Thermal Expansion (Cambridge O Level Physics)

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Dan MG

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Thermal Expansion

  • When a material is heated:
    • 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 in a Solid

Thermal expansion, IGCSE & GCSE Physics revision notes

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

Table Comparing Thermal Expansion of Solids, Liquids and Gases

State Magnitude of Expansion Explanation
Solid Expands slightly The low energy molecules cannot overcome the intermolecular forces of attraction holding them together
Liquid Expands more than solids The molecules have enough energy to partially overcome the intermolecular forces of attraction holding them together
Gas Expand significantly The high energy molecules have enough energy to completely overcome the intermolecular forces of attraction holding them together

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

Liquid-in-glass thermometer

Liquid thermometer, IGCSE & GCSE Physics revision notes

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

Bimetal thermostat, IGCSE & GCSE Physics revision notes

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

Worked example

The wooden wheels of horse-drawn carriages can be strengthened by putting a tight ring of iron around them.

Which of the following actions would make it easier to fit the iron ring over the wooden wheel whilst ensuring a tight fit?

A    Heating the iron ring

B    Heating the wooden wheel

C    Cooling the iron ring

D    Cooling the wooden wheel

 

ANSWER:  A

  • When metals are heated, they expand
  • So, heating the iron ring will cause it to expand, hence increasing its diameter
  • The expanded iron ring will fit (just) over the wooden wheel
  • When the iron ring cools, it will contract, reducing its diameter once again, and ensuring a tight fit around the wooden wheel

Examiner Tip

Remember that it is the material that expands, not the molecules. This trips up many students, losing marks.

As heat is added:

  1. The increase in temperature...
  2. Leads to an increase in kinetic energy, so that...
  3. Molecules and atoms move more quickly...
  4. And move apart
    • This separation of the molecules makes the substance bigger!

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Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.