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School-college Physics Notes: Thermal energy 5.8 Density

Density & particle theory: 5.8 Explaining thermal expansion and decrease in density with increase in temperature for gases , liquids and solids - uses of thermal expansion of materials

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INDEX physics notes: Density - particle models - factors affecting density

5.8 Thermal expansion and density


The vast majority of materials expand on heating, whether they be gases, liquids or solids.

In the case of solids, raising the temperature increase the internal energy in the form of increased thermal vibration of the atoms.

The increased amplitude of vibration of the particles causes them to be forced further apart, an effect we call thermal expansion.

Different materials expand at different rates with increase in temperature.

This effect is made use of in a bimetallic strip - illustrated below.

use of bimetal strip bimetallic strip using differential thermal expansion of metals brass steel copper aluminium

Different metals expand different amounts for the same temperature rise.

A bimetallic strip is made from two different metals joined together.

The effect of heating a bimetallic strip is shown in the diagram.

The table below shows the length increase in mm on heating 2 metre (2000 mm) strips of the metals by 10oC. [8i-94]






length increase





Suggest a pair of metals that were was used in the experiment illustrated?

Metal B must expand more than metal A

e.g. Metal B could be brass and metal A could be steel.

Uses of bimetallic strips

A change in temperature triggers some kind of physical effect to control a situation.

e.g. bimetal strips are also used in gas oven safety valves and thermostat controls.

Fluids - gases and liquids

In the case of both gases and liquids, and assuming they are not contained in a sealed container of fixed volume, they will both expand in volume on heating.

Increasing their thermal energy store, increases the average kinetic energies of the particles, which causes an increase in the force and frequency of the particle collisions.

This causes the particles to spread out more and reduce the density.

Gases expand much more than liquids or solids, because the attractive forces between the particles is very weak.

Solids expand the least because the interparticle force is the greatest.

On heating, the increase in density trend is gases >> liquids > solids.

The increase volume of a liquid with rise in temperatures is used in mercury and alcohol thermometer tubes.

Keywords, phrases and learning objectives for density

Using particle theory to explain thermal expansion decreased density on increasing temperature for gases liquids solids uses of thermal expansion


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