Thermal Expansion (Cambridge (CIE) IGCSE Physics): Revision Note

Thermal expansion

•  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 particles gain kinetic energy and vibrate or move faster

  • This increases the average distance between the particles, so the material expands

• The expansion comes from an increase in particle separation — the particles themselves do not get any bigger

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

Thermal expansion in terms of particles

Extended tier only

• 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

• This behaviour is influenced by the distances and strength of the forces between particles in each state (opens in a new tab)

Uses & consequences of thermal expansion

• 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

• A thermometer is more sensitive if the capillary tube is narrower, because the same expansion of liquid moves further along a narrower tube

  • Thinner glass around the bulb also makes the thermometer respond more quickly

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

Temperature-activated switches

• Temperature-activated switches use a bimetallic strip, made from two different metals joined together

• The two metals expand by different amounts when heated, so the strip 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 into them, to create space for the expansion to happen without causing damage