin layers, with gaps between them, to keep heat in on a cold Mars night.

Try it

  1. What does an insulator do?
  2. Why are fluffy materials good insulators?
  3. Why is there a vacuum between the walls of a flask?
  4. Why can a flask keep cold drinks cold?
Answers
  1. It slows down the flow of heat.
  2. They trap lots of still air, which conducts heat badly.
  3. Without particles, heat cannot be conducted or carried across.
  4. It slows heat getting in from the warm air, as well as heat getting out.

Common mistake

"Insulators make things warm." An insulator only slows heat flow. Wrap an ice cube in a blanket and it stays cold for longer.

Where it leads