12. Critical angle for a certain medium can be defined as the angle of incidence in a
denser medium when the corresponding angle of refraction in rarer medium
becomes 90°.
13. When the angle of incidence in a denser medium is more than the critical angle,
the ray of light reflects in the same medium. This process is called total internal
reflection of light.
14. Mirage is an optical illusion which can be observed as if there is a pond on hot
desert or coal-tarred road.
15. A mirage is formed when an inverted image of the distant object is formed on the
desert or coal-tarred road.
16. When the sunlight passes through a prism, it splits into seven colours. This process
is called dispersion of light.
17. The major cause of dispersion of light is the difference in speed of light of different
colours in the same medium. The speed of different colours in glass medium is
different.
18. The wave which is not affected by electric field and magnetic field is called
electromagnetic wave.
19. Electromagnetic waves can propagate through vacuum or without medium.
Sunlight is an example of electromagnetic wave.
20. X-rays are harmful rays for human beings having wavelength from 0.01nm to 10
nm. They cannot penetrate through bones but can easily penetrate through skin
and muscle.
21. X-rays are used to know about fracture and cracks in bones. They are used in
radiotherapy.
22. Ultraviolet rays are commonly known as UV-rays. They are harmful rays having
wave length between 10nm to 400 nm.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. The critical angle for glass medium is ..................
49° 24° 42° 50°
b. When sunlight passes through a glass prism, it splits into .............. colours.
five seven three two
c. Which of the rays has the maximum wave length?
gamma rays X-rays microwave radio wave
d. The waves which are used for transmission of television are ..............
radio waves microwaves infrared rays X-rays
GREEN Science (Physics) Book-9 101
2. Answer the following questions.
a. What is light?
b. What are the sources of light? Give any three examples.
c. Define refraction of light. Draw a figure showing this process.
d. Define the following:
i. Incident ray
ii. Refracted ray
iii. Denser medium
iv. Rarer medium
e. State the laws of refraction of light.
f. Write down the cause of refraction of light.
g. What is angle of incidence?
h. What is angle of refraction?
i. What is refractive index?
j. Write down the formula to calculate refractive index.
k. What is critical angle?
l. Write down the value of critical angle for glass, water and ice.
m. What is total internal reflection of light?
n. What is mirage? How is it formed?
o. What is dispersion of light? Draw a neat figure showing this process.
p. Write down the cause of dispersion of light.
q. What are electromagnetic waves?
r. Write down the uses of X-rays and UV-rays.
3. Give reason.
a. A pencil appears bent when it is half-immersed in water.
b. Diamond sparkles in dim light.
c. Light bends when it passes form one medium to another.
d. The depth of a pond appears less than its real depth.
e. X-rays are harmful for human health.
4. Differentiate between.
a. Incident ray and refracted ray
b. Denser medium and rarer medium
c. X-rays and UV-rays
102 GREEN Science (Physics) Book-9
5. Complete the given figures.
a. b.
c. d. 49°
Water
42°
Glass
e. f.
90°
90°
6. Draw a neat and labelled figure showing
a. Total internal reflection of light
b. Dispersion of light
7. A rainbow is seen in a rainy day. Describe its reason.
8. Numerical Problems
a. The velocity of light in air is 3 × 108 m/s, calculate the velocity of light when it
enters water. The refractive index of water is 1.33. [Ans: 2.25 × 108 m/s]
b. The refraction index of water is 4/3. If the apparent depth of water in pond is 4m,
calculate the real depth. [Ans: 5.32 m]
c. The refractive index of glass is 1.5. Calculate the value of critical angle. [Ans: 42°]
d. The real depth of a pond is 5.32 m and its apparent depth is 4m. Calculate the
refractive index. [Ans: 1.33]
e. A ray of light is passing through a medium making an angle of 45°. If the
refractive index of the medium is 1.4, calculate the angle of refraction. [Ans: 30°]
GREEN Science (Physics) Book-9 103
Grid-based Exercise 2
Group ‘A’ (Knowledge Type Questions) (1 Mark Each)
1. What is refraction of light?
2. Define emergent ray and angle of reflection.
3. State Snell’s law.
4. What is the refractive index of glass medium?
5. What is meant by denser medium? Write.
6. What is total internal reflection of light?
7. What is optical fibre?
8. What is mirage?
9. What is critical angle?
10. What is dispersion of light?
11. What is the velocity of light in air and glass medium?
12. What is endoscope? Why is it used?
13. How is light travelled in a curved pipe?
For Group ’B’ (Understanding Type Questions) (2 Marks Each)
14. The apparent depth of water in a pond appears less than the real depth. Why?
15. Light bends away from normal when it passes from the denser medium to the rarer
medium. Why?
16. Why is the refractive index of glass more than that of water? Give reason.
17. Mirage is seen in pitched road or desert during hot summer season. Why?
18. X-ray is called an electromagnetic wave. Why ? Diamond has extraordinary brilliance,
why?
19. The red ray lies at the top and violet ray lies at the bottom during dispersion of light.
What is the reason behind this fact?
20. Mention the differences between X-ray and visible ray on the basis of source and
nature.
For Group ‘C’ (Application Type Questions) (3 Marks Each)
21. Write any three uses of X-ray.
22. What is endoscope? Why is it used? What is the relationship among refractive index,
real depth and apparent depth?
104 GREEN Science (Physics) Book-9
23. Complete the given ray diagrams.
a. N b. air c. 45°
B air 50° 90°
water 90°
AM ice
24. Write down the importance of Ultraviolet ray and X-ray in the field of medical science.
25. Write down the importance of electromagnetic waves. Draw a figure showing
dispersion of light through a prism.
For Group ‘D’ (Higher Abilities Type Questions) I (4 Marks Each)
26. Study the given diagram and answer the following N
questions: i O Air
a. Which phenomenon is shown in the given r Glass
diagram?
M
b. Name the angle of incidence, angle of refraction
and emergent angle in the given diagram. R
c. Which medium is denser out of air and glass?
Explain the reason.
27. Answer the following questions on the basis of given ray Light A
diagram. Prism B
a. Name the rays A and B.
b. Which ray has more speed (A or B) in the glass
medium?
c. Which phenomenon is shown in the given diagram?
d. Ray B is bending more than the ray A. What is the reason behind it?
28. Write short notes on: b. Ultraviolet ray
a. X-ray
29. The speed of light in air is 3 × 105 km/s and the refractive index of paraffin is 1.44.
Calculate the speed of light in the paraffin. Differentiate between angle of incidence
and critical angle in two points.
30. A ray of light is passing through a medium by making an angle of 45°. If the refractive
index of the medium is 1.4, calculate the magnitude of angle of refraction. The apparent
depth of water in a pond appears less than the real depth. Why?
GREEN Science (Physics) Book-9 105
UNIT Sound
6 Weighting Distribution Theory : 10 Practical: 2
Before You Begin
In our surroundings, we see different things that produce sound.
When materials vibrate, sound is produced. So, sound is the form
of energy which is produced due to vibration of a material medium.
Sound produces sensation of hearing. The substances that produce
sound are called sources of sound. Temple bell, horn of vehicles,
loudspeaker, guitar, television, etc. are some sources of sound. Sound
waves are produced when a material vibrates. Sound propagates
through these waves.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to:
i. introduce some terms related to sound, i.e. speed, • Introduction to sound and
sound wave
frequency and wavelength.
• Wave length
ii. introduce echo and reverberation and differentiate • Frequency
between them. • Speed of sound
• Reflection of sound
iii. describe the effect of echo and reverberation. • Echo
• Reverberation
iv. solve simple numerical problems related to sound. • Simple numerical problems
related to sound
Glossary: A dictionary of scientific/technical terms
sound : the form of energy which is produced due to vibration of a body
longitudinal wave : a wave that vibrates in the direction that it is moving
frequency : the number of complete cycles made in one second
echo : the repetition of sound caused by reflection
reverberation : the prolongation of the original sound
106 GREEN Science (Physics) Book-9
Sound
There are different forms of energy. Sound energy is one of them. Sound produces the
sensation of hearing in our ears. It is produced when a body vibrates. So sound is the form
of energy produced due to vibration of a material medium. A vibrating body is a source
of sound. Radio, temple bell, horn of vehicles, guitar, etc. are some examples of sources
of sound.
Sound wave
The wave produced due to vibration of a material medium is called a sound wave. It is
produced due to vibration of solid, liquid and gas. Sound wave needs a medium (solid,
liquid or gas) for propagation. It cannot propagate through vacuum.
Fig. R CR C R CR C
8.1 C = Compression R = Rarefaction
Sound wave
When sound waves coming from a source Do You Know
propagate through air, and reach our ears,
we hear the sound. Sound propagates in the A compression is that part of a
form of longitudinal waves. A longitudinal longitudinal wave in which the particles
wave consists of alternately arranged of the medium are closer to one another.
compressions and rarefactions. When a
sound wave passes through air, the particles Rarefaction is the part of a longitudinal
of air vibrate back and forth parallel to wave in which the particles are farther
the direction of propagation forming apart.
compressions and rarefactions.
Activity 1
Take a rubber pad and a tuning fork.
Hit the rubber pad with the tuning fork. Do you hear vibrations.
Now, touch the tuning fork. What do you feel?
Write down the conclusion of this activity.
Activity 2
Take a small stone and go to a nearby pond.
Throw the stone in the pond. Be careful while throwing the stone.
Do you see ripples in the pond?
Now, throw a leaf on the surface of the ripples. What do you observe?
GREEN Science (Physics) Book-9 107
Characteristics of Sound Wave
1. Frequency of Sound
Frequency of sound is defined as the number of complete cycles produced in one second.
It is denoted by 'f'. In SI system, frequency is measured in hertz(Hz). The larger units of
frequency are kilohertz(kHz), Megahertz (MHz), etc. The relation between hertz, kilohertz
and megahertz is given below:
1000 Hz = 1 kHz Do You Know
1000 kHz = 1MHz
1 MHz = 1000000 Hz The frequency of a sound wave is 40 Hz means that
the sound wave produces 40 completes cycles or
vibrations in one second.
2. Time period
The total time taken to from a complete wave is called time period. It is denoted by T. In
the SI system, the unit of time period is second (s).
Time period is calculated by the given formula:
Time period (T) = 1
In short, frequency (f)
1
T=
f
3. Wave length
The total distance covered by a sound wave in one vibration is called wave length. It is
denoted by the Greek symbol Lambda (λ). In the SI system, wave length is measured in
meter (m).
4. Amplitude
Amplitude of a sound wave is defined as the maximum displacement of vibrating particle
from its mean position. It is denoted by 'a'. Its SI unit is metre (m).
5. Wave velocity
Wave velocity can be defined as the total distance covered by a sound wave per unit time.
Its SI unit is meter per second (m/s).
The mathematical relation between wave velocity, frequency and wavelength is called
wave equation.
Wave velocity (v) = frequency of sound (f) × wavelength (λ)
In short, v = f × λ
108 GREEN Science (Physics) Book-9
Solved Numerical: 1
The frequency of a sound wave is 60 Hz and its wave length is 85 m. Calculate the speed
of sound in the medium.
Given,
Frequency (f) = 60 Hz
Wavelength (λ) = 85 m
Speed of sound (v) = ?
We know,
v = f × λ
= 60 × 85
= 5100 m/s
\ The speed of sound (v) = 5100 m/s
Solved Numerical: 2
The speed of sound in water is 1500 m/s. If the wave length is 12.5 m, calculate the
frequency of the sound.
Given,
Speed of sound (v) = 1500 m/s
Wave length (λ) = 12.5 m
Frequency (f) = ?
We know,
v = f × λ
or, 1500 = f × 12.5
or, f = 1500
12.5
= 120 Hz
\ The frequency of sound is 120 Hz.
Relation between velocity of sound and medium
Sound wave can propagate through solids, liquids and gases. Sound travels with different
speed in different media. The speed of sound is maximum in solids and minimum in gases.
The molecules in solids are closely packed together and hence carry more vibrations. As
a result, the speed of sound is maximum in solids.
GREEN Science (Physics) Book-9 109
The speed of sound in some common media is given below:
Medium Speed of sound
1. Iron (20 °C) 5130 m/s
2. Glass (25 °C) 5500 m/s
3. Aluminium (25 °C) 5100 m/s
4. Water (20 °C) 1498 m/s
5. Alcohol (25 °C) 1210 m/s
6. Air (0 °C) 332 m/s
7. Hydrogen (0 °C) 1284 m/s
Speed of sound in Air or Gas medium
The speed of sound in air or gas medium depends on the following factors.
1. Density of air
The speed of sound is more in the object having less density. Similarly, the speed of sound
is less in the object having more density. In fact, the speed of sound is directly proportional
to the square root of the density of air.
2. Temperature of air
When the temperature of air increases, the speed of sound in air increases and vice-versa.
3. Humidity of air
The speed of sound increases when the humidity of air increases and the speed of sound
decreases when the humidity of air decreases. Humidity is the amount of water vapour
present in air.
4. Motion of air molecules
The speed of sound increases when the direction of movement of sound and air molecules
is the same. But the sped of sound decreases if the direction of the sound is opposite to
that of the direction of air molecules.
Types of Sound wave
There are three types of sound waves on the basis of their frequency. They are as follows:
(i) Ultrasound (ii) Audible sound (iii) Infrasound
(i) Ultra sound
Ultra sound is the sound having frequency more than 20,000 Hz. Ultra sound is widely
used in medical sector to examine the inner body organs. Ultrasound cannot be heard
by human ears. However, bat, insects, rats, birds can hear and produce ultrasound. This
110 GREEN Science (Physics) Book-9
Fig.sound carries more energy due to its high frequency and hence has a great penetrating
power.
Application of ultrasound
Ultrasound is the sound wave having frequency more than 20kHz. So the wave length
of ultrasound is very short. Ultrasound can travel from one place to another without
bending.
1. Doctors use ultrasound to
i. determine the sex of the child in the mother's womb.
ii. observe the growth of the embryo.
iii. kill bacteria.
iv. investigate the diseases in the inner parts of the body.
v. locate and observe tumour.
vi. do bloodless operation, etc.
2. Ultrasound is used in SONAR to measure the depth of the sea, lake, rive, etc. SONAR
stands for Sound Navigation and Ranging. A SONAR consists of a vibrator with high
frequency, a source and a detector or receiver. Ultrasound is sent towards the bottom
of the sea from its surface. The receiver gets the reflected ray from the bottom of the
sea. It also records the time taken by the sound to return the surface. Then the depth
of the sea is calculated by the given formula.
speed of sound (v) × time (t)
Depth (d) =
2
or,
v×t
d=
2
8.2
SONAR
The process of finding the depth of the sea or distance with the help of ultrasound is
called echolocation.
GREEN Science (Physics) Book-9 111
Solved Numerical: 3
A man shouts in front of a cliff and hears the echo after 0.4 seconds. If the speed of sound
in air is 330 m/s, calculate the distance between the man and the cliff.
Given,
Time (t) = 0.4 s
Speed of sound (v) = 330 m/s
Distance (s) = ?
We know that,
s = v × t
2
or, s = 330 × 0.4 m = 66 m
2
\ The distance between the man and the cliff (s) = 66 m
(ii) Audible sound
The sound which can be heard by human Do You Know
beings is called audible sound. The
frequency of audible sound ranges from 20 Rhinoceros can produce infra sound
Hz to 20,000 Hz. All sounds that we hear are of frequency 6 Hz.
called audible sound.
The frequency of sound wave ranges
from 1 Hz to 108 Hz.
(iii) Infrasound The frequency of sound of man is
about 6.5 KHz and that of woman is
Infrasound is the sound having frequency 8.5 KHz.
less than 20Hz.We cannot hear infrasound. It
is produced during earthquake and volcano
eruption. Some animals like rhinoceros, elephant, whale, etc. produce infrasound.
Activity 3
Take a sensitive stop watch and go to a nearby cliff or in front of a tall building
along with your science teacher.
Shout in front of the cliff or wall and record the time taken to hear the echo.
Now calculate the distance between you and the cliff or wall using the formula.
Reflection of sound
When a sound wave travelling in a medium strikes a surface, it returns to the same
medium. This process is called reflection of sound. So reflection of sound is defined as
returning of sound in the same medium when it strikes a hard surface. When we shout in
front of a cliff or wall or a hill, we can hear a reflected sound. The reflected sound is called
echo. Reflection of sound obeys the laws of reflection of light. We utilize the reflection of
sound in the working of megaphone, sound boards, etc.
112 GREEN Science (Physics) Book-9
Activity 4
Take a cardboard, a watch, a mirror and two plastic pipes of equal diameter. Arrange
these materials as shown in the figure.
Mirror
Plastic pipe B Plastic pipe A
Fig. Cardboard
8.3 Human Watch
Place a watch at the end of plastic pipe A and hear the sound from the end of pipe
B.
Now, move the plastic pipe B at different angles and listen to the sound.
What can you conclude from this activity?
Echo
When a sound wave strikes any hard surface, it gets reflected, which is called echo. So,
echo can be defined as the repetition of sound caused by the reflection of sound.
The conditions required for formation of echo are as follows:
1. The minimum distance between the source of sound and the reflecting surface should
be at least 17.2 metres.
2. The size of the reflector should be large
3. The loudness of the sound should be sufficient.
Let us consider the distance between the sources of sound an the reflecting surface is x
meter and the speed of sound in air is 332 m/s. Therefore, the distance covered by sound
wave in 0.1 second is 2x meter.
Given,
Speed of sound in air (v) = 332 m/s
Distance covered by sound (s) = 2xm
Time (t) = 0.1s
The distance covered by sound wave (2x) = Speed of sound × time
or, 2x = 332 × 0.1
GREEN Science (Physics) Book-9 113
332 × 0.1
or, x =
2
= 16.6 m
\ The distance covered by sound wave (s) = 16.6 m
Reverberation
If we speak in a large unoccupied hall, we can hear a number of echoes of original sound. This
phenomenon is called reverberation. So reverberation can be defined as the prolongation
of original sound due to reflection. Reverberation is the process of intermixing of original
sound with a reflected sound. A number of echoes are heard during reverberation.
Reverberation occurs when the distance between the source of sound and the reflecting
surface is less than 17.2 m. We hear reverberation in an unfurnished empty room and a
newly built room. Reverberation does not occur in an occupied room or furnished room
due to the presence of sound absorbing materials.
Activity 5
Go in a empty room or an unfurnished room and produce sound "Hello". Can you
hear reverberation?
Now, go in an occupied or furnished room of the same size and repeat the above
activity. Can you hear reverberation ? What is the reasons behind this fact?
Sound absorbing materials are kept in the walls of cinema hall to prevent reverberation so
that the audience can hear clear sound. But the sound of singer is prolonged to add melody.
Differences between echo and reverberation
Echo Reverberation
1. It is the repetition of sound caused by 1. It is the prolongation of original sound
reflection. due to reflection.
2. The minimum distance required to 2. The distance required to take place
take place an echo is 17.2 m. reverberation should be less than 17.2
3. The intensity of echo is less than that m.
of original sound. 3. The intensity of reverberation is more
than that of original sound.
Application of reflection of sound
1. Reflection of sound is used to investigate diseases like kidney stone, tumour, etc.
2. It is used to investigate the minerals.
3. It is used in SONAR to find out the depth of the sea, lake, river, etc.
4. It is used to observe the growth of the embryo inside the mother's womb.
5. It is used by soldiers to identify weapons.
114 GREEN Science (Physics) Book-9
Refraction of Sound
The bending of sound when it passes from one medium to another is called refraction of
sound. When sound passes from a denser medium to a rarer medium, it bends away from
the normal. Similarly, when sound passes from a rarer medium to a denser medium it
bends towards the normal. It shows that sound also obeys the laws of refraction of light.
Different layer of air have different temperature. Due to variation in temperature one
layer may act as a denser medium and another layer acts as a rarer medium.
Sound is heard more clear at night than that in a day due to total internal reflection of
sound waves.
Sound is heard more clear at night than in day time, why?
The surface of the land gets heated due to heat of the sun during day time. Due to this,
the temperature of land surface becomes maximum and it decreases gradually on moving
upwards. As a result, layers of air behave as denser medium while moving upwards. So,
the sound waves bend towards normal and diverge upwards from the source. Due to this
reason, less sound waves reach the listener and the sound is not heard clearly.
Cold air Hot air
Sound wave Sound wave
Fig.
Hot air Cold air
At night
8.4 At day
On the other hand, the land surface gradually cools down at night due to absence of solar
heat and the layers of air near the land surface behave as denser layer than the upper
layers. So the sound waves coming from the source get refracted away from the normal
while moving upwards. These sound waves suffer total internal reflection and reach the
listener on the surface of the land. As a result, sound is heard more clear at night than in
the day time.
Intensity of sound
Sound waves carry energy with them. The amount of energy that a sound wave carries
in one second is called intensity of sound. In simple language, the loudness of a sound is
called intensity of the sound. It is measured in decibel (dB).
The frequency of some sound /activities is given blow:
Activities/sound Intensity (in decibel)
1. Whispering 0 – 20 dB
2. Environment of library 20 – 40 dB
GREEN Science (Physics) Book-9 115
3. Sound of printing press 70 – 80 dB
4. Conversation 40 – 60 dB
5. Sound of motor car 110 – 120 dB
6. Sound due to mechanical failure 140 – 160 dB
7. Railway station 85 – 110 dB
8. Heavy street traffic 60 – 70 dB
9. Maximum hearing limit of pain 120 – 140 dB
When sound propagates, energy gets transmitted along with the sound. Every wave
carries energy from the source. When a wave carries more energy, its amplitude increases.
A sound is heard large if it has more intensity and vice-versa. The sound of more intensity
affects our ears. The sound with intensity more than 120 dB harms our ears. Deafness is
the condition of not hearing the sound of intensity upto 80 dB. The sound ranging from
60 dB to 140 dB is hard to hear.
Factors affecting the intensity of sound
1. Amplitude
The intensity of sound is directly proportional to the amplitude.
2. Density of the medium
The intensity of sound is directly proportional to the density of gas medium. The
intensity of sound in cold air is more than that of hot air because the density of cold
air is more than that of hot air.
3. Frequency of sound
The intensity of sound is directly proportional to the frequency of sound.
4. Distance from the source of sound
The intensity of a sound is more near to its. Source and the intensity gradually
decreases while moving away from the source. It is because when sound propagates
away from the source, some energy is absorbed by air molecules and some energy
gets reflected and refracted.
5. Area of vibrating body
The intensity of sound increases when the area of the vibrating increases. For example,
the intensity of the sound produced by a large temple bell is more than that produced
by a smaller one.
Pitch of sound
We hear sound of different shrillness. The shrillness of a sound is called the pitch of the
sound. The sound of a girl is more shrill as compared to that of a boy. Pitch of a sound
cannot be measured but it can be felt. The pitch of the sound helps to differentiate between
116 GREEN Science (Physics) Book-9
Fig.a flat sound and a thin or shrill sound. The pitch of a sound depends on the frequency. It
means that when the frequency of sound increases, its pitch also increases and vice-versa.
Following examples help us to get the concept of pitch of sound.
i. The pitch of the sound produced by thin string of guitar is more than the sound
produced by its thick string.
ii. The pitch of the sound of a child is more than the sound of an adult.
iii. The pitch of the sound produced by a long flute is more than that produced by a short
flute.
Noise pollution
The production of unwanted loud
sound is called noise pollution. Vehicles,
loudspeakers, crowd, musical instruments,
machineries, etc. are the sources of noise.
Causes of noise pollution
i. Automobiles like, trucks, buses, cars, 8.5
motorcycles, aeroplanes, etc. causes noise pollution.
ii. Various machines in industries and construction sites cause noise pollution.
iii. Use of loudspeakers, tape recorders, etc. causes noise pollution.
iv. Playing musical instruments in high volume and blowing pressure horns in vehicles
also cause noise pollution.
Effects of noise pollution
i. Noise pollution weakens hearing capacity.
ii. It increases irritability and loss of concentration.
iii. It increases blood pressure and mental tension.
iv. It causes insomnia, migraine and headache.
v. It may rupture the ear drum and cause deafness.
vi. Noise pollution results in psychological disorders.
Ways to reduce noise pollution
i. Avoiding the use of pressure horns in vehicles.
ii. Establishing airports away from the human settlement.
iii. Using silencers in vehicles.
iv. Using earmuffs while working in factories.
v. Plantation around industries states.
GREEN Science (Physics) Book-9 117
Key Concepts
1. Sound produces the sensation of hearing in our ears. It is produced when a body
vibrates.
2. The wave produced due to vibration of a material medium is called a sound wave.
It is produced due to vibration of solid, liquid and gas.
3. Frequency of sound is defined as the number of complete cycles produced in one
second. It is denoted by ‘f’. In SI system, frequency is measured in hertz(Hz).
4. The total distance covered by a sound wave in one vibration is called wave length.
It is denoted by the Greek symbol Lambda (λ).
5. Wave velocity can be defined as the total distance covered by a sound wave per unit time.
6. The mathematical relation between wave velocity, frequency and wavelength is
called wave equation.
7. Sound travels with different speed in different media. The speed of sound is
maximum in solids and minimum in gases.
8. The speed of sound increases when the direction of movement of sound and air
molecules is the same. But the sped of sound decreases if the direction of the sound
is opposite to that of the direction of air molecules.
9. Ultrasound is the sound having frequency more than 20,000 Hz. Ultrasound is
widely used in medical sector to examine the inner body organs.
10. The process of finding the depth of the sea or distance with the help of ultrasound
is called echolocation.
11. Reflection of sound is defined as returning of sound in the same medium when it
strikes a hard surface.
12. Echo can be defined as the repetition of sound caused by the reflection of sound.
13. Reverberation can be defined as the prolongation of original sound due to reflection.
Reverberation is the process of intermixing of original sound with a reflected sound.
14. Infrasound is the sound having frequency less than 20Hz.We cannot hear infrasound.
It is produced during earthquake and volcano eruption.
15. Reflection of sound is used to investigate diseases like kidney stone, tumour, etc.
16. The bending of sound when it passes from one medium to another is called
refraction of sound.
17. The amount of energy that a sound wave carries in one second is called intensity of
sound. In simple language, the loudness of a sound is called intensity of the sound.
It is measured in decibel (dB).
18. The shrillness of a sound is called the pitch of the sound. The sound of a girl is more shrill
as compared to that of a boy. Pitch of a sound cannot be measured but it can be felt.
19. The production of unwanted loud sound is called noise pollution. Vehicles,
loudspeakers, crowd, musical instruments, machineries, etc. are the sources of noise.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. Frequency of a sound is the number of waves formed in ........... second.
1 2 3 5
118 GREEN Science (Physics) Book-9
b. Which of the following is ultrasound?
f = 25000 Hz f = 20Hz f = 2 Hz f = 19000 Hz
c. Which of the following is the intensity of normal conversation?
20 dB 60 – 70 dB 40 – 60 dB 70 – 80 dB
d. The full from of SONAR is ..................... Sound Navigation and Ranging
Sound Navigator and Range
Sound Navigation and Range Sound Neutralization and Ranging
2. Answer the following questions.
a. What is sound ?
b. What are sources of sound? Give any two examples.
c. What is longitudinal wave? Give one example.
d. Write any three characteristics of longitudinal wave.
e. What is time period? Write its formula.
f. Define wavelength and amplitude.
g. What is wave velocity? Write down its formula.
h. Write down the relationship between speed of sound and medium.
i. Name the factors that affect the speed of sound in air.
j. What is ultrasound? Write its practical applications.
k. What is audible sound?
l. What is infrasound? When is it produced?
m. Define reflection of sound. Draw a figure to show this process.
n. What is echo? Write down the conditions required to hear an echo.
o. What is reflection of sound? Write its applications.
3. What is meant by intensity of sound ? Write down the factors affecting it.
4. Define pitch of sound with examples.
5. What is noise pollution? Write its causes and preventive measures.
6. Differentiate between:
a. Echo and Reverberation
b. Intensity and Pitch of sound
c. Frequency and Wave length
7. Give reason:
a. Sound wave is called a longitudinal wave.
GREEN Science (Physics) Book-9 119
b. Sound is heard more clear at night.
c. SONAR is useful for human beings.
d. Sound absorbing materials are kept in walls of cinema hall.
8. Numerical problems
a. The sound of wave length 0.022 m has a frequency of 15 kilohertz. Calculate the
speed of the wave. [Ans: 330 m/s]
b. If the speed of a sound wave in a medium is 5100 m/s and the frequency is 200
Hz, calculate the wave length. [Ans: 25.5 m]
c. The speed of sound in a medium is 1200 m/s and the wave length is 15.5 m.
Calculate the frequency of the sound wave. [Ans: 77.41 Hz]
d. A sound wave is sent at the bottom of a sea. If the echo is heard after 2 second,
calculate the depth of the sea. The speed of sound in water is 1500 m/s.
[Ans: 2250 m]
e. If the sound of thunderstorm in the sky is heard after 20 seconds, calculate the
height where thunder storm occurred. The speed of sound in air is 332 m/s.
[Ans: 3320 m]
f. A ship transmits a sound to the sea of depth 2250m. If the echo is received after 3
seconds, calculated the speed of sound in water. [Ans: 1500m/s]
g. The depth of a sea is 6200m and the speed of sound in water is 1500m/s. Calculate
the time at which th echo is heard after sending the sound waves. [Ans: 8.26s]
Grid-based Exercise 2
Group ‘A’ (Knowledge Type Questions) (1 Mark Each)
1. What is source of sound?
2. What is wavelength?
3. What is frequency? Write down its SI unit.
4. Define mechanical wave with one example.
5. Define longitudinal wave with one example.
6. What is meant by the sound spectrum?
7. Which type of sound can be heard by human ear?
8. What is echolocation? Write.
9. What is echo?
10. What is fathometer?
11. Write down the intensity of the sound that makes us deaf and can rupture the ear
drum.
12. What is the frequency of the sound of a man and woman?
13. What is intensity of sound? In which unit is it measured?
120 GREEN Science (Physics) Book-9
For Group ’B’ (Understanding Type Questions) (2 Marks Each)
14. Sound wave cannot propagate in a vacuum. Why?
15. Write any two differences between echo and reverberation.
16. Why the speed of sound in hot air is more than that in the cold air.
17. Why is the wavelength of girls voice is shorter than that of the boys? Give reason.
18. Why is it harmful to settle near the airport? Give reason.
19. Write any two differences between infrasound and ultrasound.
20. Sound absorbing materials are kept in the walls of cinema hall. Give reason.
For Group ‘C’ (Application Type Questions) (3 Marks Each)
21. Write any three effects noise pollution.
22. Write any three control measures of sound pollution.
23. Describe in brief the effects of sound pollution in human life.
24. Write down utility of SONAR. How is the depth of sea measured by using sound?
Write in brief.
25. If you are kept blind folded in a room, how can you find out whether the room is
occupied or blank by producing sound?
For Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)
26. The velocity of sound in three media carbon dioxide, steel (Medium) (Speed)
and water is given in the table. Answer the following A 5200 m/s
questions on this basis. B 258 m/s
C 1498 m/s
Name the media ‘A’, ‘B’ and ‘C. Write down the reason
for the fact that the speed of sound in^medium ‘A’ is
the highest. What happens to the speed of sound when
medium ‘B’ is heated? Explain the reason.
27. Differentiate between echo and reverberation. The speed of sound in a medium is
1200m/s. If the wavelength of the sound is 15.5m, calculate the frequency of the sound.
Is the sound audible to human ears?
28. A ship transmits a sound to the sea of depth 2250 m. If the echo is received after 3
seconds, calculate the speed of sound in water. Differentiate between Echolocation
and Reverberation.
29. A sound is sent at the bottom of a pond. If the echo is heard after 6 seconds, calculate
the depth of the pond. The speed of sound in water is 1498m/s. Differentiate between
audible sound and infrasound.
30. How is the density of a sea measured by using sound? Explain.
GREEN Science (Physics) Book-9 121
UNIT Current Electricity and
7 Magnetism
Weighting Distribution Theory : 10 Practical: 2
Before You Begin
When a cell is connected to a bulb with copper wires, the bulb glows due
to continuous flow of electrons through the wire. Such flow of electrons
through a conductor is called current electricity. So, the form of energy which
is produced due to continuous flow of electrons is called current electricity.
Cell, battery, dynamo and generator are the sources of current electricity.
Current electricity is a very useful form of energy. It can be converted into
heat energy, light energy, sound energy, magnetic energy. It is used to
light bulb, to operate heater, television, computer, radio, fan, telephone,
etc.
A magnet is a substance that attracts magnetic substances like iron, nickel,
etc. and rests in north-south direction when suspended freely The lodestone
is a naturally occurring magnet. The Chinese found that the lodestone,
when suspended freely, always pointed north-south direction. These days
magnets are made artificially from iron, nickel, etc. in various shapes and
sizes depending on their uses. These magnets are called artificial magnets.
A magnet has two poles at the two ends, a north pole and a south pole.
When a bar magnet is suspended by a string freely, one end of the
magnet points towards north and the other end towards south. The end
which points towards north is called north pole and another is called
south pole. The substances which are attracted by a magnet are called
magnetic substances. The phenomenon related to attractive and directive
properties of magnets is called magnetism.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to: • Ohm’s law
• Resistance
i. their measurement. • Conductivity
ii. state Ohm’s law and establish the relation among V, • Magnetic field
• Magnetic lines of force
R and I. • Terrestrial magnetism
iii. measure the conductivity of different substances.
iv. describe magnetic field and magnetic lines of force
and demonstrate them.
v. explain the elements of earth’s magnetism (angle of
dip and angle of declination).
Glossary: A dictionary of scientific/technical terms
current electricity : the form of energy which is produced due to continuous flow of electrons
through a conductor
resistance : the property of a conductor to oppose the flow of electricity through it
magnetism : the phenomenon of attracting magnetic substances
terrestrial : connected or related with the earth
122 GREEN Science (Physics) Book-9
Current Electricity
When a cell is connected to a bulb with copper wires, the bulb glows due to continuous
flow of electrons through the wire. Such flow of electrons through a conductor is called
current electricity. So, the form of energy which is produced due to continuous flow of
electrons is called current electricity. Cell, battery, dynamo and generator are the sources
of current electricity.
Current electricity is a very useful form of energy. It can be converted into heat energy,
light energy, sound energy, magnetic energy. It is used to light bulb, to operate heater,
television, computer, radio, fan, telephone, etc.
Open Electric Circuit Fig. Switch
Fig. (OFF)
The electric circuit in which a load does not
work is called open circuit. In an open circuit, Open circuit
electric current does not flow continuously. So
the load (bulb) does not work. A circuit may be
open (i) if the switch is turned 'OFF', (ii) wire
has broken or (iii)load (bulb) is damaged.
7.1
Closed Electric Circuit
The electric circuit in which a load works Switch (ON)
continuously is called closed electric
circuit. In this condition, current flows
continuously through the circuit. The
switch is turned 'ON' in a closed circuit.
7.2
Symbols Used in Circuit Diagrams Closed circuit
While drawing circuit diagrams, electrical equipment and components of electric circuit
are represented by their symbols. The main components and electrical equipments with
their symbols and functions are as follows:
S. No. Components/Devices Symbols Function
1. Wire To make circuit
2. Wires (not connected) To make electric path
3. Wires (connected) To open and close
4. Switch circuit
GREEN Science (Physics) Book-9 123
5. Electric cell Source of electricity
6. Battery Source of electricity
7. Resistor/Load
8. Bulb To convert electrical
or energy into another
form
To convert electricity
or into light
9. Ammeter A To measure the current
10. Voltmeter V To measure the
voltage
11. Fuse or To break the circuit in
case of overloading
12. Galvanometer G To detect electric
current
An electric circuit showing various components in proper order of combination is given
below:
AV
Fig.
7.3
Electric circuit
Electric current
The rate of flow of charge in an electric circuit is known as electric current. It is denoted
by I. In SI system, electric current is measured in ampere (A).
124 GREEN Science (Physics) Book-9
Electric current flowing through an electric circuit is calculated by the given formula:
Electric current (I) = Flow of charge (Q)
Q Time (t)
t
or, I =
The SI unit of flow of charge is coulomb (C) and that of time is second. So, the SI unit of
electric current is coulomb/second or ampere (A).
One ampere current
One ampere current (1A) can be defined as the rate of flow of one coulomb charge in one
second.
We know,
I = Q = 1C = 1A
t 1s
One coulomb charge consists of 6 × 1018 electrons. So 1A current is also called the rate of
flow of 6 × 1018 electrons through a conductor in 1 second.
Ammeter and Galvanometer
Ammeter is an electric equipment which is used to measure the amount of electric current
flowing through a conductor.
In order to measure the electric current flowing through an electric circuit, the positive
terminal of the cell or battery is connected to the positive terminal of the ammeter and the
negative terminal of the cell or battery is connected to the negative terminal of the ammeter.
Fig.
7.4 Galvanometer
Ammeter
Ammeter is a device having low resistance. It is always connected in series in an electric
current so that all current flowing through the circuit passes through ammeter. The device
which is used to detect the presence of electric current is called galvanometer. It is also
used to measure very small amount of electric current.
In an electric circuit, galvanometer is always connected in series.
GREEN Science (Physics) Book-9 125
Voltmeter
The electric equipment which is used to
measure the potential difference (p.d.) between
any two points of a closed electric circuit is
called voltmeter.
In an electric circuit, voltmeter is always Fig.
connected in parallel to a device, or a source
of electricity in order to find the difference in 7.5
potential between any two points of the source
or a device. Voltmeter has a high resistance. So Voltmeter
it is not connected in series.
Differences between Ammeter and Voltmeter
Ammeter Voltmeter
1. Ammeter is used to measure the 1. Voltmeter is used to measure the
magnitude of electric current flowing potential difference between any two
through a circuit. points of an electric circuit or source.
2. It is always connected in series with 2. It is always connected in parallel with
the circuit. a resistance.
Electromotive force
Metals have free electrons so they are called good conductor of electricity. But the free
electrons present in metals do not move on their own without supplying energy from an
external source. When a source (cell or a battery) is connected to a conductor, it supplies
energy necessary for flow of electrons through the conductor and during this process
some amount of work is done. So, electromotive force can be defined as the amount of
energy supplied to move unit charge (i.e. 1 coulomb charge) throughout the whole circuit.
Potential difference
Electromotive force creates potential difference across the two ends of a conductor. As a
result, an electric current flows from the positive terminal of the source to the negative
terminal. During this process, a certain amount of work is done to bring unit charge from
one point to another which is called potential difference (p.d.). So, the amount of work
done in bringing unit charge or one coulomb charge from one point to another point
of a conductor is called potential difference. It is denoted by V. In SI system, potential
difference is measured in volt (V). Potential difference is measured in closed circuit.
One volt potential difference
We know,
Potential difference (p.d.) = Charge Work done to another
moved form one point
\ 1 volt p.d. = 1 1 joule
coulomb
126 GREEN Science (Physics) Book-9
When 1 joule work is done move one coulomb charge from one point to another point of
a conductor, it is called one volt potential difference (1 V p.d).
Differences between potential difference and electromotive force.
Potential difference Electromotive force
1. It is the amount of work done in 1. It is the amount of energy supplied to
bringing one coulomb charge from carry one coulomb charge throughout
one point to another. the whole circuit.
2. It is less than electromotive force. 2. It is more than potential difference.
3. It is measured in closed circuit. 3. It is measured in open circuit.
Ohm's law
The relation between electricity flowing through a circuit (I), resistance in the circuit
(R) and the potential difference or voltage (V) was discovered by the famous German
Physicist George Simon Ohm. This relation is popularly known as Ohm’s law.
Ohm's law states that,"When the temperature and other physical conditions remain
constant, the electric current flowing through a conductor is directly proportional to the
potential difference across its two ends.”
or, I ∝ V
or, V ∝ I
or, V = RI
\ V = IR ............... (i)
Where 'R' is a constant. It is the resistance of the conductor.
Form equation (1),
I = V
R
Or, R = V .................... (2)
I
From above relation, it becomes clear that the resistance of a conductor is the ratio of the
potential difference across two ends of a conductor and the current following through it.
In SI system, resistance (R) is measured in ohm (Ω). Similarly, electric current (I) is
measured in ampere (A) and potential difference is measured in volt (V).
Experimental verification of Ohm's law Fig. A
7.6 V
● Take four dry cells of 1.5 V each, a resistor, an
ammeter a voltmeter and a switch. GREEN Science (Physics) Book-9 127
● Now, prepare an electric circuit by connecting
them as shown in the figure.
● Measure the voltage and current by connecting
four cells in the circuit and fill it in the table.
● Remove one cell from the circuit and measure the voltage and current.
● Now, measure the voltage and current by combining two cells in the circuit and then
by connecting one cell in the circuit.
● Calculate the resistance in each case and fill in the given table.
Observation Table:
S.No. Voltage (V) Current (A) Resistance (R) Conclusion
1.
2.
3.
4.
● Now, plot a graph putting the voltage (V) along the X-axis and current (I) along the
Y-axis. The graph is obtained as shown in the given figure.
Y
Current Resistance (R)
(I)
Fig.
7.7 Voltage (V) X
This graph shows the voltage (V) is directly proportional to the current (I). This activity
proves Ohm's law.
Solved Numerical: 1
A load of 6 Ω is connected to a source of 12 V potential difference. Calculate the amount
of current.
Solution:
Resistance (R) = 6 Ω
Voltage (V) = 12 V
Current (T) = ?
We know,
V = IR
or, I = V
R
128 GREEN Science (Physics) Book-9
or, I = 12
6
= 2A
\ The amount of current that flows through a circuit = 2A.
Conductors Insulators and Semi-conductors
The substances through which electric current flows very easily are called good conductors
of electricity. Examples : copper, silver, aluminium, etc.
The resistors that are sold commercially have very high resistance. They are used to
control the amount of current flowing through a circuit.
Substances like rubber, dry wood, brick, plastic, paper, etc. do not have free electrons so
they do not allow electricity to flow through them. The substances which do not allow
electric current to pass through them are called bad conductor or insulators.
Some substances like silicon, germanium, etc. contain a few free electrons. So they allow
less amount of current to flow them. They are called semi-conductors.
The substances which allow less amount of current flow through them are called semi-
conductors. More current flows through semi-conductors when their temperature
increased.
Semi-conductors pass more current than insulators and less current than conductors.
Resistance
When electric current flows through a conductor, the conductor opposes the flow of
current partially. However, the resistance of a conductor depends on various factors
like type and nature of conductor, length of conductor, thickness of conductor and so
on. Therefore, resistance vary according to nature of the conductor. The property of a
conductor due to which it opposes the flow of current through it is called resistance. It is
denoted by R. In SI system, resistance is measured in ohm (Ω).
The resistance of copper wire is very less but the resistance of nichrome wire is very high.
When one volt potential difference is produced while flowing 1A current, the resistance
of the conductor is called one ohm (1Ω).
The larger units of resistance are kiloohm (KΩ)and megaohm (MΩ) .
Relation between ohm, kiloohm and megaohm
1000 Ω = 1K Ω
1000 k Ω = 1M Ω
1M Ω = 1000000 Ω
Metals contain a large number of free electrons. Therefore, electric current flows easily
through them. But they also have resistance. The resistance of silver wire and copper wire
is very less as compared to that of nichrome wire and tungsten wire.
GREEN Science (Physics) Book-9 129
Metals like copper, aluminium, etc. have a large number of free electrons. So they
produce very low resistance. They have very less resistivity. Therefore, they are called
good conductors of electricity.
Factors affecting resistance of a conductor
1. The resistance (R) of a conductor increases with the increase in the length (l) of
conductor, i.e.
R∝l
2. The resistance of a conductor (R) is inversely proportional to its area (A) of cross-
section, i.e.
R = 1
A
3. The resistance of a conductor (R) is directly proportional to the temperature (T) of the
conductor, i.e.
R∝T
4. The resistance of the coiled wire is more than that of the straight wire.
Combination of Resistance
Generally, resistances are connected by two ways. They are:
i. Series combination of resistances
ii. Parallel combination of resistance
i. Series combination of resistances R1 R2 R3
In this type of combination, one resistance is Fig.
connected after another in series. In this type
of connection, the same current flows through
all resistances.
In figure, three resistances, R1, R2 and R3 are +–
connected in series. So, the total resistance (R) 7.8
is equal to the sum of individual resistance, i.e.
(R1 + R2 + R3), Fig: Series combination of resistance
or, R = R1 + R2 + R3
In series combination, total resistance increases with the increase in the number f resistance.
Similarly, the current in series combination decreases with increase in the number of
resistances. If one of the bulb gets fused, remaining bulbs/devices stop working as the
circuit is opened.
130 GREEN Science (Physics) Book-9
ii. Parallel combination of resistances R1
R2
In this combination, each resistance is individually R3
connected to the two terminals of the electric
source. In this type of combination, the total
potential difference across each resistance remains
the same. So, the current flowing through each
resistance also remains the same.
In parallel combination, total resistance is equal to Fig.
Fig.
the sum of reciprocal of individual resistances.
1 = 1 + 1 + 1 7.9 V
R R1 R2 R3
Do You Know
In parallel combination, the total resistance
decreases on increasing the number of In domestic electric circuits, all
resistances. In this connection, if one bulb appliances are connected in parallel
gets fused, remaining bulbs/devices work so that if one appliance goes off due to
continuously. any reason, the others may still keep
on working.
Activity 1
Prepare an electric circuit as shown in the figure by connecting two dry cells, a
switch, a bulb, an ammeter with a conducting wire (copper wire).
Leave a gap AB as shown in the figure.
1. Now, take two pieces of copper wire (one 5
cm long and another 50cm long) and connect A
the gap AB one by one by using those pieces
of wire.
Measure the amount of current flowing 7.10 AB
through the circuit.
When the circuit is completed by using the short wire (5 cm) the ammeter
shows more current flowing through the circuit than the current flown when
the circuit is completed by using long wire.
It shows that the resistance of a conductor increases on increasing its length.
In other words, resistance (R) of a conductor is directly proportional to the length
(L) of the conductor.
In short, R ∝ L
Were, R = Resistance of the wire
L = length of the wire
2. Take two wires one thick and another thin having the same length of 10cm. Complete
the electric circuit by connecting the circuit by thin and thick wire one by one.
Note down the reading of the ammeter in each case.
GREEN Science (Physics) Book-9 131
When the circuit is completed by thick wire, more current flows and when the
circuit is completed by thin wire less current flows. It shows that the resistance of
a conductor decreases if its thickness increases and vice-versa. It means that the
resistance of a conductor is inversely proportional to the thickness or cross-sectional
area of the conductor.
In short,
1
R ∝
a
Where, R = Resistance of a conductor
a = Cross-sectional area of the conductor.
3. Now, complete the circuit by using a piece of copper wire and a piece of nichrome
wire of equal length and equal thickness. Measure the current flowing through the
circuit in each case. More amount of current flows when the circuit is completed
by using copper wire and very less amount of current flows when the circuit is
completed by using nichrome wire. Copper and nichrome are two different
materials. Therefore, they have different resistance though they have the same
length and thickness.
It shows that the resistance differs according to the nature of the conductor,
4. Take a piece of nichrome wire. Complete the circuit using the nichrome wire and
measure the current.
Now, heat the piece of nichrome wire with the help of a Bunsen burner. Complete
the circuit by using hot piece of nichrome wire. Measure the current again. What do
you observe?
Less current flows when the circuit is completed with hot nichrome wire than it is
done with cold nichrome wire. It shows that the resistance of a conductor increases
when the temperature increases. The reverse of this is also true.
or, R ∝T
where, R = Resistance
T = Temperature
From the above activities, we can concludes that the following factors affect the
resistance of a conductor.
1. Length of the conductor
2. Thickness of the conductor
3. Temperature of the conductor
4. Nature of the conductor.
Conductivity
Most metals are good conductors of electricity and most non-metals are bad conductors
or insulators. However, graphite is a non-metal which can conduct electricity.
We know that the resistance of a conductor is directly proportional to the length of the
conductor, i.e.
R ∝ L ...................... (i)
132 GREEN Science (Physics) Book-9
The resistance of a conductor is inversely proportional to the thickness or cross-sectional
area of the conductor, i.e.
R ∝ 1 ......................... (ii)
a
Form equation (1) and (2), we get
R ∝ L
a
or, R = δ L ................... (3)
a
Where δ is a constant called the resistivity of a conductor. Its SI unit is Ω m (ohm meter).
So, conductivity can be defined as the reciprocal of resistivity. The conductivity of a
conductor is more if its resistivity is less. Similarly, the conductivity of a conductor is less
if its resistivity is more.
The conductivity of tungesten, nichrome, etc. is less due to more resistivity. Similarly, the
conductivity of copper, gold, aluminium, silver, etc. is more due to less reactivity.
Generally, the conductivity of metals is more than that of non-metals. On the other hand,
the resistivity of non-metals is more than that of metals. Similarly, the conductivity of
good conductors is more than that of poor conductors.
Magnetism
In previous classes, you have studied about magnet and its properties. Similarly, you have
studied the methods of making method artificially. A magnet is a substance which attracts
the substances like iron, cobalt, nickel, steel, etc. The substances like iron, cobalt, nickel,
etc. are called magnetic substances because they are attracted by a magnet.
Fig.
7.11 U-shaped magnet
Bar magnet
The property of a magnet due to which it attracts magnetic substances kept inside the
magnetic field is called magnetism. Magnetic force is developed when the molecular
magnets present in a magnetic substance align parallel to each other. The magnetic force
is maximum at the poles and minimum at the centre. A magnet has two poles. They
are north pole (N) and south pole (S). Magnets may be natural or artificial. The natural
magnets have less magnetism than the artificial magnets.
GREEN Science (Physics) Book-9 133
Magnetic Field
The attractive force of a magnet Fig. Fig.7.12
decreases when magnetic substances are
taken away from the magnet. A magnet A magnet is attracting iron dust
attracts those magnetic substance which Magnet field
are kept nearby the magnet. When we
increase the distance between a magnet
and magnetic substances there is no
influence of magnet on these substances.
So, magnetic field of a magnet can be
defined as the space around a magnet
upto where its influence on magnetic
substances can be seen.
A magnet attracts only theses magnetic
substances which are kept inside the
magnetic field.
Magnetic field is a vector quantity. The 7.13
magnetic field of a powerful magnet is
large and that of less powerful magnet is
small.
Magnetic Lines of Force
Inside the magnetic field, numerous lines of force arise form the north pole of a magnet
and these lines of force move towards the south pole in the from of curves. These curves
are known as magnetic lines of force. So, magnetic lines of force are the paths along which
unit north pole moves in a magnetic field. We can trace magnetic lines of force with the
help of a magnetic compass.
N N
N
N
EE EE
S
S
Fig. SS
7.14 (a) (b)
Magnetic lines of force
The magnetic field of a magnet is stronger in the pole than in its middle. So the magnetic
lines of force are crowded at the poles of a magnet. They always start form the north pole
of a magnet and end at south pole of the same magnet. The magnetic lines of force are the
closed and continuous curves.
The north pole of a magnetic compass kept inside the magnetic field becomes parallel to
the magnetic line of force towards the direction of magnetic line of force. When a compass
134 GREEN Science (Physics) Book-9
Fig.is moved form the north pole towards the path shown by it, a continuous and closed
curve is obtained which is known as magnetic line of force.
When magnetic lines of force are traced by pointing the north pole of a bar magnet
towards the geographical north pole, the magnetic lines of force are obtained as shown in
the figure 7.12 (a).
Similarly, when magnetic lines of force are traced by pointing the north pole of a bar
magnet towards the geographical south, the magnetic lines of force are obtained as shown
in the figure 7.12 (b). The tangent drawn to a magnetic line of force at any point gives the
direction of magnetic field at that point.
Properties of magnetic lines of force
1. Magnetic lines of force are continuous and closed curves.
2. Magnetic lines of force always start from the north pole of a magnet and end at south
pole (outside the magnet).
3. Magnetic lines of force are crowded near the poles of the magnet as magnetic field is
stronger than in the middle of the magnet.
Activity 2
Take a bar magnet and place it on the centre of a cardboard paper kept on a wooden
board.
7.15
Draw the outline of the bar magnet with a pencil.
Draw a dot at the north pole of the bar magnet.
Adjust a magnetic compass in such a way that the north pole of the compass lies on
the dot drawn.
Draw a dot on the north pole of the compass and adjust the north pole of the
compass on the second dot. Continue this process till you move to the south pole of
the magnet.
Now, connect these dots with a pencil. You will get a curve starting from the north
pole of the magnet. This curve is called magnetic line of force.
By repeating the above process, we can get numerous magnetic lines of force.
While drawing magnetic lines of force, we get two points where the magnetic compass
does not show any particular direction. These points are called neutral points. These
points are denoted by (×) are the neutral points.
GREEN Science (Physics) Book-9 135
Neutral points can be defined
as the points near magnet
where the magnetic field of
the magnet is completely
neutralized by the earth's
Fig.
Fig.magnetic field. Therefore, a
magnetic compass cannot Neutral point
show any particular direction. 7.16
At neutral points, the magnetic
field of a magnet and the magnetic field of the earth's magnet is equal but opposite to
each-other. Therefore, the resultant field at the poles is zero.
Terrestrial magnetism
The earth also exhibits magnetism. The earth itself can be taken as an extremely large magnet.
So the earth has its own magnetic field. These are magnetic lines of force in the earth's
magnetic field. A magnetic compass always remains parallel to the direction of magnetic
lines of force of the terrestrial magnet. A bar magnet when suspended freely by tieing with
a polyster or nylon thread always remains parallel to the magnetic lines of force of the
terrestrial magnet. When we trace the magnetic lines of force using a magnetic compass
form the magnetic north pole of the earth, we reach the magnetic south pole of the earth.
When an iron rod is buried in the soil facing it north south direction it develops magnetic
properties with 4 – 5 days. The magnetic north pole of the earth lies at the geographical
south pole and the magnetic south pole of the earth lies at geographical north pole, The
magnetic north pole is located at the corner of Antarctica. Similarly, the magnetic south
pole is located at Northern Canada.
Angle of declination
The magnetic poles of the terrestrial magnet and the geographical poles of the earth do
not lie in the same plane. So the imaginary line that joins geographical north - south
direction and the imaginary line that joins magnetic north - south poles cross each other
making a certain angle. This angle is called angle of declination.
A q B Magnetic south Geographical
F pole of the earth north pole
q Angle of
declination
7.17 D C Geographical
E Angle of declination south pole
Angle of declination can be defined as the angle between the magnetic meridian and geographical
meridian at a place. The value of angle of declination varies from place to place on the earth.
Angle of declination is denoted by q. It helps to find out true geographical direction.
136 GREEN Science (Physics) Book-9
The imaginary line passing through magnetic Declination
north pole and magnetic south pole of the earth
is known as magnetic meridian. Similarly, the
imaginary line passing through geographical
north pole and geographical south pole is known
as geographical meridian.
We should know the value of angle of declination Fig.7.18
of a certain place to know the exact geographical Fig.
direction. Angle of declination is used by pilots,
navigators, sailors, travellers, etc. to find out the
exact direction.
Angle of dip Horizontal Angle of
q dip
Angle of dip at a certain place is defined
as the angle made by dip needle to the S
horizontal line of that place. A magnetic
needle which rotates freely and arranged
north -south direction is called dip needle.
Dip needle remains parallel to the Vertical
magnetic lines of force of the earth's 7.19
magnet. In a certain place, a dip needle
inclines towards the ground. It means that the magnetic lines of force of the earth's magnet
also incline towards the ground.
The angle of dip varies from place to place on the surface of the earth. The value of angle
of dip at equator is zero. It is due to the equal effect of north pole and south pole of the
terrestrial magnet. The value of angle of dip in Kathmandu is 42° means that the dip
needle placed in Kathmandu makes an angle of 42° to the horizontal line of the place.
The value of angle of dip increases gradually while moving towards the north or south
form the equator and the value of angle of dip at poles is 90°.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. Electricity is the form of energy which is produced due to continuous flow of
.....................
Protons neutrons electron atoms
b. The SI unit of current is .....................
ohm ampere volt watt
GREEN Science (Physics) Book-9 137
c. The resistance of a conductor depends on .....................
length of the conductor temperature of the conductor
nature of the conductor all of th above
d. The device which is used to measure the potential difference is called .....................
ammeter voltmeter galvanometer dip needle
e. The value of angle of dip at the equator is .....................
0° 90° 45° 180°
2. Answer the following questions.
a. What is electric current?
b. Write any two sources of electric current.
c. Define electric circuit. Draw a neat figure showing a closed electric circuit.
d. What is meant by electromotive force? Define potential difference.
e. State Ohm's law and prove V = IR.
f. Define resistance of a conductor and write its three units.
g. Write any three factors that affect the resistance of a conductor.
h. What happens to the resistance of a conductor when
i. the length of the conductor is increased?
ii. thickness of the conductor is decreased?
iii. temperature of the conductor is increased?
i. What is meant by conductivity? How does it differ from resistivity?
j. What is magnetism?
k. What is magnetic field?
l. What are magnetic lines of force?
m. What is terrestrial magnetism ? Write any two evidences of existence of terrestrial
magnetism.
n. What is angle of declination? Write its advantages.
o. What is angle of dip? What is its value:
i. at equator?
ii. at north pole?
iii. at south pole?
p. What is meant by the statement that the angle of dip in Kathmandu is 42°?
q. What are neutral points?
3. Differentiate between:
a. Potential difference and Electromotive force
b. Voltmeter and Ammeter
c. Angle of dip and Angle of declination
138 GREEN Science (Physics) Book-9
4. Give reason:
a. Ammeter is always connected in series in an electric circuit.
b. Voltmeter is always connected in parallel.
c. The angle of dip at the equator is 0°.
d. The angle of dip at poles is 90°.
e. Angle of declination is very useful for us.
5. Draw a neat figure showing magnetic lines of force.
6. Numerical problems:
a. If 360 coulomb charge flows through a conductor in 3 minutes, calculate the
current flowing through a circuit. [Ans: 2V]
b. A current of 5A flows when 300 coulomb charge flows through a circuit. Calculate
the time taken to flow the charge. [Ans: 60 s]
c. Calculate the current flowing through a circuit when a resistance of 6 Ω is
connected to the potential difference of 3V. [Ans: 0.5A]
d. Calculated the resistance of a conductor when a current of 6A can be drawn by
connecting it to 12V potential difference with a wire. [Ans: 2 Ω]
e. A current of 2A flows through a circuit when a conductor of 6 Ω is connected to
a source. Calculate the potential difference or voltage. [Ans: 12V]
Grid-based Exercise 2 (1 Mark Each)
Group ‘A’ (Knowledge Type Questions)
1. What is current electricity?
2. Define electromotive force.
3. Define one ohm resistance.
4. What is an electric load? Write with an example.
5. What is the actual direction of current flow? Write.
6. What is ammeter? How is it connected in an electric circuit?
7. What is potential difference?
8. What is voltmeter? How is it connected in an electric circuit?
9. State Ohm’s law.
10. What is angle of declination?
11. What is magnetic field?
12. What is a neutral point?
13. What is the magnitude of angle of dip at the magnetic equator?
For Group ’B’ (Understanding Type Questions) (2 Marks Each)
14. Copper is called good conductor and silicon is called semiconductor. Give reason.
15. Hydroelectricity is called a renewable source of energy, why? Why is voltmeter
connected parallel with loads ?
16. Write any two differences between potential difference and electromotive force.
17. Write any two differences between Ammeter and Voltmeter.
GREEN Science (Physics) Book-9 139
18. We should know angle of declination to know the actual geographical direction of a
certain place, why?
19. Differentiate between angle of dip and angle of declination.
20. The value of angle of declination changes from place to place. Why?
For Group ‘C’ (Application Type Questions) (3 Marks Each)
21. State Ohm’s law and prove that V = IR.
22. Define series combination and parallel combination of cells. In which condition are
cells connected in series? Z
23. How is galvanometer connected in an electric circuit? Write
down the utility of angle of declination.
24. Write any three evidences that prove the magnetic property X
of the earth. Label ‘X’, ‘Y’ and ‘Z’ shown in the given diagram. Y
25. How is the angle of dip at a certain place measured? Describe
in brief with figure.
For Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)
26. Study the electric circuit shown in the given diagram and answer the following
questions:
a. What happens to the brightness of bulb when the number
of electric bulb is reduced ? Why ?
b. Write down an advantage of such type of circuit.
c. Name the type in which the bulbs are connected in the
circuit. Define the type of connection of bulbs.
27. A current of 0.33 A is flowing through a circuit. If the potential difference across two
points of a wire is 5V, calculate the resistance. Differentiate between closed circuit and
open circuit.
28. The power of the bulb in a car is 60W. Calculate the current that flows when it is
connected to a battery of 6V. Also, calculate the resistance of the bulb. Differentiate
between semiconductor and insulator.
29. Study the given figures and answer the following questions:
a. Which type of combination of loads 6V 6V
is shown in the fig. (a) and fig. (b)? 1.5 Ω 1.5 Ω 1.5 Ω
b. How much current flows in the
circuit (a) and (b)?
1.5 Ω
30. The voltage of an electric line is 220V. If 4.54 A current flows through the line when
a heater is connected to the circuit, calculate the power of the heater. Differentiate
between angle of declination and angle of dip.
140 GREEN Science (Physics) Book-9
Chemistry
UNIT Classification of
8 Elements
Weighting Distribution Theory : 9 Practical: 2
Before You Begin
Any thing having mass and volume is called matter. Different
substances around us are matter. Solid, liquid and gas are three states
of matter. Some matter can be decomposed into simple substances.
These substances are called compounds. A compound is a substance
formed by combination of two or more elements in a fixed proportion
by weight. The smallest particle of a compound is called molecule.
Water (H2O), Sodium chloride (NaCl), Carbon dioxide (CO2), etc.
are some examples of compounds. Some matter cannot be divided
into simple substance by ordinary chemical means. These matter are
called elements. The smallest particle of an element is called atom. An
element is a pure substance made of atoms. Hydrogen (H), Oxygen
(O), Carbon (C), Sodium (Na), etc. are some examples of elements.
Scientists have discovered 118 elements so far. Among them 92
elements are found in nature and the remaining 26 elements have
been synthesized in laboratory by scientists.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to: • Element and compound
i. define atom and describe the atomic structure of • Atomic structure
• Electronic configuration
elements and their electronic configuration. • Valence electron and valency
• Chemical bond
ii. introduce valency in terms of combining capacity • Molecular formula
of elements and write molecular formulae of some • Radicals and their types
common compounds. • Electrovalent bond and
iii. introduce radicals and ions with examples. covalent bond
iv. write molecular formulae of some common
compounds.
Glossary: A dictionary of scientific/technical terms
matter : anything having mass and volume
atom : the smallest particle of an element
molecule : the smallest particle of a compound
valency : the combining capacity of an atom with another atom to form a molecule
symbol : an abbreviation of the full name of an element
abbreviation : a short form of a word
GREEN Science (Chemistry) Book-9 141
Elements
Different types of substances are found in our surroundings. Among them, some cannot
be broken down into simple substances by ordinary chemical means. These substances
are called elements. An element is a pure substance which cannot be divided into two
or more simpler substances by ordinary chemical means. Hydrogen (H), Carbon (C),
Oxygen (O), Sodium (Na), Calcium (Ca), Copper (Cu), Silver (Ag), Gold (Au), etc. are
examples of elements. Elements are pure substances formed by only one kind of atoms.
Fig. Fig.8.1
Fig.
Gold Iron Silver Copper
Hydrogen, gold, iron, silver and copper are called elements because they are formed
by only one type of atoms and they cannot be broken down into simpler substances by
ordinary chemical methods. All the atoms of the same element are identical but atoms of
different elements are different.
Scientists have discovered 118 elements so far. Among them 92 are found in nature and
remaining 26 elements have been discovered by chemists in the laboratory. At normal
temperature and pressure, elements exist in all three states, viz. solid, liquid and gas. Elements
like iron, copper, gold, silver, sodium, magnesium, aluminium, etc. are found in solid state.
Elements like mercury, bromine, caesium and gallium are found in liquid state and elements
like hydrogen, helium, nitrogen, oxygen, neon, argon, etc. are found in gaseous state.
Compound
Substances like water (H2O), sodium chloride (NaCl), H H
carbon dioxide (CO2), calcium carbonate (CaCO3) etc. O
are some examples of compounds. They are formed by
combination of two or more elements in a fixed
proportion by weight. So, the chemical substance
formed by a combination of two or more elements in a 8.2 Water (H2O)
fixed proportion by weight is called a compound. A
compound is made of two or more atoms. Two or more active elements undergo chemical
reaction which results in the formation of a compound. A compound does not contain the
properties of its constituent elements.
Water is a compound formed by combination of two 8.3 OCO
atoms of hydrogen and one atom of oxygen. Carbon dioxide (CO2)
Carbon dioxide is a compound formed by combination
of one atom of carbon and two atoms of oxygen. Sodium
chloride (NaCl) is formed by combination of one atom
of sodium and one atom of chlorine.
142 GREEN Science (Chemistry) Book-9
Magnesium chloride (MgCl2) is a compound Cl Mg Cl
formed by a combination of one atom of magnesium Magnesium chloride (MgCl2)
and two atoms of chlorine. Similarly, Aluminium
chloride is formed by combination of one atom of
Aluminium and three atoms of chlorine.
Fig.8.4
Fig.
Differences between Elements and CompoundsFig.
Elements Compounds
1. An element is a pure substance formed 1. A compound is a pure substance
by only one type of atoms. formed by a combination of two or
more types of atoms.
2. The smallest particle of an element is 2. The smallest particle of a compound is
called atom. called a molecule.
3. An element cannot be split into simpler 3. A compound can be split into two or
substances by ordinary chemical more simpler substances by ordinary
means. chemical means.
Atom
An atom is the smallest particle of Shell or orbit
an element which can take part in
a chemical reaction. All atoms of Proton
an element are similar but atoms of
different elements are different. For 8.5 Neutron
example, all atoms of gold are identical Nucleus
but the atom of gold differs from that Electron
of copper or silver. Scientists have
discovered 118 types of atoms so far. An atom
Among them, most atoms cannot exist
freely in nature but atoms of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon
(Xe) and radon (Rn) exist freely in nature. Carbon (C), hydrogen (H), oxygen (O), sodium
(Na),calcium (Ca), Copper, (Cu), Silver (Ag), Gold (Au), etc. are the examples of atom.
Molecule
A molecule is smallest
particle of an element or a OO OO
compound which can exist
freely in nature. There are a C
thousand types of molecules.
The molecule of an element 8.6 Oxygen molecule Carbon dioxide molecule
consists of two or more atoms
of the same kind where as the molecules of a compound consists of two or more atoms
of different elements. Hydrogen (H2), oxygen (O2), ozone (O3), water (H2O), calcium
carbonate (CaCO3), sodium chloride (NaCl), magnesium carbonate (MgCO3), etc. are
some examples of molecules.
GREEN Science (Chemistry) Book-9 143
A molecule of oxygen consists of two atoms of oxygen. Similarly, a molecule of carbon
dioxide consists of one atom of carbon and two atoms of oxygen.
Structure of an atom Electron
An atom is made of three fundamental Fig. Shell or orbit
particles or sub-atomic particles. They Nucleus
are proton, neutron and electron. Proton
Among them, proton and neutron are
located in the nucleus whereas electrons Neutron
revolve around the nucleus in elliptical
orbits or shell.
Protons are positively charged sub- 8.7 Oxygen atom
atomic particles located in the nucleus
of an atom. The mass of one proton is equal to that of one hydrogen atom. The mass of
one hydrogen atom is considered as one amu. (atomic mass unit). The mass of one proton
is 1837 times more than that of one electron. A proton is denoted by p+.
Neutrons are chargeless sub-atomic particles located in the nucleus of an atom. The mass
of a neutron is equal to that of a proton. A neutron is denoted by n°.
Electrons are negatively-charged sub-atomic particles that revolve around the nucleus in
elliptical orbits or shells. The mass of one electron is 1837 times smaller than that of one
proton. An electron is denoted by e–.
Atomic mass unit (amu)
The mass of an atom is very small. It cannot be expressed into milligram, gram and
kilogram. So the mass of proton, neutron and electron is expressed in atomic mass unit
(amu). The mass of a hydrogen atom is 1 amu. 1 gram is equal to 6 × 1023 amu. The mass
of a proton is 1 amu. So the mass of 6 × 1023 protons is equal to 1 gram. The mass of one
proton is equal to that of one neutron. Similarly, the mass of one neutron is 1837 times
more than that of one electron.
In short,
1p+ = 1n° = 1837e– = 1 amu
Electric charge
Protons and electrons are electrically Do You Know
charged. Protons have positive charge and An atom is electrically neutral due to the
electrons have negative charge but neutrons presence of equal number of protons and
do not have any charge. The unit of electronic electrons having opposite charges.
charge is coulomb (coul). In a neutral atom,
the number of protons is equal to that of
electrons. One coulomb charge contains 6.25 × 1018 electrons. The given table shows the
comparative study of proton, neutron and electron.
144 GREEN Science (Chemistry) Book-9
S.N. Sub-atomic particles Symbol Charge Mass Location
1. Proton p+ +ve 1amu Nucleus
2. Neutron n° nil 1 amu Nucleus
3. Electron e– –ve Orbit or shell
1 amu
1837
Atomic number
An atomic number of an atom is the total number of protons present in the nucleus of
that atom. It is also defined as the total number of electrons present in a neutral atom. An
atomic number is denoted by z.
In short,
Atomic number (z) = No. of p+ = No. of e– in a neutral atom
Atomic mass or Atomic weight
The sum of number of protons and neutrons present in the nucleus of an atom is called
atomic mass or atomic weight. It is denoted by A.
Atomic mass (A) = No. of p+ + No. of n°
No. of p+ = Atomic mass (A) – No. of n°
No. of n° = Atomic mass (A) – No. of p+
No. of n° = Atomic mass (A) – Atomic number (z) [ Atomic number = No. of p+]
Worked out Numerical: 1
The atomic number of calcium atom is 20 and its atomic mass is 40. Calculate the number
of protons, neutrons and electrons.
Given,
Atomic no. (z) = 20
Atomic weight (A) = 40
We know,
Atomic no. (z) = No. of p+ = No. of e–
= 20 = 20
\ No. of p+ = 20
No. of e– = 20
Now,
Atomic mass = No. of p+ + No. of n°
GREEN Science (Chemistry) Book-9 145
or, 40 = 20 + No. of n° = 20
or, No. of n° = 40 – 20
Electronic Configuration
Electronic configuration is the systematic distribution of electrons in various orbits or
energy levels around the nucleus of an atom. The distribution of electrons around the
nucleus was proposed by Bohr and Bury in 1921 AD.
7
6
5
4
3
2
1
p+ K L M N O P Q
n° 2 8 18 32 32 18 8
Fig.
8.8
Electronic configuration in various orbits
In an atom, the shells or orbits are named as K, L, M, N, O, P and Q according to the
increasing distance from the nucleus. 2n2 rule determines the maximum number of
electrons that are accommodated in different shells of an atom. In this rule, 'n' represents
the number of shells or orbits.
Orbit/shell No. of shell Maximum number of electrons
K n=1 2n2 = 2 × 12 = 2
L n=2 2n2 × 2 × 22 = 8
M n=3 2n2 = 2 × 32 = 18
N n=4 2n2 = 2 × 42 = 32
The outermost shell of an atom cannot accommodate more than 8 electrons. So 2n2 rule
is applicable for a few elements and shells K, L, M and N. The maximum number of
electrons that can be accommodated by shells O, P and Q is 32, 18 and 8 respectively.
146 GREEN Science (Chemistry) Book-9
Atomic Name of Symbol Number of Atomic mass Electronic Valency
number elements (p+ + n°) configuration
1. Hydrogen p+ n° e– K L MN
2. Helium
3. Lithium H 1 0 1 1+0=1 1 1
4. Beryllium
5. Boron He 2 2 2 2 + 2 = 4 2 0
6. Carbon
7. Nitrogen Li 3 4 3 3 + 4 = 7 21 1
8. Oxygen
9. Fluorine Be 4 5 4 4 + 5 = 9 22 2
10. Neon
11. Sodium B 5 6 5 5 + 6 = 11 23 3
12. Magnesium
13 Aluminium C 6 6 6 6 + 6 = 12 24 4
14. Silicon
15. Phosphorus N 7 7 7 7 + 7 = 14 25 3
16. Sulphur
17. Chlorine O 8 8 8 8 + 8 = 16 26 2
18. Argon
19. Potassium F 9 10 9 9 + 10 = 19 2 7 1
20. Calcium
Ne 10 10 10 10 + 10 = 20 2 8 0
Na 11 12 11 11 + 12 = 23 2 8 1 1
Mg 12 12 12 12 + 12 = 24 2 8 2 2
Al 13 14 13 13 + 14 = 27 2 8 3 3
Si 14 14 14 14 + 14 = 28 2 8 4 4
P 15 16 15 15 + 16 = 31 2 8 5 3, 5
S 16 16 16 16 + 16 = 32 2 8 6 2, 6
Cl 17 18 17 17 + 18 = 35 2 8 7 1
Ar 18 22 18 18 + 22 = 40 2 8 8 0
K 19 20 19 19 + 20 = 39 2 8 8 1 1
Ca 20 20 20 20 + 20 = 40 2 8 8 2 2
The atomic structure and electronic configuration of the first 20 elements is given below:
1. Hydrogen (H) 1pFig.
Atomic number = 1 0n
Atomic mass = 1
8.9
Shells KL MN Hydrogen
××
No. of e– 1 ×
GREEN Science (Chemistry) Book-9 147
2. Helium (He) 2p Fig.
Atomic number = 2 2nFig. Fig. Fig. Fig. Fig.
Atomic mass = 4 8.10
Helium
Shells KL MN
×× 3p
No. of e– 2 × 4n
3. Lithium (Li) 8.11
Atomic number = 3 Lithium
Atomic mass = 7
4p
Shells KL MN 5n
××
No. of e– 2 1 8.12
Beryllium
4. Beryllium (Be)
Atomic number = 4 5p
Atomic mass = 9 6n
Shells KL MN 8.13
×× Boron
No. of e– 2 2
6p
5. Boron (B) 6n
Atomic number = 5
Atomic mass = 11 8.14
Carbon
Shells KL MN
×× 7p
No. of e– 2 3 7n
6. Carbon (C) 8.15
Atomic number = 6 Nitrogen
Atomic mass = 12
Shells KL MN
××
No. of e– 2 4
7. Nitrogen (N)
Atomic number = 7
Atomic mass = 14
Shells KL MN
××
No. of e– 2 5
148 GREEN Science (Chemistry) Book-9
8. Oxygen (O)
Atomic number = 8
Atomic mass = 16
Shells KL MN 8p
×× 8n
No. of e– 2 6 8.16 Fig. Fig. Fig.
Oxygen
9. Fluorine (F)
Atomic number = 9 9p
Atomic mass = 19 10n
8.17
Shells KL MN Fluorine
××
No. of e– 2 7 10p
10n
10. Neon (Ne) 8.18
Atomic number = 10 Neon
Atomic mass = 20
11p
Shells KL MN 12n
××
No. of e– 2 8 8.19
Sodium
11. Sodium (Na)
Atomic number = 11 12p
Atomic mass = 23 12n
Shells KL MN 8.20
1× Magnesium
No. of e– 2 8 Fig. Fig.
GREEN Science (Chemistry) Book-9 149
12. Magnesium (Mg)
Atomic number = 12
Atomic mass = 24
Shells KL MN
2×
No. of e– 2 8
13. Aluminium (Al)
Atomic number = 13 13p
Atomic mass = 27 14n
Shells KL MN 8.21
3× Aluminium
No. of e– 2 8 Fig. Fig. Fig. Fig.
14p
14. Silicon (Si) 14n
Atomic number = 14 8.22
Atomic mass = 28 Silicon
Shells KL MN 15p
4× 16n
No. of e– 2 8
8.23
15. Phosphorus (P) Phosphorus
Atomic number = 15 16p
Atomic mass = 31 16n
Shells KL MN 8.24
5× Sulphur
No. of e– 2 8
17p
16. Sulphur (S) 18n
Atomic number = 16
Atomic mass = 32 8.25
Chlorine
Shells KL MN
6×
No. of e– 2 8
17. Chlorine (Cl)
Atomic number = 17
Atomic mass = 35
Shells KL MN Fig.
7×
No. of e– 2 8
150 GREEN Science (Chemistry) Book-9
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