IGCSE PHYSICS THEMRAL EXPANSION

1. 2.2 Thermal properties and
temperature
Physics IGCSE
Madiha Shakil

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Objectives – 2.2.1
 Describe qualitatively the thermal expansion of
solids, liquids, and gases at constant pressure.
 Identify and explain some of the everyday
applications and consequences of thermal
expansion.
 Explain, in terms of the motion and
arrangement of molecules, the relative order of
the magnitude of the expansion of solids, liquids
and gases.

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Introduction
 Temperature – a measure of the internal
energy of the substance’s atoms/molecules.
 Increase in temperature – caused by the
supply of heat which increases internal energy.
 Internal energy – sum of kinetic energy and
potential energy of the atoms/molecules.

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Kinetic theory of matter
Kinetic theory – particles are constantly moving and have kinetic energy.
The higher the temperature, the faster they move, so the more kinetic
energy they contain.
Internal energy = Kinetic energy + Potential energy
Of all atoms or molecules in a matter

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Thermal Expansion
 According to the kinetic theory the molecules of solids and
liquids are in constant vibration.
 When heated they vibrate faster and force each other a
little further apart.
 Expansion results, and this is greater for liquids than for
solids; gases expand even more.
 The linear (length) expansion of solids is small and for the
effect to be noticed, the solid must be long and/or the
temperature change must be large.
Note that the cooling down of substances
tends to have the opposite effect – the
particles lose kinetic energy, come closer
together, and thus contract.

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Thermal Expansion
 When something is heated, it expands because the
molecules take up more space:
 When a solid is heated, the molecules vibrate more but stay
in place, so the relative order of magnitude of the expansion
is small.
 When a liquid is heated, it expands for the same reason as a
solid, but the intermolecular forces are less so it expands
more.
 When a gas is heated, the molecules move faster and
further apart, so the relative order of magnitude of the
expansion is the greatest.

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Uses of Thermal Expansion
1) Liquid-in-glass thermometers – uses expansion of liquid to tell
temperature.
2) Metal rods can be fitted into metal wheels, by heating wheel.
As the wheel cools, it contracts and pulls the rod and wheel
tightly together.
3) A metal lid or cap may stick on a glass jar or a bottle. Heating
the lid (for example, by running hot water over it) cause it to
expand (the glass expand much less), so the lid loosens and
can be removed.

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Uses of thermal Expansion –
Bimetal strip
 A bimetal strip is a two-thin strips of different metals
(Brass and invar mostly) welded together.
The bimetallic strips used in fire alarms, thermostat and
bimetallic thermometers.

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Bimetal Fire Alarm
 When the bimetallic strip in the fire alarm
heated up it expands and touches with contact,
so current flows through the circuit. As a result
bell rings.
https://javalab.org/en/bimetal_en/

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Bimetal Thermostat
 The bimetal thermostat are devices which
control temperature. They are fitted to
immersion heaters, ovens, and refrigerators
and to some room heaters. There are
several different designs bimetal type is very
common.
 The diagram below shows thermostat that
controls an electric heater. As the rooms
warms up, the bimetal strip expands (brass
expand more than invar), so two electrical
contact separate. This switches of the
heater. And when room cools down the
bimetal contract and again touches the
contact, so circuit completes and switch on
the heater. Temperature can be adjust by
turning control knob.

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Effects of Thermal Expansion
 On a hot day concrete runway sections in airport expands
and this cause cracking. To solve this problem we leave
small gabs between sections.
 On a hot day concrete bridges expand. To solve this problem,
we leave small gab at one end and support the other end
with rollers.
 Telephone wire contract on cold days. To solve this problem,
we leave wires slack so that they are free to change length.
 On a hot day railway lines expand. To solve this problem,
gaps are left between sections of railway lines to avoid
damage of the rails as the expand in hot weather.

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Expansion in Gases
 Gases expand when they are heated, just like solids
and liquids. We can understand this by using the
kinetic model of matter. The diagram below shows
some gas in a cylinder fitted with a piston. At first the
gas is cold and its particles press weakly on the
piston. When the gas is heated, its particles moves
faster. Now they push greater force on the piston and
push it upwards. The gas has expanded.

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Expansion in Gases.
 The upward force of the gas is balanced by downward
force of the piston. So in this situation, the pressure of
the gas is remained constant as it has expanded.
 So for a fixed mass of a gas at constant pressure, the
volume gas is directly proportional to the increasing
temperature of the gas. This is called Charles’s law.
 If the piston did not move, the volume of the gas remains
constant when it was heated but its pressure would
increase.