Thermal Expansion | Physics for Grade 10 PDF Download

Introduction

• 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

Thermal Expansion in Terms of Particles

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

Uses & Consequences of Thermal Expansion

• 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:
    (i) Metal railway tracks
    (ii) Road surfaces
    (iii) 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

Tips 

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

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