Class 10 Science Solutions Nepal

v. It is a renewable source of energy and will never get exhausted.
vi. This form of energy can easily be converted into other forms of energy.
vii. Hydroelectric power projects are multi-purpose projects as they enable us to use

their water for irrigation, industrial and domestic purposes.
QUESTIONS

# Use of hydroelectricity should be increased than the fossil fuels. Why?

Biomass and Bio-energy

Biomass is defined as the dry organic matter and its residue. It is a renewable source of
energy. The biomass includes all the plants, agricultural and forest residues like bark,
sawdust, roots, animal droppings, etc. The energy obtained from biomass is called bio-
energy.
Similarly, cow dung and other wastes of plant materials are decomposed in the
absence of oxygen to produce bio-gas. It is also known as gobar gas.

Uses of bio-gas

a. Bio-gas is used for cooking, heating and lighting purposes.
b. It is used to generate electricity.
c. The dung which is obtained after generating electricity is used as manure in the

agricultural field.
d. It does not produce smoke. So, it used as a clean source of energy.

MEMORY TIPS
Instead of burning the loose biomass fuel directly, it is more practical to compress it into
briquettes and to improve its utilities and convenience of use, too.

New Creative Science, Class 10 | 47

Wind energy

Moving air is called the wind. It possesses the kinetic
energy and is a renewable source of energy. This can be
utilized to generate electricity. Traditionally, it has been
used to drive windmills, propel sailboats, separating husk
from grains, etc. It is a renewable source of energy and can
be developed as the alternative sources of energy in the
Mahabharat region in Nepal, which is a continuous wind
blowing region in Nepal. Thus, the energy which is obtained
from the continuous blowing wind is called the wind energy.

Tidal energy

The energy derived from rising and falling ocean tides is called the tidal energy. Tides are
formed due to the gravitational pull of the moon on the water in the ocean. For using
the tidal energy, it is necessary to build a dam across the tidal region. The gates allow
water to flow in it at high tide and as the tide falls, the gates are shut and the water is
allowed to run back through turbines to generate electricity. It is an inexhaustible and
renewable pollution-free energy source.

MEMORY TIPS
High tides occur on every new moon day and full moon day and can raise the level of water
by few metres.

Geothermal energy

Geothermal energy is the heat of the earth and is the naturally occurring thermal
energy found within the earth. The temperature inside the earth’s crust is as high as
500°C near the junction between crust and mantle and further more around magmal-
zones. Water is pumped down through an injecting well where it passes through
joints and fractures in hot rocks and this water is converted into steam. This steam is
used to rotate the turbines of generators to produce electricity. It is also a renewable,
pollution-free source of energy. Thus, the energy which is obtained from the heat of the
earth is called the geothermal energy.

Nuclear energy

The energy released when the nucleus of one atom converts to another in course of nuclear
reaction is called the nuclear energy. There are two distinct ways of obtaining the nuclear
energy. They are:

a. Nuclear fission reaction
b. Nuclear fusion reaction

a. Nuclear fission reactions

In this process, an unstable nucleus of a heavy atom like uranium, polonium, thorium
etc. splits into lighter nuclei with the liberation of enormous amount of energy when

48 | Energy

it is bombarded with slow moving neutrons. Thus, the process in which an unstable
heavy nucleus of a radioactive element is broken down into two or more light nuclei is called
the nuclear fission. Nuclear fission of U-235 can be represented as,

A chain reaction showing the splitting of uranium atoms

This enormous amount of energy is used for producing steam, which rotates the
blades of turbine to generate electricity.

MEMORY TIPS

One kilogram of U-235, on fission produces as much energy as 2500 tonne of coal
produces on burning.

b. Nuclear fusion reaction

It is a process of combining two or more lighter nuclei to form a more stable heavy nucleus with
the liberation of tremendous amount of energy. The process of nuclear fusion reaction of
hydrogen to form helium nuclei is shown from the given reaction,

2 11H + 2 11H high temperature 4 He + 201e + Q (energy)
high pressure 2

Lighter nuclei Heavier nucleus

Hydrogen bomb is based on the nuclear fusion reaction and the energy generated
from this fusion reaction cannot be converted into useful energy, as the energy cannot
be controlled.

Advantages of nuclear energy

i. A small amount of uranium (U-235) like nucleus fuel can produce a large amount
of energy.

ii. It can be used as the alternative source of energy.
iii. Once the nuclear fuel is fed into the nuclear power plant, it keeps on producing

electricity for two to three years.

Disadvantages of nuclear energy

i. The initial cost of production is very high and high technology is required for it,
which cannot be afforded by developing and under-developed countries.

New Creative Science, Class 10 | 49

ii. The pollution caused by nuclear fuel is much more dangerous.
iii. The nuclear waste radiates harmful rays like α, β, γ and δ rays. When an organism

is exposed to such radiations, his biological and genetic system is disrupted.

MEMORY TIPS
Both the nuclear reactions (fusion and fission) are based on Einstein’s theory of relativity
E = ∆mc2

QUESTIONS
# In what aspects is the nuclear energy advantageous and in what aspect it is harmful?

ACTIVITY

List out the forms of energy that you use from the morning when you wake up till you reach the school. From
where do we get these different forms of energy? Can we say these are the sources of energy? Why?

Solar energy

The sun has been radiating an enormous amount of energy in the form of heat and
light at the present rate for nearly 5 billion years and will continue for about 5 billion
years more. The heat and light energy obtained from the sun is called the solar energy.

The sun is radiating energy at the rate of 4.5 × 1023 kJ/s in all directions in space. The
energy received by 1m2 of the earth’s surface is about 4 kw/h daily.

Uses of solar energy

a. The solar energy has been used for drying clothes, preserving fruits, vegetables, meat,
fish, etc.

b. It is used to dry food grains after harvesting them.
c. It is used to heat water.
d. Plants use solar energy for photosynthesis.
e. With the help of solar cookers, food is cooked by using the solar energy.
f. A solar battery converts the solar energy into an electrical energy, which is used

for various purposes. A group of solar cells connected in a particular way is called a
solar panel.

Limitations of solar energy

a. It is not available uniformly all the time in all places.
b. The solar energy which reaches our earth is in very various forms so, it cannot be

used directly.
c. A lot of costly equipment and infrastructure are required to put solar energy in use.

MEMORY TIPS
The amount of solar radiation incident is normally per second on 1m2 area of a perfectly
black surface at the mean distance of the earth from the sun. Its approximate value is 1.4
KJ per second per square metre.

50 | Energy

ACTIVITY

1. Take two conical flasks and paint one white and the other black. Fill both of them with water.
2. Place the conical flasks in direct sunlight for one hour with thermometers in their mouth.
3. Observe the temperature of the water in both the flasks. Which one is hotter? Can you think

of the ways in which this finding could be used in your daily life?

SUN: THE ULTIMATE SOURCE OF ENERGY

The sun is called the ultimate source of energy because all the other sources of energy
we have used so far are the direct or indirect consequences of the solar energy. It will
be clear from the following discussions.

Hydroelectricity as the outcome of solar energy

Water from seas, oceans and rivers evaporates due to the sun’s heat and rises up
where it condenses and forms clouds. This water returns to the earth in the form of
rain or snow which causes the water to flow in the rivers. The K.E. of the water is used
to rotate turbines and generate hydroelectricity.

Fossil fuel as the consequence of solar energy

Fossils are the remaining of dead plants and animals. The plants had used solar energy
for photosynthesis when they were alive and animals depend upon plants directly or
indirectly for their food. Therefore, fossil fuels are also the outcome of the solar energy.

Wind energy as the consequence of solar energy

Due to the heating of air above the surface of the earth, it rises being lighter. So,
these create a partial vacuum and the air pressure becomes low. Then, the air from
as the higher-pressure region blows towards the lower pressure region to balance the
pressure difference. This moving air is called the wind which can be converted into
useful work.

Similarly, biomass is the organic matter and its residue, which uses the solar energy
directly or indirectly. Due to the above reasons, the solar energy or the sun is the
ultimate source of energy and the main energy source.

About the Sun
Mass = 2 × 1030 kg (3.33 × 105 times bigger than the earth)
Diameter : 1.4 × 106 km (10 times bigger than the earth)
Surface Temperature : 5700 °C
Core Temperature : 1.5 × 107 °C
Energy Radiation : About 4 × 1026 J/S
Average Solar Energy on the Earth : 1.4 kW per square meter.

New Creative Science, Class 10 | 51

QUESTIONS
# Why is the sun called an ultimate source of energy?

MECHANISM OF ENERGY PRODUCTION IN THE SUN

The solar energy is produced due to the thermo-nuclear fusion reaction in which
hydrogen atom is continuously changing into helium nuclei liberating tremendous
amount of energy.

The sun is rich in hydrogen gas. It consists of about 75% of hydrogen and 25% of

helium. At very high temperature and pressure, hydrogen atoms split into protons

and electrons. The two protons combine due to high pressure to form a deuteron (12H)

and a positron (e+ 0 ). Now, a proton (11H) and a deuteron (21H) unite to form the isotope
1

of helium (23He). Then the two helium nuclei unite to form the stable helium nucleus (

24He). In each step of the reaction, tremendous amount of energy is released. It can be

summarized in the following reactions

i) 11H+ 11H high temperature 2 H + 10e + Q1
high pressure 1

(deuterium) (positron) (energy)

ii) 12H+ 11H high temperature 23He + Q2 (energy)
high pressure

iii) 32He + 23He high temperature 24He + 11H + 11H + Q3 (energy)
high pressure

In short,

2 11H + 2 11H high temperature 24He + 210e + Q (energy) where, Q = Q1 + Q2 + Q3
high pressure

MEMORY TIP

Positron (10e) is known as the antiparticle of electron as it has mass equal to electron but

bears the opposite charge as that of electron.

QUESTION
# How is an enormous amount of energy released from the sun?

Conditions and evidences of nuclear fusion reaction in the sun

a. The sun has a large amount of hydrogen which undergoes fusion reaction to form
helium.

b. There is a very high temperature that can split hydrogen atoms into its protons
and electrons.

c. The extreme pressure on the sun helps to unite the similar charged protons.
d. The presence of helium in the sun is the product of the nuclear fusion reaction.

52 | Energy

Present status of energy use

The type and amount of energy sources used change according to the places, living
standard of people and degree of civilization and industrialization.

About 37% of the demand of energy is fulfilled from petroleum, 25% from coal, 13%
from biomass and bio-gas, 8% from hydroelectric power, 4% from nuclear energy
(fission) and the rest are fulfilled other sources like solar energy, wind energy, tidal
energy, geo-thermal energy, etc. The demand for energy is increasing at the rate of
2.3% annually. If the sources of energy are not properly used and are not conserved
then we are likely to face energy crisis in the near future.

ENERGY CRISIS

About eighty percentage of the present demand of energy has been fulfilled by fossil
fuels, which are the non-renewable sources of energy. If these sources are exploited at
this rate then it is sure that they will be finished by 2076 A.D. But as the need of energy
is increasing by 2.3% per annum, the reserve of these sources will not last after 2037
A.D. So the world is likely to face energy problem which is called energy crisis.

The scarcity of energy sources on the earth due to the excessive use of non-renewable sources of
energy is called energy crisis.

ALTERNATIVE SOURCE OF ENERGY

Those energy sources which are used in place of non-renewable source of energy are called
alternative sources of energy. They are used to maintain the stock of the non-renewable
source of energy. They can be renewed time and again. To avoid the energy crisis, we
should develop the sources of alternative sources of energy. Some common examples
of alternative sources of energy are solar energy, wind energy, tidal energy, bio-gas,
biomass, geothermal energy, etc.

ENERGY CONSERVATION

The energy crisis may bring hazards to the existence of human beings. So, we must try
to save energy as far as possible. We must be careful to save energy.
a. The renewable sources of energy must be used as far as possible and the non-

renewable sources of energy must be used in a limited amount.
b. Alternative sources of energy should be developed which will preserve the non-

renewable sources of energy for the future.
c. Unnecessary use of energy at any place at any time must be avoided.

MODEL QUESTIONS ANSWER

1. What do you mean by energy crisis?
Ü The scarcity of energy sources on the earth due to the excessive use of non-

renewable sources of energy is called energy crisis.

New Creative Science, Class 10 | 53

2. Why are we likely to face energy crisis in the near future?

Ü In the present days, non-renewable sources (fossil fuels) are used an in excessive
amount i.e. they meet about 80% of the energy demand on the earth. The demand
for these sources is increasing 2.3% annually but their reservoir is present only
in the limited amount. So we are likely to face energy crisis in the near future.

3. Differentiate between nuclear fusion and nuclear fission reaction.

Nuclear fusion reaction Nuclear fission reaction

i. In this process, two or more i. In this process, an unstable heavy

lighter nuclei combine to form a nucleus splits into two or more lighter

stable heavy nucleus. nuclei.

ii. Hydrogen bomb is based on ii. Atom bomb is based on this principle.

this principle.

iii. The energy released by this process is more iii. The energy released by this process is

than that produced by fission reaction. less compare to fusion reaction.

iv. This energy cannot be converted iv. This energy can be converted into

into useful energy easily. useful energy.

4. Scientists are busy designing solar equipments. Why?
Ü It is because the solar energy is the perpetual means and the ultimate source of

energy. It is a pollution-free energy source. It can push the energy crisis further.

5. Coal is called the fossil fuel. Why?
Ü The coal is formed from the dead bodies of plants and animals (i.e. fossils) and

it is used to get energy. So, it is called the fossil fuel.

SUMMARY

 Energy is the capacity of doing work. Its SI unit is joule.
 A source of energy is that which is capable of providing enough useful energy.
 There are two types of sources of energy i.e. Renewable and Non-renewable sources
 The sources of energy which are exhaustible and cannot be replaced once they have been

used are called the non-renewable sources of energy.
 Coal, natural gas and petroleum are the non-renewable sources of energy.
 The sources of energy which are inexhaustible and can be replaced once they have been used

are called the renewable sources of energy.
 The solar energy, wind energy, biomass etc. are the renewable sources of energy.
 The sun is the ultimate source of energy.
 An enormous amount of energy is released from the sun due to the thermonuclear fusion

reaction.
 The heat energy of the earth is called the geothermal energy.
 The future problem of major energy sources is called energy crisis.
 Energy should be saved to push the energy crisis further.
 We should use the alternative sources of energy like hydropower, solar energy, etc. to prevent

the energy crisis.

54 | Energy

EXERCISE

1. Define the primary and secondary sources of energy.
2. Differentiate between the renewable and non-renewable sources of energy.
3. How is coal formed? Why is it called the fossil fuel?
4. Why is petroleum called the non-renewable source of energy?
5. Name three devices run by the solar energy.
6. Why is the sun called the ultimate source of energy?
7. What is the main source of the solar energy? How much energy is released from

the sun?
8. What are the conditions of the thermonuclear fusion reaction in the sun?
9. Give reasons:

(a) Hydropower is called the renewable source of energy.
(b) Energy crisis may create a big problem in the near future.
(c) Hydropower is the consequence of the solar energy.
(d) The use of fossil fuel is more than other fuels.
10. What do you think is the best alternative source of energy in Nepal? Why?
11. Define energy crisis. How can we push the energy crisis further?
12. Write the methods to save energy.

A

B GLOSSARY
C

Perpetual : continuation for a long period of time without interruption

Ultimate : happening at the end of a long process

Briquettes : small hard blocks made from coal, dust, sawdust used as fuel

Propel : to move, drive or push something forward

Impervious : not affected by something (hard & non-porous)



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UNIT

4 Heat

About the Scientist INTRODUCTION

James Prescott Joule If we touch a hot cup of tea, energy enters our hand and
(1818-1889) we feel hot. Similarly, when we touch a piece of ice, energy
transfers from our hand to the ice so we feel cold. Similarly,
James Prescott Joule was an if we place a beaker of water over a heater, the water first
outstanding British Physicist. He becomes warm and finally begins to boil being hot. This
is best known for his research in shows energy transfers from the heater to the water and
electricity and thermodynamics. causes the water to become warm. This energy is called heat.
Amongst other things, he
formulated a law for the heating Thus, heat is a form of energy which gives us the sensation of
effect of electric current. He warmth.
also verified experimentally the
law of conservation of energy Heat always flows from a hotter body to a colder body.
and discovered the value of the When the heat flows towards us, we feel hot and when the
mechanical equivalent of heat. heat flows away from our body then we feel cold. Thus,
The unit of energy and work when we touch a hot iron rod, we feel hot and we feel cold
called joule, is named after him. when we touch a piece of ice.

The substance which gives heat is known as the source of
heat, e.g. burning fuel, friction, nuclear reaction, chemical
reaction, the sun, etc.

Heat as energy

All the matters are made up of molecules and these molecules
keep on vibrating. Due to the vibration, the molecules pass
the kinetic energy. The sum of this kinetic energy is called the
internal energy of the substance. So, all the substances possess
internal energy. When the body is in contact with another
having different internal energy then this energy flows
either toward or away from that body. This internal energy
flow in the body is called heat. It is the total kinetic energy of
the molecules.

The amount of heat content in a body depends upon the
following two factors:

a. Number of molecules of the body (mass of the body)
b. Average kinetic energy of the molecules i.e.

Amount of heat ∝ mass of the body × Average K.E. of the
molecules

Different substances have different properties. So even if the
equal heat is given to the equal mass of different substances,
their average K.E. will not be the same.

56 | Heat

Unit of heat

Heat is measured by caloriemeter in Joule or Calorie. Joule is the S.I. unit of heat energy
whereas calorie is its CGS unit. The relation between Joule and calorie is,

4.2 J = 1 calorie
i.e. 4200 J = 1000 calorie
1 calorie heat is defined as the amount of heat required to raise the temperature of 1 gram of
pure water by 1°C or 1°k.

Effect of heat

Heat increases the vibration of the molecules. We cannot see the vibration but if the
molecules vibrate more, then it brings changes to the physical and chemical properties of
the substances which we call the effects of heat. The effects of heat can be summarised as,

a. change in temperature of the body
b. change in the state of the body
c. change in the size or the volume of the body
d. change in the solubility of the body
e. change in the chemical properties of the body

MEMORY TIPS
When a hot body is kept in thermal contact with the cold body then heat flows from a
hotter body to a colder body until they have the same temperature. This is called the zeroth
law of thermodynamics.

QUESTIONS
# We feel cold when we touch ice. Why?
# Why do we feel hot on a sunny day?

ACTIVITY

Take two beakers of the same size. Pour 50 gm water into one of the beakers and 100 gm water
into another. Measure the temperature of both the beakers. Now, by using the same size of the
spirit lamp, heat these two beakers. Note their temperature in every two minutes.
Which beaker’s temperature increases faster? Explain the phenomenon.

Thermometer

Water

Spirit lamp

New Creative Science, Class 10 | 57

TEMPERATURE

Temperature is the measure or degree of hotness or coldness of a body.

A body having a high temperature is hot and the body with a low temperature is cold.
When these bodies are kept in contact, heat flows from the hotter body to the colder
one. So, we can say temperature is the factor which determines the flow of heat. A
hotter body is said to possess a higher temperature than a colder one. It is the average
rate of vibration of molecules due to heat.

Measurement of Temperature

Temperature is measured by thermometers in centigrade, kelvin or Fahrenheit scale.
Kelvin is the SI unit of temperature.

A thermometer is constructed on the principle that, “the substance expands on heating
and contracts on cooling.”

Liquid thermometers are more commonly used in which the liquid like mercury or
alcohol is used as the thermometric substance.

THERMOMETERS

A thermometer is an instrument that measures the temperature of
a body. It is based on a principle that, ‘a substance expands
on heating and contracts on cooling and increase in volume
of the substance is directly proportional to the rise in
temperature.’

Different substances are used in a thermometer, which show
the temperature. They are known as thermometric substances. Generally, liquids like
alcohol and mercury are used as the thermometric substance.

Mercury is used as the thermometric liquid because of the following properties.
i) It is a good conductor of heat.
ii) It expands and contracts uniformly.
iii) It doesn’t wet the glass capillary. So, the reading is correct.
iv) It has high density. So, a small bulb or a short glass tube is sufficient to
contain much mercury.
v) Its temperature changes faster due to its low specific heat capacity.
vi) It can measure a wide range of temperature as its freezing point is -39°C and
boiling point is 357°C.
vii) It has silvery colour. Thus, it can be read easily inside the glass capillary.

However, despite these advantages, mercury has some disadvantages also. As we have
noticed its freezing point is –39°C. So, it is not suitable to measure the temperatures in
cold regions when the temperature is far below –39°C. Another disadvantage is that
the mercury is highly toxic. So, it should be handled with great care.

58 | Heat

QUESTIONS
# Why is mercury used as the thermometric liquid?
# Why is mercury thermometer not suitable to measure the temperature of very cold

regions?

Alcohol as the thermometric substance

Another commonly used thermometric substance is
alcohol. It is used in thermometers due to the following
properties.

i) Its freezing point is very low i.e. – 117°C. So,
it is suitable to measure the temperature of
very cold places.

ii) It expands seven times more than that of
the mercury. So, the reading is more correct.

But it has also many disadvantages like:
i) It is a colourless liquid and is a bad conductor of heat.
ii) It sticks on the wall of the capillary tube.
iii) It doesn’t expand uniformly.
iv) It has a very low boiling point i.e. 78°C. So, it is not suitable even to measure
the boiling point of water.
v) It has low density.

QUESTIONS
# Why do we use alcohol thermometers in a very cold regions?

Water as the thermometric liquid

Water is not suitable to use as a thermometric liquid because of the following reasons:
i) The pure water is the bad conductor of heat.
ii) It sticks on the wall of the capillary.
iii) It has low density.
iv) Its expansion rate is not uniform.
v) Its specific heat capacity is very high. So, the rise in its temperature is very low.

MEMORY TIPS
The more the specific heat capacity of a substance, the less will be its rate of change in
temperature. Water (sp. heat capacity 4200 J kg–1°C–1) gets heated or cooled slower
than the mercury (sp. heat capacity 138 J Kg–1°C–1).

New Creative Science, Class 10 | 59

QUESTIONS
# Why can't water be used as the thermometric liquid?

Types of thermometers

Thermometers are classified according to their thermometric substances like i) liquid
thermometer ii) gas thermometer, iii) radiation thermometer, iv) thermoelectric
thermometer, v) resistance thermometer, etc. Similarly, according to their purposes,
thermometers are classified into:

i) Laboratory thermometer
ii) Clinical thermometer
iii) Maximum and minimum thermometer
i) Laboratory thermometer
It is used to measure the temperature of the air, water and other
various substances in the laboratory. It is mainly round and
long. The mercury rises and falls automatically in this
thermometer after being hot and cold respectively. The
wall of the bulb is made thin and the capillary is made
narrow for the quick response to the temperature change.
The stem is graduated from –10°C to 110°C. For measuring the temperature, the bulb
is kept in contact with the substance whose temperature is going to measure. Now,
the change in mercury level is noticed. There is a vacuum above the mercury surface
in the capillary tube which makes it easier for the mercury to rise.

QUESTIONS
# Why is the bulb of a laboratory thermometer made thin?

ii) Clinical thermometer
The thermometer which is used to measure the body temperature is called a clinical thermometer
or doctor’s thermometer. It is comparatively shorter than the laboratory thermometer
and is prismatic in shape. So that we can get the magnified and clear view of the
mercury level inside the capillary. There is a constriction above the bulb of mercury
in the capillary. This helps to increase the mercury level when heated but doesn’t
allow the mercury level to fall even when the temperature falls. So, it helps to take the
correct temperature of the body sometime after taking it out from the human body.

60 | Heat

The stem is graduated from 35°C to 42°C or, 94°F to 108°F. There is a mark at 37°C or
98.6°F, which is the normal human body temperature. For measuring temperature, the
bulb is kept in contact with the human body by keeping it in the armpit or below the
tongue and it should be taken out after about 2 minutes to read the body temperature.
It should be jerked before using again to put the mercury level in the initial position
because the constriction does not allow it to fall automatically.

QUESTIONS
# Why must a clinical thermometer be jerked before using it?

Differences between clinical and laboratory thermometre.

Clinical thermometer Lab thermometer

1. It is used to measure the body 1. It is used to measure the temperature

temperature. of various substances in the lab.

2. It has constriction in its stem. 2. It doesn’t have construction in its
stem.

3. The stem is graduated from 35°C to 3. The stem is graduated from –10°C to

42°C. 110°C.

4. It is prismatic in shape. 4. It is round and cylindrical in shape.

iii) Maximum and minimum thermometer

It is used to measure the maximum and minimum temperature in a certain place within 24 hrs.
It consists of a U-shaped tube partly filled with mercury and partly filled with alcohol.
One arm of this thermometer is connected to a large bulb filled with mercury and the
other arm is connected to a small bulb, which is also filled with alcohol. One arm shows
the maximum temperature and the other arm shows the minimum temperature. Each
arm has an index to show the temperature.

New Creative Science, Class 10 | 61

When the temperature increases, the alcohol in the large bulb expands. Due to this,
the mercury level along with the pointer moves upward in the other arm to show
the maximum temperature. When the temperature falls, the alcohol contracts and
the mercury level along with pointer moves upward in the same arm and shows the
minimum temperature. As the pointers do not fall themselves once, they are set to the
maximum and minimum temperatures so, to reset the thermometer, both the indexes
have to be pulled down with the help of a magnet. So, that they are just above the
mercury surface.

MEMORY TIPS
The normal body temperature of human body is 37°C or 98.6°F.

Relation between the temperature scales

If C, F and K are the temperature in Celsius, Fahrenheit and Kelvin scales then the
relation between these temperature scales is given as,

given temperature – lower fixed point in the particular scale
number of divisions between upper fixed points and lower fixed point

C – O F – 32 K – 273
i.e. 100 = 180 = 100

MEMORY TIPS
The upper fixed point and lower fixed points in Celsius scale are 100°C and 0°C, that in
Fahrenheit scale is 212°F and 32°F and that in kelvin scale is 373K and 273K respectively.

Relation between heat and temperature

Heat is a cause and temperature is the result of the heat content in the body. The
higher the heat content in a body the more will be the temperature and vice-versa.

The more the heat on a body, the more will be the average kinetic energy of the molecules
and hence will have more temperature. Thus, we can say that the temperature is
directly proportional to the amount of heat contained in a body.

For example, if we pour a cup of water from a kettle containing warm water then both
the water in the cup and in the kettle will have the same temperature. However, the
amount of heat contained in the water of the kettle will be more. Since, it contains
more water and thus, the total kinetic energy of water molecules is more in the kettle
than in the cup.

62 | Heat

Differences between heat and temperature.

Heat Temperature

1. It is a form of energy which gives us 1. It is the measure of degree of hotness
the sensation of warmth. and coldness of the body.

2. It is the total kinetic energy of all 2. It is the average K.E. of the molecules
molecules of a body. of the body.

3. It is the cause of temperature. 3. It is an effect of heat.

4. It is measured by a calorimeter. 4. It is measured by a thermometer.

5. Its SI unit is joule. 5. Its SI unit is kelvin.

QUESTIONS
# In the winter season, we feel cold when we get out of bed. But as we return to the same

room after staying outside for some time, we feel warm in the same room. Why?

HEAT EQUATION

It is an equation which shows the relation between the amount of heat gained or lost
by a body having a mass and change in temperature.

Let, the initial temperature of the body of mass ‘m’ is t1 and Q amount of heat is
supplied or taken out from the body. So, that its final temperature reaches t2, then it
is found that,

amount of heat gained or lost (Q) ∝ mass of the body (m)

amount of heat gained or lost (Q) ∝ change in temperature (t2 – t1) or dt
i.e. Q ∝ m .................... (i)

Q ∝ dt .................... (ii)

Combining equation (i) and (ii), we get,

Q ∝ mdt

∴ Q = msdt .................... (iii)

Where, S is a proportionality constant known as the specific heat capacity of the
substance. The relation given by eqn (iii) is called the heat equation.

Thus, the amount of heat gained or lost by a body is equal to the product of mass, specific heat
capacity and change in temperature. It is called heat equation.

New Creative Science, Class 10 | 63

ACTIVITY

Take two identical beakers of the same size. Pour equal masses of water and oil in these
beakers separately. Observe their initial temperature by using a thermometer. Now heat
the beakers gently by using the similar spirit lamp. Note the temperature of both the oil
and the water in every 2 minutes.
You will find the temperature of oil raised faster than water though mass and amount of
heat given to them are the same.
It is because of the specific heat capacity. The specific heat capacity of oil is less than
that of water. So, it gets heated faster.

SPECIFIC HEAT CAPACITY

From the heat equation, we have,

Q = msdt

Q
∴s = mdt
When m = 1 kg and change in temperature (dt) = 1°C or 1°k,
then,
s=Q

Thus, specific heat capacity can be defined as the amount of heat gained or lost by a substance
of mass 1 kg to change its temperature by 1°C or 1°k.

Its S.I. unit is J/kg°C.
Specific heat capacity of some substances

S.N. Materials Specific heat S.N. Materials Specific heat
capacity in J/kg°C capacity in J/kg°C
1. Lead
2. Gold 130 11. Aluminium 900
3. Mercury 134
4. Silver 138 12. Air 1010
5. Brass 234
6. Copper 380 13. Petrol 1670
7. Steel 400
8. Iron 447 14. Water vapour 1974
9. Glass 460
10. Sand 670 15. Olive oil 2000
800
16. Ice 2100

17. Paraffin 2200

18. Alcohol 2400

19. Water 4200

64 | Heat

From the table, it is clear that different substances have different specific heat capacities.
So, some substances get heated or cooled faster and some take a longer time. The
substances with low specific heat capacity get heated and cooled faster and vice-versa.

Water has the specific heat capacity of the highest value i.e. 4200 J/kg°C. It means
4200 J heat is absorbed or evolved by 1 kg water to change its temperature by 1°C.
Due to this, water is used as a cooling or heating agent.

MEMORY TIPS

Latent heat of a substance is defined as the amount of heat required to convert 1kg of a
substance of one state to another state (solid, liquid or gas) without the rise in temperature.

QUESTIONS

# The specific heat capacity of water is 4200J/kg°C. What do you mean by it?
# Why is water used to cool the engines of vehicles?

Principle of calorimeter

It states that, ‘The amount of heat lost by a hotter body is equal to the amount of heat
gained by a colder body when they are kept in thermal contact.

i.e. heat lost = heat gained by a colder body from a hotter body

If m2, s2, dt2 be those of colder body, Mix both
water
then according to this principle, m1,s1,t1 m2,s2,t2

then, Hot Cold
water water
Q1 = Q2

where, Q1 and Q2 are the heat lost
and heat gained respectively.

i.e. m1s1dt1 = m2s2dt2 t
or , m1s1(t – t1) = m2s2(t – t1)

SOLVEDNumerical

1. How much heat is required to raise the temperature of 0.1kg water from 10°C
to 95°C?

Solution:
Given, mass of water (m) = 0.1 kg
Initial temperature of water (t1) = 10°C

New Creative Science, Class 10 | 65

Final temperature of water (t2) = 95°C
Specific heat capacity (S) = 4200 J kg–1°C–1

Amount of heat supplied (Q) = ?

We have, from heat equation,

Q = msdt

= 0.1 × 4200 × (95 – 10)

= 0.1 × 4200 × 85

= 35700 J

∴ The amount of heat supplied is 35700 J.

2. For taking a bath, water at 40°C is required. Calculate the mass of cold water
at 15°C, which is to be added in 60kg water at 100°C for bathing purpose.

Solution:

Given, mass of hot water (m1) = 60 kg
Temperature of hot water (t1) = 100°C
Final temperature (t) = 40°C

Temperature of cold water (t2) = 15°C
Mass of cold water (m2) = ?

We have, from principle of calorimeter,

Heat lost by hot water = Heat gained by cold water

m1s1dt1 = m2s2dt2 [ s1 = s2]
or, 60 × (t – t1) = m × (t – t2)

or, 60 × (100 – 40) = m2 × (40 – 15)

or, m2 = 3600
25

∴ m2 = 144 kg

Hence, 144 kg of cold water should be added.

MODEL QUESTIONS ANSWER

1. When we get out of the bed in the winter season, we feel cold but when we come
back to the same room after staying outside for sometime, we feel warm. Why?

Ü When we get out of the bed, our body is hot and the heat flows from our body to
the surrounding in the air. So we feel cold. But after staying in the air for some
time, our body acts as a colder body and heat from the air of the room flows
towards us. So, we feel warm.

66 | Heat

2. The glass filled with water at 4°C overflows whether it is cooled or heated.
Why?

Ü As we know that water has the least volume at 4°C. So, if it is heated then it
expands normally and if cooled also, it expands due to its anomalous expansion.
So, it overflows whether it is heated or cooled.

3. Honey and ghee solidify from the bottom but water freezes from top to
bottom. Why?

Ü It is due to the anomalous expansion of water as water of 4°C remains at the
bottom and that of 0°C remains at the top. So, it freezes from top to bottom. But
the layers of ghee and honey having the lowest temperature lie at the bottom of
the vessel. So, they solidify from bottom to top.

4. Water is used to cool the engines of vehicles. Why?
Ü Water has the highest value of specific heat capacity. So, it absorbs a large

quantity of heat i.e. 4200J per degree rise in temperature. So, it absorbs a large
amount of heat from the hot engines without the appreciable increase in its own
temperature which helps to cool the engines.

5. Which one will heat faster, 1 kg of mercury or 1 kg of water if a equal amount
of heat is supplied to them?

Ü Since, the specific heat capacity of mercury is less (i.e. 138 J kg–1 °C–1) than that of
water (i.e. 4200 J kg–1 °C–1), mercury will get heated faster than water.

6. A thick glass tumbler cracks when hot tea is suddenly poured into it. Why?
Ü When hot tea is suddenly poured into a thick glass tumbler, its inner surface

expands more quickly than the outer surface. Due to this non-uniform expansion,
the inner surface exerts pressure on the outer surface and the glass cracks.

SUMMARY
 Heat is a form of energy which gives us the sensation of warmth.
 Heat always flows from a hotter body to a colder body.
 Temperature is the degree of the measure of the hotness or coldness of a body.
 Heat is the total kinetic energy and temperature is the average kinetic energy of the molecules.
 Heat is measured by a calorimeter and temperature is measured by a thermometer.
 Water at 4°C has a maximum density and a minimum volume.
 Water expands on cooling from 4°C to 0°C and contracts on heating from 0°C to 4°C. This

type of peculiar behaviour of water is known as the anomalous behaviour of water.
 The amount of heat gained or lost by a 1kg mass to change its temperature by 1°C is called

its specific heat capacity.
 The principle of the calorimeter states that, ‘the amount of heat lost by a hotter body is equal

to the amount of heat gained by a colder body.

New Creative Science, Class 10 | 67

EXERCISE

1. Define heat. Write its SI unit.
2. Write the effects of heat.
3. Differentiate between heat and temperature.
4. Define heat and temperature on the basis of their molecular motion.
5. Write down the advantages and disadvantages of the mercury thermometer.
6. Write down the advantages and disadvantages of the alcohol thermometer.
7. Write down any two differences between the clinical thermometer and the

laboratory thermometer.
8. Write down any two differences between heat and temperature.
9. Describe the structure and function of the maximum-minimum thermometer.

10. Define:

(a) One calorie heat (b) Specific heat capacity

(c) Thermometer (d) Thermometric substances

11. The specific heat capacity of water is 4200 J kg–1°C–1. What do you mean by it?

12. Give reasons:

(a) Water is used to cool the engines of vehicles.

(b) Fishes can live inside a frozen pond.

(c) The days in the desert are very hot while the nights are very cold.

(d) Mercury gets heated faster than water.

(e) Water is used in hot water bags.

(f) Honey solidifies from the bottom but water freezes from the top.

(g) Water pipes burst in cold countries.

(h) A mercury thermometer is suitable to measure a high temperature.

(i) An alcohol thermometer is suitable to measure a very cold temperature.

13. Answer the following questions on the basis of the given table.

A 2100J/Kg°C
B 910J/Kg°C
C 138J/Kg°C

(a) If equal masses of all these substances are heated with an equal amount of
heat, which one will get heated faster?

68 | Heat

(b) If all of them are heated at the same temperature, which one will have more
amount of heat?

(c) Which of them is used as a cooling agent and which of them is used as a
thermometric liquid?

Numerical problems

14. Calculate the amount of heat required to change the temperature of an iron ball
of mass 3 kg from 30°C to 200°C. (Specific heat capacity of iron is 460 J/kg°C)
[Ans: 234600 J]

15. Calculate the final temperature when 2400 joules of heat is given to an iron of

mass 2kg at 20°C. (Specific heat capacity = 460 J/kg°C) [Ans: 22.6°C]

16. If 50 kj of heat is supplied to 10kg of water, what is the rise in temperature?

(Given specific heat capacity of water = 4200 J kg–1 °C–1) [Ans: 1.19°C]

17. Hot water at 100°C is added to 300 g of water at 0°C until the final temperature is

40°C. Find the mass of hot water added. [Ans: 0.2 kg]

18. The temperature of water is 2°C in winter. If 25 litres of water is to be heated to

35°C. Calculate the amount of heat required for it. (Given specific heat capacity of

water = 4200 J kg–1 °C–1). Mass of 1 litre water = 1 kg [Ans: 3465000J]

19. What is the specific heat capacity of water if 2.1 × 105 joule heat is required for
2kg of water to raise its temperature from 40°C to 65°C? [Ans: 4200 J kg–1 °C–1]

20. Calculate the temperature of a pressure cooker if its mass is 3 kg and its initial

temperature is 25°C supplied with the heat of 85000J. (sp. heat capacity of pressure

cooker = 1000 J/ kg°C). [Ans: 53.9°C]

A

B GLOSSARY
C

Kinetic energy : energy due to the motion of the body

Peculiar : strange or unusual

Density : mass per unit volume

Volume : total space occupied by a body



New Creative Science, Class 10 | 69

UNIT

5 Light

About the Scientist INTRODUCTION

Galileo Galilee Light is a form of energy which gives us the sensation of vision.
(1564-1642) Due to this, we are able to see around and enjoy the beauty
of nature. During the day we are able to see due to the sun’s
Galileo Galilee was born on 15 light and at night, we need other sources of light to see
February 1564 AD in Pisa, Italy. around such as glowing bulbs, tube light, burning candles,
He was a remarkable craftsman. etc. Things that produce light are called its sources. The sun
He developed a series of telescopes is the main source of light on the Earth. Light always travels
whose optical performance was in a straight line in one medium. In this chapter, we shall
much better than that of other study the phenomena of refraction of light by lenses and
telescopes available during those their application in real life situation.
days. Around 1640, he designed
the first pendulum clock. In his LENS
book “starry messenger” on his
astronomical discoveries, Galileo Lens is a piece of a transparent refracting medium (usually glass),
claimed to have seen mountains which is bound by two unparalleled surfaces.
on the moon, the Milky Way Some of us wear spectacles. We often see watchmakers and
made up of tiny stars, and four jewelers using a magnifying glass to see the small parts.
small bodies orbiting Jupiter. The spectacles and magnifying glasses are the examples
In his books ‘Discourse on of lenses. They are also used in a cameras, microscopes,
Floating Bodies’ and ‘Letters telescopes, light fibres/optic fibres etc. The lenses are of two
on the Sunspots’ he disclosed his types.
observations of sunspots using
his own telescopes and through i) Convex lens or converging lens
his observations on Saturn and ii) Concave lens or diverging lens
Venus. Galileo argued that all
planets must orbit the sun and i) Convex lens
not the earth, contrary to what
was believed at that time. The lens which is thick at the centre and thin at the sides is
called the convex lens. It is also known as the converging
lens as it converges the beam of light falling to it to a point,
e.g. lens used in magnifying glass, human eye lens, etc.

Convex lens

70 | Light

ii) Concave lens

The lens which is thinner at the centre and thicker at the sides is called the concave
lens. It is also known as the diverging lens as it diverges the light falling to it.

Concave lens

MEMORY TIPS

There are 3 types of convex lens:
i) Biconvex or double convex
ii) Plano convex
iii) Concavo convex lens

Similarly, biconcave or double concave, plano-concave and convexo concave are the
three types of concave lens.

QUESTIONS

# Define lens.
# Define convex and concave lens.

ACTIVITY

1. Hold a convex lens in your hand. Direct it towards the sun. Focus the light from the sun on a sheet
of paper. Obtain a sharp bright image of the sun.

2. Hold the paper and the lens in the same position for a while. Keep observing the paper.
3. Will it burn? Do the same process with a concave lens to find out if the paper burns.

Lens is a combined form of Prisms Convex lens
New Creative Science, Class 10 | 71
A lens is formed by the combination of many
prisms. The upper and lower part of a convex
lens is made up of prism whereas the middle
part of the lens is made up of thin a glass slab.
In a convex lens, the base of the upper prism
is facing downwards and the base of the lower
prism is facing upwards. In a prism, the rays

of the light always bend towards the base, but the light rays passing through the glass
slab finally become straight. This is a reason why a convex lens converses the rays of
light after refraction.

A concave lens is also formed by the

combination of many prisms. The upper

and the lower part of a concave lens is made

up of a prism whereas the middle part of

the lens is made up of a thin glass slab. In a

concave lens, the base of the upper prism is

facing upwards and the base of lower prism Concave lens
is facing downwards. In prism, the rays of

light always bend towards the base, but the light rays passing through the glass slab

finally become straight. This is a reason why a concave lens diverges the rays of light

after refraction.

Some terms related to lens

(1) Centre of curvature: (c or 2F)

A lens (whether convex or concave) has two spherical surfaces. Each of these surfaces
forms a part of a sphere.

The centre of these spheres from which a lens is made is called the centre of curvature. It is
represented by c or 2F.

Principal axis Centre of Centre of Principal axis Centre of Centre of
curvature curvature curvature curvature

2F 2F 2F 2F

(2) Optical centre

The central point of the lens is called the optical centre. If a ray passes through an optical
centre, it does not deviate at all.

Convex lens Concave lens

72 | Light

(3) Principal axis

The straight line which passes through two centres of the curvature and the optic centre is
called the principal axis. In the figures, XY and X’Y’ are the principal axis of the convex
and the concave lens respectively.

O ptica l plane Princi pal axis Princi pal axis

Convex lens Concave lens

(4) Focus (or, Principal Focus)

It is a point on the principal axis at which the parallel beam of the incident light meet or appear.

Convex lens Concave lens

In a convex lens, the refracted rays meet actually at a point. So, the focus of the convex
lens is real. But in a concave lens, the refracted rays don’t actually meet but only appear
to meet at the focus, so the focus is virtual.

MEMORY TIPS
There are two focuses on either side of the lens at an equal distance from the optical
centre.

(5) Focal length (f)

The distance between the focus (F) and the optical centre (O) of a lens is called the
focal length.

(6) Radius of curvature (R) or (2f)

The distance between the optical centre (O) and the centre of the curvature of the lens is called
the radius of the curvature.

New Creative Science, Class 10 | 73

The relation between the radius of the curvature and the focal length is,
Radius of curvature = 2 × focal length
i.e. R = 2f

MEMORY TIPS
The focal length of the convex lens is regarded as positive (+) and that of concave lens is
regarded as negative (–).

Finding the focal length of a convex lens

When the parallel rays from a distant object incident on a convex lens then its image is
formed at the focus of the lens after the refraction through it. This is used to determine
the rough focal length of the lens. For this, put a convex lens in a holder and keep it
in front of a distant object like a window or a tree. Then put a paper screen behind the
convex lens.

Move the paper screen towards or away from the convex lens till a clear inverted
image of the distant object is obtained on the screen. Calculate rough focal length of
this lens between lens and paper.

MEMORY TIPS
The process of making a sharp image on a screen with a convex lens by changing the
distance between lens and its screen is called focusing.

Formation of an image by a lens

To find the position and nature of the image of an object, the following things should
be considered.

i) A ray parallel to the principal axis after refraction passes through the focus
in the case of a convex lens or appears to proceed from the focus in case of a
concave lens. As in figure (a).

ii) A ray passing through the optical centre goes straight without deviation as
in figure (b).

iii) A ray passing through the focus of a convex lens or appearing to pass
through the focus in case of a concave lens after refraction becomes parallel
to the principal axis as in figure (c).

OF O FO
(a) (b) (c)

74 | Light

(v) If the two refracted rays actually meet at a point, it forms a real image and
if they appear to proceed from a point, it forms a virtual image.

MEMORY TIPS
The real image is inverted and virtual image is erected.

Formation of images by a convex lens Parallel rays

(1) When the object is at infinity

When the object is at infinity, the image is formed at F F
the focus on the other side of the lens. O B' Image

A'

It is real, inverted and diminished.

(2) When the object is beyond 2F

A FO F B' 2F Image
Object Image Position: Between F and
2F
B 2F A' Size: Diminished
Nature: Real and
inverted

(3) When the object is at 2F

A

Object F 2F Image
2F B' Position: At 2F
F O Image Size: Same size as the
B object
A' Nature: Real and
inverted

(4) When the object is between F and 2F

A F 2F B' Image
Object Image Position: Beyond 2F
2F B F O Size: Enlarged
Nature: Real and
inverted

A'

New Creative Science, Class 10 | 75

(5) When the object is at F

A

Object F Image
F Position: At infinite
O Size: Highly enlarged
B Nature: Real and
inverted
X
Y Image
Position: Beyond object
(6) When the object is between F and O Size: Enlarged
Nature: Virtual and
A' erect

Image A F
B'
Object B O
2F F

Formation of images by a concave lens

(1) Object in between infinity and focus
Image position – between focus and optical centre on the same side of the object
Nature – virtual, erect and diminished

Object O Image
2F F Image Position: Between focus and
optical center
F Size: Diminished
Nature: Virtual and erect

(2) Object at infinity

FO F Image
Position: At focus (F)
Size: Highly diminished
Nature: Virtual and erect

76 | Light

MEMORY TIPS

A lens kept on any page of a book can be distinguished as a convex or a concave, without
touching it. If the letters appear enlarged through the lens then the lens is a convex and if
they appear shrunk then it is a concave.

Differences between convex and concave lens.

Convex lens Concave lens

1. It is thick in the middle but thin at the 1. It is thin in the middle but thicker at

edges. the edges.

2. It has a real focus. 2. It has a virtual focus.

3. It converges the parallel beams of 3. It diverges the parallel beams of light

light falling on it. falling on it.

4. It forms both real and virtual images. 4. It always forms virtual images.

5. Its power is considered to be positive. 5. Its power is considered to be negative.

6. The images may be magnified or 6. The images are always diminished.
diminished.

Power of a lens

The ability of a convex lens to converge and that of a concave lens to diverge the rays falling on
it is called its power.

Mathematically,

it is the reciprocal of focal length of a lens expressed in metres.

Power (P) = 1

∴P=1 focal length (f)

f

Its SI unit is dioptre (D).

Since the focal length of a convex lens is taken positive, its power is also considered
to be positive and the focal length of a concave lens is taken negative. So, its power is
also considered to be negative.

SOLVEDNumerical

1. Sophia wears a pair spectacles of power –1.5D. Calculate the focal length and
the type of lens she wears.

Solution:

Given, power of lens (P) = – 1.5D
Focal length (f) = ?

New Creative Science, Class 10 | 77

We have,

P=1
f

or, – 1.5 = 1
f
f= 1
– 1.5

∴ f = – 0.66 m

Hence, the focal length is – 0.66m and since the focal length is –ve, the lens is
concave.

2. Find the power of the lens having the focal length 20cm.

Solution:

Given, focal length (f) = 20 cm = 100 m = 0.2 m

Power of lens (P) = ?

We have,
P=1
f

or, P = 1
0.2

∴ P=5D

Hence, the power of the lens is 5 Dioptre.

MEMORY TIPS
When a number of thin lenses are placed in contact with one another, the power of the
combination is equal to the algebraic sum of the powers of individual lenses.

Magnification (m)

The magnification produced by a lens is defined as the ratio of the size of the image (I) as formed
by the refraction through the lens to the size of object (O).

Size of image (I)
Thus, magnification (m) = Size of object (O)
It is a unitless quantity.

If the magnification (m) < 1, then the size of the image is smaller than the size of the
object. If m = 1, then the size of the image and the size of the object is equal. Similarly
if m > 1, then the image is larger than the object.

78 | Light

A

Object FO B'
B 2F F Image 2F

A'

In the above figure, AB is the object and A’B’ is its real image. Let the distance of the
object from its centre OA be (u) and the distance of its image from its centre OB is (v).
Then from the figure, it is clear that,

i) OAB = OA’B’ [both are 90°]
ii) AOB = A’OB’ [vertically opposite angles]
iii) ABO = A’B’O [remaining angles of each triangle]
∴ ABO ~ A’B’O Thus, we can write,

Then, we can write,
A'B' OA'
AB = OA

size of image (I) image distance (v)
i.e. size of object (O) = object distance (u)

∴ magnification (m) = I = v
O u

MEMORY TIPS

● Magnification of the concave lens is always less than one and that of the convex lens

may be less or more or equal to one.

● The formula 1 = 1 + 1 is called the lens formula.
f u v

where, f = focal length of lens, u = object distance and v = image distance from the

optical centre of lens.

QUESTIONS

# Define the magnification of a lens. Why is the magnification of the concave lens always less
than one?

New Creative Science, Class 10 | 79

Differences between real image and virtual image.

Real image Virtual image

1. It is formed when the rays intersect at 1. It is formed when the rays of the light

a point after refraction or reflection. appear to meet at a point.

2. It can be obtained on a screen. 2. It cannot be obtained on a screen
but can be seen through the optical
instruments.

3. It is always inverted. 3. It is always erect.

Uses of lenses

1. Our eye consists of a convex lens. It helps to form image of an object on the
retina and we are able to see the object.

2. They are used in the spectacles for the correction of vision defects.
3. They are used in optical instruments like telescope, microscope, camera,

projector, etc.

OPTICAL INSTRUMENTS

The instruments which can produce the image of an object with the help of lenses, prisms and
mirrors are called optical instruments. The optical instruments may form a real or virtual
image.

A camera and a human eye can form a real image whereas a microscope, telescope
and binoculars form a virtual image.

Camera

It is an optical instrument which uses a convex lens to take the photographs of the
objects and people in a photographic film.

Human eye

Human eyes are the natural optical instruments that consist of a convex lens. An eye
consists of three layers: i) Sclera ii) Choroid iii) Retina

The sclera is the outermost layer which provides fixed shapes to the eyeball. It also
forms a layer which is transparent. The choroid is the middle layer. It helps to form
a clear image by absorbing light. It consists of the iris which regulates the size of the
pupil. The pupil on the other hand controls the passage of light in the eye.

The human eye lens is a convex lens made up of proteins. It is supported and adjusted
by ciliary muscles. The image of an object is formed in the retina which is made up
of two types of cells, rods and cones. These are the light sensitive cells which are
connected with the brain through the optic nerves.

80 | Light

Struct ure of an eye

Working

When the light reflected by an object enters the eye, it gets refracted through the lens.
Due to this ,a real inverted and diminished image of the object is formed on the retina
where the optic nerves carry the impulse to the brain. The brain interprets these signals
so that the object is seen erect.

MEMORY TIPS
● The blind spot is the region in the retina where the image is not seen as it doesn’t consist

of an optic nerve.
● An image formed in the yellow spot is seen more clear than in other places.
● The two eyes on the opposite sides of the head in some animals give the widest

possible field of a view.

QUESTIONS
# What are the similarities and differences between an eye and a camera?

Power of Accommodation

The eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified
to some extent by the ciliary muscles. The change in the curvature of the eye lens can
thus change its focal length.
The ability of the eye lens to adjust its focal length is called accommodation.
However, the focal length of the eye cannot be decreased below a certain minimum
limit. To see an object comfortably and distinctly, you must hold an object at about
25cm from the eyes.
The minimum distance at which the object can be seen must distinctly without strain, is called
the least distance of distinct vision (LDDV). It is also called the near point of the eye. For a
young adult with normal vision, the near point is about 25cm.

New Creative Science, Class 10 | 81

The farthest point upto which the eye can see objects clearly is called the far point of the eye. It
is infinity for a normal eye. Thus, we can see the objects clearly that are between 25cm
and infinity.

Near point
25 cm

Retina it ar i t

Retina

MEMORY TIPS
The impression of an image lasts on the retina for about 0.1 second even after the removal
of the object. It is called the persistence of vision.

Defects of vision

Sometimes, the eye may gradually lose its power of accommodation. In such condition,
the person cannot see the objects distinctly and comfortably. It is called the defect of
vision.
There are mainly two most common defects of vision. They are:

i) Myopia or near sightedness or short sightedness
ii) Hypermetropia or long sightedness or far sightedness

(i) Myopia or near sightedness or short sightedness
It is a defect of vision in which a person can not see the distant objects clearly but can see the
objects near to the eye. A person with the defect has the far point nearer than infinity. In
a myopic eye, the image of a distant object is formed in front of the retina but not at
the retina itself as shown in the figure.

Conca ve lens

A n eye suf f ering f rom myopia Correct ion of myopia

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Causes
This defect may arise due to,

i) Excessive curvature of the eye lens or shortening of the focal length of the lens.
ii) Elongation of the eyeball.
Remedy
A concave lens of suitable power will bring the image back onto the retina and thus
the defect is corrected as shown in the figure.

(ii) Hypermetropia or long sightedness or far sightedness
It is a defect of vision in which a person can see distant objects clearly but cannot see the nearby
objects distinctly. A person with this defect has the near point further away from the
normal near point (25cm). Such a person has to keep a reading material much beyond
25cm from the eye for comfortable reading. In a hypermetropic eye, the light rays
from a closer object are focused at a point behind the retina as shown in the figure.

Convex lens

e e s fferi r h er etr ia Correct ion of hypermetropia

Causes

This defect is caused due to:
i) Elongation of focal length.
ii) Shortening of eyeball.

Remedy

Eyeglasses with convex lenses provide the additional focusing power required for
forming the image on the retina as shown in the figure.

MEMORY TIPS

Our eyes can live even after our death. By donating our eyes after we die, we can give light
to the life of a blind person.

QUESTIONS

# A student has difficulty reading the writing on the board while sitting in the last row.
What could be the defect the child is suffering from? How can it be corrected?

# An old person reads a newspaper by holding it far from his eyes. Why?

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Microscope

It is an optical instrument which is used to see the magnified image of a small object.
There are two types of microscopes i.e. (i) simple microscope and ii) compound
microscope.

i) Simple microscope
It is also known as a hand lens or eye lens or magnifying glass. It is actually a convex lens.

Uses
a) It is used in laboratories to see the different parts of plants, small insects, etc.
b) It is used by watchmakers to observe the small parts of a watch clearly.

ii) Compound microscope

It is a combination of two convex lenses, objective and an eyepiece. The lens, which is
closer to our eye is known an eyepiece. It has a larger focal length. On the other hand,
the objective lens is the one which lies closer to the object. It has a smaller focal length.

Uses
a) It is used to see the minute unicellular and multicellular organisms and plants.
b) It is used in clinics to examine stool, blood, urine, etc.
c) It is used to observe different types of protozoa.
Differences between simple and compound microscopes.

Simple microscope Compound microscope

1. It consist of only one convex lens. 1. It consists of two convex lenses.

2. Its final image is virtual and erect. 2. Its final image is virtual and inverted.

3. Its magnification power is less. 3. It has high magnification power.

4. Its magnification power is only due to 4. Its magnification power is the product

one lens. of both lenses.

Telescope

It is an optical instrument used to see the distant objects clearly. There are two types of
telescopes; i) Astronomical telescope and ii) Terrestrial telescope.

i) Astronomical telescope

It is used to see the heavenly bodies of the sky. It also consists of two lenses eyepiece
and objective. The objective lens has larger focal length than the eyepiece.

MEMORY TIPS

The large focal length of the objective lens collects a maximum amount of light coming
from a far off object.

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Difference between a microscope and a telescope.

Microscope Telescope

1. It is used to see tiny objects. 1. It is used to see distant objects.

2. The focal length of the objective is 2. The focal length of the objective is

shorter than that of the eyepiece. longer than that of the eyepiece.

3. The final image is formed beyond the 3. The final image is formed at infinity.
objective lens.

MEMORY TIPS

A terrestrial telescope is used to view distant objects on the earth’s surface. The image
formed by which a telescope is virtual, erect and magnified.

QUESTIONS

# Why is the focal length of the objective lens of a microscope is small whereas that of a
telescope is larger?

MODEL QUESTIONS ANSWER

1. Why is a convex lens called a converging lens whereas concave lens is called
a diverging lens?

Ü A convex lens is called a converging lens because it converges the parallel beam
of light falling to it to a fixed point but a concave lens diverges the beam of light
from a fixed point. So, it is also called a diverging lens.

2. Differentiate between the camera and the human eye.

Eye Camera

1. The eye lens has a variable focal 1. It has a fixed focal length

length.

2. The distance between the eye lens 2. The distance between the lens and

and the retina is fixed. the photographic film is varied.

3. It is a natural optical instrument. 3. It is an artificial optical instrument.

4. An image is formed on the retina. 4. An image is formed on a photographic

film.

3. The objective lens of a microscope has small aperture. Why?

Ü A microscope is used to see very tiny objects. So, the objective lens should collect
the light reflected from these objects only. Hence, it should have a small aperture
to see tiny objects clearly.

4. The objective lens of the telescope has a large aperture. Why?

Ü A telescope is used to see distant objects and it should therefore collect more
light from those objects. Hence, its objective should have a large aperture to see
distant objects clearly.

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5. Sanjay sees distant objects comfortably but he wears spectacles while reading
a book.

a) What defect of vision does he have?
Ü Since, he can’t see the nearer objects, he must be suffering of farsightedness

or hypermetropia.

b) Which type of spectacles should he use?
Ü He should use a convex lens with a suitable focal length.

SUMMARY
 There are two types of lenses namely the convex lens and the concave lens.
 The lens which is thick on the middle and thin the edges is called a convex lens. It is also known

as converging lens.
 The lens which is thin in the middle and thick at the edges is called a concave lens. It is also

known as a diverging lens.
 The reciprocal of the focal length expressed in metre is called the power of the lens.
 The focal length of a convex lens is positive and that of a concave lens is negative.
 The power of a convex lens is expressed as positive and that of a concave lens is expressed

as negative.
 The ratio of the size of the image to the size of object is called a magnification. It is also the

ratio of image distance to the object distance.
 A camera is an optical instrument which uses a convex lens. It gives the real and inverted

image of an object.
 A human eye is a natural optical instrument.
 The two most common defects of vision are myopia and hypermetropia.
 Myopia or short sightedness is caused by the shortening of the focal length and elongation of

the eyeball.
 Hypermetropia or long sightedness is caused by the elongation of the focal length and

shortening of the eyeball.
 Myopia is corrected by using a concave lens and hypermetropia is corrected by wearing a

convex lens of a suitable focal length.
 A microscope is an optical instrument used to see tiny objects and a telescope is an optical

instrument used to see distant objects clearly.

EXERCISE

1. Define a lens. Write its types.

2. What are the different types of lens?

3. Why is a convex lens called a converging lens and a concave lens is called a
diverging lens?

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4. Define:

(a) power of the lens with SI unit (b) near point of eye

(c) focus (d) far point of eye

(e) real image (f) accommodation of eye

(g) virtual image (h) optical centre

5. Differentiate between:

(a) Convex lens and concave lens (b) Camera and eye

(c) Real image and virtual image (d) Telescope and microscope

(e) Far point and near point of eye

6. Draw a ray diagram for a convex lens and give the nature and position of the
image when the object is,

(a) beyond 2F (b) between 2F and F (c) at F

(d) between F and O (e) at 2F

7. Draw a diagram to show a convex lens forming a magnified and erect image.
Give one application of this condition.

8. Write the function of the following parts:

(a) Pupil (b) Ciliary muscle (c) Iris

(d) Optic nerve (e) Shutter in camera

9. What is the defect of vision of the eye in the given figure? What is to be done
to correct this defect of vision? And why?

10. Write the causes and remedy of:

(a) Myopia (b) Hypermetropia

11. What is the defect of the vision of the eye in the given figure? What is to be done
to correct this defect of vision? And why?

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12. A student sitting in the back row of the class is not able to see the writing on the
board. What type of defect is he suffering from?

13. Sabin wears a lens of power + 1.5 D. Answer the following questions on the
basis of this fact.
(a) What is the defect of the vision called?

(b) How is it removed? Explain with a suitable diagram.

(c) Name the lens used.

(d) How should he hold a newspaper while reading without using this lens?

Numerical problems

14. A convex lens has a focal length 0.2m. What is the power of the lens? [Ans: 5 D]

15. Determine the size of the real image of an object of 2cm high placed 40cm away from

a convex lens. The real image is formed 80 cm away from the lens. [Ans: 4 cm]

16. Prem wears spectacles of power + 1.5 D, calculate the focal length and find the

nature of the lens used. [Ans: 0.66 m, convex]

17. An object is placed at a distance of 20m from a convex lens of focal length 15
cm.

(a) Find the image distance.

(b) Is the image real or virtual?

(c) Is the image erect or inverted?

A

B GLOSSARY
C

Incident : something that happens unusual or unpleasant

Converge : to move towards and meet at the same place or point

Diverge : to move away from a same point or place

Magnification : the degree to which something is magnified

Accommodation : the process of adopting, adjustment



88 | Light

UNIT

6 Electricity and Magnetism

About the Scientist INTRODUCTION

Michael Faraday The discovery and utility of electricity have revolutionized
(1791-1867) the homes and industries of the civilized world. Life today
cannot be thought without it. Several devices like lights,
Michael Faraday was an television, radio, motors, various machines of industries, etc.
experimental physicist. He had of this modern life are operated on the basis of electricity.
no formal education. He worked
in a bookbinding shop during Electricity is a form of energy produced due to the change in the
his early years. He used to read number of electrons in a body or the flow of electrons through a
books that come for binding. In conductor.
this way Faraday developed his
interest in science. He got an There are two kinds of electricity: i) static electricity and ii)
opportunity to listen to some current electricity. Electricity produced due to the change in
public lectures by Humphrey number of electrons in a body which cannot be transferred
Davy of Royal Institute. He from one place to another is the static electricity and the
made careful notes of Davys electricity produced due to the flow of electrons through a
lectures and sent them to Davy. conductor is the current electricity.
Soon he was made an assistant
in Davy’s laboratory at the Mathematically,
Royal Institute. Faraday made total charge (Q)
several path-breaking discoveries
that include electromagnetic Current (I) = time taken (t)
induction and the laws of
electrolysis. Several universities i.e. I = Q
conferred on him the honorary t
degree but he turned down
such honours. Faraday loved ELECTRIC CIRCUIT
his science work more than any
honour. The devices like lamps, heaters, motors, etc. which operate
with the help of electricity or by consuming electricity are
called the electric loads. In order to work, these loads should
be connected to a source of electricity through a conducting
wire.

The conducting path of electricity consisting of loads, switches,
sources of electricity etc. is called the electric circuit.

There are two types of electric circuits:
i) Open electric circuit
ii) Closed electric circuit

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Open electric circuit Source

It is the electric circuit in which electric current is not –+
flowing. It may be due to the breaking of wire or off state
of the switch. So, the electric loads are not working. Switch

Closed electric circuit Bulb

It is the electric circuit, in which the electric current is flowing through it. Here, the
switch is in the on state and the electric loads are working.

Source

–+

Switch

Bulb

MEMORY TIPS
It is wrong to think that electrons are consumed in an electric circuit when current flows
through it. In fact, we provide energy to set these electrons into motion through the electric
circuit and we pay for it to the electricity board.

QUESTIONS
# Differentiate between an open circuit and a closed circuit.

CONNECTION OF ELECTRIC LOADS

It is often necessary for us to connect more than one loads in a circuit. There are two
ways to connect the loads in a circuit i.e.

a) Series combination
b) Parallel combination

Series combination of resistance –+

In this combination, two or more loads are connected in

such a way that the positive terminal of one is joined with

the negative terminal of another. The value of the electric

current is the same in each load but the value of potential

difference (pd) is different in them. The rest loads do not work continuously, if any

one of them fails to work. If the loads used are the lamps then the brightness of the

lamps increases with the reduction in the number of lamps.

The total resistance (R) = R1 + R2 + R3 + ...

90 | Electricity and Magnetism

Parallel combination of resistance –+

In this combination, two or more loads are connected in

such a way that all their positive terminals are joined at

one point and all their negative terminals are joined at

another point. The value of the electric current in each

load is different but the value of pd remains constant. The

rest of the loads also work even if any one of them fails to work. The change in the

number of lamps doesn’t alter the brightness of the bulb.

( )The total resistance1 = 1 + 1 + 1 + ...
R R1 R2 R3

COMBINATION OF CELLS

A cell is a source of electric current which converts chemical energy to electric energy.
A single cell produces about 1.5V of electric current which becomes insufficient for
many cases. So, for those situations, the grouping of cells be done. The grouping or
combination of cells is done in the following two ways.

a) Series combination of cells
b) Parallel combination of cells

Series combination of cells

If the positive terminal of one cell is connected to the negative terminal of another,
then the combination is known as series combination.

–v1 + –v2 + –v3 +

Features of series combination

i. In this combination, the total pd across the external resistance is equal to the sum
of the pd of all the resistances i.e.V = V1 + V2 + V3

ii. The current flowing through the circuit increases with the increase in the number
of cells. However, the life of the cells remains constant. So, the bulb glows more
brightly when the number of cell increases.

Parallel combination of cells

If the cells are connected parallel to each other then such a combination of cells is
called the parallel combination.

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– v3+

–+
– +v2

v1

Features of parallel combination

i. In this combination, the total voltage is equal to the individual voltage of each cell.

i.e. V = V1 = V2 = V3
ii. The current flowing through the circuit doesn’t increase with the increase in the

number of cells but the lifetime of the cell increases. That’s why the bulb glows for

a longer time with the same brightness.

MEMORY TIPS

In the parallel combination of loads of electrical devices, the rest of the appliances continue
to work, even one of them fails to work. So, in our home, a parallel combination of loads is
done.

ACTIVITY

1. Connect three bulbs in series with a cell and observe its brightness. Now, remove one of the
bulbs and again observe its brightness. What change do you observe and why?

2. Again connect these bulbs parallel to the cell. Observe the brightness. Now, again remove
one of the bulbs. How does the brightness of the bulb change? Explain it.

QUESTIONS

# The brightness of the bulb increases with the decrease in the number of bulbs in the
series combination. Why?

# In the parallel combination of cells, the brightness doesn’t get affected by the number of
cells used, then why is it used?

ELECTRIC POWER

Electricity is the most versatile and flexible energy which can be easily converted into

another forms. The rate at which the electric device converts electrical energy into other forms

of energy is called its electric power.

i.e. electric power (P) = transformation of energy (E)
time taken (t)

or, electric power (P) = electric energy supplied (E)  transformation of energy
time taken (t) = electric energy supplied

Its SI unit is Js–1 or watt (W)

92 | Electricity and Magnetism

We see the bulb marked 60W, 100W etc. It means that the bulb can convert 60J of
electrical energy into heat and light energy in one second (for 60W bulb). The electric
power (P) of a device is the product of potential difference (V) across its ends and
current (I) supplied in it.

i.e. P = I × V

SOLVEDNumerical

1. In a room a bulb of 100 watt is connected to 220 V supply. Find the current
passing in it.

Solution:

Given,

Power of bulb (P) = 100 W

Voltage (V) = 220 V

Current (I) = ?

We have,

P=I×V

or, 100 = I × 220

or, I = 100 = 0.45 A
220
Hence, the current passing in the bulb is 0.45 A.

DC AND AC

If the polarity of electricity doesn’t change with time, it is called the direct current (DC).
Batteries, DC generators are the main sources of direct current. This current cannot be
used in transformers.

If the polarity of electricity changes periodically and its magnitude changes continuously then
it is called alternating current (AC). The sources of AC are the AC generators or dynamo.

R R current

current + + + + Time
Time ––– –
(a) D.C. circuit (b) Graph of direct current (c) A.C. circuit
(d) Graph of alternating current

MEMORY TIPS

In Nepal, the AC of frequency 50 Hz is supplied. It means that the polarity of the AC changes
50 times in one second.

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Differences between AC and DC.

AC DC

a. Its magnitude changes continuously a. Its magnitude as well as direction

and direction changes periodically. remain constant with time.

b. It can be altered by transformers. b. It cannot be altered by transformers.

c. Its main sources are dynamo and AC c. Its main sources are battery and DC

generators. generators.

QUESTIONS
# Why can't DC be changed by transformers?

HOUSE WIRING SYSTEM (DOMESTIC ELECTRIFICATION)

In the domestic electrification, we use AC. The electric circuit used in houses and
industries is called domestic electrification.

All the appliances are connected in parallel in the house wiring system. So, if any one
device fails, other devices may not be affected.

The current is transferred in high voltage from the power stations to the houses and
it is stepped down to decrease its voltage before distributing to the house. Now the
electricity is fed into the house by a heavy cable containing two wires i.e. live wire (L)
and neutral wire (N). These two wires that are well insulated enter the meter box that
contains the main fuse in live wire and a meter. The wires from the meter box now let
to enter the main switch box. The main switch box is earthed. The earthing prevents us
from electric shock by sending the excess current to the ground through a conductor.

The fuse used in the main switch box is called co nsumer’ s f usaend that in the meter box
is called authority’s main fuse. Now, the wires are connected to the distribution board
from where current is distributed to all parts of the house. The systematic diagram of
domestic electrification is shown in the following figure.

94 | Electricity and Magnetism

MEMORY TIPS
There are two circuits i.e. the lighting circuit and power circuit. The lighting circuit uses the
fuse of 5A whereas power circuit uses the fuse of 15A.

QUESTIONS
# Why are fuses and switches connected to the live wire of the electric circuit?

Points to be remembered in household wiring

The following points should be remembered in household wiring:
i) Colour code of the wires, brown/red coloured insulation is for live wires, blue/

black insulation for neutral wire and green or yellow stripped insulation for
earth wire.
ii) Switches and fuses are connected in the live wire, so that if they are switched off
or blown up, the current may not flow through the load.
iii) The fuses should be of appropriate rating i.e. slightly excess than the current
consumed.
iv) Separate fuses should be used for each room or floor.
v) Wiring should not be done is damp places.
vi) Earthing should be done for heavy electrical appliances.
vii) There should be separate power circuit and light circuit.

EFFECTS OF CURRENT ELECTRICITY

Electricity can be easily converted into other energy forms and can bring the following

changes in a substance.

i) Heating effect ii) Lighting effect

iii) Magnetic effect iv) Chemical effect

i. Heating effect of current

No material is 100% resistance-free. So, if current passes through a resistance, it gets
heated up, which is called the heating effect of current. This principle is used to construct
electric heater, irons, toasters, rice cookers, electric kettles, etc. The heating element
used in these devices must have high resistance and melting points.

Heating devices

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Nichrome is commonly used as a heating element. It is an alloy of 60% nickel and 40%
chromium. It has high resistance (resistivity) and high melting point of about 900°C
and it does not corrode either. But it may get destroyed by water.

QUESTIONS
# Why is the nichrome wire in an electric heater covered with a metallic sheet?

ii. Lighting effect

When the current is used to produce light it is called the lighting effect of the current.
When the current is passed through a load of high resistance power and high m.p, it
first gets heated and glows to emit light. This is called the lighting effect of the current.
It is used to construct electric bulbs, lamps, and fluorescent lamps.

Electric bulb (filament lamp)

The main lighting element in the bulb is the coiled F ilament
tungsten wire known as the filament. The filament Supporting wire
is very thin and highly coiled to increase its

resistance. The m.p. of the filament is about 3400°C. Nitrogen gas

The filament bulb is filled with inert gases like Glass stem

nitrogen, neon, argon etc. to prevent the oxidation Thick wire

of the filament. Its average life is of about 1000 A luminium co vering
Pin
hours and it converts only 10% of electrical energy
Terminals

into light and other 90% of energy is wasted in the E lect ric b ulb

form of heat.

Fluorescent lamp (tube light)

It consists of a long discharge tube having two Starter
electrodes at its ends. It is filled with mercury F luoresce nt power
vapour and coated with fluorescent powder. A
choke provides the high voltage to the circuit. It Tub e light
produces light by producing the ultra violet rays
and changing it into visible light by the fluorescent Mercu ry
powder. Its average life is about 3000 hours and
it converts 30% of electrical energy into light and E lect rode vapour
70% of it is wasted in the form of heat.
Choke co il A .C. 220v

F luoresce nt lamp

96 | Electricity and Magnetism