Thermal Expansion (Oxford AQA IGCSE Physics)
Revision Note
Thermal Expansion
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
Heating a solid
When this happens, the space taken up by the substance increases
The molecules themselves remain the same size
The space in between them increases
Thermal Expansion of Solids, Liquids and Gases
State | Expansion | Explanation |
|---|---|---|
Solids | Expand slightly | Low-energy molecules cannot overcome the intermolecular forces of attraction holding them together |
Liquids | Expand more than solids | Molecules have enough energy to partially overcome the intermolecular forces of attraction holding them together |
Gases | Expand significantly | Higher energy molecules have enough energy to completely overcome the intermolecular forces of attraction holding them together |
Hazards 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
Things that are prone to buckling in this way have gaps built in, this creates space for the expansion to happen without causing damage
Useful thermal expansion
Thermometers rely 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
Temperature-activated switches work when a bimetallic strip (a strip made from two types of metal)
The two metals expand at different rates so the strip bends by a predictable amount at a given temperature
When the bimetallic strip is cool, the circuit is not connected
When the bimetallic strip is heated, it bends and closes the circuit
Temperature activated switches
Worked Example
The wooden wheels of horse-drawn carriages can be strengthened by putting a tight iron ring 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, causing them to lose marks.
As the substance is heated:
This 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 molecules makes the substance bigger!
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