NOTE:
The ball should pass through the ring when
both are at room temperature
·
Heat the ball and try to
pass it through the ring. Observe what happens.
·
Leave it for sometime
OBSERVATION
·
When both the ball and
the ring are at the same 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 sometime, it goes
through.
EXPLANATION
·
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.
WHY
SOLIDS EXPANDS ON HEATING
The molecules of a solid are closely
packed together and are continuously vibrating in their fixed positions
When a solid is heated the molecules gain
more kinetic energy and therefore make larger vibrations about their fixed
positions. This increase in vibration means that the molecules collide with
each other with larger forces and the molecules increases and so the solid
expand.
LINEAR
EXPANSIVITY
The measure of the tendency of a
particular material to expand is called its expansivity e.g. aluminium expands more than iron thus aluminium
has higher expansivity than iron.
|
material
|
Linear
exapnsivity (K-1) x 10-6
|
|
Aluminium
|
26
|
|
Brass
|
19
|
|
Copper
|
16.8
|
|
Iron
|
12
|
|
Concrete
|
11
|
|
Steel
|
11
|
|
Glass
|
9
|
The knowledge of linear expansivity values
is applied in the designing of materials to ensure that they are able to
operate well under varying thermal conditions.
Ordinary glass expands at a higher rate than
Pyrex glass. When hot water is poured into a tumbler made of glass it breaks
but does break in Pyrex glass.
Concrete and steel are reinforced together
because they are of the same linear expansivity. Hence cannot crack under
varying thermal conditions.
The
bimetallic strip
When two metals of different linear
expansivity are riveted together they form a bimetallic strip.
Brass and iron are used to make bimetallic strip as shown below.
On heating the bimetallic strip, brass
expands more than iron. The brass thus becomes longer than the iron for the
same temperature range. Hence, the bimetallic strip bends with brass on the
outside of the curve as shown in (b) below
On cooling, the brass contracts more than
iron. It therefore becomes shorter than the iron and thus ends up being on the
inner side of the curve as shown in (c) above
APPLICATIONS
OF EXPANSION AND CONTRACTION IN SOLIDS
1. RAILWAY LINES
Gaps are left between the rails. Expansion
for the rail is provided by overlapping the plane ends using overlapping joints
as shown in the figure below
If these gaps for the expansion are not
provided then during hot weather, they rails may buckle out, bend and cause
derailment of the train leading to destruction and accidents.
1 2. STEAM
PIPES
Pipes carrying steam from boilers are
fitted with loops or expansion joints to allow pipes to expand and contract
easily when steam passes through and when it cools down.
1 3. TELEPHONE WIRES
They are loosely fixed to allow for
contraction and expansion. During cold weather, they contract and when it is
warm they expand.
Telephone or electricity wires appear to
be shorter and taut in the morning. However in hot afternoons, the wires appear
longer and slackened.
2 4. STEEL BRIDGES
In bridges made of steel girders, one end
is fixed and the other end placed on rollers to allow for expansion as shown
5. RIVETS
Thick metal plates, sheets and girders in
ships are joined together by means of rivets.
The rivet is fitted when hot and then
hammered flat. On cooling, it contracts, pulling the two firmly together as
shown
1 6. ELECTRIC THERMOSTAT
A thermostat is used to maintain a steady
temperature in some devices such as electric iron box, refrigerators, fire
alarm and flashing unit for indicator lamp in motor cars.
EXPANSION
AND CONTRACTION IN LIQUIDS
The experimental set up below can be used
to demonstrate expansion of a liquid.
Heat
A glass flask is
filled with coloured water and heated as shown above
OBSERVATION
Immediately the
level of coloured water on the tube drops slightly at first and then starts
rising.
EXPLANATION
The initial fall
of the level of the water is due to the expansion of the glass flask which gets
heated first. The water starts expanding when heat finally reaches it and it
rises up the tube.
NOTE: The water expands
faster than the glass.
QUESTION
Explain why there
is a drop in the level of the water initially followed by a steady rise in the
level of water.
Different liquids expand more than others for a given
temperature as shown in the diagram
In this case,
methylated spirit expands most, followed by alcohol and finally water.
EXPANSION IN GASES
The experiment
below can be used to demonstrate expansion of air.
Invert the flask
with glass tube dipped into the water as shown.
Warm the flask
with your hands for some time and note what happens.
Remove your hand
and let the flask cool while the tube is still inserted in water.
OBSERVATION AND EXPLANATION
When the flask is
warmed the level of water column inside the glass tube drops indicating air
expands. When the flask is warmed further, some bubbles are seen at the end of
the glass tube.
On cooling the air
inside the flask contracts and water rises up the glass tube.
THE ANOMALOUS (UNUSUAL) EXPANSION OF WATER
Solids, liquids and
gases expands when heated and contracts when cooled. Water however shows an
anomalous (unusual) behaviour in that it contracts
when it is temperature is raised from 0oC to about 4oc.
When ice is heated
from say -20oC, it expands until its temperature reaches 0oC
and it melts with no change in temperature. The melting is accompanied by contraction. The water formed will
still contract as its temperature rises from 0oC as shown
Above 40C,
the water expands with increase in temperature. Since volume of a given mass of
water is minimum at 4oC, water at this temperature has a maximum
density, slightly higher than 1g/cm3.
A sketch of the variation of density with temperature
At the melting
point of water (o0C) there is a drastic increase in the volume, resulting
in a large decrease in density as the ice forms.
EFFECTS OF ANAMALOUS EXPANSION OF WATER.
1 1. Freezing of lakes and
ponds
Water in lakes and
ponds usually freezes in winter. Ice is less dense than water and floats on
water.
Since ice a bad
conductor of heat it insulates the water below against heat losses to the cold
air above.
Water remains at 40C
being the most dense, remains at the bottom of a lake while ice being less
dense floats on layers of water at different temperatures as shown.
Fish and other
aquatic animals and plants can therefore survive by living in the liquid layers
below the ice.
1 2. Icebergs
Since the density
of ice (0.92g/cm3) is slightly less than that of water it floats
with only a small portion above the water surface. The rest and bigger portion
rests under water. A big mass of such submerged ice is known as an iceberg.
It poses a great
danger to ships as navigators cannot see the submerged part.
2 3. Weathering of Rocks
When water in a
crack in a rock freezes, it expands. This expansion breaks the rock into small
pieces.
3 4. Water pipes
Water pipes bursts
when the water flowing through the pipes freezes
MEASURING TEMPERATURE
THERMOMETER
A thermometer is
an instrument used for measuring temperature. There are various types of
thermometers in use.
The common types
of thermometer include;
- Liquid-in-glass
thermometer.
- Clinical thermometer
- Six’s maximum and minimum
thermometer
LIQUID-IN-GLASS THERMOMETER
A liquid-in-glass
thermometer commonly in use is mercury
or coloured alcohol as the thermometric substance.
The volume of the
liquid changes uniformly with the change in temperature.
The liquid in the
bulb must; (characteristics)
- Be easily seen (visible).
- Expand or contract
uniformly and by a large amount over a small range of temperature.
- Not stick to the inside
of the tube. (Should not wet the inside of the tube.
- Have a wide range of
temperature.
THERMOMETRIC LIQUIDS-the
most common in use is mercury and alcohol.
Mercury freezes at
-39oC and boils at 357oC while alcohol freezes at -115oC
and boils at 78oC. Alcohol is therefore suitable for measuring
temperatures below -39oC.
PROPERTIES OF THE TWO THERMOMETRIC LIQUIDS
|
Alcohol
|
Mercury
|
|
Low boiling point, 78oC
|
High boiling point, 357oC
|
|
Low melting point, -115oC
|
Relatively higher melting point, -39oC
|
|
Poor thermal conductor
|
Good thermal conductor
|
|
Expansion slightly irregular
|
Expands regularly
|
|
Wets glass
|
Does not wet glass
|
|
Coloured to make it visible
|
Opaque and silvery
|
QUESTION
Explain why water
is not used as a thermometric liquid?
TEMPERATURE SCALE
A scale of
temperature is obtained by selecting two temperatures known as fixed points
The range between
this two fixed points is divided into a number of equal divisions.
On the celcious
scale, the lower fixed point is the temperature of pure melting ice and is
taken as 0oC. The impurities in the ice would lower its melting
point.
The upper fixed
point is the temperature of steam above water boiling at normal atmospheric
pressure of 760mmHg and is taken as 100oC.
The temperature of
boiling water itself is not used because any impurities in water would raise
its boiling point. The temperature of steam is not affected by impurities in
water.
When these points
have been marked, the range between them is divided into 100 equal divisions.
Each division is called degree.
FEATURES OF A COMMON THERMOMETER
The basic features
of a common laboratory are as shown below.
Bulb- Carries the liquid
in the thermometer. It has a thin glass wall for effective heat transmission
between the liquid and body whose temperature is taken.
Capillary bore – Liquid
expands and contracts along the capillary tube. It is narrow for high degree of
accuracy.
Glass stem –Is
a thick wall surrounding the capillary bore. It also serves as a magnifying
glass for easy reading of scale.
CELCIOUS AND KELVIN SCALE
They are the
commonly used temperature scale.
The celcious scale
has the fixed points at 0oC and 100oC.
In Kelvin scale,
the temperature of pure melting ice is 273K while that of pure boiling water at
normal atmospheric pressure is 373K.
The lowest
temperature in the Kelvin scale (0K) is referred as absolute zero. This is the temperature at which the energy of the
particles in material is zero.
To change oC to Kelvin
T = (ѳ – 273) K where
ѳ is the temperature in oC
Example
Convert 25oC in Kelvin
T = (25 + 273)
= 298 K
To change Kelvin to oC
Ѳ = (T- 273) 0C where T is the temperature in
Kelvin
Example
Convert 1 K
Ѳ = 1-273
=-272oC
NOTE: Temperature in
Kelvin scale cannot have a negative value because the absolute zero, (0 K), is
the lowest temperature attainable.
CLINICAL THERMOMETER
A clinical
thermometer is an instrument used to measure the temperature of a human body.
It uses mercury as
its thermometric substance and has a narrow constriction in the tube just above
the bulb.
The diagram below
shows the main features of a clinical thermometer.
The constrictionprevents the mercury level
from falling down when it contacts with the human body.
The clinical
thermometer has a short scale of temperature from 35oC to 43oC
spread over its entire level. This is because the human body temperature falls
slightly above or below 37oC which is the temperature of a normal
and healthy person.
SIX’S MAXIMUM AND MINIMUM THERMOMETER
This thermometer
is used to record the maximum and minimum temperature of a place during a day.
The thermometer
consists of a U-tube connected to two bulbs. The U-tube contains mercury.
The two bulbs
contain alcohol.
The figure below
shows the main features of a six’s maximum and minimum thermometer.
Working of the thermometer
When temperature
raises alcohol occupying volume of bulb A expand and forces mercury in the
U-tube to rise on the right hand side.
The mercury in
turn pushes the steel index A upwards. The maximum temperature can be noted
from the lower end of the steel index A.
On the other hand
when the temperature falls, alcohol in the bulb A contracts and the mercury is
pulled back rising u the left hand side of the U-tube. The index B is then
pushed up. During contraction of the alcohol, index A is left behind ( in the
alcohol) by the falling mercury.
The minimum
temperature is then read from the lower end of index B.
NOTE: To reset the
thermometer, a magnet is used to return the steel indices to the mercury
surfaces.
The bimetallic thermometer
