Thermal Expansion (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

•  When a material is heated at constant pressure:
  • Its temperature increases
  • Its overall volume increases (it expands)
  • Its density decreases

• This expansion happens because:
  • The molecules start to move around (or vibrate) faster as they gain kinetic energy
  • This causes them to collide with each other more often and push each other apart

Thermal expansion diagram

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

• 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

Extended tier only

• The thermal expansion of materials can have some useful applications as well as some undesirable consequences

Applications of thermal expansion

• Useful applications of thermal expansion include:
  • Liquid-in-glass thermometers
  • Temperature-activated switches

Liquid-in-glass thermometer

• A liquid-in-glass thermometer relies on the expansion of liquids to measure temperature
• It consists of a thin glass capillary tube containing a liquid that expands with temperature
  • At one end of the tube is a glass bulb, containing a large volume of the liquid which expands into the narrow tube when heated
  • A scale along the side of the tube allows the temperature to be measured based on the length of liquid within the tube

Liquid-in-glass thermometer

As the bulb is heated, the liquid expands and moves along the capillary tube

Temperature-activated switches

• Temperature-activated switches utilise a bimetallic (made from two types of metal) strip
• It consists of two metals that expand at different rates and bends by a predictable amount at a given temperature

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

Consequences of thermal expansion

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

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