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Effects of Thermal Energy Expansion & Contraction

Notes

1. Introduction

Generally, most bodies increase in size in all directions when heated (i.e. expand), anddecrease in size in all directions when cooled (i.e. contract).Using kinetic theory, theexpansion can be explained in the following way.

When a body is heated, the molecules gained kinetic energy, they vibrate morevigorously and are further apart from one another. Therefore the body increases in sizeor expand.

Conversely, when a body cools down, the molecules lost kinetic energy, they vibrate lessvigorously and are closer to one another. Therefore the body decreases in size or

contracts.

2. Demonstration of expansion in solids

Metal ball and ring experiment

The metal ball slips through the metal ring easily at room temperature. When the metalball is heated, it expands and can no longer pass through the ring. When it cools down toroom temperature, it will slip through the metal ring again.

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3. Precautions against expansion in solids

a) Expansion of bridges

In building bridges, engineers must account for expansion in their design. Gaps mustbe left at one end, and that end supported by rollers. Otherwise, large stresses would

arise and the concrete would crack.

b) Expansion and contraction of wires

Allowance is made for the expansion or contraction so that the wires do not sag toomuch during the day or snap at night.

c) Large bends in pipes

Pipes carrying hot or cold liquids and gases often have large bends in them. The

bends allow the pipes to expand or contract without cracking.

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4. Applications of expansion in solids

a) Hot riveting process

b) Tyre fitting process

c) Bimetallic Strip

A bimetallic strip is made of two different metals, in which one metal expands orcontracts more than the other for the same rise in temperature. The two metals arewelded or riveted together.

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Devices which make use of a bimetallic strip:

Electric bell

Electric iron

How the bimetallic thermostat works in the electric iron:

5. Demonstrating expansion in liquids

Almost all liquids expand when heated. Different liquids expand by different amountswhen heated through the same change in temperature.

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6. Anomalous expansion of water

As water is cooled, it contracts until it reaches 4C. However as it cooled further,

from 4C to 0C, it expands.

Volume of water is at its minimum at 4C, the density of water is at its maximum at4C.

When a pond is freezing over, the densest water at 4C remains at the bottom ofthe pond. The less dense (at lower temperature of between 3C and 0C) water floatsin layers above it. The water on the surface is frozen, but floats because it is lessdense than the water below it. The different density layers stop convection currentsspreading the thermal energy.

Ice is a bad thermal conductor so that the layer of ice on the top of a pond acts likean insulating blanket and slows down further loss of thermal energy from the water below.

Aquatic animals and plants make use of this phenomenon, by living in the liquidlayers when the water freezes over in the winter.

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7. Applications of Expansion of Liquids

Liquid-in-glass thermometer use thermal expansion and contraction of a liquid to measuretemperature. Most liquid-in-glass thermometers are filled with either mercury or alcohol.

Think About this...why is water not used in thermometers? (due to the anomalous behaviourof water!)

8. Demonstrating expansion in gases

When the air is warmed up by placing both hands on the flask, the liquid thread isseen to rise very quickly.

This shows that the air in the flask expanded by a large amount.

9. Applications of expansion of gas

a) Rising of dough

Breads and cakes rise when baked because the gases inside the dough expandwhen heated.

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b) Motor Engines

The high pressure produced by the expansion of hot gases is used in internalcombustion engines of motor vehicles and in explosives.

c) Hot air balloons

Thermal expansion and contraction in a gas causes its density to change. Anincrease in the volume of a fixed mass of gas reduces its density.

A hot air balloon will rise when the air inside it is heated and becomes less densethan the surrounding air.

It will fall when the air inside it is cooled.

END