Thermal Expansion (Cambridge (CIE) O Level Physics): Revision Note
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
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
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
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 Tips and Tricks
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 molecules makes the substance bigger!
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