2.2 Thermal Properties and Temperature
Chapter 2: Thermal Physics
2.2.1 Thermal Expansion of Solids, Liquids and Gases
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
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.
Thermal Expansion in terms of molecules:
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.
Uses of Thermal Expansion:
Liquid-in-glass thermometers – uses expansion of liquid to tell temperature.
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.
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.
Bimetal Strip:
A bimetal strip is a two-thin strips of different metals (Brass and invar mostly) welded together.
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The bimetallic strips used in fire alarms, thermostat
and bimetallic thermometers.
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
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.
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 they expand in hot weather.
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.
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.
2.2.2 Measurement of Temperature
It is the measurement of the hotness or coldness of an object.
Heat – Refers to the amount of thermal energy that is being transferred from a hotter to a colder region.
Temperature is measured using thermometers.
The unit of temperature is: Kelvin (K), degree Celsius (°C) and Fahrenheit (F)
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Thermometric substance:
Substances (can be solids, liquids or gas) that have physical properties that vary continuously and linearly with
temperature.
Thermometric property
Examples of thermometers
Volume of a fixed mass
of liquid (e.g. mercury
or alcohol)
Mercury
Alcohol
Clinical Thermometer
Electromotive force (e.m.f)
(between hot and cold
junctions of two different
metals joined together)
Thermocouple
Resistance of metal
e.g. Platinum
Resistance thermometer
Pressure of a fixed mass
of gas at constant volume
Constant-volume gas thermometer
Scale of Temperature:
A scale and unit of temperature are obtained by choosing two temperatures,
called the fixed points, and dividing the range between them into a number
of equal divisions or degrees.
On the Celsius scale, the lower fixed point (ice point) is the temperature of
pure melting ice and is taken as 0°C.
The upper fixed point (steam point) is the temperature of the steam above
water boiling at normal atmospheric pressure, and is taken as 100°C.
When the fixed points have been marked on the thermometer, the distance
between them is divided into 100 equal degrees. The thermometer now has
a linear scale, in other words it has been calibrated or graduated.
Liquid-in-glass Thermometer
In this type the liquid in a glass bulb expands up a capillary tube when the
bulb is heated. The liquid must be easily seen and must expand (or contract)
rapidly and by a large amount over a wide range of temperature.
It must not stick to the inside of the tube or the reading will be too high when the temperature is falling.
Mercury and colored alcohol are in common use.
Mercury freezes at −39°C and boils at 357°C. Alcohol freezes at −115°C and boils at 78°C and is therefore more
suitable for low temperatures.
Clinical Thermometer
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A special type of mercury-in-glass thermometer used by doctors and nurses.
Its scale only extends over a few degrees on either side of the normal body temperature of 37°C, i.e. it has a
small range.
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