STD VIII PHYSICS
HEAT
CONCEPT OF HEAT (HEAT : A FORM OF ENERGY)
We have already studied that heat is a form of energy which cannot be seen, but can only be
felt. During winter we sit in the sun or near the fire to feel warm. In this case, we cannot see
heat coming from the sun or the fire , but we can experience it.
We also experience that on rubbing our palms, they get heated or on passing electric current
in a metallic wire, the wire gets heated.
Each body is made up of molecules which are in the state of random motion .Each molecule
exerts a force of attraction on the other molecules. The heat energy of a body is the internal
energy of the molecules of the body.
DEFINITION OF HEAT
Heat is a form of energy which flows.It is the energy of random motion of molecules
constituting the body.
UNITS OF HEAT
S.I.unit – joule (J)
C.G.S. unit – erg
1 J = 107 erg
Other units : calorie (cal) and kilocalorie (kcal)
1 kilocalorie = 1000 calories
1 cal = 4.186 J (or nearly 4.2 J )
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FLOW OF HEAT
Consider two bodies A and B .Let the temperature of body A be 80°C and that of body B be
30°C. On bringing bodies A and B in contact , heat will flow from hot body A to the cold
body B.Heat will continue to flow till both the bodied attain the same temperature.
The body which loses heat is at a higher temperature than the body which receives heat. Thus
, the temperature determines the direction of flow of heat.
DEFINITION OF TEMPERATURE
Temperature is a quantity which tells us the thermal state of a body(i.e., the degree of
hotness or coldness of the body). It determines the direction of flow of heat when two
bodies are placed in contact.
If there is no transfer of heat between the two bodies placed in contact , they are said to be at
the same temperature.
The amount of heat energy contained in a body depends on :
1.Mass of the body.
2.Temperature of the body
3.Material of the body.
UNITS OF TEMPERATURE:
S.I.unit : kelvin(K)
Common unit: degree celsius (°C)
Relation between kelvin and degree Celsius
T K = 273 + t°C
Thus by adding 273 to the temperature in degree Celsius, we get the temperature in kelvin.
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DIFFRENCE BETWEEN HEAT AND TEMPERATURE
HEAT
1.Heat is a form of energy obtained due to
random motion of molecules in a substance.
TEMPERATURE
1.Temperature is a quantity which
determines the direction of flow of heat on
keeping the two bodies in contact.
2.The S.I. unit of temperature is kelvin(K).
3.The temperature of the body depends on
the average kinetic energy of molecules due
to their random motion in it.
4.Temperature is measured by a
thermometer.
2.The S.I. unit of heat is joule(J).
3.The amount of heat contained in a body
depends on mass , temperature and material
of the body.
4.Heat is measured by the principle of
calorimetry.
THERMAL EXPANSION
Most substances expand when heated and contract when cooled.
The change in length / area or volume (due to contracting/expanding) is directly related to
temperature change.
The expansion of a substance on heating is called the thermal expansion of that substance.
A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., the
length, area and volume, all increase on heating.
The liquids and gases do not have a definite shape, so they have only the cubical or volume
expansion.
Gases expand much more than the liquids, while liquids expand more than the solids.
Expansion of a substance: gas > liquid > solid
DEMONSTRATION OF THE THERMAL EXPANSION IN SOLIDS
Gravesand’s ball and ring experiment : Take a metallic ring and a metallic ball.The
diameters of the ring and the ball are such that the ball just slips through the ring at room
temperature. Heat the ball on a burner and make an attempt to pass the heated ball through
the ring.
A cold ball
easily passes
through the
ring.
The ball when
heated undergoes
volumetric
expansion and
does not pass
through the ring.
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DEMONSTRATION OF THE CUBICAL EXPANSION IN LIQUIDS
Experiment: Take an empty bottle with a tight fitting cork having a hole drilled in its middle
.Fill the bottle completely with water and add a few drops of ink to make it coloured . Put the
cork in the mouth of the bottle and fix a drinking straw through the hole in the cork.Pour
some more water into the drinking straw so that the level of water in the straw becomes
visible.Now heat it by means of a burner and look at the level of water in the straw.
You will notice that the level of water in the drinking straw has risen.This shows that water
expands on heating (i.e., its volume increases ).
Now remove the burner and allow the water to cool. You will notice that after sometime the
level of water in the straw falls back to its initial level. This shows that water contracts on
cooling (i.e., its volume decreases).
DEMONSTRATION OF THE CUBICAL EXPANSION IN GASES
Experiment : Take an empty bottle .The bottle contains air. Attach a rubber balloon to its
mouth as shown in fig.(a).Place the bottle in the bowl of hot water, let it stand for a few
minutes and watch what happens.
(a)
(b)
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What's happening?
After some time , you will notice that the balloon gets inflated [fig.(b)].The reason is as the
air inside the balloon heats up it starts to expand. The molecules begin to move faster and
further apart from each other. This is what makes the balloon stretch. There is still the same
amount of air inside the balloon and bottle, it has just expanded as it heats up.
EXPLANATION OF THERMAL EXPANSION
When a substance is heated, the average kinetic energy of molecules increases and they start
moving with an increased speed. As a result the mean separation between the molecules also
increases which results in an increase in the length, area and volume of the substance. On
cooling the substance, the mean separation between the molecules decreases which results in
the contraction of length, area and volume of the substance.
THERMAL EXPANSION OF SOLIDS
A solid has a definite shape, so when a solid is heated, it expands in all directions i.e., the
length, area and volume, all increase on heating.
The increase in the length is called the
linear expansion.
The increase in the area is called the
superficial expansion.
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The increase in the volume is called the
cubical expansion.
On heating a rod, the increase in its length
(i). depends on the material of the rod
For e.g., If two identical rods of iron and aluminium are heated to the same rise in
temperature, it is found that the aluminium rod expands nearly twice the iron rod.
(ii).is directly proportional to its original length
When two rods of same material, but of unequal lengths are heated through the same rise in
temperature , they expand through unequal lengths.
(c).is directly proportional to the increase in its temperature
When rods of same material and of equal lengths are heated through different rise in
temperatures, it is found that they expand through unequal lengths.
EXAMPLES OF THE USES OF THERMAL EXPANSION OF SOLIDS
1.Loosening the metal lid of a glass jar:
If the lid of a glass jar is tight, holding it
under hot water for a short while or
pouring hot water over the lid will make it
easier to open. This is because on
warming the lid , it expands more than the
glass and so gets loosened.
2. Rivetting:
Rivets are used to join two steel plates
together. Hot rivet is driven through the
hole in the plates. One end of the rivet is
hammered to form a new rivet head. When
cool, the rivet will contract and hold the
two plates tightly together.
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3. Thermostat in electrical appliances:
Thermostat is a temperature control
device. It
is a bi-metallic strip made up of two
different metals like steel/iron and brass
which expand and contract by different
amounts. Brass expands more than steel.
As
the temperature rises, due to unequal
thermal expansion, the strip bends. Due to
this
the contact breaks and the circuits gets
disconnected.
Bimetallic strips are used in refrigerators,
ovens, irons and heaters to regulate the
temperature of these appliances.
4. Fitting the iron rim on the wooden
wheel:
The radius of the iron ring is slightly less
than that of the wooden
wheel. It, therefore, cannot be easily
slipped on to the rim of wooden wheel.
The iron
ring is, therefore, first heated to a higher
temperature so that it expands in size and
the
hot ring is then easily slipped over to the
rim of the wooden wheel. Cold water is
now
poured on the iron ring so that it contracts
in size and holds the wooden wheel tightly
4.Glassware used in kitchen are generally made up of pyrex glass. The reason is that the
pyrex glass do not expand much on being heated and therefore they do not crack.
EXAMPLES OF DISADVANTAGES OF THERMAL EXPANSION OF SOLIDS
1.Railway tracks :
While laying the railway tracks on wooden
plank, a small gap is left between the successive
lengths of rail as shown in the figure.if no gap is
left, the expansion of rails due to rise in
temperature in summer will cause them to bend
sideways which may cause derailment.
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2.Construction of a bridge:
In construction of a bridge , steel girders are used
.One end of girder is fixed in the concrete, but
the other end is supported on rollers and not
fixed into the pillar.This is done so that if there is
any rise or fall in temperature during summer or
winter, girders may expand or contract without
affecting the pillar.
3.Cracking of a thick glass tumbler:
Glass is a poor conductor of heat.When hot
liquid is poured into the tumbler, the inner
surface of the tumbler becomes hot and expands
while the outer surface remains at the room
temperature and does not expand.Due to this
unequal expansion, the tumbler cracks.
4.Joints in metal pipes:
A metal pipe line expands in summer and
contracts in winter due to change in atmospheric
temperature. The pipe line may get deformed due
to this.To avoid it, loops are joined with the pipe
line so that expansion in pipe line during summer
increases the size of the loop, while contraction
in pipe line during winter decreases the size of
the loop.
5. Electric cables and telephone wires:
Telephone wires and cables contract on cold
days and sag in summers. To solve this
problem, we leave wires slack so that they are
free to change length.
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