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1
Temperature of a body
This is the degree or extent of coldness or
hotness of a body on some chosen scale.
Temperature is measured using an
instrument called thermometer.
It is a basic quantity and is measured in
degrees Celsius (0C) or Kelvin (K)
Kelvin (K) is the SI unit of temperature.
Temperature is a scalar quantity.
Expansion and Contraction of Solids
Metals and other solids expand when
heated and contract when cooled.
In simple terms,
Expansion is the increase in size of an
object when heated.
Contraction is the decrease in size of an
object when cooled.
Demonstration of expansion in solids
a.) The ball and ring experiment
Heat the ball and try to pass it through the
ring.
Leave it there for some time.
Observation
When both the ball and the rind are
at room temperature, the ball just
passes through the ring.
When the ball is heated it does not
go through the ring.
But when left there for some time,
it goes through.
Why?
When heated, the ball expands so that it
cannot go through the ring.
When left on the ring for some time, the
temperature of the ball decreases and it
contracts. At the same time, the
temperature of the ring increases and it
expands so that the ball goes through it.
b.) The bar and gauge apparatus
To investigate the force due so
expansion and contraction
Fit the tensioning nut and cast-iron pin in
the inside position of a bar-breaking
apparatus shown in figure (a). Tighten the
tensioning unit until there is no room for
expansion of the steel rod.
Heat the steel rod strongly and note what
happens to the cast iron pin.
Allow the apparatus to cool and repeat the
experiment with the cast iron pin on the
outside as shown in figure (b).
Withdraw the flame and allow it to cool.
You may pour cold water on it to cool it
faster. What happens lo the pin?
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The cast iron pin breaks in both cases
Conclusion: The experiment shows that
very strong forces occur when metals
expand or contract due to heating and
cooling, respectively.
Expansion in gases also can result in very
strong forces, for instance the internal
combustion engines’ pistons are powered
by the expansion resulting from burning
fuel-air mixtures.
Applications of the expansion of solids 1. Construction of railway lines– an
expansion joint is allowed between any
two rails to accommodate expansion. A
fish plate is used to join two rails. Modern
railway system use the overlapping joint
at the end of rails.
2. Construction of bridges and roof tops
(steel girders)– for bridges one side has
rollers while the other is fixed to allow for
expansion. Concrete slabs are also laid on
the ground leaving space filled with pitch
to allow for expansion.
3. Hot water pipes– pipes carrying hot
water (steam) from boilers are fitted with
expansion joints for expansion.
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4. Riveting – used to join two pieces of
metal together i.e. bimetallic strips, car
bodies, drums etc. Fitting rail cart wheel
using heat uses the principle of rivets.
Bimetallic
Bimetallic Strip.
ANOMALOUS EXPANSION OF
WATER
Solids and liquids expand on heating and
contract on cooling. Unlike other
substances, water has unusual behaviour.
As the temperature of water rises from 00C
to 40C, the volume of water falls showing
that water contract on heating from 00C to
40C.
At 40C, water has minimum volume.
After 40C, the volume of water rises as
heating continues.
A graph of volume against Temperature
Can you explain the meaning of
mass of a body?
Do you think the mass of water
change as its volume changes?
Write the equation relating mass,
volume and density.
As the volume of water reduces
then rises, does this have an effect
to its density?
Draw the graph of density against
temperature.
Effects of anomalous expansion of water.
Freezing of lakes and ponds.
In cold weather, when the
temperature of water falls below
00C, water in the lakes freezes and
ice is formed. So, how do fish
survive in frozen water? The
density of ice is less than that of
water and therefore stays at the top
and the water on the bottom remain
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at 40C. This assists the aquatic
animals and plants to survive in
cold water.
Bursting of water pipes.
Water pipes can burst when water
flowing through the pipes freezes.
This is because on freezing the
volume of water increases.
Weathering of rocks. When
water freezes in the cracks of a
rock the volume of water increases.
This causes the rock to break into
small pieces resulting into
weathering of rocks.
Icebergs.
When water freezes to form ice, a
large mass of ice is immersed in
water and only its tip can be seen.
This can cause an accident to the
navigators.
Temperature
What is temperature?
Which instrument is used to measure
temperature of a body?
There are different types of a thermometer.
1. Liquid-in-thermometer
a.) Mercury thermometer
b.) Alcohol thermometer
c.) Clinical thermometer
2. Six’s maximum and minimum
thermometer
Liquid-in-glass thermometer
It uses either alcohol or mercury as
thermometric liquid.
Properties of thermometric liquid.
Visible.
Has a wide range of temperature
i.e. low freezing point and high
boiling point.
Do not stick on the walls of the
thermometer.
Sensitive to temperature change.
Expand uniformly.
Question
What are the advantages of using mercury
as a thermometric liquid than using
alcohol?
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Calibration of a thermometer.
Thermometers must be calibrated before
they can be used to measure temperatures.
The calibration of an instrument refers to
the process of marking-up a scale on the
instrument to be used as measurement.
To produce a scale on a thermometer, two
fixed points must be determined first. Then
the two points must be the temperatures
which can easily and correctly be
reproduced in any part of the world.
On the Celsius scale, the two fixed
points are the ice point (0°C) and the
steam/boiling point (100°C).
The ice point (0°C), lower fixed point, is
the melting temperature of pure ice at
standard atmospheric pressure (760 mm
Hg).
The steam point (100°C), upper fixed
point, is the temperature of steam at
standard atmospheric pressure (760 mm
Hg).
After obtaining, the highest point and the
lowest point. We divide the length
between them to equal parts / scale.
Features of a thermometer
Bulb- the walls of the bulb is made
thin to make the thermometer more
sensitive to temperature change.
Capillary tube- it is made thin or
narrow to make the thermometer
more accurate i.e any slight
expansion or contraction of the
thermometric liquid is measurable.
Glass wall- it has a glass wall to
ensure:
a.) The reading is magnified
b.) Mechanical protection
c.) Easy reading because it is
clear.
During construction of the thermometer,
the following should also be considered:
The quantity of mercury in
the bulb is small so that the
mercury takes little time to
warm up.
The capillary tube is of
uniform cross-section so that
the mercury level changes
uniformly along its length.
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Question
To determine the upper fixed point of a
thermometer, the thermometer is not put
in boiling water but in the steam.
Explain.
Clinical Thermometer.
Unlike ordinary thermometers which have
only the bulb, capillary tube and the stem;
clinical thermometers have constriction.
Constriction break the mercury thread
after the reading has been taken hence the
mercury in the tube stays back. After use,
the mercury in the tube can be forced
through the constriction back to the bulb
by flicking the thermometer.
The normal human body temperature is
370C .
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A thermometer calibrated from 00C to
1000C is not required in the hospital.
Temperature scale conversion
The most commonly used temperature
scales are Celsius and Kelvin scales.
Celsius scale is also known as Centigrade
scale.
Fixed points:
For Celsius scale: 00C & 100
0 C
In Kelvin scale, the temperature of pure
melting ice is 273 K while that of pure
boiling water at normal atmospheric
pressure is 373 K.
Absolute zero temperature is the
temperature at which the energy of the
particles in a material is zero.
The lowest temperature in Kelvin scale is
0 K.
Change 0C to Kelvin.
T = Ɵ + 273
Where T= temp in kelvin
Ɵ = temp in 0C
Convert the following temperatures into
Kelvin.
a. 250C
b. 1000C
c. 00C
d. -2340C
e. 5600C
Convert the following temperatures into 0C
a. 350 K
b. 100K
c. 1K
d. 0K.
Six’s maximum and minimum
thermometer
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It is used to record minimum and
maximum temperatures of a given place.
When the temperature of the surrounding
rises, the oil in the bulb A expands pushing
the mercury which in turn pushes up the
oil in the other arm. This compresses the
vapour above the oil and the maximum
index is pushed up to the maximum
position. This is the maximum
temperature.
When the temperature falls, the oil
contracts back into the bulb. Mercury
flows back pushing the minimum index to
the minimum position. This gives the
minimum temperature.
After taking the readings, the indices are
pulled down to the level of the mercury
using a magnet.
A bimetallic thermometer
It is made up of a coiled bimetallic strip
whose one end is fixed and the other end
connected to a pointer. Commonly used
metals are brass and invar. When the
temperature rises brass expands more than
invar. The strip thus curls forcing the
pointer to move over a calibrated scale.