SuMMarY
• The machine which is simple in structure and makes work easier, faster and
more convenient is called a simple machine.
• We use simple machines to multiply the force applied, change the direction of
the force applied and to apply force at a convenient point.
• A lever is a rigid bar which moves freely about a fixed point called the fulcrum.
• The lever in which the fulcrum is situated between the effort and the load is
called the first class lever.
• The lever in which the load is situated between the fulcrum and the effort is
called the second class lever.
• The lever in which the effort is situated between the fulcrum and the load is
called the third class lever.
• A pulley is a simple machine having a grooved circular disc over which a rope
passes.
• A wheel and axle is a simple machine having two cylinders of different radius.
• Inclined plane is a sloping surface or a wooden plank which is used to push
things upward.
• A screw is a modified inclined plane with grooves cut in it.
• Wedge is a simple machine having two or more sloping surfaces that taper
either to form a sharp edge or pointed edge.
• A wedge is a piece of metal, wood, etc. with one thick end and another sharp
pointed end.
PHYSICS Oasis School Science and Environment - 8 45
exercise
1. Choose the best answer from the given alternatives.
a. Crowbar is an example of ................................ .
i. first class lever ii. second class lever
iii. pulley iv. third class lever
b. The distance between the fulcrum and the load is called ........................... .
i. load arm ii. effort arm
iii. fulcrum iv. effort distance
c. A force applied to a machine to do work is called .............................. .
i. output ii. input iii. load iv. effort
d. The doorknob of a house is an example of .............................. .
i. wedge ii. pulley iii. wheel and axle iv. screw
e. An axe is an example of .............................. .
i. lever ii. pulley iii. wedge iv. screw
2. Tick (√) the correct statement and cross (x) the incorrect one.
a. Beam balance is an example of a simple machine.
b. Crowbar helps us to multiply the force.
c. Wheel-barrow is an example of first class lever.
d. In a fixed pulley, the pulley moves with the load.
e. The work done by the machine is called input work.
f. In a wheel and axle, effort is applied on the wheel.
3. Fill in the blanks with appropriate words.
a. A simple machine makes our work easier, .................. and more convenient.
b. The fixed point about which a lever can rotate is called .................. .
c. Fishing rod is an example of .................. class lever.
d. A pulley is a simple machine which helps to change the .................. of the
force.
e. The distance between two screw threads is called a .................. .
46 Oasis School Science and Environment - 8 PHYSICS
4. Answer the following questions.
a. What is a simple machine? Give any three examples.
b. Write down any three advantages of using simple machines.
c. Define mechanical advantage, velocity ratio and efficiency of a machine.
d. What is a lever ? State the principle of lever.
e. What is first class lever ? Give any two examples.
f. What is second class lever? Give any three examples.
g. Define third class lever with two examples.
h. What is a pulley? Why is it used in our daily life ?
i. What is a wheel and axle? Give any three examples.
j. What is an inclined plane? Write down its uses.
k. What is a screw ? What is meant by pitch and thread of a screw ?
l. What is a wedge? Give any three examples.
m. What is meant by input work and output work?
5. Differentiate between:
a. First class lever and Third class lever
b. Input work and Output work
c. Fixed pulley and Movable pulley
6. Show the relation among MA, VR and η of a machine.
7. Give reason.
a. A bottle-opener is called the second class lever.
b. A pulley is used to lift water from well.
c. A screw-driver is used to unscrew the rusted nut.
d. The steering of a car is called a wheel and axle.
e. The ramp is used to lift a heavy load.
f. The hill roads are built to have gradual slopes.
8. Classify the given levers. Draw their diagrams showing fulcrum, points of
application of effort and load.
i) Scissors ii) Fire tongs (iii) Nail cutter
iv) Wheel-barrow v) Nut-cracker vi) Forceps
PHYSICS Oasis School Science and Environment - 8 47
9. Describe the importance of simple machines in our daily life.
10. Numerical Problems
a. A load of 1000 N can be lifted by applying an effort of 250 N. If the load arm
is 25 cm, calculate the effort arm. [Ans: 100 cm]
b. An effort of 20 N is applied to lift a load. If the load arm and effort arm are
15 cm and 60 cm respectively, calculate the load. [Ans: 80 N]
c. Neha and Reha are playing see-saw. Neha is sitting 60 cm away from
the fulcrum and Reha is sitting 40 cm away from the fulcrum. Calculate
the effort that Reha should apply to lift Neha. The weight of Neha is
360 N. [Ans: 540 N]
d. Study the given figure and calculate the load distance. [Ans: 20 cm]
40 cm 325 N
650N
?
e. An effort of 4 N is applied to lift a load of 20 N. If the effort arm is 5m,
calculate the following:
(i) load arm (ii) input work (iii) output work
[Ans: (i) 1m (ii) 20 Nm (iii) 20 Nm
f. In a block and tackle system of 3 pulleys, a load of 900 N is lifted by applying
an effort of 600 N. Calculate MA, VR and η of the pulley system.
[Ans : MA = 1.5, VR = 3, η = 50%]
g. A load of 100 N is lifted using an effort of 40 N by using a wheel and axle. If
the radius of the wheel is 15 cm and that of the axle is 5 cm, calculate the MA,
VR and ɳ of the wheel and axle.
[Ans : MA = 2.5, VR = 3, η = 83.33%]
500N
h. In an inclined plane, a load of 1000 N is lifted by 4m 12m 100L0oNad
applying an effort of 500N. Study the given figure and
calculate MA, VR and efficiency.
[Ans : MA = 2, VR = 3, η = 66.66%]
48 Oasis School Science and Environment - 8 PHYSICS
4uNIt Estimated teaching periods : Th Pr
4 1
Blaise Pascal
PreSSure
Objectives
After completing the study of this unit, students will be able to:
• introduce atmospheric pressure and explain its importance.
• introduce liquid pressure.
• derive the formula to calculate liquid pressure and solve simple numerical
problems related to pressure.
Course of Study
• Introuduction to pressure
• Atmospheric pressure
• Importance of atmospheric pressure
• Liquid pressure
• Measurement of liquid pressure
• Characteristics of liquid
• Density and relative density
• Floating and sinking
Points to be Focused/Questions to be Discussed
• What is pressure? What is its SI unit?
• What is atmospheric pressure?
• What is liquid pressure? What is its importance?
• What are the characteristics of liquid pressure?
• What is density? What is relative density?
• What is meant by floating and sinking?
PHYSICS Oasis School Science and Environment - 8 49
4.1 Introduction
We prefer a sharp knife for cutting vegetables
to a blunt one. A sharp knife cuts vegetables
better due to its thin edge. The force of our hand
falls over a small area of the object producing
a bigger pressure which cuts the object easily.
But a blunt knife does not cut an object easily
due to its thick edge. The force of our hand falls
over a large area of the object and produces less
pressure which cuts the object with difficulty. Fig. 4.1
Thus, the effect of the same force on different
areas is different. The force acting perpendicularly on a unit area of a surface is called
pressure. In SI system, pressure is measured in newton per square metre (N/m²). It is also
called pascal (Pa). Pressure is a scalar quantity.
4.2 Measurement of Pressure
The value of pressure is obtained by dividing the force acting perpendicularly on an object
by the area of the object on which the force acts. The total force acting perpendicularly on
a given surface is called thrust. So pressure can also be represented as follows:
Pressure (P)= Thrust (F)
Area (A)
From the above relation, it can be concluded that the pressure depends on the following
two factors:
(i) Force applied or the thrust (ii) Area over which the force acts
The same force can produce different pressures depending on the area over which it acts.
When a force acts over a large area of a surface, it produces a small pressure. But if the
same force acts over a small area, it produces a large pressure. When the force acting on a
surface increases, the pressure also increases and vice-versa.
Differences between force and pressure
Force Pressure
1. Force is the push or pull which changes or 1. The force acting perpendicularly on a unit
tries to change the position of an object. area of a surface is called pressure.
2. Its SI unit is N. 2. Its SI unit is Pa or N/m2.
pressure /ˈpreʃə(r)/ - the force acting perpendicularly on a unit area of a surface
50 Oasis School Science and Environment - 8 PHYSICS
Worked out Numerical 1
A wooden block of 800N occupies 2m² surface area. Calculate the pressure exerted.
Solution:
Given,
Force (F) = 800N
Area (A) = 2m²
Pressure (P) = ?
According to the formula,
P = F
A
P = 800
2
P = 400 N/m²
∴ The pressure exerted (P) = 400 Pa.
4.3 Atmospheric Pressure
The earth is surrounded by a thick layer of air which is called atmosphere. Air has
weight and it exerts pressure. The pressure exerted by atmosphere due to its weight is
called atmospheric pressure. The atmospheric pressure at the sea level is called standard
atmospheric pressure which is about 101300 N/m2 or 760 mmHg. The atmospheric
pressure decreases with an increase in altitude. So, the atmospheric pressure is less at the
top of Mt. Everest than that of the Terai.
Atmospheric pressure acts on each and every living organism including different objects
on the earth surface. But we do not feel the atmospheric pressure because it is neutralized
by the blood pressure. Thus, the pressure exerted by blood in our body is almost equal to
the atmospheric pressure.
Nose bleeding occurs as we go to a higher altitude since the pressure in our body remains
constant but the external atmospheric pressure decreases. Due to this reason, body
pressure is more than that of the atmospheric pressure and hence bleeding of nose starts.
Different altitudes have different atmospheric pressure. This helps blowing of air from
one place to another. The pressure in the aeroplanes and jet planes is adjusted so that
the passengers feel comfortable and they can breathe easily though they fly in a higher
altitude.
PHYSICS Oasis School Science and Environment - 8 51
Activity 1 Fig. 4.2
• Take a water trough and dip an empty glass as shown in
the figure. Tilt the glass gradually. Observe the activity.
Air bubbles come out of the glass. The glass which
is dipped into the water trough contains air though it
seems empty.
Activity 2
• Take a tin can and fill it Water Lid Atmospheric
with water leaving some vapour Partial pressure
space. Boil the water for few Water vaccum
minutes and close the lid.
Allow it to cool by pouring Tin
cold water over it. What do
you observe? Fig. 4.3
When the water in the tin can boils, it drives the air out of the can. On cooling the
can by closing the lid, a partial vacuum is formed inside the can. As a result, the
tin can shrinks due to the atmospheric pressure.
Activity 3
• Take a glass. Fill it with water up to the brim so Water
that there is no space for air. Cover the glass with a Glass
postcard. Invert the glass as shown in the figure. Now,
remove the hand from the postcard gently. What do Postcard
you observe?
Fig. 4.4
It is found that water does not fall from the glass immediately. The atmospheric
pressure helps to hold water in the glass by pressing the postcard upward.
Activity 4
• Take water in a glass. Suck the water slowly with a straw pipe.
How is it possible to draw water from the glass? Think!
When air in the straw is sucked, it creates partial vacuum inside
the pipe. So, it helps to reduce the pressure in the straw and
atmospheric pressure helps to raise the water level upward.
A similar process takes place while filling the ink in a fountain pen.
Fig. 4.5
52 Oasis School Science and Environment - 8 PHYSICS
Reasonable fact-1
Air blows from one part to another on the surface of the earth.
Air blows from one part to another on the surface of the earth due to change in
atmospheric pressure of earth's surface and due to effect of gravity.
Reasonable fact-2
A glass does not immerse in water when dipped straight downwards.
When a glass dipped straight downwards in water, the air present inside the glass
cannot flow outside the glass. Therefore, a glass does not immerse in water when
dipped straight downwards.
4.4 Importance of Atmospheric Pressure
Atmospheric pressure is very important for our day to day activities. We are able to use
various equipment due to the presence of atmospheric pressure. It is important to:
i. fill ink in a fountain pen.
ii. fill medicine in a syringe.
iii. fill air in a bicycle tube or tube of vehicles.
iv. lift water by using a water pump.
v. draw soft drinks through a straw.
vi. use rubber suction pads on the walls to hang clothes, calenders, etc.
4.5 Liquid Pressure
Liquids stored in vessels have their own weight. The weight acts on
the bottom of the vessel. Thus, liquids exert pressure at the bottom of
the vessel due to their weight. The thrust exerted by a liquid per unit
area of the surface is called liquid pressure.
The total force exerted by a liquid perpendicularly on a surface is h
called thrust of the liquid. If a thrust (F) is acting on a surface area A
(A) in contact with a liquid, the pressure exerted by the liquid on the Fig 4.6
surface is given by
Pressure (P) = Thrust (F)
Area (A)
∴ P = AF
The SI unit of liquid pressure is N/m2 or Pa (pascal).
PHYSICS Oasis School Science and Environment - 8 53
Pressure exerted by liquid column
Let us consider a cylindrical vessel of base area 'A' is filled with a liquid of density 'd'.
The pressure is exerted at the bottom of the vessel due to the weight of the liquid which
is given by
Pressure (P) = Force (F)
= Area (A)
Weight of the liquid (F)
Base area of the container (A) [∵W = F]
m.g m - mass of liquid
= A g - acceleration due to gravity [∵F = mg]
d.V.g ∵ density (d) = mass (m)
= A volume (V)
A.h.d.g
= A (∵ V = l × b × h = A × h)
∴ P = hdg Proved
From the formula P = hdg, it can be noted that liquid pressure depends on the following
factors:
(i) depth of the liquid from its free surface (h)
(ii) density of the liquid (d)
(iii) acceleration due to gravity (g)
Worked out Numerical 2
Calculate the pressure exerted by water at a depth of 2 m. The density of water is
1000 kg/m3.
Solution:
Density (d) = 1000 kg/m3
Depth (h) = 2 m
Acceleration due to gravity (g) = 9.8 m/s2
Pressure (P) = ?
We have,
P = hdg
54 Oasis School Science and Environment - 8 PHYSICS
= 2×1000×9.8
= 19600 Pa
∴ The pressure exerted by the water is 19600 Pa.
Worked out Numerical 3
A tank of dimensions 3m × 2m × 1m is filled with water. Calculate the pressure exerted
by water at the bottom of the tank.
Solution:
Depth (h) = 1m [ ∵ The height of the tank is 1m.]
Density (d) = 1000 kg/m3
Acceleration due to gravity (g) = 9.8 m/s2
Pressure (P) = ?
We have,
P = hdg
= 1 × 1000 × 9.8 = 9800 Pa.
The pressure exerted by water is 9800 Pa.
Worked out Numerical 4
A drum is filled with a liquid of depth 2m. Calculate the density of the liquid if the
pressure exerted by it is 5000 N/m2.
Solution:
Depth (h) = 2m
Pressure (P) = 5000 N/m2
Acceleration due to gravity (g) = 9.8 m/s2
Density (d) = ?
We have,
P = hdg
or, d = P = 25×0090.8 = 255.1 kg/m3
hg
∴ The density of the liquid is 255.1 kg/m3.
PHYSICS Oasis School Science and Environment - 8 55
4.6 Properties of Liquid Pressure
1. Liquid pressure is transmitted equally in all directions, if pressure is applied to
the liquid kept in a closed vessel.
Activity 5
• Take a plastic bag and make holes on it with a pin. Fill Fig. 4.7
the bag with water and close the lid. Press the bag as
shown in the figure and observe the flow of water through
the holes. What do you know from this activity ?
We can see that the force of water coming out of each
hole is the same. This is due to the property of liquid
that liquid pressure is transmitted equally in all
directions. This law is called Pascal's law. This law was
propounded by Blaise Pascal.
2. Liquid pressure increases with increase in depth.
Compare this property with the above property. If the liquid is acted by external
pressure, liquid pressure is transmitted equally in all directions. If not so, liquid
pressure increases with increase in depth. Liquid pressure (P) is directly proportional
to the height (h) of the liquid column, i.e. P ∝ h. Therefore, water dams are made
thicker at the base so that the bottom can withstand more pressure due to water.
Activity 6
• Take a bottle and make 4 different holes A, B, C and D at
different heights as shown in the figure. Close the holes by A
using cork and fill the bottle with water. Open all these holes B
simultaneously and observe the flow of water.
• As we go from top to bottom of the bottle, i.e. A to D, the C
pressure of the flow of water increases. D
• Due to this property, the bucket in downstairs tap fills Fig. 4.8
faster than upstairs tap. Also, the base of a dam is made
wider to hold more pressure given by the water.
But it is to be noted that if the bottle is pressed, the rate of flow of water through
all holes remains the same.
density /ˈdensəti/ - the mass per unit volume of a substance
56 Oasis School Science and Environment - 8 PHYSICS
3. Liquid determines its own level.
Activity 7
• Take a Pascal tube and fill it with water. Observe the level in all the tubes.
What do you find?
You will find the same height of liquid in all the tubes.
Fig. 4.9 Liquid maintains its own level
4. Liquid pressure is directly proportional to the density of the liquid. Liquid having
less density exerts less pressure and liquid having more density exerts more pressure.
So, if mercury and water are kept in two similar vessels up to the same height, the
pressure exerted by mercury is more than that by the water.
4.7 Density of a Substance
Density of a substance is defined as the mass per unit volume of the substance. Generally,
we say that iron is heavier than plastic. It means that the density of iron is more than
that of the plastic. Substances differ from one another in their densities. In other words,
the same mass of some substances occupy less space and others occupy more space. For
example, one kilogram of iron occupies less space but cotton of the same mass occupies
much more space.
1 kg cotton 1 kg iron
Fig. 4.10
If 'm' is the mass of an object and 'V' is its volume, the density (d) of the object is calculated by
Density (d) = Mass (M)
Volume (V)
∴ d = m
V
PHYSICS Oasis School Science and Environment - 8 57
The density of a substance is measured in kilogram per cubic metre (kg/m3), gram per
cubic centimetre (g/cm3), etc. The density of pure water is 1g/cm3 or 1000 kg/m3 at 40C.
Activity 8
• Take a cuboid of tin and calculate its volume by measuring its length, breadth
and height.
• Fill the cuboid completely with sand and measure the weight of the sand.
• Remove the sand from the cuboid. Fill the cuboid completely with water and
measure its weight.
Fig. 4.11
• Calculate the density of sand and water. What can you conclude from this
activity?
4.8 Relative Density
We know that iron is heavier than wood or wood is lighter than iron. It means that the
density of iron is more than that of the wood. Similarly, the density of iron is more than
that of water. But the density of water is more than that of ice. In the first case, we said
that density of iron is more than that of water and in the second case, we said that density
of water is more than that of ice. It creates confusion. In order to avoid this confusion, the
density of different substances is compared to the density of pure water. Relative density of a
substance is defined as the ratio of the density of the substance to the density of pure water.
Density of the substance
Relative density (R.D.) = Density of pure water
Relative density is the ratio of the density of two substances. So relative density has no
unit. The concept of relative density tells us how many times the density of a substance is
less or more than that of pure water at 40C.
The relation between density and relative density of a substance can be given as:
Density of a body = R.D. of the body × Density of pure water
If the relative density of a body is less than that of water (i.e. less than 1 g/cm3), the
substance will float. If the relative density is more than that of water (i.e. more than
1g/cm3), the body will sink.
58 Oasis School Science and Environment - 8 PHYSICS
Differences between density and relative density
Density Relative Density
1. It is the mass per unit volume of a 1. It is the ratio of the density of a substance
substance. to the density of pure water.
2. Its SI unit is kg/m3. 2. It has no unit.
Activity 9
• Repeat the activity 6 and calculate the relative density of sand. Calculate the
relative density of soil, cotton and salt solution by the same method.
Worked out Numerical 5
The mass of a wooden block is 64 kg. If its density is 800 kg/m3, calculate its volume.
Solution:
Given, Mass (m) = 64 kg
Density (d) = 800 kg/m3
Volume (V) = ?
We know, m
d = V
or, 800 = 64
V
64
or, V = 800 = 0.08 m3
∴ The volume of the wooden block is 0.08m3.
Worked out Numerical 6
The density of mercury is 13.6 g/cm3. If the density of water is 1g/cm3, calculate the
relative density.
Solution:
Given,
Density of mercury = 13.6 g/cm3
Density of water = 1 g/cm3
Relative density = ?
PHYSICS Oasis School Science and Environment - 8 59
We know,
Density of mercury
Relative density = Density of water
Relative density = 13.6 g/cm3
1 g/cm3
= 13.6
∴ Relative density = 13.6.
4.9 Floating and Sinking
When a piece of stone is thrown in water, it sinks. But a piece of ice cube floats. If the
density of a body is less than that of water, it floats and if the density of the body is
more than that of water, it sinks. For example, if a piece of iron is kept in water, it sinks.
Because, the density of iron (i.e. 7.8 g/cm3) is more than the density of water (i.e. 1 g/
cm3). But when the same piece of iron is kept in mercury, it floats because the density of
mercury is more (i.e. 13.6 g/cm3) than that of iron (i.e. 7.8 g/cm3). Therefore, the floating
or sinking of a body depends on the density of the body and the density of the liquid in
which the body is kept.
Water Iron nail Ice cube
Iron nail Mercury Water
Fig. 4.12 Sinking and floating objects
The substances whose density is more than that of a liquid sink in the liquid and the
substances whose density is less than that of a liquid, float on the liquid.
Reasonable fact-3
A piece of plastic floats on water but a piece of iron sinks.
A piece of plastic floats on water because the density of a piece of plastic is less than
that of water but the density of piece of iron is more than that of water. So, a piece of
iron sinks in water.
60 Oasis School Science and Environment - 8 PHYSICS
Reasonable fact-4
A balloon bursts when filled with more air.
When a balloon is filled with more air, the pressure of air inside the balloon is greater
than that of the atmospheric air pressure and that air tries to escape out from the
balloon. As a result, balloon bursts when filled with more air.
Activity 10 egg
• Take a beaker and fill it with water. egg
Keep an egg in the beaker. Does the (a) (b)
egg sink? Now, dissolve some salt in
the beaker and form a concentrated Fig. 4.13
salt solution. Does the egg float? What
can you conclude from this activity?
SuMMarY
• The force acting perpendicularly on a unit area of a surface is called pressure.
• The total force acting perpendicularly on a given surface is called thrust.
• The earth is surrounded by a thick layer of air which is called atmosphere. Air
has weight and it exerts pressure.
• The thrust exerted by a liquid per unit area of the surface is called liquid
pressure.
• Density of a substance is defined as the mass per unit volume of the substance.
• Liquid pressure is directly proportional to the density of the liquid.
• Relative density of a substance is defined as the ratio of the density of the
substance to the density of pure water.
• The floating or sinking of a body depends in the density of the body and the
density of the liquid in which the body is kept.
• The substances whose density is more than that of a liquid sink in the liquid
and the substances whose density is less than that of a liquid, float on the liquid.
• If the relative density of a body is less than that of water (i.e. less than 1 g/cm3), the
substance will float. If the relative density is more than that of water (i.e. more than
1g/cm3), the body will sink.
PHYSICS Oasis School Science and Environment - 8 61
exercise
1. Choose the best answer from the given alternatives.
a. The atmospheric pressure at sea level is .................... .
i. 11300 N/m² ii. 101300 N/m2
iii. 30110 N/m iv. 110300 N/m2
b. The formula to calculate liquid pressure is .................... .
i. P = hdg ii. P = Adg
iii. P = hdv iv. P = dAhg
c. Liquid exerts pressure in .................... .
i. upward direction ii. downward direction
iii. all directions iv. one direction
d. The SI unit of density is.................... .
i. kg/m–3 ii. kg/cm3
iii. g/cm3 iv. kg/m3
e. Which of the following bodies floats on water?
i. plastic ii. iron nail iii. stone iv. brick
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. The atmospheric pressure is maximum at sea level.
b. The SI unit of pressure is newton.
c. Liquid pressure increases with depth.
d. Mass per unit volume of a body is called density.
e. The substances whose density is more than a liquid float on
the liquid.
3. Fill in the blanks with appropriate words.
a. The pressure exerted by .................... is called atmospheric .................... .
b. Liquids exert pressure equally in all .................... .
c. The CGS unit of density is .................... .
d. .................... is the ratio of density of a substance to the density of pure water.
e. The substance whose density is .................... than that of water floats.
62 Oasis School Science and Environment - 8 PHYSICS
4. Answer the following questions.
a. What is pressure? Write down its SI unit.
b. What is atmospheric pressure? Write down its importance.
c. What is liquid pressure? Write down the factors that affect the liquid pressure.
d. Write down the characteristics of liquid pressure.
e. What is density? Write down its SI unit.
f. What is meant by relative density of a substance?
g. What type of objects float and sink in water? Write.
5. Differentiate between:
a. Atmospheric pressure and Liquid pressure
b. Density and Relative density
6. Give reasons.
a. Atmospheric pressure decreases with increase in altitude.
b. The base of a dam is made wider.
c. A balloon bursts when more air is filled in it.
d. Relative density has no unit.
e. An egg sinks in fresh water but floats on concentrated salt solution.
7. Describe an experiment to demonstrate that air exerts pressure.
8. Describe an experiment to show that liquid pressure increases with increase in
depth.
9. Describe an activity to find the relative density of soil.
10. Numerical problems:
a. The mass of a box is 300 kg and its base area is 15 m2. Calculate the pressure
exerted by the box on the ground (g = 9.8 m/s2). [Ans: 196 N/m2]
b. Calculate the force applied if 100 N/m2 pressure is exerted over the area of 0.2
m2. [Ans: 20 N]
c. Calculate the pressure of water in a well if the depth of the water is 10 m.
[Ans: 98000 Pa]
PHYSICS Oasis School Science and Environment - 8 63
d. Calculate the pressure exerted at the bottom of the given container.
[Ans: 8820 Pa]
d = 300 kg/m3
3m
A
e. If a force of 2000N acting on a surface exerts 200 N/m2 pressure, calculate the
area of the surface. [Ans: 10 m2]
f. Calculate the pressure exerted on the ground by a boy of mass 60 kg if he
stands on one foot. The area of the sole of his shoe is 150cm2.
[Ans: 39200 Pa]
g. Calculate the mass of an ice block of volume 12 m3. The density of ice is 920
kg/m3. [Ans: 11040 kg]
h. The dimension of a water tank is 5m×4m×2m. Calculate the pressure at the
bottom of the tank when it is half-filled. The density of water is 1000 kg/m3
and acceleration due to gravity at that place is 9.8 m/s2. [Ans: 9800 Pa]
i. The density of water is 1000 kg/m3 and that of mercury is 13600 kg/m3.
Calculate the relative density. [Ans: 13.6]
64 Oasis School Science and Environment - 8 PHYSICS
5uNIt Estimated teaching periods: Th Pr
3 0
Crane
ENERGy, WORk AND
POWER
Objectives
After completing the study of this unit, students will be able to:
• differentiate between energy, work and power and explain relationship
among them.
• describe the transformation of energy and demonstrate it.
• write the formula to calculate work, energy and power and solve simple
numerical problems.
Course of Study
• Energy and kinds of energy
• Mechanical energy
• Work and its types
• Transformation of energy
• Power
• Relation between energy, work and power
Points to be Focused/Questions to be Discussed
• What is energy? What are its types?
• What is mechanical energy?
• Define potential energy and kinetic energy.
• What is work done?
• What are the types of work done?
• What is meant by transformation of energy?
• What is power? What is its SI unit?
• What is the relation among energy, work and power?
PHYSICS Oasis School Science and Environment - 8 65
5.1 Introduction
Plants and animals need energy to survive. Food provides necessary energy to our body.
The capacity or ability of a body to do work is called energy. Work is said to be done when
a body moves in the direction of the force applied. Both energy and work are measured in
joule (J). The amount of energy possessed by a body is equal to the amount of work it can
do when energy is released. Similarly, the rate of doing work is called power. The SI unit
of power is watt (W). Energy, work and power are closely related to each other.
5.2 Energy
When a body is capable of doing a work, it is said to possess energy. The reverse is also
true. Thus, energy is defined as the capacity of doing work. Energy is a scalar quantity.
The SI unit of energy is joule (J) and its CGS unit is erg. The amount of energy possessed
by a body is equal to the amount of work it can do when the energy is released.
Activity 1
• We use many sources of energy like firewood, biogas, petrol, diesel, etc. Enlist
work isst hodeuo rsncoeeu,s re ocnefe ser ngwyehr igiscy hc oa yrnoesu ud meupseeed na.dt eynotu orn h tohme es.u Dn.i sPcruespsa irne sam sahlol rgt rroeupposr th oonw i tc. ertain
5.3 Different Forms of Energy
There are several forms of energy. Some of them are explained below:
1. Mechanical Energy
The energy possessed by a body by virtue of its motion or position or configuration
is called mechanical energy. Mechanical energy is of two types:
a. Kinetic energy b. Potential energy
a. kinetic Energy (kE)
The energy possessed by a body due to its motion is called kinetic energy. The bullet
fired from a gun, a moving ball, a flying bird, a rolling stone, etc. are some examples
of the objects possessing kinetic energy.
Activity 2
• Take a heavy ball. Drop it on a thick bed of sand, a wet bed of sand would be
better. Drop the ball on the bed from the height of about 25 cm. The ball creates
a depression.
• Repeat this activity from the heights of 50 cm, 1 m and 1.5 m. Ensure that all the
depressions are distinctly visible. Compare the depth in each case. Which one
of them is the deepest and which one is the shallowest? Why?
capable /ˈkeɪpəbl/ - having the ability for doing sth
66 Oasis School Science and Environment - 8 PHYSICS
Derivation of the formula of kE
Let us consider a body of mass 'm' initially at rest (u = 0) is moving with an acceleration
'a'. Let 'v' be the final velocity acquired by the body in moving distance 's'.
According to the equation of motion,
We have, u=0 a
v2 = u2 + 2as F
s
or, v2 = 02 + 2 as [since, u = 0] Fig. 5.1
or, s = 2va2 …………….. (i)
Again,
KE = Work done
= F× s
= m.a × 2va2 [∵ F = ma, s = 2va2 ]
1
= 2 mv2
∴ KE = 21 mv2
From the above formula, it becomes clear that:
i. the kinetic energy (KE) of a body is directly proportional to the mass (m) of the
body, i.e. KE ∝ m,
ii. the kinetic energy (KE) of a body is directly propertional to the square of the
velocity (v), i.e. KE ∝ v2.
Worked out Numerical 1
Calculate the kinetic energy of a body of mass 100 g moving with the velocity of 20 m/s.
Solution: Mass of the body (m) = 100 g = 100 kg = 0.1 kg
Velocity (v) = 1000
20 m/s
Kinetic energy (KE) = ?
We have, KE = 11
2 mv2 = 2 × 0.1×202 = 20 J
∴ The kinetic energy (KE) = 20 J
b. Potential Energy (PE)
The energy possessed by a body resting at a certain height due to its position or
configuration is called potential energy. For example, a stretched rubber, compressed
spring, leg lifted to kick, water stored in a dam possess the potential energy.
PHYSICS Oasis School Science and Environment - 8 67
Differences between Potential energy and kinetic energy
Potential energy kinetic energy
1. Potential energy is the energy possessed 1. Kinetic energy is the energy possessed by
by a body by virtue of its position or a body by virtue of its motion.
configuration.
2. Potential energy is calculated by PE = mgh. 2. Kinetic energy is calculated by KE = 1/2
mv2.
Reasonable fact-1
The potential energy of a stone kept on the earth's surface is zero.
The potential energy of a stone kept on the earth's surface is zero because the distance
between stone and earth's surface is zero. Since, potential energy is the product of
mass, acceleration due to gravity and height. So, if height is zero, then potential energy
is also zero.
Activity 3
• Take a slinky. Ask your friend to hold one of its ends. You hold the other end
and move away from your friend. Now, you release the slinky. What happens?
How does the slinky acquire energy when it is stretched?
Derivation of the formula of PE h
Let us consider a body of mass 'm' is raised to a height of 'h'. Then, Fig. 5.2
PE = Work done (W)
= F× s [∵ W = F×s]
= m.g × h [∵ F = mg and s = h]
∴ PE = mgh
This energy is also called gravitational potential energy.
Worked out Numerical 2
Calculate the potential energy of a body of mass 15 kg if it is raised to a height of 10 m.
Solution: = 15 kg
Mass of the body (m) = 10m
Height (h) = ?
Potential energy (PE)
We have, = mgh
PE = 15 × 9.8 × 10 [∵ g = 9.8 m/s2]
= 1470 J
∴ The potential energy of the body (PE) = 1470 J.
68 Oasis School Science and Environment - 8 PHYSICS
2. Heat Energy
The energy produced due to the vibration of molecules Fig. 5.3 Electric heater
of a body is called heat energy. For example, an electrical
heater, burning coal, etc. possess heat energy. The heat
produced by burning petrol in a motorcycle engine
provides the energy needed to run the motorcycle.
3. Light Energy
When a body is extremely heated, it emits light. Light is a form
of energy which produces the sensation of vision. For example,
energy possessed by a glowing bulb, a burning candle, etc. The
sun is the natural source of light energy. The solar energy is
used by green plants for photosynthesis. Fig. 5.4 Sun
4. Sound Energy
The form of energy produced due to the vibration of
a body is called sound energy. It travels only through
the material medium. An electric bell, a temple bell,
an earphone, etc. produce sound. Hence, these are
some sources of sound energy.
5. Electrical Energy Fig. 5.5 Temple bell
The energy which is generated due to the flow of electrons
through a conductor is called electrical energy. When
electrons flow through a conductor, current is produced
which is called electrical energy. There are different
sources of electrical energy such as hydroelectricity, cell,
battery, dynamo, generator, etc. Fig. 5.6 Lighting bulb
Reasonable fact-2
Electrical energy is widely used among the various types of energy.
Modern life is not possible without electrical energy. Electrical energy is used to run
machines in factories. It is used to operate computer, television, mobile phone, electric
trains, trolley buses, lift, fans in summer and heater in winter, motor to pump water
from a well, etc. Since, electrical energy has more advantage than other forms of
energy, electrical energy is widely used among the various types of energy.
6. Chemical Energy
Different substances such as food, coal, petrol, firewood, diesel, kerosene, etc. store
chemical energy. The energy stored in these substances is released when a chemical
change takes place. This type of energy is called chemical energy.
vibration /vaɪˈbreɪʃn/ - a continous shaking movement
PHYSICS Oasis School Science and Environment - 8 69
Fig. 5.7. Petrol
7. Magnetic Energy
The energy possessed by a magnet is called magnetic
energy. It can be used to run electric bell, to produce
hydroelectricity, etc.
Fig. 5.8 Magnetic energy
8. Nuclear Energy
The energy produced from the nucleus of an atom is
called nuclear energy. It can be produced when two
or more light nuclei combine to form a single heavy
nucleus (nuclear fusion) or when a heavy nucleus
splits into two or more light nuclei (nuclear fission).
This energy is also called atomic energy.
Fig. 5.9 Nuclear energy
5.4 Work
In general, the term work is used to describe any activity in which muscular or mental
effort is exerted. But in physics, work is said to be done only when a body moves in the
direction of the force applied. Work done on a moving body is equal to the product of
force exerted on the body and the distance covered by the body in the direction of force
(displacement), i.e.
Work done (W) = Force (F) × Displacement (s)
∴ W = F × s
When we sit on a chair and study for 2-3 hours, we are not doing work from a physicist's
point of view. Similarly, no work is done by a person standing on a place for a long time.
fission / ˈ f ɪ ʃ n / - the process of splitting the nucleus of an atom
70 Oasis School Science and Environment - 8 PHYSICS
Work is said to be done when the force applied to a body succeeds in moving it. If a horse
pulls a cart and covers some distance, it does some work. Thus, the work done by a body
depends on following two factors:
i) the magnitude of the force applied (F)
ii) the displacement produced by the force (s)
No work is done Fig. 5.10 Some work is done
The SI unit of force is newton (N) and that of displacement is metre (m). So the SI unit
of work done is newton metre (Nm). It is also called joule (J). In CGS system, work is
measured in erg. Work is a scalar quantity.
One joule work
The work done is said to be 1 joule if 1 N force can displace a body through 1 m distance
in the direction of the force applied.
Since, W = F × s Initial position Final position
1 J = 1 N × 1m 1m
1N
Relation between joule and erg Fig. 5.11 One joule work done
We have,
Reasonable fact-3
W = F × s
No work is done when a person is
1 J = 1 N × 1 m standing by carrying a load of 50
kg for one hour.
= 105 dyne × 100 cm [∵ 1 N = 105 dyne]
No work is done when a person is
= 107 dyne. cm standing by carrying a load of 50 kg
for one hour because for the work
= 107 erg. 107 erg to be done by a person there should
be some distance covered by the
∴ 1 J = person because work done is the
product of force and displacement.
gravity /ˈɡrævəti/ - the force with which a body is pulled towards the earth's surface
PHYSICS Oasis School Science and Environment - 8 71
5.5 Types of Work mg
h
1. Work done against gravity
Fig. 5.12
2. Work done against friction
1. Work done against gravity
Gravity is the force by which a body is pulled towards the
centre of the earth. The work done by lifting a body from
the earth's surface is called work done against gravity. For
example, the work done by lifting an object. Let us consider a
body of mass 'm' is lifted to a height 'h'. Then,
Work done against gravity (W) = Force × displacement
or, W = Weight × height
or, W = m.g × h
∴ W = mgh
Reasonable fact-4
2. Work done against friction Work cannot be done for a long
The force which opposes the motion of a body time without having food.
moving on the other body when they are in For work to be done we have to
contact is called friction. The work done by apply certain force on the body.
dragging a body over the surface of another If we don't eat food for long time
body is called work done against friction. For the potential energy stored in our
example, a body dragging over a surface of muscle will be finished. As a result,
no energy is available to do any
another body. work. So, work cannot be done for
a long time without eating food.
Worked out Numerical 3
Calculate the work done if a force of 30 N is applied to move a body through 15 m.
Solution:
Force (F) = 30 N
15 m
Displacement (s) = ?
Work (W) = F × s
30 × 15
We have, W = 450 J
=
=
∴ The work done (W) = 450 J.
72 Oasis School Science and Environment - 8 PHYSICS
Worked out Numerical 4
Calculate the work done when a person of 40 kg climbs a 12 m tall tree.
Solution:
Mass of the person (m) = 40 kg
Displacement (s) = h = 12 m
Work done (W) = ? [∵ F = mg]
We have, [∵ 1 kg = 9.8 N]
W = F × s
= mg × h
= 40 × 9.8 × 12
= 4704 J
∴ The work done (W) = 4704 J.
5.6 Transformation of Energy Fig. 5.13
Transformation of energy is the process in which one form
of energy is converted into another form. For example, the
conversion of electrical energy into light energy and heat energy
by using an electric bulb. Some examples of transformation of
energy are given below:
Transformation of energy while using torchlight
Battery Copper wire Electric bulb
Heat and light energy
Chemical energy Electrical energy
Transformation of energy in hydroelectric project
Water stored in a dam Falling water Rotation of turbine Electricity in coil
Electrical energy
Potential energy Kinetic energy Kinetic energy
Transformation of energy while using an electric fan
Current in wire Formation of electromagnet in the coil Movement of fan blades
Kinetic energy
Electrical energy Magnetic energy
PHYSICS Oasis School Science and Environment - 8 73
Some devices and objects with their energy conversion
a. Solar cell : Light energy into electrical energy
: Sound energy into electrical energy
b. Microphone : Chemical energy into electrical energy
: Electrical energy into heat energy
c. Battery or Cell : Mechanical energy into electrical energy
: Mechanical energy into electrical energy
d. Heater : Electrical energy into light energy and heat energy
: Electrical energy into mechanical energy
e. Turbine : Electrical energy into sound energy
: Electrical energy into magnetic energy
f. Dynamo/Generator : Electrical energy into light and sound energy
g. Electrical bulb
h. Electric motor
i. Loudspeaker
j. Electromagnet
k. Television
l. Burning of fuels : Chemical energy into heat and light energy
m. Explosion of crackers/bombs : Chemical energy into heat, light and sound energy
5.7 Principle of Conservation of Energy
According to this principle, "Energy can neither be created nor be destroyed but it can be
changed from one form to another."
For example, if 100 J of electrical energy is supplied to an electric bulb, it converts 10 J
of electrical energy into light energy and remaining 90 J of electrical energy into heat
energy, i.e.
Electrical energy = Heat energy + Light energy
100 J = 90 J + 10 J
5.8 Power
Suppose an old man takes 10 minutes to do a particular work whereas a young man takes
only 5 minutes to do the same work. It is clear that the rate of doing work of the young
man is more than that of the old man. So, the rate of doing work is called power. It is a
scalar quantity. Power is measured in watt (W) or J/s or horsepower (h.p.) The SI unit of
power is watt (W). Power is calculated by the given formula.
Power (P) = Work done (W)
Time taken (t)
∴ P = Wt
explosion /ɪkˈspləʊʒn/ - the sudden violent bursting and loud noise of sth
conservation /kɒnsəˈveɪʃn/ - the act of preventing sth from being lost, wasted, damaged or destroyed
74 Oasis School Science and Environment - 8 PHYSICS
From the above formula, it becomes clear that power of a body depends on two factors,
viz. (i) the amount of work done and, (ii) the time taken to do the work.
1. When a body does larger amount of work in a particular time, its power is said to be
greater and vice-versa, i.e. P ∝ W.
2. When a body takes lesser time to do a particular amount of work, the power is said
to be greater and vice-versa, i.e. P ∝ 1 .
t
One watt power
The power is said to be 1 watt if 1 joule work is done in 1 second.
Since, P = Wt
1J
1 J = 1s
Watt (W) is an important unit of power as it is widely used in electrical work. A bulb
of 60W consumes electrical energy at a rate of 60 joules per second. Another unit of
power is horse power (h.p.) which is equal to 746 W. The relation among different
units of power is given below:
1000 W = 1 kilowatt (kW)
1000 kW = 1 mega watt (MW)
746 W = 1 horse power (h.p.)
Meaning of 100 W written on an electrical bulb
Since, P = Wt
∴ 100 W = 100 J
1s
Hence, 100W written on an electric bulb means that the bulb converts 100 J of electrical
energy into heat energy and light energy in 1 second.
Differences between Work and Power
S.N. Work S.N. Power
1. Work is the product of force and 1. Power is the rate of doing work.
displacement in the direction of force.
2. Its SI unit is joule (J). 2. Its SI unit is watt (W).
3. It is not affected by the time taken. 3. It is affected by the time taken.
PHYSICS Oasis School Science and Environment - 8 75
Worked out Numerical 5
Calculate the power of a man of mass 40 kg if he can climb a staircase of height 10m in 5
seconds.
Solution:
Given,
Mass of the man (m) = 40 kg
Height of the staircase (h) = 10 m
Time (t) = 5 s
Power (P) = ?
We have,
P = Wt
= F×s [∵ W = F × s]
= t
= m.g.×h [∵ F= mg and s = h]
= t
∴ The power of the man (P) =
40×9.8×10 [∵ g = 9.8 m/s2]
5
3920 = 784
5
784 W.
Worked out Numerical 6
There are 15 steps in a staircase and the height of each step is 12 cm. A person of mass 40 kg
can climb up the staircase in 9 seconds. Find the power of the person.
Solution:
Height of each step of the stair case = 12 cm
No. of steps = 15
∴Total height of the staircase (h) = 12 × 15
= 180 cm = 1.8 m [∵1m = 100cm]
Mass of the person (m) = 40 kg
Time taken (t) = 9 s
Power (P) = ?
We have,
P = W = F×h [∵distance travelled (s) = h]
tt
76 Oasis School Science and Environment - 8 PHYSICS
= m×g×h
t
= 40×9.98×1.8
= 78.4 W
∴The power of the person is 78.4 W.
Worked out Numerical 7
A crane lifts a load of 75000 N to a height of 20 m in 100 s. Calculate the power of the crane.
Aslo, find out the power in horse power.
Solution:
Load (F) = 75000 N
Height (h) = 20 m
Time taken (t) = 100 s
Power (P) = ?
We have,
P = W = F×h = 75000×20 = 15000 W
tt 100
The power of the crane = 15000 W = 17540600 h.p. [ ∵ 1 h.p. = 746 W]
= 20.1 h.p.
∴ The power of the crane (P) = 20.1 h.p.
Worked out Numerical 8
A porter carries 40 bricks, each of weight 10N, to a distance of 75m in 50s. What is his power ?
Solution:
Total bricks = 40
Weight of each brick = 10 N
∴ Total weight of the bricks (F) = 40 × 10 = 400 N
Distance (s) = 75 m
Time taken (t) = 50 s
Power (P) = ?
We have,
P = Wt = F×th = 400 × 75 = 600 W
∴The power of the porter is 600 W. 50
PHYSICS Oasis School Science and Environment - 8 77
5.9 Relation Between Energy, Work and Power
Energy, work and power are interrelated to each other. The capacity to do work is called
energy. We need energy to do work. We eat food to get energy. The food consists of
chemical energy. The chemical energy present in the food changes into muscular energy
after digestion. We use muscular energy to do work. If we do not eat food, we feel weak
and cannot do work properly.
The rate of doing work is called power. Two persons that do the same amount of work
may have different power. Power depends on time taken but the work done does not
depend on time. The capacity to do work is called energy and the rate of doing work is
called power. Hence, energy, work and power are closely related to each other.
SuMMarY
• The capacity or ability of a body to do work is called energy. Its SI unit is joule (J).
• The energy possessed by a body by virtue of its motion or position or configuration
is called mechanical energy.
• The energy possessed by a body resting at a certain height due to its position or
configuration is called potential energy.
• The energy produced due to the vibration of molecules of a body is called heat energy.
• Light is a form of energy which produces the sensation of vision.
• The energy which is generated due to the flow of electrons through a conductor is
called electrical energy.
• The energy possessed by a magnet is called magnetic energy. It can be used to run
electric bell, to produce hydroelectricity, etc.
• The energy produced from the nucleus of an atom is called nuclear energy.
• The work done by lifting a body from the earth's surface is called work done against
gravity.
• The work done by dragging a body over the surface of another body is called work
done against friction.
• Transformation of energy is the process in which one form of energy is converted
into another form.
• The rate of doing work is called power. It is a scalar quantity. Its SI unit is watt (W).
78 Oasis School Science and Environment - 8 PHYSICS
exercise
1. Choose the best answer from the given alternatives.
a. The SI unit of energy is ............................ .
i. joule ii. newton iii. watt iv. pascal
b. The formula to calculate potential energy is ........................... .
i. PE = dgh ii. PE = mgh
iii. PE = 1 mv2 iv. PE = Wt
2
c. Which of the following has kinetic energy?
i. stretched rubber ii. running water
iii. water in a dam iv. stretched catapult
d. Electric fan converts ........................... .
i. chemical energy into mechanical energy
ii. electrical energy into kinetic energy
iii. magnetic energy into electrical energy
iv. kinetic energy into electrical energy
e. The rate of doing work is called ........................... .
i. force ii. energy iii. work iv. power
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. The capacity to do work is called energy.
b. The water stored in a dam contains kinetic energy.
c. Light bulb converts electrical energy into light and heat energy.
d. The rate of doing work is called energy.
e. The SI unit of power is watt (W).
3. Fill in the blanks using appropriate words.
a. The energy contained in a body at a certain height is .................... .
b. .................... is measured in joule.
c. Flying bullet contains .................... energy.
PHYSICS Oasis School Science and Environment - 8 79
d. Generator converts .................... energy into electrical energy.
e. The rate of doing .................... is called power.
4. Answer the following questions.
a. What is energy? Write down its SI unit.
b. What is mechanical energy?
c. Define potential energy with any two examples.
d. What is kinetic energy? Give any two examples.
e. Name the factors on which the potential energy and kinetic energy of a body
depend.
f. Define light energy and magnetic energy.
g. What is meant by work against friction? Give one example.
h. What is meant by work against gravity? Give one example.
i. What is transformation of energy? Give two examples.
j. State the law of conservation of energy.
k. What is power? Write its SI unit.
l. Describe in brief the relationship among energy, work and power.
m. Define 1 joule work and 1 watt power.
n. What is meant by 60 W written on an electric bulb?
5. Differentiate between:
a. Potential energy and Kinetic energy
b. Work against friction and Work against gravity
c. Work and Power
6. Name the form of energy present in each of the following:
a. Running water
b. Stretched catapult
c. Bullet fired from a gun
d. Battery
e. Bread
f. Burning candle
g. Lighting bulb
h. Ringing bell
i. Water stored in a dam
80 Oasis School Science and Environment - 8 PHYSICS
7. Name the equipment that transforms energy as follows:
a. Light energy into electrical energy
b. Mechanical energy into electrical energy
c. Electrical energy into heat energy
d. Electrical energy into light energy
e. Chemical energy into heat and light energy
f. Mechanical energy into sound energy
g. Sound energy into electromagnetic energy
h. Electromagnetic energy into sound energy
8. Numerical Problems
a. An object can be displaced to 5 m by the application of 20 N force. Calculate
the work done. [Ans: 100 J]
b. Calculate the work done if a boy of mass 20 kg is taken to a height of 2 m.
(take g = 9.8 m/s2) [Ans: 392 J]
c. Calculate the potential energy contained by the water of 2500 kg in a dam if
the height of the water column is 12m. [Given : density of water = 1000 kg/m3
and acceleration due to gravity = 9.8 m/s2.] [Ans : 294000J]
d. An object of mass 20 kg is moving with the velocity of 15 m/s. Calculate the
kinetic energy. [Ans: 2250 J]
e. A crane can lift a load of 3000 N to a height of 10 m in 10 seconds. Calculate
the power of the crane in horse power. [Ans: 4.02 h.p.]
f. A porter carries 30 bricks to a distance of 50 m in 25 s. If the weight of one
brick is 12N, calculate the work done and power. [Ans: 18000J, 720W]
PHYSICS Oasis School Science and Environment - 8 81
6uNIt Estimated teaching periods : Th Pr
4 1
Celsius
Heat
Objectives
After completing the study of this unit, students will be able to:
• define heat and temperature and differentiate between them.
• explain the determination of units of temperature (Celsius and Fahrenheit)
and show the relationship between these units.
• explain the structure and working mechanism of simple and clinical
thermometer.
Course of Study
• Heat and temperature
• Thermometer and its structure
• Thermometric liquids (mercury and alcohol)
• Calibration of thermometer
• Transformation of units of temperature
• Types of thermometer
Points to be Focused/Questions to be Discussed
• What is meant by heat and temperature?
• What is a thermometer?
• What are thermometric liquids?
• What is meant by calibration of thermometer?
• What are different units of temperature?
• What are different types of thermometer?
82 Oasis School Science and Environment - 8 PHYSICS
6.1 Introduction
Heat is a form of energy. It produces the sensation of warmth. When two bodies, one hot
and another cold, come into contact, heat flows from the hot body to the cold one. So, it is
to be noted that heat always flows from the hot body to the cold body. Thus, we feel cold
when we touch the ice due to the flow of heat from our body to the ice. Similarly, we feel
hot when we touch the boiling water due to the flow of heat from the hot water to our body.
Matter is made up of different molecules. Heat produces a kind of vibration in these
molecules. If the vibration of the molecules of a body increases, heat produced from it
also increases. The sum of the kinetic energy of molecules of a body is called heat energy.
The SI unit of heat is joule (J) and CGS unit is calorie (cal.). Heat of a body is measured
by calorimeter.
Calorimeter Thermometer
Fig. 6.1
The degree of hotness or coldness of a body is called its temperature. It is the average kinetic
energy of molecules of a body. The SI unit of temperature is kelvin (K) and some other
units of temperature are °C (degree Celsius), and °F (degree Fahrenheit). Temperature of
a body is measured by thermometer.
6.2 Differences between Heat and Temperature
S. N. Heat S. N. Temperature
1.
It is a form of energy which 1. It is the degree of hotness or
2. produces the sensation of warmth. coldness of a body.
It is the cause of change in 2. It is the effect of heat.
temperature.
3. Heat always flows from a hot 3. Temperature gives the direction of
body to the cold body. flow of heat.
4. The SI unit of heat is joule (J). 4. The SI unit of temperature is kelvin
(K).
sensation /senˈseɪʃn/ - a feeling that we get when something affects our body
PHYSICS Oasis School Science and Environment - 8 83
Activity 1
• Take two beakers of the same size and keep 50 ml of water in one and 100 ml in
another. Heat these beakers with the same-sized spirit lamps. Observe the reading
of the thermometer of both after equal interval of time. What do you observe?
The beaker with less amount of water shows higher temperature if equal
amount of heat is supplied to both beakers for equal duration of time.
When a body is heated, its temperature rises. It is because the molecules of the body
vibrate faster which helps to increase the average kinetic energy of each molecule. So, the
temperature of a body depends on the average kinetic energy of molecules of the body.
6.3 Thermometer
To know the condition of hotness or coldness of a body, its temperature should
be measured. A device which is used to measure the temperature of a body is called
thermometer. When a body is heated, it expands and when it is cooled, it contracts. This
is the working principle of the thermometer. Generally, thermometers are made by using
gas and liquid because they expand more than the solids. In our daily life, mostly we use
liquid thermometer.
Fig. 6.2 Thermometer
Liquid Thermometer
The thermometer in which a liquid is used as a thermometric liquid is called liquid
thermometer. In a liquid thermometer, mercury or alcohol is used as a thermometric
liquid. So, if mercury is used in a thermometer, it is called a mercury thermometer and if
alcohol is used, it is called an alcohol thermometer. Both of them have the same structure.
Thermometer consists of a capillary tube made up of glass having a bulb at one end
and its other end is closed. The bulb of the thermometer is filled with mercury or alcohol
according to the type of thermometer. The capillary tube is sealed in a cylindrical glass tube.
To measure the temperature of a body, the bulb of the thermometer is kept in close contact
of the body. The bulb is heated due to the contact with the body and the liquid kept in
the bulb expands. The liquid starts to flow into the capillary tube and gives a constant
level. This constant level is read with the scale provided on the thermometer and the
temperature of the body is noted. When the temperature of a cold body is measured, the
liquid contracts and gives the reading.
6.4 Thermometric Liquids
The liquids which are used in a thermometer are called thermometric liquids. Two liquids,
i.e. mercury and alcohol are commonly used as thermometric liquids.
84 Oasis School Science and Environment - 8 PHYSICS
Advantages of Mercury as a Thermometric Liquid
1. Mercury is a silvery white liquid. So it can be seen easily in the capillary tube.
2. It is a good conductor of heat as it is a liquid metal.
3. Its rate of expansion and contraction is uniform.
4. It does not stick to the inner wall of the capillary tube.
5. It remains in liquid state in a wide range of temperatures. The boiling point of
mercury is 357°C and its freezing point is - 39°C.
Disadvantage of Mercury as a Thermometric Liquid
The freezing point of mercury is –39°C. So, it cannot measure the temperature below
– 39°C. Due to this reason, mercury thermometer cannot be used in very cold regions to
measure very low temperature.
Advantages of Alcohol as a Thermometric Liquid
1. The freezing point of alcohol is –117°C. So, it can be used to measure very low
temperature in cold regions.
2. Its expansion rate is six times more than that of mercury. So, it is more sensitive to
the change in temperature.
3. It is cheaper than mercury.
Disadvantages of Alcohol as a Thermometric Liquid
1. Alcohol has no uniform rate of expansion and contraction.
2. The boiling point of alcohol is 78°0C. So, it is not suitable to measure the temperature
above 78°0C.
3. It sticks to the inner wall of the capillary tube. So, it cannot measure accurate
temperature.
4. It is a bad conductor of heat.
6.5 Calibration of Thermometer
The process of determining the scale in a thermometer is called calibration of thermometer.
To determine the scale, we need to find the two points in the thermometer. They are -
upper fixed point and lower fixed point.
a. Upper fixed point: The temperature of the boiling water at the standard atmospheric
pressure is considered as the upper fixed point. It is 1000C.
b. Lower fixed point: The temperature of the pure melting ice at standard atmospheric
pressure is considered as the lower fixed point. It is 00C.
The distance between the upper fixed point and lower fixed point is divided according to
the type of scale. For example, if the scale is Celsius, the space is divided into 100 equal
divisions and each division is called 10C.
contraction /kənˈtrækʃn/ - the process of becoming smaller
PHYSICS Oasis School Science and Environment - 8 85
Activity 2
Determination of upper fixed point of the thermometer
• Take a round bottom flask with some water. Insert the thermometer and a glass
tube in the flask as shown in the figure. Ensure that the bulb of the thermometer
should be above the level of the water. Then heat the water with the help of a
Bunsen burner until it boils.
• Observe the temperature in the thermometer at which it shows a constant
value. This is the boiling point of water. This gives the upper fixed point of the
thermometer, i.e. 100°C.
Stand 1000C
Thermometer
Glass tube
Cork
Water vapour
Round bottom flask
Water (boiling)
Tripod stand
Bunsen burner
Fig. 6.3 Determination of upper fixed point
Activity 3
Determination of lower fixed point of Thermometer
the thermometer
• Take some ice cubes in a funnel. Put
a beaker under the funnel. Insert the 0 0C
bulb of the thermometer in the ice Melting ice cubes
Glass
of the funnel as shown in the given Stand
diagram.
• The level of mercury goes on de-
creasing and gives a constant read-
ing after some time. The constant Beaker
temperature is the melting point of Water
ice which is called lower fixed point
of the thermometer, i.e. 0°C. Fig. 6.4 Determination of lower fixed point
86 Oasis School Science and Environment - 8 PHYSICS
The calibration of the thermometer is carried out at the standard atmospheric pressure.
ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ę¡ȱ ȱ ȱ ȱ
ę¡ȱȱȱȱǯ
ěȱ ȱ ȱę¡ȱȱȱ ȱę¡ȱ
ȱę¡ȱ ȱę¡ȱ
1. The temperature of the hoiling water at 1. The temperature of the pure melting ice
the standard atmospheric pressure is con- at the standard atmospheric pressure is
ȱȱȱȱę¡ȱǯ ȱȱȱ ȱę¡ȱǯ
2. Its value is 1000C. 2. Its value is 00 C.
ŜǯŜȱȱȱ ȱ
ȱȱ ȱȱ ȱę¡ȱȱȱȱę¡ȱȱȱȱȱȱ
called fundamental interval. The fundamental interval is divided into a number of equal
divisions. Each division is taken as 'one degree' or one unit.
There are three fundamental scales related to measurement of temperature. They are:
i. Celsius (or Centigrade) scale
ii. Fahrenheit scale
iii. Kelvin (or absolute) scale
ǯȱ ȱ DZȱThis is the popular scale given by Celsius. In this scale, the melting
ȱ ȱ ȱ ǻǯǯȱ Ŗȱ ǚ Ǽȱ ȱ ȱ ȱ ȱ ȱ ę¡ȱ ȱ ȱ ȱ ȱ ȱ ȱ
ǻǯǯŗŖŖȱǚ Ǽȱȱȱȱȱȱę¡ȱǯȱ ǰȱȱȱŗŖŖȮŖȱƽȱŗŖŖȱȱǯȱ
Each division is called 1Ĉ0C. It is to be noted that if this thermometer is used to
ȱȱȱ ȱȱ ȱę¡ȱȱǻǯǯȱ ȱŖĈ0C), the scale is
continuously marked with minus sign.
ǯȱ ȱ DZȱIn this scale, the temperature of melting ice is 32Ĉ0F and that of
boiling water is 212Ĉ0F. The fundamental interval is divided into 180 divisions. Each
division is called one degree Fahrenheit, i.e. 1°F.
ǯȱ
ȱ DZȱ ȱȱǰȱȱ ȱę¡ȱȱȱŘŝřȱ
ȱȱȱȱę¡ȱȱȱ
373K. The interval between these two points is divided into 100 equal sections. Each
division is called one kelvin, i.e.1K.
ȱȱ ȱȱ ȱ ȱ ȱȱȱȱȱȱȱȱ ȱȱȱȱȱȱȱȱȱȱ
Melting point of ice 0Ĉ0C 32Ĉ0F 273 K
Boiling point of water 100Ĉ0C 212Ĉ0F 373 K
ȱ
PHYSICS Oasis School Science and Environment - 8 87
Relation among Celsius, Fahrenheit and kelvin Scales
Scale–Lower fixed point
Upper fixed point–Lower fixed point
C – 0 F – 32 K – 273
=100 – 0 2=12 – 32 373 – 273
=C10–00 F=1–8032 K – 273
100
373 K 1000C 2120F Water boils
310 K 37.00C100 degree intervals98.60F Normal body
273 K 00C 100 degree intervals320 F temperature
100 degree intervals Water freezes
Kelvin scale Celsius scale Fahrenheit scale
Fig. 6.5 Different temperature scales
Differences between Celsius scale and Fahrenheit scale
Celsius scale Fahrenheit scale
1. In this scale, the melting point of ice (00C) 1. In this scale, the melting point of ice (320F)
is taken as a lower fixed point and boiling is taken as a lower fixed point and boiling
point of water (1000C) is taken as upper point of water (2120F) is taken as upper
fixed point. fixed point.
2. In this scale, the fundamental interval is 2. In this scale, the fundamental interval is
divided into 100 equal divisions. divided into 180 equal divisions.
Worked out Numerical 1
Convert 37°0C into degree Fahrenheit (0F).
Solution: C = 37
Given,
F = ?
88 Oasis School Science and Environment - 8 PHYSICS
We have,
C–0 = X–0
100 100
or, 37 = F – 32
100 180
or, F - 32 = 37 ×180
100
or, F = 37 ×18 + 32
or, 10
F = 66.6 + 32
or, F = 98.6
∴ 37°0C = 98.6 0F
Worked out Numerical 2
Calculate the temperature at which both Fahrenheit and Celsius scales show the same
reading.
Solution:
Let the temperature be x0.
Now, we have
or, C–0 = X–0
100 =
100
X–0
100 x – 32
180
or, x ×180 = x - 32
100
or, 18x = x-32
or, 10
18x = 10 x – 320
or, 18x - 10x = – 320
or, 8x = – 320
or, –320
or, x = 8
x = – 40
or, –40°0C = – 40°0F
∴ At - 40° both Fahrenheit and Celsius scale show the same reading.
PHYSICS Oasis School Science and Environment - 8 89
6.7 Types of Thermometer
There are different types of thermometer but the following thermometers are commonly
used in our daily life.
a. Clinical Thermometer: The clinical thermometer is constructed for measuring the
temperature of human body. That is why it is also called doctor's thermometer. A
clinical thermometer is shown in the following figure.
Fig. 6.6 Clinical thermometer
The clinical thermometer has a constriction near the bulb. It allows the mercury
to rise up in the capillary tube when the thermometer is in contact with the body.
But it does not allow the mercury to fall back into the bulb after the removal of
thermometer from the body. Due to the constriction in the clinical thermometer, the
temperature of the human body can be taken easily and accurately.
To measure the temperature of the human body, the bulb of the clinical thermometer
is kept under the tongue or in the armpit of the person at least for two minutes. The
mercury of the bulb in the thermometer absorbs the heat energy of the human body
and expands. As a result, the level of the mercury rises in the capillary tube showing
the body temperature of the person.
When the thermometer is taken out of the mouth or armpit of the person, the sudden
cooling and contraction of the mercury in the bulb breaks the thread of mercury at the
constriction. Hence, the mercury stays at the original level and the temperature can
be noted easily without any difficulty. The mercury in the stem of the thermometer
must be returned to the bulb by shaking it before its reuse. The thermometer must
be washed with water before using it.
b. Laboratory Thermometer: A laboratory thermometer consists of a thick walled
capillary tube made up of glass in which one end is closed and its other end has a
cylindrical bulb. The cylindrical bulb and a small portion of the capillary tube are
filled with pure mercury. The air from the capillary tube is completely taken out
before closing the upper end of the capillary tube. In this thermometer, the scale
ranges from – 10°C to 110°C. The vacuum made above the mercury column makes it
easy to expand the thermometric liquid.
Fig. 6.7 Laboratory thermometer
90 Oasis School Science and Environment - 8 PHYSICS
The basic differences between laboratory thermometer and the clinical thermometer are:
(i) The clinical thermometer has the graduations from 35°C to 42°C or 95°F to
107.6°F because the temperature of the human body varies only in this range.
But the laboratory thermometer has the range from -10°C to 110°C.
(ii) The clinical thermometer has a constriction near the bulb but not in the
laboratory thermometer.
c. Maximum and Minimum min. max. Y
Thermometer: The thermometer that 0C 0C
is used to measure the maximum and - 20 –
minimum atmospheric temperature of -10 – – + 50
twenty four hours of a particular place – –
is called maximum and minimum 0– – +40
thermometer. –
X– – +30
It has a U-tube which is partially –
filled with mercury and the rest of the +10 – – +20
U– tube is filled with alcohol keeping +20 – –
small vacuum for expansion. The +30 – – +10
limb X is completely filled but limb Y +40 – –
is partially filled for vacuum. When +50 – –0
mercury – -10
–
– -20
the temperature of the surroundings magnet
increases, the alcohol in the limb X
expands. This expansion pushes the Fig. 6.8 Maximum-minimum thermometer
mercury and the mercury pushes the
index in the limb. When the temperature
falls, the alcohol in the limb contracts. As a result, the mercury flows towards the
alcohol of limb X. This movement helps to displace the index in the limb X. The
index of Y gives the maximum temperature and the index of X gives the minimum
temperature. We need to note the reading of the lower point of the index.
Once the reading is taken, the indices are drawn down to the mercury surface by
using a C magnet because the index is made up of magnetic substance.
Differences between Clinical thermometer and Simple thermometer
Clinical thermometer Simple thermometer
1. It is constructed for measuring the 1. It is constructed for measuring the
temperature of human body. temperature of different bodies in
laboratory.
2. It has a constriction near the bulb. 2. It has no constriction near the bulb.
3. In this thermometer, the scale ranges from 3. In this thermometer, the scale ranges
350C–420C. from –100C to 1100C.
PHYSICS Oasis School Science and Environment - 8 91
Reasonable fact-1
Mercury thermometer is not suitable to measure the temperature of very cold place.
The freezing point of mercury is –390C. So, it cannot measure the temperature below –390C. Hence,
mercury thermometer is not suitable to measure the temperature of very cold place.
Reasonable fact-2
Mountain climbers do not use mercury thermometer.
The freezing point of mercury is –390C. So, it cannot measure the very cold temperature below
–390C. Therefore, mountain climbers do not use mercury thermometer.
Reasonable fact-3
The temperature of boiling water cannot be measured by alcohol thermometer.
The boiling point of water is 1000C whereas the boiling point of alcohol is only 780C. So, the
temperature of boiling water cannot be measured by alcohol thermometer.
Reasonable fact-4
Mountain climbers use an alcohol thermometer instead of mercury thermometer.
Mercury thermometer cannot measure the temperature below –390C whereas alcohol thermometer
can measure the temperature down to –1170C. So, mountain climbers use an alcohol thermometer
instead of mercury thermometer.
Reasonable fact-5
A narrow constriction is kept near the bulb of the clinical thermometer.
A narrow constriction is kept near the bulb of the clinical thermometer because while measuring
the temperature of the human body, the bulb does not allow the mercury to fall back into the bulb
after the removal of thermometer from the body.
SuMMarY
• Heat is a form of energy which produces the sensation of warmth. Its SI unit is J.
• The degree of hotness or coldness of a body is called its temperature. The SI unit of temperature is K.
• When a body is heated, it expands and when a body is cooled, it contracts. It is the principle of
thermometer.
• Thermometer is a device which is used to measure the temperature of a body.
• The main types of temperature scales are:
(i) Celsius (ii) Fahrenheit (iii) Kelvin
• Alcohol and mercury are used as thermometric liquid.
• The process of determining the scale in a thermometer is called the calibration of thermometer.
• Mercury can measure the temperature in the range of –39 °C to 357 °C.
• Alcohol can measure the temperature in the range of –117 °C to 78.3 °C.
• The normal body temperature of the human body is 37 °C or 98.6 °F.
• A clinical thermometer has a constriction near the bulb to stop the backward flow of the mercury so
that the reading can be taken easily.
• Maximum - minimum thermometer is used to measure the maximum and minimum atmospheric
temperature of 24 hours of a place.
92 Oasis School Science and Environment - 8 PHYSICS
exercise
1. Choose the best answer from the given alternatives.
a. Heat is a form of energy which produces the sensation of ........................ .
i. warmth ii. sound iii. light iv. vision
b. The SI unit of temperature is ........................ .
i. 0C ii. °0K iii. K iv. 0F
c. The melting point of ice is ........................ .
i. 100°0C ii. 0°0C iii. 0 0F iv. 0 0K
d. The boiling point of water is .................
i. 0°0C ii. 212 0F iii. 32 0F iv. 100 0K
e. The temperature of human body is measured by ........................ .
i. calorimeter ii. clinical thermometer
iii. laboratory thermometer iv. alcohol thermometer
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. In CGS System, heat is measured in calorie.
b. Heat gives the direction of flow of temperature.
c. The boiling point of water is taken as a lower fixed point.
d. The boiling point of water is 373 K.
e. The clinical thermometer has a constriction near the bulb.
3. Fill in the blanks using appropriate words.
a. The degree of hotness or coldness of a body is called .................... .
b. Mercury and .................... are the thermometric liquid.
c. The melting point of ice is ....................°0F.
d. .................... thermometer is suitable for very cold regions.
e. The boiling point of water is taken as .................... fixed point.
4. Answer the following questions.
a. What is heat? Write its SI unit.
b. Define temperature and write its SI unit.
c. What is thermometer? Write down the working principle of thermometer.
PHYSICS Oasis School Science and Environment - 8 93
d. What are thermometric liquids?
e. Write down two advantages and one disadvantage of mercury as a
thermometric liquid.
f. Write down the boiling point and freezing point of mercury and alcohol.
g. What is meant by calibration of thermometer?
h. Define upper fixed point and lower fixed point.
i. What is meant by fundamental interval in context of temperature scale?
j. What is clinical thermometer? Draw its figure.
5. Differentiate between:
a. Heat and Temperature
b. Alcohol and Mercury
c. Upper fixed point and Lower fixed point
d. Clinical thermometer and Laboratory thermometer
6. Give reason.
a. Mercury thermometer cannot be used to measure the temperature in a very
cold region.
b. Mercury is used as a thermometric liquid.
c. The clinical thermometer has a constriction near the bulb.
d. The clinical thermometer has graduations from 35°C to 42°C.
7. Show the relation among °0C, °0F and k.
8. Describe an experiment to show the determination of upper fixed point and
lower fixed point.
9. Draw a neat and labelled figure of laboratory thermometer and maximum–
minimum thermometer.
10. Numerical Problems
a. Convert 100°C into °F and K. [Ans : 212°F, 373 K]
b. Convert 200°C into °F. [Ans : 392°F]
c. Convert 90° F into °C. [Ans : 32.22°C]
d. Convert 273 K into °C. [ Ans : 0°C]
e. Convert 373 K into °F. [Ans : 212°F]
94 Oasis School Science and Environment - 8 PHYSICS
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