Thermal Expansion (CIE IGCSE Physics)

• When materials are heated, they expand
• This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other apart

When a solid is heated, the molecules vibrate more, pushing each other apart

Note: When this happens, it is the space taken up by the molecules that increases. The molecules themselves remain the same size. 

• Thermal expansion occurs in solids, liquids and gases

    

• When temperature is increased (at constant pressure);
  • Solids will tend to expand the least
  • Gases expand the most
  • Liquids fall in between the two

EXTENSION

• Molecules do not expand, but the space in between them does
  • When solids, liquids and gases are heated:

• The thermal expansion of materials can have some useful applications, but also has some undesirable consequences

 

• Applications:
  • Thermometers rely on the expansion of liquids to measure temperature
  • Temperature-activated switches work when a bimetallic strip, consisting of two metals that expand at different rates, bends by a predictable amount at a given temperature

The bimetallic strip will bend upwards when heated, closing the circuit

 

• Consequences:
  • The expansion of solid materials can cause them to buckle if they get too hot
  • This could include:
    • Metal railway tracks
    • Road surfaces
    • Bridges

  • Things that are prone to buckling in this way have gaps built in, this creates space for the expansion to happen without causing damage