Modern Concept Science and Environment – 8 43
d) Mechanical advantage of a simple machine is 4, what does it mean?
e) What is an ideal simple machine?
f) Write any three methods to increase efficiency of a simple machine.
g) What is a lever? Write down its types.
h) State the principle of lever.
i) Define first class lever, second class lever and third class lever.
j) Which kind of lever are the followings:
i) Nutcracker ii) Pliers iii) A hockey stick
iv) Fire tong v) A broom vii) Nail cutter
viii) Human-arm ix) Wheelbarrow x) Fishing rod
k) Two persons are not equally strong. One is weaker than another. They are going
to carry a wooden pole. Suggest them a way to carry the wooden pole in such a
way that the stronger person needs to apply more effort and the weaker person
needs to apply less force. Also explain the scientific reason behind your answer.
8. Numerical problems
a) An effort of 200 N is used to lift a load of 800 N by using a lever. If the load is at
a distance of 40 cm from the fulcrum then find the effort distance. [Ans: 160 cm]
b) Ram with the weight of 400 N and Shyam with the weight of 500N are willing to
play on a see-saw. If Ram is at a distance of 2m from the fulcrum then how far
from the fulcrum should Shyam sit to balance Ram? [Ans: 1.6 m]
c) The mechanical advantage of a simple machine is 3 and its velocity ratio is 4. Find
its efficiency. [Ans: 75%]
d) A person lifts a load of 400 N with the help of a lever by applying an effort of
100N. The load is kept at a distance of 40 cm and the effort is applied at a distance
of 2m from the fulcrum. Find the value of MA, VR and efficiency?
[Ans: MA = 4, VR = 10, efficiency = 40%]
e) A man uses a long crowbar to lift a load of 1000 N as
shown in the figure. Find
i) the effort needed to lift the load
ii) the mechanical advantage of the crowbar
[Ans: 250 N, 4]
f) Find the MA, VR and efficiency of the lever shown in the 80 N
given figure. [Ans: 1.25, 2, 62.5%]
g) A force of 50 N applied in a simple machine displaces 100 N
it downward by 0.25 m in rising a load of 100 N 10 m
through 10 cm. Calculate MA, VR, and efficiency.
[Ans: 2, 2.5, 80%] 5m
9. Draw the diagrams.
a) Draw a diagram to show the position of fulcrum, load and effort in a lever when
it is used as a force multiplier.
b) Draw a diagram to show the position of fulcrum, load and effort in a lever when
it is used as a speed multiplier.
44 P r e ss u r eEstimated teaching periods Theory Practical
UNIT 4 1
4 Pressure
Syllabus issued by CDC Blaise Pascal
Introduction to pressure
Atmospheric pressure
Importance of atmospheric pressure
Liquid pressure
Measurement of liquid pressure
Characteristics of liquid
Density and relative density
Floating and sinking
LEARNING OBJECTIVES
At the end 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.
Key terms and terminologies of the unit
1. Pressure : The force acting perpendicularly on a unit area of a surface is called
pressure.
2. One pascal : When a force of 1N is applied normally on an area of 1 m2, then the
pressure acting on the surface is called 1 pascal.
3. Liquid pressure : The force applied by a liquid on per unit area of the walls of the container
is called liquid pressure.
4. Atmospheric pressure : The force exerted by the weight of the atmosphere on per unit area of
the earth’s surface is known as atmospheric pressure.
5. Standard atmospheric pressure : The atmospheric pressure at the sea level is called standard atmospheric
pressure which is about 101300 N/m2 or 760 mm Hg.
6. Density : The mass in per unit volume of a substance is called density.
7. Relative density : Relative density is the ratio of density of a substance to the density of
water at 4oC.
8. Barometer : An instrument that measures atmospheric pressure is called a barometer.
4.1 Introduction Force
Area
In our daily life, we prefer sharp and pointed objects for cutting and piercing.
It is our common experience that we feel easy to cut vegetables and fruits
with the help of a sharp knife but different with the blunt one. It is because
Pressure
Modern Concept Science and Environment – 8 45
the sharp knife has less area in the cutting edge. The force exerted per unit area is more in case
of the sharp objects and less in case of the blunt one. Thus, the force acting perpendicularly on
a unit area of a surface is called pressure.
Pressure (P) = Force (F)
Area (A)
Pressure has a magnitude but no direction associated with it. So, pressure is a scalar quantity.
4.2 Relation among Force, Area and Pressure
The same force can produce different pressure depending on the area. For example, when we
stand only one foot, more pressure is exerted on the ground. But, if we stand on both feet, less
pressure is exerted on the ground. Therefore, pressure depends upon following two factors:
Force applied
Pressure increases with increasing force and decreases with decreasing force. In other words,
pressure exerted is directly proportional to the force applied.
i.e. P α F
Surface area
Pressure decreases with increasing area and increases with decreasing area. In other words,
the pressure exerted is inversely proportional to the surface area.
i.e. P α 1
A
ACTIVITY 1
OBJECTIVE : To observe that pressure is directly proportional to the surface area.
METHODS :
1. Put your hand with open palm on a table.
2. With the help of another hand, gently put a
brick over your palm in two different positions as shown in the figure.
3. What do you observe? Do you feel more pain when the brick is kept vertically?
EXPLANATION :
The weight of the brick is same in both positions. But, in vertical position, the weight acts on less area
and exerts more pressure.
4.3 Measurement of Pressure
Units of pressure
The SI unit of pressure is newton per square metre (Nm-2), or pascal (Pa). The CGS unit of
pressure is dyne per square centimetre (dyne cm−2), or barye (Ba).
46 Pressure
Relation between the SI unit and CGS unit of pressure: 1 Pa = 10 barye
One pascal pressure
We know that, Pressure (P) = Force (F) or, 1Pa = 1 N
1 m2
Area (A)
When 1N force is applied normally on an area of 1 m2, then the pressure acting on the surface
is called 1 pascal.
Differences between force and pressure
S.N. Force S.N. Pressure
1 Force is a push or pull which changes 1 Force acting normally on per unit area
or tries to change the state of a body. is called pressure.
2 The SI unit of force is newton (N). 2 The SI unit of pressure is pascal (Pa).
Solved Numerical 4.1
A plywood cupboard of 400N weight occupies 2m² surface area. Calculate the pressure exerted.
Solution:
Given, Force (F) = 400N Area (A) = 2m² Pressure (P) = ?
According to the formula, pressure (P) = Force (F) = 400 = 200
Area (A) 2
∴ The pressure exerted (P) = 200 Pa.
4.4 Atmospheric Pressure
The earth has its own force of attraction called gravity. Due to this force of gravity,
a thick layer of air is present around the earth’s surface called atmosphere. The
huge mass of the atmosphere exerts force on the earth’s surface and creates a large
amount of pressure. Thus, the force exerted by the weight of the atmosphere on
per unit area of the earth’s surface is known as atmospheric pressure. Earth and its atmosphere
ACTIVITY 2
OBJECTIVE : To show that atmosphere exerts pressure.
METHODS :
1. Take a tin can and pour some water in it.
2. Heat the tin can until water starts to boil.
3. Allow water to boil until the steam pushes out all the air form the tin can.
4. Close the lid and stop heating.
5. Pour cold water over the hot tin can. Do you notice that the tin can gets crushed?
EXPLANATION :
When air inside the tin can comes out, there is formation of a partial vacuum. So, the high atmospheric
pressure from outside exerts a great force to crush the can.
Modern Concept Science and Environment – 8 47
ACTIVITY 3
OBJECTIVE : To show that atmosphere exerts pressure.
METHODS : Atmospheric pressure
1. Take a glass tumbler and fill it completely with water.
2. Cover the tumbler with a thick card-board.
3. Use a hand to invert the glass tumbler pressing the card-board by
the palm of another hand.
4. Remove your palm from the card-board. Does it fall down?
EXPLANATION :
Card-board is pressed up by the atmosphere. The force caused by the atmospheric pressure acts on the
surface of the cardboard is greater than the weight of the water in the glass tumbler. This shows that the
atmosphere exerts pressure.
FACT WITH REASON
We are not crushed by atmospheric pressure, why?
At sea level, atmospheric pressure is about 105 Pa. Blood flows through blood vessels with its own
pressure. Blood exerts pressure from inside and atmosphere applies pressure from outside of our body.
Blood pressure balances the atmospheric pressure. So, we are not crushed by atmospheric pressure.
Units of atmospheric pressure
a) The SI unit of atmospheric pressure is N/m2 or pascal (Pa).
b) Atmospheric pressure is generally expressed in millimeter of mercury column.
At sea level, 1 atm = 760 millimeters of mercury (mmHg) = 1.013 ×105 Pa
c) Another unit used for atmospheric pressure is torr.
1 torr = 1 mmHg = 133.3 Pa and 1 atm = 760 torr
e) The unit of pressure used in meteorology is bar.
i) 1 bar = 105 Pa and 1 atm = 1.013 bar
ii) 1 milibar (mb) = 1 bar = 100000 Pa = 100 Pa
1000 1000
Measurement of atmospheric pressure
The instruments that measure atmospheric pressure are called barometers. Mercury barometer
and aneroid barometer are the main two types of barometers. Here
you will learn about mercury barometer.
Mercury Barometer
Mercury barometer consists of liquid metal called mercury. A simple Barometer
mercury barometer can be made with a clean, dry, thick-walled glass
tube about 1 m long. The tube is sealed at one end and filled with
mercury. The tube is inverted over a trough with mercury.
48 Pressure
At sea level, the mercury level in the tube drops until it is Memory Tips
about 760 mm or 76 cm vertically above the mercury level
in the trough. This height of the mercury column in the The mercury barometer was
tube gives the atmospheric pressure. invented in 1643 by Evangelista
Torricelli (1608-1647), who was a
Standard atmospheric pressure student of Galileo.
The atmospheric pressure is more at sea level and decreases as we go up. Thus, the atmospheric
pressure at the sea level is called standard atmospheric pressure which is about 101300 Pa, or
760 mmHg.
Solved Numerical 4.2
A wall is of height 3 meters and length 7 meters. Find the force exerted by the atmosphere
on its sides if standard atmospheric pressure is 1.013 × 105 Pa.
Solution:
Here, standard atmospheric pressure (P) = 1.013 ×105 Pa
Height of wall (h) = 3 m Memory Tips
Length of wall (l) = 7 m Altimeter is an instrument in air
craft to measure altitude. It is
Area of the wall (A) = l × h = 3 m × 7 m = 21m2 based on the principle that the
atmospheric pressure decreases
Now, the force on the wall is given by the formula with the height above the sea level.
∴ F = PA = 1.013 × 105 × 21 = 2.13 × 106 N
Variation of atmospheric pressure with altitude
Effect of gravity is less at high altitude and there is thin Very low
atmosphere compared to the sea level. This reduces Low
density of the air as well as the atmospheric pressure. So, High
atmospheric pressure decreases with increase in altitude.
While flying aeroplanes and jet planes at high altitude,
the air pressure inside them is maintained for comfortable
breathing of the passengers.
FACT WITH REASON
At high altitudes, it is a common experience to have nose bleeding, why?
At high altitude, the external atmospheric pressure is lower than the pressure within our body. This
results bursting of the thin capillaries in the nose and bleeding occurs from the nose. So, at high
altitudes, it is common experience to have nose bleeding.
Astronauts wear specially designed pressure suits in space, why?
There is no atmosphere in the space. The internal blood pressure of the astronauts becomes higher
than the external pressure. It may cause blood vessel to rupture. The continuous bleeding may lead to
the death. So, astronauts wear specially designed pressure suits in space.
Modern Concept Science and Environment – 8 49
Importance of atmospheric pressure Memory Tips
i) Atmospheric pressure is used for filling ink in a A fan inside the vacuum cleaner
fountain pen.
lowers the air pressure inside it.
ii) Atmospheric pressure is used for filling medicine in a The air rushes inside the vacuum
syringe. cleaner carrying dust and dirt
iii) It is used for filling air in a bicycle tube, or tube of with it. The filter bag in the cleaner
vehicles. filters the dust and dirt from air
iv) It is used for lifting water by using a water pump. which is cleaned regularly.
v) It is used to draw soft drinks through a straw.
a) Drinking through a straw Atmospheric Pressure less than
Pressure atmospheric pressure
For sucking soft drink through a straw, the lower end of the
drinking straw is dipped into the soft drink. When we suck Liquid being pushed
through the upper end of the straw, it reduces the air pressure up in straw
inside the straw. As a result, the high atmospheric pressure on
the surface of the liquid pushes the liquid up through the straw. Drinking through a straw
FACT WITH REASON
Atmospheric pressure helps in sipping soft drinks from the bottle, how?
One end of the straw is kept in the soft drink and air is sucked through another end. Sucking removes
air from the straw tube and the air pressure inside it is reduced. The air pressure acting on the surface
of the soft drink forces it into the straw tube and the drink rises into the mouth.
Piston
Atmospheric Low pressure
Pressure
b) Filling medicine in a syringe
When we dip needle of the syringe in medicine and its piston is
withdrawn, the pressure inside the syringe becomes low. The
high atmospheric pressure acting on the surface of the medicine
pushes the medicine up into the syringe.
c) Lift pump Filling medicine in a syringe
A lift pump has a cylindrical barrel with a spout near the top. An air tight piston moves
Down Up
up and down the barrel with the help of a handle. A valve
‘V1 ‘ is at the bottom of the barrel, called foot valve. It is made H
up of rubber or leather flap. Another valve ‘V2 ‘, called piston
valve which moves up and down with the help of handle as H
shown in the given figure.
Working BB
Atmospheric
i) When we push the handle down, the valve ‘V1 ‘ opens pressure
and the valve ‘V2 ‘ remains closed. Water enters into the Lift pump
barrel due to the atmospheric pressure.
50 Pressure
ii) When we pull the handle up, the valve ‘V2 ‘ opens and the valve ‘V1 ‘ remains
closed. Water comes out through the spout.
Metal bar
d) Filling ink in a pen
A pen has a rubber tube in it. When the rubber is pressed by pushing Filling ink in a pen
the clip, the air inside the tube comes out and pressure becomes less.
On releasing the clip, the greater atmospheric pressure on the surface
of the ink in the inkpot pushes the ink up into the tube of the pen.
4.5 Liquid Pressure
A liquid does not have its own shape. It gains the shape of the container in which it is kept. A
Liquid applies thrust on the wall of the container. This thrust causes pressure on the wall of
the container. Thus, the force applied by liquid on per unit area of the walls of the container is
called liquid pressure. Liquid also exerts pressure on any object in the liquid.
Expression for liquid pressure
Let us consider a regular liquid container having area of cross-section ‘A’. If h
the container contains a liquid of density ‘d’ up to the height ‘h’, then the force A
at the bottom of the container is the same as the weight of the liquid. Therefore,
the liquid pressure is
P = F = Weight of liquid (W) = mg
A A A
P = Vdg [Since, mass of liquid (m) = volume of liquid × density]
A
P = Ahdg [Since, volume of liquid (V) = Areas of cross section × height]
A
P = hdg
Factors affecting liquid pressure
Liquid pressure depends on three factors. They are:
a) The depth of the liquid column
Liquid pressure at a point is directly proportional to the depth of the point from the free
surface of the liquid.
FACT WITH REASON Low pressure Thin wall
The base of a dam is made thicker, why? High pressure Thick wall
Water is collected in the dam. According to the formula P = hdg,
the water pressure increases with depth. Therefore,the wall of dam
is made thicker at the bottom to withstand the higher pressure of
water.
Modern Concept Science and Environment – 8 51
The deep sea divers wear a special suit, why?
Water pressure increases with depth. Deep sea divers feel more pressure on their body while swimming
deeper. This may cause bursting of the blood vessels resulting bleeding. So, to overcome the high water
pressure, the deep sea divers wear a special suit.
In case of the water supplies in buildings, the pressure of water on the ground floor is higher than that
in upstairs, why?
Water tanks are kept on the roof of the buildings. According to the formula P = hdg, the water pressure
on a tap increases with the depth of water column in the pipe. So, the pressure of water on the ground
floor is higher than that in upstairs.
b) The density of the liquid
Liquid pressure at a point is directly proportional to the density of the liquid. High
density liquid exerts more pressure at bottom of the container.
c) The acceleration due to gravity
The liquid pressure at a point is directly proportional to the value of acceleration due
to gravity at that place.
Solved Numerical 4.3
Calculate water pressure at the bottom of a swimming pool of 2 m depth. (Density of water
= 1000 kg/m3)
Solution:
Depth of swimming pool (h) = 2 m Density of water = 1000 kg/m3
Acceleration due to gravity (g) = 9.8 m/s2 Pressure at the bottom (P) = ?
According to the formula, P = hdg = 2 × 1000 × 9.8 = 19,600 Pa
∴ The pressure at bottom of the swimming pool is 19, 600 Pa.
Properties of liquid pressure
a) Liquid applies pressure in all directions
Liquids molecules have weak force of attraction and they push the walls of the
container or surfaces in contact. If liquid is kept in a close container,it applies pressure
in all direction.Thus, liquid pressure is transmitted equally in all directions, if pressure
is applied to the liquid kept in a closed vessel.
ACTIVITY 4
OBJECTIVE : To show that liquid pressure is transmitted equally in all direction.
METHODS :
1. Take a polythene bag or balloon and make holes on it with a pin.
2. Fill it with water.
3. Now, hold the balloon tightly on its mouth.
52 Pressure
4. Press water down with your hand and observe the out flow of water from these holes.
5. Observe the effect of pressure on the flowing of water through all holes?
EXPLANATION : When pressure is applied in a liquid at a point in a closed container, the pressure
transmits equally in all direction. This is Pascal’s law of liquid pressure.
b) Liquid pressure increases with the depth of liquid column
Liquid pressure increases with increasing depth of the liquid column. This results into
high pressure. For example, water pressure on the ground floor tap is more than that on
the top floor tap, to withstand high pressure deep inside the sea, the sea divers wear a
special suit, etc.
ACTIVITY 5 A
B
OBJECTIVE : To show that liquid pressure increases with the depth of liquid column. C
D
METHODS :
1. Take a cylinder and make holes on a side of the bottle as shown in the given figure.
2. Fill the bottle completely with water and observe the outflow of water from
different holes.
3. Do you observe the maximum pressure of water on the bottom hole?
EXPLANATION :
In case of the cylinder, the hole present at the bottom has maximum depth. So, the water
pressure is also maximum at the bottom hole.
c) Liquid pressure depends on the density of liquid
Liquid pressure increases with increasing density of the liquid. For example, if oil and
water are collected in two different containers and a hole is made at the same depth on
the both containers then the water with high density comes out with more pressure.
Oil Water
density = 0.93g/cm3 density = 1g/cm3
h h
Pressure depends on the density of liquid
d) Liquid pressure does not depend on the shape of the container.
Whatever the shape or width, the liquid pressure at a particular
depth is the same.
4.6 Similarity and Differences between the Liquid Pressure and Atmospheric Pressure
Similarities
i) Air pressure applies in all direction like the liquid pressure.
ii) Air pressure becomes less with increasing altitude similarly liquid pressure decreases
with decreasing depth.
Modern Concept Science and Environment – 8 53
Differences
S.N. Liquid pressure S.N. Atmospheric pressure
1 Liquid pressure cannot compress 1 Atmospheric pressure compresses the
the liquid in lower layers. air in lower layers
2 Liquid is denser than air and applies 2 Gas is very less dense than liquid and
more pressure. applies less pressure.
4.7 Density
One kilogram iron occupies less space but cotton of the same mass occupies much more space.
So, iron has more density. Similarly, mercury is heavier than water if both of them have
the same volume. It means that mercury has more weigh than water. So, instead of
saying that mercury is heavier than water, we say that mercury has more density
than water. Thus, the mass per unit volume of a substance is called density.
Mathematically, Density (d) = Mass (m)
Volume (V)
Units of density
The SI unit of density is kilogram per cubic meter. It is denoted by kg/m3 or kg m-3.
Similarly, CGS unit of density is gram per cubic centimeter. It is denoted by g/cm3 or g cm-3.
Densities of the common substances
Substance Density (in kg m-3) Substance Density (in kg m-3)
Aluminium 2700
Alcohol 790 Steel 7800
Mercury 13600
Petrol 800 Gold 19300
Ice (at 0oC) 920
Water (at 4oC) 1000
Measurement of density
a) Density of regular bodies
Mass of regular bodies is measured by a beam balance and their volume is calculated by
using a formula to calculate density. For example:
i) Density of a cube = Mass of the cube = mass = m
Volume of the cube (side)3 l3
ii) Density of a cuboid = Mass of the cuboid = length × mass × height = l × m h
Volume of the cuboid breadth b×
iii) Density of a sphere = Mass of the sphere = m , where ‘r’ is radius of the
sphere. Volume of the sphere 4 πr3
3
54 Pressure
Solved Numerical 4.4
Density of aluminium is 2700 kg/m3 and that of steel is 7800 kg/m3. Calculate mass of the
steel having same volume as 1350 kg of aluminium.
Solution:
For aluminium Memory Tips
Density (d) = 2700 kg/m3
Mass (m) = 1350 kg Hide the letter under your figure
According to formula, that you need to find. It gives you
Density (d) = Mass (m) a formula.
Volume (V)
or, V = m = 1350 = 0.5 m3
d 2700
For steel
Density (d) = 7800 kg/m3.
According to formula, m = d × V = 7800 × 0.5 = 3900 kg
∴ The mass of the steel is 3900 kg.
b) Density of irregular bodies 75 ml
50 ml
Mass of the irregular body is measured by a beam balance
and their volume is measured by liquid displacement Irregular shaped
method to calculate density. Afterwards, the density of an object
irregular body is calculated by the same formula:
Density (d) = Mass (m)
Volume (V)
4.8 Relative Density (R.D.)
Relative density is the measurement which shows how many times more dense
the substance is than water at 4oC. It is the ratio of density of a substance to the density
of water at 4oC.
Relative density = Density of the substance .......... (i) Memory Tips
Density of water at 4°C
The relative density of a substance
Mass of the substance tells you how many times more
dense the substance is than water
or, R.D. = Volume of the substance at 4°C.
Mass of water
Volume of water at 4°C
When volume of the substance = volume of water at 4oC, then:
Relative density (R.D.) = Mass of the substance
Mass of the same volume of water at 4°C
Modern Concept Science and Environment – 8 55
Thus, relative density of a substance is the ratio of mass of a substance to the mass of the
same volume of water at 4°C.
The relation between density and relative density of a substance can be expressed as:
Density of a body = R.D. of the body × Density of pure water at 4°C.
FACT WITH REASON
Relative density is a unitless quantity, why?
Relative density is a ratio between densities or masses of the substances and water at 40C, therefore,
it has no units.
4.9 Floating and Sinking
When a piece of stone is kept in water, it sinks. But, a piece Memory Tips
of wooden log floats. It is because the density of stone is
greater than the density of water but the density of wooden 1. Density of an object > density
log is less than the density of water. Similarly, iron nail of the liquid; object sinks in the
sinks in water but the rubber cork floats in water. In the liquid.
same way, an iron nail sinks in water but floats in mercury 2. Density of an object < density
because the density of iron (i.e. 7.8 g/cm3) is more than of the liquid; object floats in the
that of water (i.e. 1.0 g/cm3) and less than that of mercury
(i.e. 13.6 g/cm3).Therefore, the floating or sinking of a body liquid.
depends on the density of the body and the density of the liquid in which the body is kept.
For floatation, the density of a body must be less than that of the liquid. For example, kerosene
has less density than that of water so it floats in water. For sinking, the density of the body
must be more than that of the liquid. For example, a stone has more density than that of water,
so it sinks in water.
ACTIVITY 6
OBJECTIVE : To show that object with less density floats in water and the object with more density sinks
in water
METHODS : Pure water Salt water
1. Take a beaker and fill two third of it with water.
2. Keep an egg in water and observe. Does egg sink in water?
3. Now, make saturated solution of salt in water and again repeat the
process. Does egg float in salt solution?
EXPLANATION :
Pure water has less density but the salt solution has more density. Therefore, an egg sinks in pure water
but floats in salt solution.
FACT WITH REASON
An ice cube floats in water, why?
Ice has less density (i.e. 0.9 g/cm3) than that of water (i.e. 1.0 g/cm3). So, an ice cube floats in water.
56 Pressure
ANSWER WRITING SKILL
1. What is atmospheric pressure? Write down the value of standard atmospheric pressure.
Ans: The force exerted by the weight of the atmosphere on per unit area of the earth’s surface is known
as atmospheric pressure. The atmospheric pressure at the sea level is called standard atmospheric
pressure which is about 101300 Pa, or 760 mm Hg.
2. A balloon filled with more air bursts, why?
Ans: If air pressure inside the balloon is more than the atmospheric pressure, it gets stretched
beyond its limit and bursts. So, a balloon filled with more air bursts.
3. Gold has a relative density of 19.3. What does it mean?
Ans: Gold has a relative density of 19.3. It means that gold is 19.3 times more denser than water.
4. What is liquid pressure? Write down the formula to calculate it.
Ans: The force applied by a liquid on per unit area of the wall of the container is called liquid pressure.
It can be calculated by using the formula P = hdg.
5. Write down the properties of liquid pressure.
Ans: The properties of liquid pressure are:
i) Liquid applies pressure in all directions
ii) Liquid pressure increases with the depth of liquid column
iii) Liquid pressure depends on the density of liquid
iv) Liquid pressure does not depend on the shape of the container
6. Liquid pressure changes according to the depth, how?
Ans: From the formula P = hdg, we know that liquid pressure (P) is directly proportional to the height (h)
of the liquid column(i.e. P ∝ h). So, at more depth there is more pressure of liquid and at less depth
there is less pressure of liquid.
7. At which condition an object sinks and floats in liquid?
Ans: 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.
8. Nose bleeding occurs at high altitude, why?
Ans: At high altitude, there is less atmospheric pressure compared to the blood pressure. This results in
burst of the thin capillaries in the nose and bleeding occurs from the nose.
9. A water tank has its height 1 m. Calculate the pressure at its bottom when it is completely filled with
water. (g = 9.8m/s².)
Solution:
Height of water column (h) = 1m
Density of water (d) = 1000kg/m³
Acceleration due to gravity (g) = 9.8m/s²
Liquid pressure = ?
Now, the pressure at the bottom of the tank (P) = hdg = 1×1000×9.8 = 9800 Pa
Thus, the pressure at bottom of the water tank is 9800 Pa.
Modern Concept Science and Environment – 8 57
10. The mass of the brick shown in the given figure is 2.4 kg. Find its density.
Solution:
Mass of the brick (m) = 2.4 kg = 2400 g
Length of the brick (l) = 20 cm
Breadth of the brick (b) = 10 cm 5 cm
10 cm
Height of the brick (h) = 5 cm 20 cm
From the formula, volume of the brick (V) = l × b × h
or, V = 20 × 10 × 5 = 1000 cm3
Now, the density of the brick is given by d = m = 2400 = 2.4 g/cm3
v 1000
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) The force acting perpendicularly on a unit area of a surface is called …….
b) The SI unit of pressure is …… .
c) The instrument used to measure atmospheric pressure is called…… .
d) Liquid pressure increases on ……….. depth of liquid.
e) At higher altitude, atmospheric pressure is…………..
f) A body with ……... density floats in water.
g) 1 cm3 of copper has a mass of 8.9 g. The density of copper is ……. g/cm3.
2. Write True for the correct and False for the incorrect statements.
a) Pressure is directly proportional to the surface area.
b) The cutting edges are made sharp to increase pressure.
c) The foundation of a building is made wider to increase pressure on the ground.
d) A liquid in a container exerts pressure in all directions.
e) Air exerts pressure because it occupies space and has weight.
f) The atmospheric pressure is minimum at the sea-level.
g) Inside a bottle filled with water, pressure is maximum at the surface.
STEP 2
3. Answer the following questions in one word.
a) Which physical quantity represents the mass in a unit volume?
b) What is the instrument used to measure atmospheric pressure?
c) What is the layer of air above the earth surface called?
d) In which condition an object floats in water?
e) What is the value of normal atmospheric pressure at sea level?
f) What is the CGS unit of pressure?
58 Pressure
4. Write any two differences between:
a) Force and pressure
b) Liquid pressure and atmospheric pressure
c) Density and relative density
5. Give reasons.
a) A sharp knife cuts objects more effectively than a blunt knife.
b) Camel can walk easily in the desert than the horses.
c) Blood pressure in human body is greater at the feet than at the brain.
d) Deep sea divers have to wear specially designed air filled suits for their protection.
e) Air exerts pressure.
f) Air pressure is more near the earth’s surface and it goes on decreasing as we go up.
g) Mountaineers usually suffer from nose bleeding at high altitudes.
h) Relative density has no unit.
i) An egg sinks in pure water but floats in salt water.
6. Answer the questions with the help of the given figure. AB C
a) Three tiny holes are made in an empty can at different
levels, one over the other. The can is filled completely
with water.
i) Which property of liquid is shown in the given
figure?
ii) Why does the water flowing from the lowest hole
goes farthest and that from the uppermost hole
nearest to the can?
b) The piece of a post card does not fall off the glass though
the glass is full of water and its whole weight is exerting
pressure on the card, why?
c) The tin can with water in its bottom is heated to boil
water and the stem is allowed to escape for some time.
The open mouth is sealed with air-tight cap and cooled
under tap-water. The tin can gets crushed, why?
STEP 3
7. Answer the following questions.
a) Define:
i) pressure ii) 1 pascal pressure iii) liquid pressure
iv) atmospheric pressure v) standard atmospheric pressure
b) Explain the relationship among force, area and the pressure.
Modern Concept Science and Environment – 8 59
c) How would pressure change if:
i) area is doubled by keeping force constant
ii) force is doubled by keeping area constant.
d) Write the properties of liquid pressure.
e) Explain the factors that affect liquid pressure.
f) Derive the expression for liquid pressure, P= hdg. Where the terms have their
usual meaning.
g) What is the cause of atmospheric pressure?
h) Define standard atmospheric pressure.
i) What makes a balloon get inflated when air is filled in it?
j) Explain any four importance of atmospheric pressure.
k) Write down the relation of sinking and floating with the density of a liquid.
8. Numericals:
a) A ground water tank has its height 2m. Calculate the pressure at its bottom when
it is completely filled with water. ( g = 9.8 m/s2) [Ans: 19600 Pa]
b) A liquid of density 1200 kg/m3 is filled in a beaker. The pressure at the bottom of
the beaker is 5880 Pa. Find the height of the liquid column. (g= 9.8 m/s2)[Ans: 0.5m]
c) The pressure at the bottom of a drum in which a liquid is filled upto the height of
2m is 9800 Pa. Find density of the liquid filled in that drum. [Ans: 500 kg/m3]
d) The mass of an iron object is 16,000 kg and its volume is 2 m3. Find density of
iron. [Ans: 8000 kg/m3]
e) A golden ring of mass 38.6 g has volume 2 cm3. Calculate its density.
[Ans: 19.3 g/cm3 ]
f) 10 kg petrol has density of 800 kg/m3. Find its volume in cm3. [Ans:12500 cm3]
g) The density of copper is 8900 kg/m3. If the volume of a piece of copper is 2 m3, find
its mass. [Ans: 17800 kg]
h) Density of aluminium is 2700 kg/m3 and that of steel is 7800 kg/m3. Calculate mass
of the steel having the same volume as 675 kg of aluminium. [Ans: 1950 kg]
i) The density of mercury is 13600 kg/m3 and that of water is 1000 kg/m3. Calculate
the relative density of mercury. [Ans: 13.6]
9. Draw the diagram:
a) to show the variation of liquid pressure with depth.
b) to show the mechanism of filling medicine in a syringe.
60 W ork , E nEesrtimg yataedn tdeaPcohwinegrperiods Theory Practical
UNIT 3 0
5 Work, Energy and Power
Syllabus issued by CDC Crane
Energy and different types of energy
Mechanical energy and its types
Work and its types
Transformation of energy
Power
Relation between energy, work and power
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
differentiate between energy, work and power and explain the 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.
Key terms and terminologies of the unit
1. Energy : The capacity of a body to do work is called energy.
2. Work : Work is said to be done when a body moves in the direction of the force applied.
3. Power : The rate of doing work is called power.
4. One joule work : One joule work is done when a force of 1N displaces an object through a
distance of one meter in the same direction of the force.
5. One watt power : Power is said to be one watt when one joule of work is done in one second.
6. Work done against friction : The work done in pushing or pulling a load by applying force against friction
is called work done against friction.
7. Work done against gravity : The work done in lifting the weights by applying force against gravity is called
work done against gravity.
8. Mechanical energy : The energy of an object due to its motion or position or configuration is called
mechanical energy.
9. Kinetic energy : The energy possessed by a body by virtue of its motion is called kinetic energy.
10. Potential energy : The energy possessed by a body by virtue of its position or configuration is
called potential energy.
11. Heat : Heat is a form of energy which produces sensation of warmth in our body.
12. Light : Light is a form of energy which produces sensation of vision.
13. Sound : Sound is a form of energy which is produced by the vibrating bodies.
14. Electrical energy : he energy which is generated due to the flow of electrons through a conductor
is called electrical energy.
15. Magnetic energy : The energy possessed by a magnet is called magnetic energy.
Modern Concept Science and Environment – 8 61
16. Nuclear energy : The energy released during nuclear reactions (nuclear fusion and nuclear
fission) is called nuclear energy.
17. Energy transformation : The process of change of energy from one form to another is called
energy transformation.
18. Law of conservation of energy : Law of conservation of energy states that energy can neither be created
nor be destroyed but it can be changed from one form to another.
5.1 Introduction
All animals do various activities in their daily life. Plants also perform different kinds of
metabolic process in their body. To do these internal and external activities living organisms
need energy. This energy comes from food. Thus, the capacity of a body to do work is called
energy. It is measured in joule (J). In physics, work is said to be done when a body moves in
the direction of the force applied. It is also measured in joule (J). Here, energy and work are
complement of each other. So, both are measured in joule (J).In case of machines, they need
different sources of energy like electricity, petrol, diesel, etc. to do work. Different machines
have different capacity to do work which is known as power. Thus, the rate of doing work is
called power. It is measured in watt (W). In this unit, we will discuss about, work, energy and
power and their relation. We will also discuss about the principle of conservation of energy
along with some examples of transformation of energy.
5.2 Work
Generally, reading, writing, playing, cooking, loading, unloading, guarding, etc. are called
work. In physics, the word ‘work’ is used only in those cases where there is force acting on
a body and the body moves by a certain distance. Work is done when force acting on a body
results displacement in the direction of force. For example,
i) When a ball is thrown horizontally, the ball covers some distance along the direction of
force applied. Here, work is done on rolling the ball.
ii) When we pull a box on the ground, the box covers some distance along the direction of
force applied. Here, work is done on moving the box.
Work is said to be done when a body moves in the direction of the force applied. Mathematically,
the work done is defined as the product of force and the displacement in the direction of force.
i.e. W = F × s s F
Where, F = Force applied, s = Displacement in the direction of the force
θ = 0°
Work
The meaning of work in our daily life is quite different than that in physics. For example, a
watchman standing in front of a gate, a person standing at a place by holding some load on
his head, a dog tied to a gate, etc. don’t do any work in the language of physics.
In physics, the two main conditions to do work are:
i) Force must be applied on a body
ii) The body should get displaced in the
direction of force
Pushing a wall Pushing a car
62 Work, Energy and Power
FACT WITH REASON
If we push a wall for a long time, there is no work but we get tired, why?
If we push a wall for a long time, we expend calories and get tired. This is because our muscles
need continuous production and supply of energy to maintain a force even when there is no net
displacement. The energy produced is converted into heat. Thus, no work but we get tired.
Work is called a scalar quantity, why?
Although, work is the product of two vector quantities (force and displacement), it has only magnitude
but no direction. So, work is called a scalar quantity.
Factors affecting the Work
A) Magnitude of the applied force
Work done is directly proportional to the force applied. Greater the force, greater will
be the work done.
B) Magnitude of displacement
Work done is directly proportional to the displacement. Greater the displacement,
greater will be the work done.
Units of work Memory Tips
a) The SI unit of work is joule (J). The word erg is derived from the
b) The CGS unit of work is erg. Greek word for work; ergon.
Relation between joule and erg.
Since, W = F × s
or, 1 J = 1 N × 1m
or, 1 J = 105 dyne × 100 cm
∴ 1 J = 107 erg
Other units of work b) 1 megajoule (MJ) = 106 joule (J)
a) 1 kilojoule (kJ) = 103 joule (J)
c) 1 gigajoule (GJ) = 109 joule (J)
Solved Numerical 5.1
The mass of a car along with the driver is 1000 kg and it accelerates with 6 m/s2 within the
distance of 500 m. Calculate the work done by the engine of the car.
Solution:
Displacement of car (s) = 500 m Mass of the car (m) = 1000 kg
Acceleration (a) = 6m/s2
Work done (W) = ?
Now, work done (W) = Force × displacement = ma × s =1000 × 6 × 500 J = 3000000 J = 3000 KJ
Modern Concept Science and Environment – 8 63
1 joule work
1 joule work is done when a force of 1N displaces an object through a distance of one meter in
the same direction of the force. 1 J of work
Mathematically, W = F × s
or, 1 J = 1 N × 1s 1N
Types of Work 1m
1 J work
a) Work done against friction
b) Work done against gravity
a) Work done against friction
The opposing force between two surfaces in relative Applied force Friction force
motion is called frictional force and the work done
against this force is called work done against friction.
Thus, the work done in pushing or pulling a load by Work against friction
applying force against friction is called work done against friction. Examples: pushing
a book on a table, pulling a box on the ground, etc.
If ‘F’ is the force of friction and ‘s’ is the displacement of a body then the work done
against friction is given by W= F × s
ACTIVITY 1
Fix a hook of a spring balance on a block. Pull the block up to certain distance
on the surface of the table. Note the reading on the spring balance which gives
the effort applied while pulling the block. Multiply the distance covered and
the effort applied. This gives the work done against friction.
Solved Numerical 5.2
A box is pushed 3 meters on a horizontal surface and the frictional force experienced by the
box is 100N. Calculate the work done to overcome this friction.
Solution:
Frictional force (F) = 100 N
Displacement (s) = 3 m
Work done = ?
Now, work done to overcome friction (W) = F× s = 100 × 3 = 300 J
b) Work done against gravity
Each and every object is pulled towards the center of the earth
due to force of gravity. This force of gravity is called weight of
the object. In order to lift the object, an upward force equal to the
weight must to be applied on the object. The work done in lifting
the weights by applying force against gravity is called work Work against gravity
done against gravity. For example, throwing a ball up, lifting a load, climbing stairs, etc.
64 Work, Energy and Power
The work done against gravity is equal to the product of the weight of the body and the
vertical distance through which the body is raised.
If a mass ‘m’ is lifted to a height ‘h’ then the work done against gravity is given by
Work done against gravity (W) = Force of gravity × displacement
or, W = weight × height
∴ W = m g h
ACTIVITY 2
Hang a mass in the hook of a spring balance. Lift the mass up to a certain height. Read the
reading on the spring balance which gives the effort applied while lifting the load. Multiply
the height of the body and the effort you applied. This gives the work done against gravity.
Solved Numerical 5.3
Calculate the work done by a cat of 5 kg when it jumps to a height of 4 m.
Solution:
Mass of the cat (m) = 5 kg
Displacement (s) = 4 m
Work done = ?
Now, the work done against gravity (W) = mgh = 5× 9.8× 4 = 196 J
Thus, the work done by the cat is 196J.
5.3 Energy
Survival of living beings is impossible without energy. To perform various life process and
daily activities we need energy. The capacity of reading, writing, walking, playing, etc. is only
possible due to energy. This energy comes from the food we eat. In a machine, there is fuel
which supplies energy for their working. No work can be done without energy. Thus, the
capacity of a body to do work is called energy.
Units of Energy
The SI unit of energy is joule (J) and its cgs unit is erg.
Unit Symbol Equivalence in joule
erg erg 1 erg = 10-7 J
Calorie Cal 1 Cal = 4.2 J
Kilowatt hour kWh
1kWh = 3.6× 106 J
FACT WITH REASON
The unit of energy is the same as that of work, i.e. joule (J),why?
Energy is the capacity of doing work. It is equal to the amount of work done by the body. For example,
1 J energy is the capacity to do 1J work. So, the unit of energy is the same as that of work.
Modern Concept Science and Environment – 8 65
5.4 Forms of Energy
1. Mechanical Energy
The energy of an object due to its motion or position or configuration is called mechanical
energy. For example, a bird flying at certain height has mechanical energy. There are two
types of mechanical energy. They are:
a) Kinetic energy b) Potential energy
a) Kinetic energy
Moving objects can do work. For example, blowing air can rotate Kinetic energy
blades of a windmill; flowing water can rotate the turbines, etc.
Thus, the energy possessed by a body by virtue of its motion is called
kinetic energy. A rolling football, a falling apple, a running athlete, a
moving vehicle, etc. possess kinetic energy. The formula to calculate
kinetic energy is KE = 12mv2
Solved Numerical 5.4
A bullet having mass 150g is shot with a velocity of 960m/s, calculate kinetic energy of the
bullet?
Solution:
Mass of the bullet (m) = 150g = 150 = 0.15 kg
Velocity (v) = 960 m/s 1000
Kinetic energy = ?
Now, kinetic energy of the bullet is given by KE = 1mv2
2
or, KE = 1 × 0.15 × (960)2 = 69,120 J Memory Tips
2
If mass of a body is doubled, its
Factors affecting kinetic energy
i) Mass kinetic energy also gets doubled
The kinetic energy of a body is directly proportional and if mass of the body is halved;
to the mass of the body i.e., KE α m (when ‘v’ remains its kinetic energy also gets halved.
constant) Memory Tips
ii) Velocity If the velocity of a body is doubled,
The kinetic energy of a body is directly proportional its kinetic energy becomes four
to the square of the velocity of the body. i.e. KE α v2 times and if the velocity of a body
is halved, then the kinetic energy
b) Potential energy gets reduced to one-fourth.
Potential energy is a stored energy in the body. The energy possessed by a body by virtue
of its position or configuration is called potential energy. For example, a compressed spring,
stretched catapult, water in a dam, etc. possess potential energy.
66 Work, Energy and Power
Potential energy of an object at certain height h
When a body is lifted vertically upward against the gravitational force,
some work is done on the body. This kind of work done on the body is Ground
stored as a potential energy.
Let a body of mass ‘m’ be placed at height ‘h’ above the ground. Then, Potential energy
Potential energy of the body = Work done against gravity
∴ Potential energy = weight × h = mgh
FACT WITH REASON
The potential energy, 'mgh' is also called gravitational potential energy, why?
The potential energy, 'mgh' is due to work done against gravity. So, the potential energy, 'mgh' is called
gravitational potential energy.
Solved Numerical 5.5
A student has a bag of mass 3 kg. He lifts it from the floor to the table of height 1 m. Find
the potential energy gained by the bag. (g = 9.8 m/s2)
Solution:
Mass of the student (m) = 3 kg
Height of the table (h) = 1 m
Acceleration due to gravity (g) = 9.8 m/s2
Potential energy = ?
According to the formula, potential energy = mgh = 3 × 9.8 × 1 = 29.4 J
Differences between kinetic energy and potential energy
S.N. Kinetic energy S.N. Potential energy
1 The energy possessed by a body by 1 The energy possessed by a body by
virtue of its motion is called kinetic virtue of its position or configuration
energy. is called potential energy.
2 Greater the velocity, greater will be 2 Greater the height, greater will be the
the kinetic energy. potential energy.
3 Kinetic energy is given by: 3 Potential energy is given by:
KE = 21mv2. PE = mgh.
2. Heat Energy
Heat is a form of energy which produces sensation of warmth in our body. It is the sum of
kinetic energy of the particles present in a substance. Heat always flows from a body at high
temperature to a body at low temperature. Heat energy can be converted into other forms of
energy. The heat produced by burning petrol in an automobile engine provides the energy to
run the vehicles.
Modern Concept Science and Environment – 8 67
3. Light energy
Light is a form of energy which produces sensation of vision. It can be converted into electrical
energy by the use of photoelectric cells. Green plants directly use light energy to prepare food
during photosynthesis. Light energy also causes chemical change to take place. When light
energy falls on the photographic film, the image is recorded on the photographic film due to
the chemical change.
4. Sound energy
Sound is a form of energy which is produced by the vibrating bodies. The eardrum of the ear
vibrates when sound produced by a vibrating body falls on it. As a result, we hear sound.
We can experience it as a form of energy. When a stone hits the floor, a part of its mechanical
energy gets converted into the sound energy and heat energy.
5. Electrical energy
The energy which is generated due to the flow of electrons through a conductor is called
electrical energy. We obtain it from different sources like, dry cell, solar panels, etc.
Electrical energy can be used to run a number of electrical devices like fan, bulbs, washing
machines, etc. It is possible to convert electrical energy into other forms of energy easily.
6. Magnetic energy
A magnet attracts magnetic substances. So, the energy possessed by a magnet is called
magnetic energy. Magnetic energy is used to make an electromagnet which is widely used in
electrical devices. It is also used to separate magnetic substances from a mixture.
7. Chemical energy
Chemical energy is caused by chemical reaction. Petrol, coal, kerosene, etc. have stored energy
in a chemical form. Green plants prepare food by the photosynthesis. Energy stored in the
digested food gets released during respiration.
8. Nuclear energy
The nucleus of an atom has a vast amount of energy. This stored energy gets released during
nuclear reaction. The energy released during nuclear reactions (nuclear fusion and nuclear
fission) is called nuclear energy.
5.5 Energy Transformation
The process of change of energy from one form to another is called energy transformation.
Under suitable conditions, energy can be transformed from one form into another.
Examples of energy transformation
a) In photosynthesis, plants convert solar energy into chemical energy.
Solar Energy Chemical Energy
68 Work, Energy and Power
b) When hands are rubbed,mechanical energy changes into heat energy due to friction.
Mechanical Energy Heat Energy
c) When a torch light is switched on,the chemical energy of the cells is converted into
electrical energy. The bulb converts electrical energy into light energy.
Chemical Energy Electrical Energy Light Energy
d) Generation of electricity from a bicycle dynamo: When wheel of a bicycle rotates the
cap of the dynamo, then the magnet inside the dynamo rotates and coil intersects
the magnetic lines of force. During this process, mechanical energy is converted into
electrical energy.
Mechanical Energy Electrical Energy
e) Generation of electricity in hydropower station: Water stored in a high dam has plenty
of potential energy. This potential energy is converted into kinetic energy when the
stored water allowed falling through a tunnel. The kinetic energy of water is used to
rotate the turbines connected with generators. Thus, electricity is produced from the
potential energy present in the sorted water.
Potential Energy Kinetic Energy Electrical Energy
f) Generation of electricity in a thermal-power station: Coal is used as a fuel in the thermal-
power plant. When coal is burnt, its stored energy is converted into heat energy. This
heat energy converts water into steam. Now,steam runs the turbines connected with
generators to generate electricity.
Chemical Energy Heat Energy Mechanical Energy Electrical Energy
g) Heat engine: A heat engine is a device which converts heat energy into mechanical
energy. For example:
i) Steam engine : We keep coal in steam engine. Coal contains chemical energy. This
coal is burnt liberating heat energy. This heat energy is used to convert water into
steam. Steam at high pressure pushes the piston of the engine and wheels come
in motion.
Chemical Energy Heat Energy Mechanical Energy
ii) Automobile engine: In automobile engine, fuel is burnt to change chemical energy
into heat energy and finally into mechanical energy.
Chemical Energy Heat Energy Mechanical Energy
Some examples of the devices that convert energy from one form to another form:
a) Electrical bulb It converts electrical energy into light energy and heat energy.
b) Television It converts electrical energy into light and sound energy.
c) Heater It converts electrical energy into heat energy.
Modern Concept Science and Environment – 8 69
d) Solar cell It converts light energy into electrical energy.
e) Microphone It converts sound energy into electrical energy.
f) Battery or cell It converts chemical energy into electrical energy.
g) Turbine It converts mechanical energy into electrical energy.
h) Dynamo or generator It converts mechanical energy into electrical energy.
i) Electric motor It converts electrical energy into mechanical energy.
j) Loudspeaker It converts electrical energy into sound energy.
k) Electromagnet It converts electrical energy into magnetic energy.
l) Burning of fuels It converts chemical energy into heat and light energy.
5.6 Law of Conservation of Energy
If energy of one form disappears, an equal amount of energy appears in another form. Similarly,
if one form of energy appears, an equal amount of energy from another form disappears. The
energy, which appears or disappears, is actually the transformation of energy from one form
to another. Hence, the law of conservation of energy states that energy can neither be created
nor be destroyed but it can be changed from one form to another. For example, in a torch light,
the chemical energy stored in the dry cells gets transformed into electric energy in the wire
and heat energy and light energy in the bulb filament.
5.7 Power
Time required to complete a work varies as per the worker or the machines. For example, an
adult can do a work in two hours whereas the same work can be done by an old in three hours.
Here, the rate of doing work is more in adult than the old man. In this case, it is also said that
the power of an adult is more than the old man. Thus, the rate of doing work is called power.
It is also defined as the rate at which energy is transferred.
Mathematically, Power (P) = Work done (W)
Time taken (t)
Units of power
The SI unit of power is joule per second or watt (W).
Unit Symbol Equivalence in watt (W)
Horse power h.p. 1 h.p.= 746 W
Kilowatt kW 1kW = 1000W
Megawatt MW 1MW = 106 W
1 watt power
We know that,
Power (P) = Work done (W) or, 1 W = 1 J
Time taken (t) 1s
Thus, when one joule work is done in one second it is called one watt.
70 Work, Energy and Power
Meaning of power of electrical appliances
a) 60 W bulb: It means that the bulb converts 60 J of electrical energy into heat and light
energy in one second.
b) 150 W fan: It means that the fan converts 150 J electrical energy into mechanical energy
in one second.
Solved Numerical 5.6
A person applies constant force of 200 N to pull a crate for 10 m distance along a level floor
in 1 minute. What is the power used by this person?
Solution:
The force applied (F) = 200 N
Displacement (s) = 10 m
Time taken (t) = 1 minute = 60 s
Power (P) = ?
According to the formula, power (P) = Work done (W) = F × s = 200 × 10 = 2000 = 33.33 W
Time taken (t) 60 60 60
The power used by the person is 33.33 W
Solved Numerical 5.7
An electric motor lifts an elevator weighs 1.2 × 104 N up to a distance 9 m in 15 seconds. Find
the power of the motor .
i) in kilowatt (kW) ii) in horse power (h.p.)
Solution:
Force (F) = 1.2 × 104 N
Distance (d) = 9 m
Time taken (t) = 15 s
Power (P) = ?
According the formula, oower (P) = Work done (W)
Time taken (t)
or, P = F × s = 1.2 × 104 × 9 = 7.2 × 103 W
t 15
i) In kilowatt ii) In horse power (h.p.)
1000 W = 1 kW 746 W = 1 h.p.
∴ 7.2 × 103 W = 7.2 × 103 kW = 7.2 kW ∴ 7.2 × 103 W = 7.2 × 103 h.p. = 9.65 h.p.
1000 746
Factors affecting power
a) Work done (W): Power is directly proportional to the work done.
b) Time taken (t): Power is inversely proportional to the time taken.
Modern Concept Science and Environment – 8 71
Differences between work and power
S.N. Work S.N. Power
1 Work is done when force acting on 1 Power is the rate of doing work.
a body causes displacement in the
direction of force.
2 The SI unit of work is joule (J). 2 The SI unit of power is watt (W)
3 Work = Force × displacement 3 Power = Work
Time
5.7 Relation between Work, Energy and Power
Energy, work and power are interrelated. Energy is the capacity of doing work. Similarly,
energy changes from one form to another form while doing work. For example, 20 joule work
done by an electric bulb means that the bulb converts 20 joule electrical energy into heat and
light energy. The rate of doing work or energy conversion is called power. If a bulb does
20 joule work in 1 second then its power is 20 W. In short, work is the amount of energy
converted and energy converted per unit time is power.
ANSWER WRITING SKILL
1. Define work and write down its SI unit.
Ans: Work is said to be done when a body moves in the direction of the force applied. The SI unit of work
is joule (J).
2. What is energy? Write down the types of mechanical energy.
Ans: The capacity of a body to do work is called energy.
There are two types of mechanical energy. They are: a) Kinetic energy b) Potential energy
3. Define power. Write down the SI unit of power. How much watt is present in 1 h.p.?
Ans: The rate of doing work is called power. The SI unit of power is joule per second or watt (W). In one
horse power (h.p.) there is 746 watt.
4. An electric bulb has power 100 watt. What does it mean?
Ans: An electric bulb has a power of 100 watt. It means that the bulb converts 100 joule electrical energy
into heat and light energy in one second.
5. What is energy transformation? Give an example.
Ans: The conversion of one form of energy into another form is called energy transformation. For
example, the chemical energy stored in a dry cell is converted into light energy using torchlight.
6. What is the relation between work, energy and power?
Ans: Energy is the capacity of doing work. Similarly, energy changes from one form to another form
while doing work. The rate of doing work or energy conversion is called power. Thus, work, energy
and power are interrelated.
7. Without food we cannot live for long time, why?
Ans: We need energy to do daily activities as well as different life process. The energy required for these
activities comes from the food that we eat. So, without food we cannot live for long time.
72 Work, Energy and Power
8. Write down the energy conversion while using battery in a torchlight.
Ans: When a torchlight with battery is switched on, the chemical energy of the cells is converted into
electrical energy. The bulb converts electrical energy into light energy.
Chemical energy Electrical energy Light energy
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) The kinetic energy possessed by a body is directly proportional to its ……. .
b) The energy possessed by a body by virtue of its position is called ……..
c) The falling water contains …….. energy.
d) Nuclear energy is released in the form of …….… during nuclear fission.
e) An electric bulb converts ……. energy into light energy.
f) The relation between horse power and watt is..……. h.p.
g) An electric motor of 1 h.p. converts …….. J electrical energy in 1 s.
2. Write True for the correct and False for the incorrect statements.
a) The capacity of doing work is called energy.
b) A compressed spring has potential energy.
c) The potential energy of a body is inversely proportional to the mass of the body.
d) A loudspeaker converts sound energy into electrical energy.
e) A dry cell stores chemical energy.
f) The source of energy in stars is nuclear energy.
g) One watt is equivalent to746 h.p.
3. Match the following.
a. Work i. W
t
b. Kinetic energy ii. F × s
c. Potential energy iii. 1 mv2
2
d. Power iv. m g h
e. Erg v. unit of power
f. H.p. vi. unit of energy
STEP 2
4. Answer the following questions in one word.
a) What is the SI unit of work?
b) Write down the SI unit of power.
Modern Concept Science and Environment – 8 73
c) What is called to the rate of doing work?
d) Which device converts mechanical energy into electrical energy?
e) Which energy is present in a stone kept on the top of a tower?
5. Write any two differences between:
a) Kinetic energy and potential energy
b) Work done against gravity and work done against friction
c) Work and power
6. Give reasons.
a) No work is done if you push against the wall of a building.
b) We cannot work for a long time without food.
c) A watchman does no work but gets tired.
d) A parachutist has more potential energy when he is in air than he is just above
the ground.
e) A horse possesses more kinetic energy if a horse and a dog are running at the
same speed.
7. Answer the questions with the help of the given figure.
a) Name the energy possessed by the followings:
i) ii) iii) iv) v)
b) Write the formula to calculate energy in the following cases:
h s F
θ = 0°
i) Ground iii)
ii)
STEP 3
8. Answer the following questions.
a) What is work? When is work said to be done?
b) Explain two factors on which work done depends upon.
74 Work, Energy and Power
c) Define:
i) Chemical energy ii) Heat energy iii) Light energy
iv) Sound energy v) Electrical energy vi) Magnetic energy
vii) Nuclear energy viii) One watt power ix) One joule work
d) Write down the factors on which the amount of kinetic energy depends upon.
e) Write down the factors on which potential energy depends upon.
f) What types of energy is possessed by the following?
i) Stretched spring ii) A stone kept on the roof of a building
iii) A running car iv) Water stored in the reservoir of a dam
v) A compressed spring vi. A battery
g) Write down the meaning of the following statements:
i) Power of an electric bulb is 60 W.
ii) Power of an electric fan is 150 W.
iii) Power of an electric motor is 1 h.p.
iv) Power of an electric heater is 1 kW.
h) Explain the relation between work, energy and power with an example.
i) What is meant by energy transformation? Write with an example.
j) Explain the energy transformation that takes place in a torch light using dry cells.
k) State the principle of conservation of energy.
l) State the energy changes which occur in the following cases:
i) dry cell in a circuit ii) electric heater iii) fan
iv) dynamo v) photo cell vi) charging a battery
vii) burning of coal viii) loudspeaker x) microphone
m) Name a device which does following energy transformation:
i) Chemical energy into electrical energy
ii) Electrical energy into heat energy
iii) Electrical energy into light energy
iv) Nuclear energy into heat energy
v) Electrical energy into mechanical energy
vi) Mechanical energy into electrical energy
vii) Electrical energy into mechanical energy
viii) Electrical energy into sound energy
ix) Sound energy into electrical energy
9. Numerical problems.
Modern Concept Science and Environment – 8 75
a) A stone of 0.5 kg mass is thrown with 5 m/s. Find the kinetic energy of the stone.
[ Ans: 6.25 J]
b) Find the kinetic energy of a ball of mass 200 g moving at a speed of 20 m/s.
[Ans: 40 J]
c) Find the velocity of a body of mass 100 g having kinetic energy of 20 J.[Ans: 20 m/s]
d) How much energy is required to lift a mass of 5 kg to a height of 3 m? (g = 9.8 m/s2 )
[ Ans: 147 J]
e) How much energy is required to lift a mass of 25 kg to a height of 10 m? (g = 9.8 m/s2 )
[Ans: 2450J ]
f) Calculate the potential energy of a stone of mass 10 kg placed at a height of 4 m
above the ground. (g = 9.8 m/s2) [Ans: 392J ]
g) A child of mass 30 kg is at the top of a sliding surface of height 10 m. Find the
potential energy stored in him. [Ans: 2940J ]
h) A block of wood is placed on a rough horizontal plane. If the friction between the
block and the plane is 60 N, what is the work done to move it through a distance
of 15 m? [Ans: 900J ]
i) A collie carries a weight of 1000N to a distance of 50m. Find the power of the
collie. [Ans: 50000J ]
j) Ram having mass 50 kg climbs on a staircase of height 20m. Find the work done
by him. [ Ans: 9800J]
k) A cat having mass 5 kg jumps to a height of 4 m. Find the work done by the cat.
[Ans: 196J]
l) A boy of mass 50 kg can run up to a staircase in 10 s. If there are 30 steps in the
staircase each having a height of 16 cm, then calculate power of the boy.
[Ans: 23,520 W]
m) A potter caries 20 bricks each of weight 10 N to a height of 100 m in 50s. Find
power of the potter. [Ans: 400W ]
n) A water pump can lift 500 kg water to a tank situated at a height of 10 m in 10 s.
Calculate power of the pump.(g = 9.8 m/s2) [4900 W]
10. Draw the diagram:
a) to show the work done against gravity.
b) to show the work done against friction.
76 Heat Estimated teaching periods Theory Practical
UNIT 4 1
6 Heat
Syllabus issued by CDC Celsius
Heat and temperature
Thermometer and its structure
Thermometric liquids (mercury and alcohol)
Calibration of the thermometer
Transformation of units of temperature
Types of thermometers
LEARNING OBJECTIVES
At the end 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 the simple and clinical thermometer.
Key terms and terminologies of the unit
1. Heat : Heat is a form of energy which gives us a sensation of warmth.
2. Temperature : The degree of hotness or coldness of a substance is called
temperature.
3. Heat (in terms of molecular motion) : The sum of kinetic energy of all the molecules in a substance
is called heat.
4. Temperature (in terms of molecular motion) : The average kinetic energy of all the atoms or molecules
present in a substance is called temperature.
5. Thermometer : The instrument which is used to measure temperature is
called thermometer.
6. Thermometric substance : The liquid which is used in the bulb of a thermometer is
called thermometric liquid or thermometric substance.
7. Principle of thermometer : A thermometer is based on the principle that matter expands
on heating and contracts on cooling.
8. Lower fixed point : The melting point of pure ice at a pressure of 760 mm Hg is
taken as the lower fixed point. Its value is 00C.
9. Upper fixed point : The boiling point of pure water at a pressure of 760 mm Hg
is taken as upper fixed point. Its value is 1000C.
10. Calibration : The process in which a number of equal divisions are made
in between the upper fixed point and lower fixed point of a
thermometer is called calibration.
11. Clinical thermometer : A thermometer which is used to measure our body
temperature is called clinical thermometer.
Modern Concept Science and Environment – 8 77
12. Kink : A bend near the bulb of a clinical thermometer is called kink.
13. Laboratory thermometer : A thermometer which is used to measure temperature of the different substances
in the laboratory is called a laboratory thermometer.
6.1 Introduction
Sometimes we feel hot and sometimes cold. The hot and cold is due to the difference of heat
between the body and the environment. If we gain heat, we feel hot and if we lose heat, we
feel cold. Thus, heat is a form of energy which gives us a sensation of warmth. The substances
or media which give us heat are called source of heat. Some important sources of heat are sun,
bio-fuel, fossil fuel, electricity, etc.
Heat is very important form of energy. It is required for various activities. It is an essential
source of energy for all plants and animals to perform various metabolic activities. We require
heat to cook food, to light lamp, to iron clothes, to have hot water bath, etc. Goldsmith uses
heat from a flame to design an ornament. In most of the industries and factories, heat is the
main source of energy to run them.
In our daily life we feel cold when we touch a bottle of cold Calorimeter Thermometer
drink, a milk pouch, etc. which are kept in the refrigerator. On
the other hand, we feel hot when we touch a cooking utensil
on fire, boiled water, etc. The degree of hotness or coldness of a
substance is called temperature. The SI unit of heat is joule and
the CGS unit of heat is calorie. Heat is measured by a calorimeter
and temperature is measured by a thermometer.
6.2 Effects of Heat
Heat causes the following effects:
a) Heat causes change in state of matter.
b) Heat causes change in temperature.
c) Heat causes change in volume.
d) Heat changes the solubility of a substance.
e) Heat causes change in the rate of a chemical reaction.
6.3 Heat and Kinetic Energy
A body is made up of large number of particles called molecules. The arrangement of molecules
in solid, liquid and gases is different. These molecules possess translational, vibrational and
rotational motion. Due to the motion of the molecules, the
substances have kinetic energy. When kinetic energy of the
atoms or molecules in a substance increases, it becomes warmer.
Thus, the sum of kinetic energy of all the molecules in a substance
is called heat. For example, a bucket of hot water has more Heat and kinetic energy
molecules than that in a cup of hot water. More molecules means
more kinetic energy and more heat.
78 Heat
When we rub two bodies, the kinetic energy increases. This results increase in heat as well as
temperature. For example, molecules in our skin move faster when we rub two hands together
and hands become warmer. Memory Tips
In this unit we will discuss about the flow of heat, applications When matter is heated, the atoms
of different mode of transmission of heat, temperature and or molecules in the matter move
measurement of temperature. faster.
FACT WITH REASON
A match stick produces flame when its head is rubbed sharply against the side of a matchbox, why?
When the head of a match stick is rubbed sharply against the rough surface on the side of a matchbox,
the kinetic energy of the molecules on the surface increases due to friction. The surface gets heated
and produces a tiny spark. This spark ignites the chemical substance on the head of the match stick.
Thus, a match stick produces flame when its head is rubbed sharply against the side of a matchbox.
6.4 Temperature and Kinetic Energy
The degree of hotness or coldness is proportional to the Memory Tips
average kinetic energy of the molecules in a substance. The Faster the motion of the molecules
change in average kinetic energy of the particles causes an higher the temperature .
effect of change in degree of hotness or coldness of a body.
When the average kinetic energy of the particles in a substance increases, its degree of hotness
also increases. So, the average kinetic energy of all the atoms or molecules in a substance gives
its temperature. The average speed of the molecules in a hot substance is fast and the average
speed of the molecules in a cold object is slow.
6.5 A Case with More Heat but Low Temperature
There are more molecules in a bucketful of luke warm water than More heat Less heat
that in a cupful of boiling water. As the sum of kinetic energy
is more, there is more heat in the bucketful of luke warm water. but low but more
However, the average kinetic energy of the molecules in a
bucketful of luke worm water is less than that in a cupful of boling temperature temperature
water. So, the temperature of bucketful of luke warm water is less
than the cupful of boiling water.
FACT WITH REASON
One liter of boiling water has more heat than that in a red hot iron nail, why?
Heat is the sum of kinetic energy of all the molecules in a substance. There are more molecules in one
liter of water than that in an iron nail. Due to this, the sum of kinetic energy of all the molecules in one
liter of boiling water is more than that in a red hot iron. So, one liter of boiling water has more heat
energy than that in a red hot iron nail.
Modern Concept Science and Environment – 8 79
Differences between heat and temperature
S.N. Heat S.N. Temperature
1 The sum of kinetic energy of all the 1 The average kinetic energy of all the
molecules in a substance called heat. molecules in a substance is called
temperature.
2 It is the cause of change in 2 Change in temperature is the effect of
temperature. heat.
3 It depends upon the kinetic energy of 3 It depends upon kinetic energy of the
the molecules and mass of the body. molecules only.
4 The SI unit of heat is joule (J). 2 The SI unit of temperature is Kelvin (K).
6.6 Measurement of Temperature
The property of expansion of matter due to heat is used to measure temperature. It is measured
generally in three different scales with the help of thermometers.
6.7 Thermometer
The instrument which is used to measure temperature is called a thermometer. It is used to
measure the condition of hotness and coldness of a body. Generally, thermometer is named
according to the thermometric substance used in it. If gas is used in the thermometer, it is
called a gas thermometer. Similarly, if liquid is used in the thermometer, it is called a liquid
thermometer. The commonly used thermometer is a liquid thermometer.
Thermometer
Thermometric liquid
The liquid which is used in the bulb of a thermometer is called thermometric liquid or
thermometric substance. Usually mercury or coloured alcohol is used in a thermometer.
Properties of the thermometric liquid
The properties of a good thermometric liquid are:
a) Thermometric liquid should be a good conductor of heat.
b) It should be opaque for clear visibility.
c) It should expand uniformly throughout the measuring range.
d) It should be in the liquid state throughout the measuring range.
Principle of thermometer
Thermometer is based on the principle that 'matter expands on heating and contracts on
cooling'.
80 Heat
Construction of a thermometer
a) Capillary tube : A thermometer is consist of a very fine glass tube having a very small
bore called capillary tube.
b) Stem : The capillary tube is protected by a thick glass tube called stem.
c) Bulb : At one end of the capillary tube in a thermometer, a very thin walled tube is
provided called bulb. It is filled with thermometric liquid.
Lower fixed point
The melting point of pure ice at a pressure of 760 mm Hg is taken as the lower fixed point. Its
value is 0°C.
ACTIVITY 1 Stand Thermometer
OBJECTIVE : To find the lower fixed point of a thermometer. 0 0C
Melting ice cubes
Immerse the bulb of a thermometer into a funnel containing pure Glass
melting ice at normal atmospheric pressure (1 atm). When the mercury
level in the thermometer becomes stable, make a mark at that level as Beaker
0°C. This is called the lower fixed point or ice point. Water
Upper fixed point
The boiling point of pure water at a pressure of 760 mm Hg is taken as the upper fixed point.
Its value is 100°C.
ACTIVITY 2 Stand 1000C
Thermometer
OBJECTIVE : To find the upper fixed point of a thermometer.
Glass tube
Insert the thermometer into the apparatus such that the bulb should Cork
be just above the boiling water at normal atmospheric pressure (1
atm). When the mercury level in the thermometer becomes stable, Water vapour
make a mark at that level as 100°C. It is called the upper fixed point or Round bottom flask
boiling point.
Water (boiling)
Tripod stand
Bunsen burner
Calibration
The process in which a number of equal divisions are made in between the upper fixed point
and lower fixed point is called calibration. The markings on the stem are called graduations
or degrees.
6.8 Measurement of Temperature by a Thermometer
One of the most familiar thermometers is mercury thermometer. In this thermometer, pure
mercury is filled in the bulb. When the bulb comes in contact with a hot surface, the mercury
in the bulb expands and its volume increases. The increased volume of mercury is forced to
rise in the capillary tube. Increasing the level of mercury depends upon the degree of hotness.
Temperature is measured by looking the level of mercury column in the capillary tube.
Modern Concept Science and Environment – 8 81
Advantages of mercury as a thermometric liquid
a) Mercury is a good conductor of heat. So, it needs very little heat to expand.
b) It does not wet the glass tube. So, it helps in recording accurate temperature.
c) Expansion of mercury is uniform over a wide range of Memory Tips
temperatures. Boiling temperature of mercury is
357°C. So, a temperature as high
d) It is opaque and silvery in colour. So, it can be clearly as 357°C can be recorded with the
seen in the capillary tube.
Disadvantage of a mercury thermometer help of a mercury thermometer.
a) Mercury is very expensive liquid metal.
b) Freezing temperature of mercury is -39°C. So, a mercury thermometer can record only
up to -39°C.
c) Mercury is a poisonous substance. It is dangerous if the bulb of thermometer is broken.
FACT WITH REASON
A mercury thermometer cannot measure very low temperatures, why?
The freezing temperature of mercury is -39°C. It remains in liquid state only upto -39°C. So, a mercury
thermometer cannot measure very low temperatures.
Advantages of alcohol thermometer
Melting point of alcohol is -117°C. So, an alcohol thermometer can record a temperature as
low as -117°C.
a) Alcohol thermometer can record the temperature in Arctic and Antarctic region.
b) Alcohol expands six times more than mercury Memory Tips
for the same rise in temperature. Thus, an alcohol Alcohol thermometer is used rather
thermometer can measure temperature more
than a mercury thermometer in
accurately than a mercury thermometer.
very cold region.
c) It is cheaper than mercury.
FACT WITH REASON
The Mt. Everest climbers use alcohol thermometer, why?
Freezing temperature of alcohol is -117°C. Alcohol remains liquid up to -117°C. So, an alcohol
thermometer can measure very low temperature in the Mt. Everest.
Disadvantages of an alcohol thermometer
a) Alcohol is a bad conductor of heat.
b) It does not have uniform rate of expansion and contraction.
c) It sticks to the wall of the capillary tube. So, it is difficult to measure accurate temperature.
d) The boiling point of alcohol is 78°C. So, alcohol thermometer cannot measure a high
temperatures above 78°C.
82 Heat
FACT WITH REASON
An alcohol thermometer cannot measure the temperature of boiling water, why?
Boiling point of alcohol is 78°C. It remains in liquid state only upto 78°C. So, an alcohol thermometer
cannot measure the temperature of boiling water which is 100°C.
6.9 Thermometric Scales
There are three thermometric scales. They are Degree Celsius, Degree Fahrenheit and Kelvin scales.
a) Celsius or centigrade scale: It is one of the most widely used temperature scale. This
scale was introduced by a Swedish astronomer Anders Celsius (1701- 1744) and is
known after his name. In this scale, there are 100 divisions between the upper fixed
point, 100°C (i.e. the temperature at which water boils) and lower fixed points, 0°C (i.e.
the temperature at which water freezes). Each division on this scale is called one degree
centigrade or Celsius.
b) Fahrenheit scale: This scale was introduced by German physicist Daniel Gabriel
Fahrenheit (1686-1736) and is known after his name. In this scale, there are 180 divisions
between the upper fixed point, 212oF and lower fixed points, 32oF. Each division on this
scale is called one degree Fahrenheit.
c) Kelvin scale: Kelvin scale is named after the British physicist William Thomson, Baron
Kelvin (1824-1907). It is the scale of temperature used in the scientific research. In this
scale, there are 100 divisions between the upper fixed point, 373 K and lower fixed point
273 K. Each division on this scale is called one Kelvin.
373 K 1000C 2120F Water boils
310 K 100 degree intervals37.00C 98.60F Normal body
100 degree intervals temperature
100 degree intervals
273 K 00C 320 F Water freezes
Kelvin scale Celsius scale Fahrenheit scale
Thermometric scales
Modern Concept Science and Environment – 8 83
6.10 Conversion of Celsius Scale, Fahrenheit Scale and Kelvin Scale
The following relation can be used to find any conversion relation:
Reading – lower fixed point = Constant (for all scales)
Upper fixed point – Lower fixed point
The relation between Celsius, Fahrenheit and Kelvin is
C–0 = F – 32 = K – 273
100 – 0 212 – 32 373 – 273
or, C – 0 = F – 32 = K – 273
100 180 100
1. Conversion of Celsius scale into Fahrenheit scale
C – 0 = F – 32 or, C = F – 32 or, C = 5(F – 32)
100 180 5 9 9
2. Conversion of Celsius scale into Kelvin scale
C – 0 = K – 273 or, K = C + 273
100 100
3. Conversion of Fahrenheit into Kelvin scale
F – 32 = K – 273 or, F – 32 = K – 273
180 100 95
Solved Numerical 6.1
Convert 37°C into (a) °F and (b) K
Solution:
Given, Celsius scale temperature C= 37°C
a) To convert into °F b) To convert into K
From the relation, From the relation,
C = 5(F – 32) C – 0 = K – 273
9 100 100
or, 37 = 5 (F – 32) or, K = C + 273 = 37 + 273 = 310 K
9
or, 37 × 9 = F – 32
5
or, 66.6 = F – 32
∴ F = 66.6 + 32 = 98.6 °F
Solved Numerical 6.2
A temperature 'x' is recorded both on Celsius and Fahrenheit scale. Find the value of x.
Or, calculate the temperature at which the Fahrenheit and Celsius scales show the same reading.
Solution:
Here, Celsius temperature (C) = Fahrenheit temperature (F) = x
From the relation,
84 Heat
C = F – 32 (Since F = C = x)
59
or, x = x – 32
59
or, 9x = 5x – 160
or, 4x = – 160
or, x = – 160 = – 40
4
Therefore, at -40, both Celsius and Fahrenheit scale show the same reading.
6.11 Types of Thermometers
a) Clinical thermometer Memory Tips
A thermometer which is used to measure our body Nowadays, digital clinical
temperature is called clinical thermometer. It is also called thermometers are used widely to
doctor's thermometer. Clinical thermometer has a prismatic measure our body temperature.
shape. This prismatic shape helps to magnify the mercury Digital thermometers do not use
column inside the capillary tube. So, recording of the exact mercury and alcohol.
temperature becomes easy. A small bend is present near Mercury Thread
the bulb of the clinical thermometer. It is called kink. Kink
prevents the back flow of mercury. Clinical thermometer is Bulb Kink
jerked before its use to return the mercury back into the bulb.
Clinical thermometer
The range of the clinical thermometer is from 35°C to 43°C.
FACT WITH REASON
The bulb of a thermometer is made thin, why?
The bulb of a thermometer consists of thermometric liquid. A thin wall of the bulb helps to conduct
heat from outside to the thermometric liquid. So, the bulb of a thermometer is made thin.
Doctor's jerk the clinical thermometer before its use, why?
A small bend is present near the bulb of a clinical thermometer. It is called kink. Kink prevents the
back flow of mercury. So, a clinical thermometer is jerked before its use to return the mercury back
into the bulb.
b) Laboratory thermometer
A thermometer which is used to measure the temperature Mercury
of different substances in a laboratory is called laboratory
thermometer. A laboratory thermometer does not consist Bulb Thread
kink. A common laboratory thermometer can measure the Laboratory thermometer
temperature from -10°C to 110°C.
Modern Concept Science and Environment – 8 85
Differences between clinical thermometer and laboratory thermometer
S.N. Clinical thermometer S.N. Laboratory thermometer
1 Clinical thermometer is used to 1 Laboratory thermometer is used to
measure human body temperature. measure the temperature of different
substances in a laboratory.
2 It has a prismatic shape. 2 It has a cylindrical shape.
3 It has a kink near the bulb. 3 It does not have kink.
4 It ranges from 35°C to 43°C. 2 It ranges from -10°C to 110°C.
6.12 Human Body Temperature
The normal human body temperature is 37oC (or 98.6oF). In summer season, the temperature
of our surrounding may be more than 37oC and in winter season, the temperature of our
surrounding may fall below 37oC. But the temperature of our internal organs like the heart,
lungs, brain, etc. does not change.
To find our body temperature the bulb of the clinical thermometer is placed under the tongue
or armpit for a few minutes. The expanded mercury column inside the capillary tube in the
thermometer determines the body temperature.
ANSWER WRITING SKILL
1. What is heat? Define heat based on kinetic theory.
Ans: Heat is a form of energy which gives us the sensation of the warmth. According to kinetic theory,
the sum of kinetic energy of all the molecules in a substance is called heat.
2. What is temperature? Define temperature based on kinetic theory.
Ans: The degree of hotness or coldness of a substance is called temperature. According to kinetic theory,
the average kinetic energy of all the atoms or molecules in a substance is called temperature.
3. What are thermometric substances? Write down the common thermometric substances.
Ans: A liquid which is used in the bulb of a thermometer is called thermometric liquid or thermometric
substance. Mercury and coloured alcohol are common thermometric substances.
4. What are lower and upper fixed points?
Ans: The melting point of pure ice at a pressure of 760 mm Hg is taken as the lower fixed point. Its value
is 00C. The boiling point of pure water at a pressure of 760 mm Hg is taken as upper fixed point. Its
value is 1000C.
5. Write down the formula to convert temperature scale from one to another.
Ans: The formula to convert temperature scale from one to another is given below:
C–0 = F – 32 = K – 273
100 180 100
6. Why does clinical thermometer have a prismatic shape?
Ans: Clinical thermometer has a prismatic shape. This prismatic shape helps to magnify the mercury
column inside the capillary tube. So, recording of the exact temperature becomes easy.
86 Heat
7. Convert 100oC into oF.
Ans: Solution:
Here, Celsius scale temperature, C= 100 0C
To convert into 0F scale we use the formula
C = F –932 or, 100 × 9 =F – 32 or, 180 = F – 32
5 5
or, F = 180 + 32 = 212 °F
8. Write two differences between Celsius scale and Fahrenheit scale.
S.N. Celsius scale S.N. Fahrenheit scale
1 In this scale, there are 100 divisions 1 In this scale, there are 180 divisions
between the upper fixed point and between the upper fixed point and lower
lower fixed points. fixed points.
2 Each division in this scale is called 2 Each division in this scale is called one
one degree centigrade or Celsius. degree Fahrenheit.
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) The degree of …….. or ……..of a body is called temperature.
b) There are …… equal divisions in between upper fixed point and lower fixed point
in Kelvin scale.
c) Mercury boils at …… °C.
d) Kink is present in a ………… thermometer.
e) …… thermometer can measure very low temperature.
f) The range of a clinical thermometer is from………to………
g) A laboratory thermometer can measure the temperature from …….0C to ……0C.
2. Write True for the correct and False for the incorrect statements.
a) Heat always transfers from a body at low temperature to a body at high
temperature.
b) Fahrenheit thermometers are calibrated from 32 oF to 212 oF.
c) Freezing temperature of alcohol is -39 0C.
d) Alcohol thermometer can measure a very low temperature.
e) The lower fixed point in Kelvin scale is 273 K.
f) The normal human body temperature is 37 oC.
g) There are 180 equal divisions in between the upper fixed point and lower fixed
point in Kelvin scale.
Modern Concept Science and Environment – 8 87
STEP 2
3. Answer the following questions in one word.
a) What is the cause of change in temperature of a body?
b) What is the SI unit of heat?
c) Name the thick glass tube which protects the capillary tube of a thermometer.
d) What is the boiling point of alcohol?
e) What are the markings on the stem of a thermometer called?
4. Write any two differences between:
a) Heat and temperature b) Upper and lower fixed point
c) Clinical and laboratory thermometer d) Celsius scale and Fahrenheit scale
5. Give reasons.
a) Mercury thermometer can record a high temperature.
b) Alcohol thermometer can record very low temperature.
c) We cannot measure the temperature of the boiling water by using an alcohol
thermometer.
d) Doctors jerk a clinical thermometer for a new record of temperature.
e) Clinical thermometer is provided with a kink.
f) Clinical thermometer is not used to measure temperature of boiling water.
g) The shape of a mercury used clinical thermometer is made prismatic.
h) The bulb of a thermometer is made from a thin glass.
STEP 3
9. Answer the following questions.
i) Write the effects of heat.
j) Write the properties of a good thermometric liquid.
k) What is the principle of a thermometer?
l) Explain the construction of a thermometer.
m) What is meant by calibration of a thermometer?
n) Write the advantages and disadvantages of a mercury thermometer.
o) Write the advantages and disadvantages of an alcohol thermometer.
p) Among the mercury thermometer and alcohol thermometer which one will you
suggest to the Mt. Everest climbers for measurement of temperature there? Write
with a reason.
q) Write the relation between Celsius, Fahrenheit and Kelvin scale.
6. Numerical problems
a) Convert 100oC into oF. [Ans: 212oF]
b) Convert 37 oC into oF [Ans: 98.6oF]
c) Convert 98.6oF into oC. [Ans: 37oC]
d) Convert 32oF into oC. [Ans: 0oC]
88 Light Estimated teaching periods Theory Practical
UNIT 4 1
7 Light
Syllabus issued by CDC Image
Introduction to light, ray and beam of light
Mirror and its types
Characteristics of images formed by plane mirror
Real and virtual image
Images formed by concave and convex mirror
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
introduce mirror and its types (plane and spherical) and demonstrate reflection of light from
spherical mirrors.
introduce real image and virtual image and demonstrate them.
draw and demonstrate the images formed by spherical mirrors.
explain the uses of spherical mirrors.
Key terms and terminologies of the unit
1. Light : Light is a form of energy which makes objects visible to us.
2. Ray of light
: The narrow path of the light which is represented by a straight line with an
arrow is called a ray of light.
3. Beam of light : A collection of rays of light in a certain pattern is called a beam of light.
4. Reflection of light
: The process of returning light to the same medium after striking a surface is
5. Incident ray called reflection of light.
6. Reflected ray : The light ray from the source which falls upon the reflective surface is called
7. Normal incident ray.
8. Angle of incidence : The light ray which bounces away from the reflective surface is called reflected ray.
: A perpendicular line drawn at the point of incidence on the reflective surface is
called a normal.
: The angle made by an incident ray with normal is called angle of incidence.
9. Angle of reflection : The angel made by a reflected ray with normal is called angle of reflection.
10. Regular reflection of light : When a parallel beam of light, coming from a source, strikes a surface and reflects
in a parallel way, such type of reflection is called regular reflection of light.
11. Irregular reflection of light : When a parallel beam of light strikes a surface and reflects in different
directions, such type of reflection is called an irregular reflection of light.
12. Image : When a number of rays, starting from a point after reflection or refraction, meet
together, then image is formed.
13. Real image : The image which is formed by actual meeting of two or more rays after
reflection or refraction is called a real image.
Modern Concept Science and Environment – 8 89
14. Virtual image : The image which does not form by actual meeting of two or more rays after reflection
or refraction is called virtual image.
15. Mirror : A smooth polished surface from which a regular reflection takes place is called mirror.
16. Plane mirror : The mirror which has flat and smooth reflective surface is called a plane mirror.
17. Lateral inversion : The phenomenon due to which the image of an object turns through an angle of 1800
about a vertical axis is called lateral inversion.
18. Spherical mirror : The mirror which is a part of a glass sphere is called a spherical mirror.
19. Concave mirror : In concave mirror, the polishing or silvering is done on the outer surface of the cut
part of a hollow sphere.
20. Convex mirror : In convex mirror, the polishing or silvering is done on the inner surface of the cut part
of a hollow sphere.
21. Pole : The geometric center of a spherical mirror is called pole.
22. Center of curvature : The center of the hollow glass sphere of which the spherical mirror is a part is called
center of curvature.
23. Radius of curvature : The radius of a hollow glass sphere of which the spherical mirror is a part is called
radius of curvature.
24. Principal axis : The line passing through the center of curvature and pole of the mirror is called
principal axis.
25. Principal focus : The point on the principal axis at which all rays, parallel to the principal axis meet or
appear to meet after reflection from a mirror is called principal focus.
26. Focal length : The distance between pole and principal focus of a spherical mirror is called focal length.
27. Medium : The substance through which light propagates or tends to propagate is called a
medium.
28. Rarer medium : The medium in which light travels faster is called optically rarer medium
29. Denser medium : The medium in which light travels relatively slower is called optically denser medium.
30. Refraction of light : The phenomenon of bending of light as it passes obliquely from one optical medium
to another is called refraction of light.
31. Lateral shift : The perpendicular distance between incident ray and emergent ray is called lateral
shift or lateral displacement.
7.1 Introduction
We cannot see objects in a dark room. However, if we switch on a bulb, the objects become
visible. When light falls on the objects, it bounces from their surfaces and enters into our eyes. As
a result, image of the objects forms on the retina of the eyes and we can see objects present in the
room. Thus, light is a form of energy which makes objects visible to us. It means that objects are
not visible without light. The study of light and vision is called optics. Light travels at a speed
of 3 × 108 m/s in vacuum. In this unit we will discuss about the reflection of light from a plane
reflective surface and spherical reflective surface. We will also discuss about the phenomenon of
bending of light when it passes from one optical medium to another optical medium.
90 Light
7.2 Source of Light
Those bodies which emit light are called sources of
light. For example, the sun, electric bulb, lamp, etc. The
sources of light are of two types. They are self-luminous
and non-luminous. Light that enters into our eyes can
come directly from a self-luminous object or it may be Source of light
the reflected light from a non-luminous object. Self-
luminous sources of light emit their own light. For example, the sun, stars, firefly, etc. On the
other hand, objects like moon, rocks, plastics, glass, etc. do not emit their own light. They are
called non-luminous objects.
7.3 Ray of Light
The narrow path of the light which is represented by a straight line A ray of light
with an arrow is called a ray of light. The arrow head of a ray gives
its direction. Rays are produced when light passes through a small hole.
7.4 Beam of Light
You might have seen a beam of sunlight when it enters into a room through a narrow opening
or a hole. A collection of rays of light in a certain pattern is called a beam of light. A beam of
light is produced when light passes through a larger collecting hole. There are three types of
beam of light. They are:
a) Convergent beam
b) Divergent beam
c) Parallel beam Parallel beam of light Convergent beam of light Divergent beam of light
7.5 Reflection of Light
When a beam of light travelling through a medium falls on
the surface, then there occurs three possibilities. They are:
a) A part of light may be absorbed
b) A part of light may return back to the same medium,
which gives the phenomenon of reflection of light.
c) A part of light may get transmitted through
the medium, which gives the phenomenon of
refraction of light.
Reflection is a phenomenon by virtue of which, incident Reflection of light
light is partly or completely return back into the same medium. The ray of light which returns
to the same medium after striking on the surface is called reflected ray of light. This reflected
ray of light enters into our eyes and makes the things visible. Thus, the process of returning
light to the same medium after striking a surface is called reflection of light.
Modern Concept Science and Environment – 8 91
7.6 General Terms Used in Reflection of Light
a) Incident ray : The light ray from a source which falls upon the reflective surface is called
incident ray. IO is an incident ray in the given figure. N
b) Point of Incidence : The point on the reflective I R
surface where an incident ray strikes is called point
of incidence. In the given figure, 'O' is the point of ir
incidence. P OQ
c) Reflected ray : The light ray which bounces away Reflection of light on a plane mirror
from the reflective surface is called reflected ray. OR
is the reflected ray in the given figure.
d) Normal : A perpendicular line drawn at the point of incidence on a reflective surface is
called normal. In the given figure ON is a normal.
e) Angle of incidence : The angle made by an incident ray with normal is called angle of
incidence. The ∠ION is the angle of incidence in the given figure. It is represented by ∠i.
f) Angle of reflection : The angel made by a reflected ray with normal is called angle of
reflection. The ∠RON is the angle of reflection in the given figure. It is represented by ∠r.
Laws of reflection Memory Tips
a) The incident ray, the reflected ray and the normal at i. A smooth surface shines due to
the point of incidence all lie in the same plane. the regular reflection from it.
b) The angle of incidence is equal to the angle of ii. There is irregular reflection
reflection, i.e. ∠i = ∠r. from a rough surface. So it
Types of reflection doesn't shine.
There are two types of reflection of light: regular reflection and irregular reflection.
a) Regular reflection
In regular reflection, light bounces back in well-defined Regular reflection
direction. When a parallel beam of light, coming from a source,
strikes a surface and reflects in parallel way, such type of
reflection is called regular reflection of light. It takes place on
smooth surfaces. For example, there is a regular reflection on a
plane mirror, water surface, polished metallic surface, etc.
b) Irregular reflection Irregular reflection
In irregular reflection, light bounces back in different directions.
When a parallel beam of light strikes a surface and reflects in
different directions, such type of reflection is called irregular
reflection of light. This type of reflection takes place when the
surface is not smooth. For example, wall, tree, etc.
92 Light
7.7 Image
When a number of rays, starting from a point after reflection or refraction, meet together, then
image is formed. There are two types of images. They are:
Real Image
The image which is formed by actual meeting of two or more rays after reflection or refraction
is called a real image. A real image is inverted. It can be obtained on a screen.
Virtual Image
The image which is not formed by actual meeting of two or more rays after reflection or
refraction is called a virtual image. When reflected or refracted rays appear to meet, a virtual
image is formed. A virtual image is erect and it cannot be obtained on the screen.
Differences between real image and virtual image.
S.N. Real image S.N. Virtual image
1 Real image is formed when the reflected 1 Virtual image is formed when the
or refracted rays intersect at a point. reflected or refracted rays appear to meet.
2 Real image is always inverted. 2 Virtual image is always erect.
3 It can be obtained on a screen. 3 It cannot be obtained on a screen.
7.8 Mirror
A smooth polished surface from which a regular reflection takes place is called mirror.
Characteristics of mirror
a) The surface of mirror shines.
b) It forms an image due to regular reflection.
Types of Mirror
There are two types of mirror. They are plane mirror and spherical mirror.
1. Plane Mirror
We look our face on a plane reflective surface. It is called a plane mirror. Thus, the mirror
which has flat and smooth reflective surface is called a plane mirror.
Polished surface
Reflective surface
Mirror Reflection on a mirror
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