motion. The material in which a wave is traveling
is called medium of the wave. Water of the pond is
the medium. The wave travels due to the repeated
vibrations of the particles of the medium. When a
piece of stone is thrown into the water, the energy
of the stone disturbs the water molecules close to
the stone. The stone transfers energy to the nearby
molecules of the water and the water molecules
start vibrating. The vibrating molecules transfer
some of the energy to the neighbouring molecules Fig. 6.1 Ripples produced in a pond
which also start vibrating and so on. In this way,
all the water molecules start vibrating and give rise to a water wave. So, a wave motion is a
vibratory disturbance travelling through a medium in which energy is carried from one place
to another. Wave motion transmits energy from one place to another by means of a periodic
disturbance between the two points. Water wave, sound wave, light wave are some examples
of waves. In a sound wave, the sound energy carried out by speech (vocal chord) disturbs the
air molecules. The air molecules start vibrating and carry energy from one place to another.
When the vibrating air molecules strike the ear drum, it also vibrates and stimulates the
sensory cells located in the inner ear. These sensory cells transmit sound impulses to the brain
through auditory nerves. As a result, we hear the sound.
6.3 Characteristics of Wave Motion
1. Wave motion is a periodic disturbance produced by a vibrating body.
2. In a wave motion, the particles of a medium do not travel from one place to another.
They only vibrate about a fixed position called mean position.
3. A wave motion travels with the same speed in all directions.
4. A wave transfers energy from one point to another but not the matter or particles.
5. The velocity of a wave depends only on the nature of the medium.
Activity 1
• Let us put some pieces of paper on the surface of still water. Drop a piece of stone into
the water. Observe the motion of the pieces of paper.
When the water waves reach the paper, the paper starts vibrating up and down with
the water surface. The paper remains where it was placed and is not carried away to
one side of the pond. It proves that water waves do not carry water from one place to
another but only energy.
6.4 Types of Wave
Some waves can travel only through a material medium while some can travel even through
vacuum. On the basis of requirement of medium, waves are classified into two types.
They are:
PHYSICS Oasis School Science - 9 93
1. Mechanical wave
2. Electromagnetic wave
1. Mechanical wave
The wave which requires material medium for its propagation is called mechan-
ical wave. This wave is also called elastic wave because it depends on the elastic
nature of the medium. Sound wave, water wave, etc. are mechanical waves as they
cannot travel through vacuum.
2. Electromagnetic ave
The wave which does not require any material medium for its propagation is called
electromagnetic wave. This wave can travel even through vacuum. Electromagnetic
wave produces electric and magnetic field when it travels. X-ray wave, light wave,
UV-ray wave, etc. are some examples of electromagnetic waves.
6.5 Classification of Wave
On the basis of the direction of vibrating particles of a medium, there are two types of
waves. They are:
1. Transverse wave
2. Longitudinal wave
1. Transverse wave
The wave in which the particles of a medium vibrate perpendicular to the direction of
propagation of the motion of the wave is called transverse wave, e.g. light wave, water
wave, wave on a string if one end is fixed, radio wave, etc. The water waves produced
on the surface of water in a pond when a stone is dropped in water are also transverse
waves. When the stretched wire of a sitar is plucked, transverse waves are produced in
the wire itself.
Direction of vibration of particles
P
AB
Direction of propagation of wave
Q
Fig. 6.2 Vibration of medium particle in a transverse wave
When a transverse wave travels horizontally in a medium, the particles of the medium
vibrate up and down in the vertical directions. When the vibrating particles move upward
above the line of zero disturbance, they form an elevation or hump which is called crest
94 Oasis School Science - 9 PHYSICS
and when the vibrating particles move downward below the line of zero disturbance,
they form a depression which is called a trough. So a transverse wave consists of crests
and troughs. The graphical representation of a transverse wave showing crests and
troughs is given below:
Displacement Wavelength Crest Crest
Crest
Trough Trough Direction of propagation
Wavelength
Fig. 6.3 Graphical representation of a transverse wave
Terminology
Amplitude : The maximum displacement of vibrating particle from its mean position
is called amplitude. Its SI unit is metre (m).
Crest : The elavation or the peak point of a transverse wave is called crest.
Trough : The depressed point of a transverse wave is called trough.
Wave length (λ) : The distance between any two consecutive crests or troughs is called
wavelength. It is measured in metre in SI system.
Frequency (f) : The number of complete cycles made in one second is called frequency. It
is measured in hertz (Hz).
Direction of wave motion
Direction of vibration
of rope particles
Fig. 6.4 A transverse wave on a string (rope)
amplitude /ˈæmplɪtjuːd/ - the maximum displacement of vibrating particle from its mean position
PHYSICS Oasis School Science - 9 95
2. Longitudinal wave
The wave in which particles of a medium vibrate along the direction of propagation of
the wave is called longitudinal wave, e.g. sound wave, wave in coiled spring when it is
pushed in or pulled out.
The waves which travel in a spring when it is pulled or pushed are called longitudinal
waves. Similarly, the sound waves in air are longitudinal waves. When a sound wave
passes through air, the particles of air vibrate back and forth parallel to the direction of
sound wave. Thus, when a sound wave travels in the horizontal direction, the particles of
the medium also vibrate back and forth in the horizontal direction. The waves produced
in air when a sitar wire is plucked are longitudinal waves. We can produce longitudinal
waves in solid, liquid and gas.
Particles vibrating along the direction of motion of wave
C R C R C Direction of propagation
Fig. 6.5 Particles of medium vibrating in a longitudinal wave
When a longitudinal wave travels in a medium, the particles of the medium vibrate
back and forth in the same direction in which the wave travels. When the vibrating
particles come closer to one another than they normally are, then a compression is
formed. Similarly, when the vibrating particles move further from one another than they
normally are, then a rarefaction is formed.
A compression is that part of a longitudinal wave in which the particles of the medium
are closer to one another than normal. Similarly, a rarefaction is that part of a longitudinal
wave in which the particles of the medium are farther apart than normal. A longitudinal
wave consists of alternately arranged compressions and rarefactions. The diagrammatic
representation of a longitudinal wave is given below:
Diagrammatic representation of longitudinal wave
Tuning Greater Less Greater Less Greater Less
fork density density density density density density
C = Compression
R = Rarefaction
Fig. 6.6 Longitudinal sound wave propagation in air
96 Oasis School Science - 9 PHYSICS
Activity 2
• Take a spring and fix its one end to the wall. Now, pull out another end of the spring
and release it. What do you observe? Can you see waves produced by the spring?
Observe compressions and rarefactions by repeating this activity.
Terminology Related to Longitudinal Wave
Amplitude : The maximum displacement of vibrating particle from its mean
position is called amplitude. Its SI unit is metre (m).
Compression : The region of the longitudinal wave in a medium having higher
density is called compression.
Rarefaction :
The region of the longitudinal wave in a medium having lower
density is called rarefaction.
Wave length (λ) : The distance between any two consecutive compressions or
rarefactions is called wavelength. It is measured in metre in SI system.
Frequency : The number of complete cycles made in one second is called frequency.
It is measured in hertz (Hz).
Differences between Transverse wave and Longitudinal wave
S.N. Transverse wave S.N. Longitudinal wave
1. The wave in which the particles of a 1. The wave in which particles of a
medium vibrate perpendicular to the medium vibrate along the direction
direction of propagation of the motion of propagation of the wave is called
of the wave is called transverse wave. longitudinal wave.
2. These waves can be produced only in 2. These waves can be produced in all
solids and surface of liquids (as they the three media, i.e. solid, liquid and
have rigidity). gas.
3. These waves have crests and troughs. 3. These waves have compressions and
rarefactions.
4. There is no pressure and density 4. There is pressure and density
variation. variation.
6.6 Sound
Sound is a form of energy which produces the sensation of hearing. Sound is produced due to
the vibration of a body. It requires material medium to travel. So, it cannot travel in vacuum.
Sound travels in the form of longitudinal waves. The object that produces sound is called the
source of sound, e.g. loudspeaker, temple bell, radio, etc.
Activity 3
• Make a list of different types of instruments which produce sound. Note the part of
the instrument that vibrates during the production of sound.
PHYSICS Oasis School Science - 9 97
6.7 Spectrum of Sound Wave
The group of various types of sound waves is called spectrum of sound wave. The frequency
of sound wave ranges from 1 Hz to 108Hz. The frequency of sound wave differs according to
the source of sound. The sound waves having the same speed may have different frequencies.
The sound of children and girls is shrill due to high frequency. But the sound of boys becomes
dull (hoarse) after getting maturity. The frequency of sound of adult men is about 6.5 KHz and
that of adult women is about 8.5 KHz. Human ear cannot hear all the sounds having different
frequencies. Human ear can hear the sound of frequencies 20 Hz to 200000 Hz.
On the basis of frequency, the sound is classified into three types. They are:
1. Infrasound 2. Audible sound 3. Ultrasound
1. Infrasound
The sound having a frequency less than 20 Hz is called infrasound. This sound is also
called sub-sound or infrasonic sound. It is produced during the earthquake and volcanic
eruptions. Similarly, simple pendulum and animals like whale, elephant also produce
infrasonic sound. Rhinoceros also produce infrasound of less than 6Hz. This sound
cannot be heard by human beings but can be felt by touching.
2. Audible sound
The sound having a frequency between 20 Hz and 20 kHz is called audible sound. This
sound is audible to human ear and hence is called audible sound. The range of frequency
from 20 Hz to 20 kHz is known as the frequency range of hearing in human beings.
The sound produced while talking, tuning radio, playing guitar, singing song, etc. are
some examples of audible sound. The frequency range of hearing in different animals is
shown below:
20–20,000 Hz 20–20,000 Hz 15–50,000 Hz 60–65,000 Hz 40–12,000 Hz
Fig. 6.7
3. Ultrasound
The sound having frequency more than 20 kHz is called
ultrasound. It is also called ultrasonic sound. This sound is
not heard by human ear. But some animals like whale, bat,
mouse, insects and dolphin can produce, detect and use
ultrasound. Similarly, cat and dog can also hear ultrasound.
The ultrasound produced by bat is reflected back by other Fig. 6.8
objects in their path, and the echo thus formed is detected
by bats. As a result, bat can fly at night without colliding
98 Oasis School Science - 9 PHYSICS
with other objects. Due to very high frequency, i.e. greater than 20000 Hz, ultrasound
has a greater penetrating power than ordinary sound. So, it can be used to detect objects
under the sea and organs inside human body. These days ultrasound is widely used for
various purposes. Some of them as mentioned below:
Uses of Ultrasound
1. It is used in SONAR to find the depth of oceans, seas, lakes, etc. and to locate
shoals of fish, shipwrecks, submarines, icebergs in sea, etc.
2. It is used to find tumours and foreign objects inside the human body.
3. It is used by some animals like dolphin, bat, etc. for locating their body.
4. It is used to kill bacteria and to detect the fault in metals or rocks.
5. It is used in the treatment of muscular pain and arthritis.
6. It is used by doctors to determine the sex of embryo in mother's womb and its
growth.
Activity 4
• Take a drum and beat it. Observe the surface of the drum. What do you find?
You will notice the vibrations of the surface of the drum. So, sound is produced by
the vibration of the body.
6.8 Speed of Sound in Different Media
Sound travels with different velocity in different media. Solids are more elastic in nature than
gases and liquids, i.e. Egas < Eliquid < Esolid. The velocity of sound is maximum in solids and minimum
in gases. This is because the molecules are packed closer in solids and liquids than in gas. Since
molecules carry the vibrations, they do so more effectively when they are close together.
One can press his ears against the railway track to find whether a train is approaching or not. This
is because we know that the speed of sound in solids is much more than that in gases. The sound
produced by the moving wheel of the train travels much faster along the steel track than through
the air. So, the sound is heard through the steel track before it is heard through the air.
The speed of sound (v) can be calculated by the given formula :
speed of sound (v) = frequency (f) wavelength (λ)
∴ v=f×λ
shoal /ʃəʊl/ - a large number of fish swimming together as a group
shipwreck /ˈʃɪprek/ - a ship that has been lost or destroyed at sea
- a disease that causes pain and swelling in joints
arthritis /ɑːˈθraɪtɪs/
PHYSICS
Oasis School Science - 9 99
Worked out Numerical 1
The speed of sound in a medium is 330 m/s and its wave length is 3.3 m. Calculate the frequency
of the sound.
Solution:
Speed of sound (V) = 330 m/s
Wave length (λ) = 3.3 m
Frequency (f) = ?
We know,
f = v = 330 = 100 Hz
λ 3.3
∴ The frequency of the sound is 100 Hz.
Speed of sound in some media
S.N. Medium Example Speed of sound
1. Gases Air (0 0C) 332 m/s
Air (20 0C) 344 m/s
2. Liquids Carbon dioxide 258 m/s
Hydrogen (0 0C) 1270 m/s
3. Solids Distilled water (200C) 1500 m/s
Alcohol (25 0C) 1210 m/s
Turpentine (25 0C) 1325 m/s
Sea water (25 0C) 1533 m/s
Aluminium (25 0C) 5100 m/s
Copper (25 0C) 3560 m/s
Glass (0 0C) 5000 m/s
Glass (25 0C) 5500 m/s
Steel (00 C) 5200 m/s
Iron (200C) 5130 m/s
Granite (00 C) 6000 m/s
6.9 Factors Affecting the Speed of Sound in Gases
The speed of sound in any gaseous medium is affected by a number of factors. These factors
are discussed below:
1. Density (d)
The velocity of a sound in a gas is inversely proportional to the square root of density of
the gas. For example, the density of oxygen is 16 times greater than that of hydrogen. So, the
velocity of sound in hydrogen is 4 times greater than the velocity of sound in oxygen, i.e.
100 Oasis School Science - 9 PHYSICS
Velocity of sound ∝ 1
density
2. Temperature (T)
The velocity of sound is directly proportional to the square root of its absolute
temperature, i.e.
Velocity of sound ∝ T
This is due to the reason that with an increase in temperature, the density will decrease
and the speed of sound increases.
3. Humidity
The amount of water vapour present in the air is called humidity. The density of the
gaseous medium changes due to the water vapour. The water vapour reduces the density
of air, i.e.
Density of dry air > Density of moist air
Therefore, velocity of sound in moist air is higher than that of the velocity of the sound
in dry air. That is why sound travels faster on a humid day than on a dry day.
4. Direction of motion of air
The velocity of sound is higher in the direction of motion of air than in the opposite
direction.
But the following factors do not affect the speed of sound in the gaseous medium.
1. Change in frequency
2. Change in wavelength
3. Change in factors like phase, loudness, pitch, etc.
4. Change in pressure
6.10 Application of reflection of sound
i) Reflection of sound can be used to find out the depth of lake, river, sea, etc.
ii) Reflection of sound is used to investigate brain tumour, kidney stone, etc.
iii) It is used to examine the development status of embryo in the mother's womb.
iv) It is used by geologists to investigate minerals.
v) It is used by security personnel to find out enemy's weapons.
vi) It is used in auditoriums to produce reverberation.
PHYSICS Oasis School Science - 9 101
6.11 Refraction of Sound
Like light rays, sound waves also refract while passing from one medium to another. The
speed of sound wave changes when it passes from one medium to another. As a result, re-
fraction of sound wave takes place. Air can form the layers having different densities due to
variation in temperature where refraction of sound occurs. The bending of sound waves while
passing from one medium to another is called refraction of sound waves. Sound waves bend
away from normal while passing from a denser to a rarer medium and towards the normal
while passing from a rarer medium to a denser medium.
Sound is heard clearer at night than during day time
During day time, the land surface gets heat-
ed due to solar radiation, the temperature of
land surface becomes maximum and it de-
creases gradually while moving upwards.
As a result, layers of air gradually behave
as denser medium while moving upwards.
Thus, the sound waves bend towards nor-
mal and diverge upwards from the source.
Therefore, less sound waves reach the listen-
er and sound is not heard clearly.
At night, the land surface gradually cools
down due to the absence of solar radiation
and the layers of air near the land surface
behave as denser layers than the upper
layers. So, the sound waves coming from the
source get refracted away from the normal Fig. 6.9
while moving upwards and finally these
waves suffer total internal reflection and
reach the listener on the surface of the land. As a result, sound is heard clearly at night than
during the day time.
102 Oasis School Science - 9 Fig. 6.10
PHYSICS
6.12 Reflection of Sound
When the sound traveling in a medium strikes on the other surface, it returns to the same
medium. This process is called reflection of sound. The bouncing back of sound when it strikes
a hard surface is called reflection of sound. Hard surfaces like walls, metal sheets, plywood,
etc. reflect sound waves. The laws of reflection of light are obeyed during the reflection of
sound. Sound can be reflected from any hard surface whether it is rough or smooth. The
reflection of sound is utilized in the working of megaphone, sound boards and ear trumpet.
The reflection of sound causes echoes.
Activity 5 1 2
• Take two plastic tubes. Take a mirror and a card- Fig. 6.9
board and arrange them as shown in the figure.
Now, keep a watch at the outer end of tube 1.
Keep tube 1 constant with watch and move the
second tube in different angles. Find the angle
between the cardboard and these tubes. What
can you conclude from this activity?
6.13 Echo
If we stand near a cliff or at one corner of a big empty hall and shout the word "HELLO!" we will
hear the word "HELLO!" coming from the cliff or empty hall in the form of reflected sound. This
sound is called echo. It is heard after reflection of the sound from a cliff or wall or any other hard
surface. So, the repetition of sound caused by the reflection of sound waves is called echo. The
minimum distance from a sound reflecting surface to hear an echo is 17.2 metres.
Conditions for formation of echo
i) The minimum distance between the source of sound and the reflector should be at
least 17.2 m.
ii) The size of the reflector must be large.
iii) The intensity or loudness of the sound should be sufficient.
Calculation of Minimum Distance to Hear an Echo
Scientists have found that the human ear can hear two sounds separately only if there is a
1
time interval of 10 th of the a second (or more) between the two sounds. It means that we can
hear the echo separately only if there is a time gap of at least 1 th of
the original sound and 10
a second between them. Knowing the minimum time-gap required for an echo to be heard
and the speed of sound in air, we can calculate the minimum distance from a sound reflecting
surface which is required to hear an echo as follows:
PHYSICS Oasis School Science - 9 103
Solution: Speed of sound in air = 344 m/s
Time taken = 1 S = 0.1 s
10
Distance travelled = ?
We know, Speed = Distance travelled
Time taken
or 344 = Distance travelled
0.1
or, Distance travelled
= 344 × 0.1 = 34.4 m
∴ The distance from the sound reflecting surface to hear an echo should be half of 34.4m
which is 34.4 = 17.2 m. From this, we can conclude that the minimum distance from a sound
2
reflecting surface is 17.2m to hear an echo when the sound travels in air. However, when the
sound travels in water or any other solid, the minimum distance for hearing the echo will be
different because the speed of sound in water and other solids is different.
6.14 Reverberation
The intermixing of original sound with a reflecting sound is called reverberation. In other
words, the prolongation of the original sound is called reverberation. This phenomenon occurs
when the distance between the source of sound and the reflecting surface is less than17.2 m.
A number of echoes of original sound are heard during a reverberation. Thunder that follows
lightning flash is an example of reverberation. Reverberation is heard in a newly built hall
or an empty room but not in a furnished or occupied room because the materials kept in a
furnished room absorb the sound.
Difference between Echo and Reverberation
S.N. Echo S.N. Reverberation
1 The repetition of the sound is called 1. The prolongation of sound is called
echo. reverberation.
2. The distance for an echo to take 2. The distance for reverberation to take
place is 17.2m or greater. place is less than 17.2 m.
6.15 Echolocation
Echolocation is the process of locating an object with respect to the surroundings. Different
animals like whales, dolphins, bats and some birds use echo to locate their body, to find their
prey and to navigate without the use of eyes.
reverberation /rɪˌvɜːbəˈreɪʃn/ - the prolongation of the original sound
thunder /ˈθʌndə/ - a loud noise that you hear after a flash of lightning
104 Oasis School Science - 9 PHYSICS
Fig. 6.10 (a) Measuring depth of sea by (b) Bats and dolphins make high pitched sounds while flying
using SONAR or moving which bounce off objects in the form of echoes
The depth of the ocean can also be calculated with the help of this method. The method of
finding the depth or position with respect to the surrounding objects is called Sound Navigation
and Ranging (SONAR). The device which is used to find the depth is called Fathometer. It has
a high frequency vibrator, a source and reciever or a detector. Ultrasound is sent towards the
bottom of the sea and the reflected sound waves are obtained by the reciever. The time taken
by those waves to reach the receiver is recorded and the depth of the sea can be calculated by
the following formula:
Depth (s) = Speed of sound in the given medium × Time
2
∴ s = v×t
2
Worked out Numerical 2
Calculate the depth of the sea if the echo is heard after 10 seconds. The speed of sound in
water is 1500 m/s.
Solution:
Speed of sound in water (v) = 1500 m/s Remember
Time taken (t) = 10 s Physical quantity SI unit
Depth (s) =? Speed m/s
We have, Time s
s = v×t Depth m
2
v×t
= 1500×10 s= 2
2
= 7500 m.
∴ The depth of the sea is 7500 m.
PHYSICS Oasis School Science - 9 105
6.16 Characteristics of Sound
All sounds which produce the sensation of hearing may be divided into two categories, i.e. (i)
musical sound and (ii) noise. However, the distinction between a musical sound and a noise is
subjective, i.e. a sound which is musical for someone may be a noise to others.
i) Musical sound
A continuous and uniform sound produced by regular and periodic vibrations that
produces pleasing effect on our ears and mind is called musical sound, e.g. the sound
produced by piano, violin, flute, harmonium, etc. The graphical representation of a
musical sound is given below:
Amplitude
Time
ii) Noise Fig. 6.11 Graphical representation of a musical sound
A discontinuous and non-uniform sound produced by irregular and non-periodic
disturbances that produce unpleasant effect on our ears and mind is called noise, e.g. the
sound produced by horns of vehicles, engines in factories, a falling brick, the sound of a
crowd, etc. The graphical representation of a noise is given below:
Amplitude
Time
Fig. 6.12 Graphical representation of noise
Differences between Musical sound and Noise
S.N. Musical sound S.N. Noise
1. It produces pleasing effect on ears 1. It produces unpleasant effect on ears
and mind. and mind.
2. It produces regular wave form. 2. It produces irregular wave form.
3. The frequency of musical sound is 3. The frequency of noise is low.
high.
4. There is no sudden change in 4. There is a sudden change in amplitude.
amplitude.
106 Oasis School Science - 9 PHYSICS
Characteristics of musical sound
A musical sound has three basic characteristics, viz. pitch, loudness and quality.
i) Pitch: The shrillness of a sound is called pitch. It enables us to differentiate between
two sounds with equal loudness coming from different sources and having different
frequencies. If the pitch is high, the sound is said to be shrill and if the pitch is low,
the sound is flat (hoarse). Pitch is a sensation which depends upon the wavelength
or frequency. The pitch has subjective existence while the frequency has an objective
existence. The pitch is purely a physical sensation while the frequency is a measurable
quantity. The voice of a woman has a higher pitch than that of a man because the
frequency of a woman's voice is greater than that of a man's voice. So, the woman's
voice is shriller than the man's voice. Pitch of a sound depends on the frequency or
wavelength. Thus, the higher the frequency of a sound, the higher is its pitch and vice-
versa. The thinner strings of a guitar give out sound of higher pitch than the thicker
strings because the thin strings vibrate with higher frequency than the thick strings.
ii) Loudness: Loudness is the property by virtue of which a loud sound can be
distinguished from a faint one, both having the same pitch. Loudness depends on the
intensity or the amplitude of the wave. The intensity is a measurable quantity while
loudness is a sensation. The intensity at a point is defined as the amount of vibrational
energy associated per unit area per unit time. Its SI unit is watt per square metre (W/m2).
It can be calculated by the following formula.
Sound energy
Intensity of sound (I) = Time × area
90 dB 84 dB 78 dB
Source of sound 2m 4m
1m Fig. 6.13
a) Factors Affecting the Loudness of Sound
i) Amplitude of the vibration of the source: The loudness or intensity of sound
is directly proportional to the square of the amplitude of the sound waves at a
given point, i.e.
∴ Intensity ∝ (amplitude)2
PHYSICS Oasis School Science - 9 107
ii) Distance from the vibrating body: Loudness decreases with an increase in the
distance between the source and the listener. The loudness of sound is inversely
proportional to the square of the distance, i.e. loudness ∝ 1 .
(distance)2
iii) Surface area of the vibrating body: The greater the surface area of the vibrating
body, the greater is the loudness of the sound. It means that loudness increases
with an increase in the area of the vibrating body, e.g. a larger drum produces
louder sound than a smaller one.
iv) Density of the medium: The loudness of sound is directly proportional to the
density of the medium, i.e. the greater the density of the medium, the louder is
the sound.
v) Motion of the medium: If wind is blowing in the direction of propagation of
sound, loudness is increased and vice-versa.
vi) Presence of resonant bodies: The loudness of sound increases due to the presence
of sound reflectors and decreases due to the presence of sound absorbers.
b) Measurement of loudness of sound
The loudness of sound is measured in decibels (dB). The zero point on the decibel scale
is the lowest sound that can be heard by the human ear. It is also called the threshold
of hearing. The loudness of a whisper is 20 dB. The sound of conversation, i.e. 60 dB is
about 10,000 times louder than the whisper. The sound wave of loudness more than 120
dB damages human ears. If the sound wave of loudness 40 dB to 65 dB is not heard by
human ear it is called hearing impairment and if the sound wave of 80 dB is not heard by
human ear, it is called deafness.The sound levels of different sounds is tabulated below:
Sounds Sound level in dB
1. Threshold of hearing 0
2. Rustling of leaves 0-20
3. Whisper 0-20
4. Normal conversation 40-60
5. Moving vehicles 60-80
6. Factory noise 80-90
7. Loud music in disco 130
8. Jet aircraft 140
9. Noise of railway station 85-110
10. Sound of printing press 70-80
11. Sound of motor car 110-120
12. Heavy street traffic 60-70
13. Environment in library 20-40
14. Listening limit of hurt 120-140
15. Sound produced due to mechanical failure 140-160
108 Oasis School Science - 9 PHYSICS
iii) Quality
The quality is the property by which two sounds of the same pitch and possibly of the
same intensity given by two different instruments can be distinguished from each other.
It is also called timbre.
The quality of musical sound depends on the waveform of the sound. The waves
produced by different instruments differ in their waveforms.
6.17 Noise Pollution
Unwanted harsh and loud sound from any source that causes discomfort of any kind is called
noise pollution. It is undesirable and can cause irritability, loss of concentration, stress, sleep
disturbance and can even damage hearing.
Effects of noise pollution
i) Noise pollution may result in the loss of hearing to deafness.
ii) It reduces concentration, increases stress and causes headache.
iii) It increases blood pressure and nervous tension.
Ways to reduce noise pollution
To minimize noise pollution, we need a certain degree of discipline on the part of all of us. The
following are the measures one should adopt to keep noise pollution under control:
i) The use of loud speakers at functions, programmes, etc. should be avoided or stopped.
ii) People should not talk too loudly or tune the music too loudly.
iii) People working in factories, etc. where they are subjected to constant loud noise of
machinery should take special precaution to protect their ears.
iv) Traffic noise could be reduced to a great extent by instilling traffic discipline among bus
and automobile drivers. Pressure horns should not be used in automobiles.
v) Explosion of fire crackers should be avoided as far as possible in different festivals.
timber /ˈtɪmbə/ - the quality of sound that is produced by a particular voice or musical instrument
instill /ɪnˈstɪl/ - to gradually make sb feel, think or behave in a particular way over a period of time
PHYSICS Oasis School Science - 9 109
SUMMARY
• Sound is a form of energy which produces the sensation of hearing.
• The disturbance which carries energy from one place to another place is
called wave.
• The wave which requires material medium for its propagation is called
mechanical wave.
• The wave which does not require material medium for its propagation is
called electromagnetic wave.
• The wave in which the particles of a medium vibrate perpendicular to the
direction of propagation of the motion of the wave is called transverse wave.
• The wave in which particles of a medium vibrate along the direction of
propagation of the wave is called longitudinal wave.
• The sound having the frequency of less than 20 Hz is called infrasound.
• The sound having the frequency between 20 Hz and 20 KHz is called
audible sound.
• The sound having frequency more than 20 KHz is called ultrasound.
• When sound traveling in a medium strikes the other surface, it returns to
the same surface. This process is called reflection of sound.
• Loudness, intensity and amplitude determine the characteristics of a
sound.
• Unwanted sound from any source that causes discomfort of any kind is
called noise pollution.
110 Oasis School Science - 9 PHYSICS
Exercise
Group-A
1. What is sound? How is it produced?
2. What is source of sound?
3. What is a wave? How is sound transmitted?
4. Define mechanical wave.
5. What type of wave is called transverse wave?
6. Define longitudinal wave.
7. What is frequency? Write down its SI unit.
8. What is the relation between speed of sound, frequency and wavelength?
9. What is sound spectrum?
10. Define infrasound and ultrasound.
11. What is fathometer? Name any three animals which can hear ultrasound.
12. What is echolocation?
13. What is the frequency of the sound of a man and woman?
14. What is audible sound?
15. What is pitch of sound? Write.
16. What is the relation between thickness of wire and pitch of the sound?
17. What is amplitude?
18. What is intensity of sound? In which unit is it measured?
19. Define reflection and refraction of sound?
20. What is echo? In which condition can we hear echo?
21. What is reverberation?
22. What is sound pollution?
23. Write down the intensity of the sound that makes us deaf and can rupture the ear drum.
24. What should be the distance between the source of sound and reflecting surface to
produce an echo?
25. Which animal can produce and hear ultrasound?
26. What is noise?
27. What is SONAR?
PHYSICS Oasis School Science - 9 111
Group-B
1. Differentiate between transverse wave and longitudinal wave in any points.
2. Write any two differences between audible sound and infrasound.
3. Ceiling and walls of an auditorium hall are fitted with sound absorbing materials, why?
4. The speed of sound in solids is more than that in liquids. Give reason.
5. The sound is heard more clearly at night than in the day time, why?
6. The sound of a girl is different from that of a boy. Give reason.
7. Differentiate between infrasound and ultrasound in any two points.
8. The sound of girls is shapr but the sound of boys is hoarse, why?
9. Echo is not heard in small rooms, why?
10. It is harmful to settle near the airport. Justify the statement.
Group-C
1. Write any three applications of reflection of sound.
2. Describe in brief the effect of sound in human life.
3. Write any three effects of noise pollution.
4. Write any three ways to reduce noise pollution.
5. What is the relation between thickness of wire and pitch of the sound? Explain.
6. If you are kept blindfolded in a room, how can you find out whether the room is
occupied or blank by producing sound?
7. Write down the utility of SONAR. How is the depth of sea measured by using sound?
Write in brief.
Group-D
1. What is longitudinal wave? Write with a figure. Differentiate between fathometer and
sonometer.
2. 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? [Ans: 77.41 Hz]
3. 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. [Ans: 1500 m/s]
4. 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 1498 m/s. Differentiate between
audible sound and infrasound. [Ans: 4494 m]
5. A man claps and the echo produced from the wall of a tall building, reaches our ears
after 0.24 seconds. If the speed of sound in air is 333.3 m/s, calculate the distance between
man and the wall. Differentiate between infrasound and ultrasound. [Ans: 39.99 m]
112 Oasis School Science - 9 PHYSICS
7UNIT Estimated teaching periods
Theory 10
Practical 2
CURRENT ELECTRICITY
AND MAGNETISM Andre Ampere
Objectives
After completing the study of this unit, students will be able to:
• describe units of current, voltage, power and resistance with their
measurement.
• explain Ohm's law and establish the relation among V, R and I.
• compare and measure the conductivity of different substances.
• demonstrate and describe magnetic field and magnetic lines of force.
• describe the elements of earth's magnetism (angle of dip and angle of
declination).
A. Current Electricity
7.1 Introduction
In the present day world, electricity is the most important source of energy. It is used in
industries, offices, homes and many other places. Electricity is used for obtaining light, heat,
operating computers, fans, machines and many other equipment. Every atom contains two
types of charged particles called protons and electrons. When electrons flow continuously
through a conductor, current is produced. So, the electricity produced due to the continuous
flow of electrons through a conductor is called current electricity.
7.2 Source of Electricity
The objects which can produce electric current are called sources of electricity. Cell, battery,
dynamo, generator, etc. are the sources of electricity. Electric current is of two types, viz. direct
current (DC) and alternating current (AC). We obtain direct current from the cell and battery.
Similarly, we obtain alternating current from dynamo and generator.
electricity /ˌɪlekˈtrɪsəti/ - the form of energy produced from charged particles or flow of electrons through a conductor
PHYSICS Oasis School Science - 9 113
a. Cell Positive terminal
Cell is one of the sources of electricity which produces direct current.
Specially, the dry cell contains carbon rod and zinc container which
act as positive and negative terminal respectively. Cells are cheap and
portable. They are used in watches, cameras, radios, walkman, torch- Negative terminal
light, etc. A group of cells is called a battery.
b. Hydroelectricity Fig. 7. 1 cell
It is another source of electricity in which a big turbine is rotated by the water stored
in a dam. The current is induced in the coil when the magnet attached to the turbine is
rotated. The current produced in the coil is alternating current since the polarity of the
current changes alternately. It is a type of renewable source of energy. Now-a-days, the
development and use of hydroelectricity is spreading throughout the world.
7.3 Electric Circuit
The essential components for the flow of current electricity are source, load and conducting
wires. The path made by connecting a load, a switch and a source with a good conducting
wire in which electric current can flow is called electric circuit. The electric equipment that
works with the help of current electricity is called electric load. Electric bell, radio, fan, etc. are
some examples of electric load. There are two types of electric circuits. They are :
i. Open electric circuit ii. Closed electric circuit
i. Open electric circuit
The electric circuit in which the electric current is not flowing is called open electric circuit.
In such a circuit, current does not flow when the switch is turned off or due to the breakage
of any other part of the electric circuit. So, the electric load does not work in an open circuit.
Battery Battery Break in the
circuit
Closed
Switch
Open
Switch
Electric load Electric load
(a) Fig. 7.2 Open electric circuit (b)
ii. Closed electric circuit Battery Closed Switch
The electric circuit in which electric current flows Electric load
continuously is called closed electric circuit. In such
a circuit, current is flowing and the switch is on. So, Fig. 7.3 Closed electric circuit
an electric load is in working condition.
In our homes, schools and offices, electric circuits
are used to glow the light bulb and to operate other
electrical appliances. They are closed (on) or opened
(off) with the help of switches fixed on switch boards
in every room.
114 Oasis School Science - 9 PHYSICS
7.4 Direction of Flow of Current
In the beginning, physicists had no idea about the kind of charges which flow through a
circuit. They assumed the direction of flow of positive charge as the direction of current which
is called the direction of conventional current. About 100 years later, when the structure of
an atom became clearly known it was, however, found that the flow of negatively charged
electrons constitutes the electric current and not the flow of positively charged particles.
But still physicists prefer not to change their earlier assumption as various laws in electricity
and magnetism follow conventional direction of current, i.e. flow of positive charge.
Flow of electrons Flow of current
Flow of electrons
Copper wire
Conventional
direction
Fig. 7.4 Conventional direction of flow of current
Electric current is a scalar quantity. Why?
Current is the rate of flow of charge. In any closed circuit, the direction of current is known
which may be clockwise or anticlockwise. Is the current really a vector quantity? No, it is
not a vector quantity as it simply represents the rate of flow of charge and charge itself is
not a vector quantity. We can always turn or twist a wire carrying the current (as normally it
is done in homes while using an extension cord) without affecting the working of electrical
appliances. Moreover, the laws of vector addition do not hold good for the current flowing
in two branches. Hence, currents are added like scalars.
7.5 Conductors and Insulators
Those substances which conduct electric current through them are called conductors. All
metals are conductors of current electricity, e.g. copper, silver, gold, iron, etc. Metals conduct
electricity due to the presence of free electrons in them. Those electrons which are free to move
in the entire volume of the body of a metal and do not revolve round the nucleus of atom are
called free electrons.
Those substances which do not conduct electric current through them are called insulators.
Most of the non-metals are insulators except graphite, e.g. rubber, plastic, dry wood, glass,
leather, pure water, etc.
conventional /kənˈvenʃ(ə)n(ə)l/ - following what is traditional
PHYSICS Oasis School Science - 9 115
7.6 Electric Current Physical Quantity SI unit
Charge coulomb (C)
The rate of flow of charge in a circuit is called electric Time second (S)
current. The SI unit of current is ampere (A). If 'Q' is
the amount of charge flowing through a conductor Current ampere (A)
in time 't', current (I) is given by
Charge (Q)
Current (I) = Time (t)
Q
∴ I = t
In SI system, electric current is measured in ampere (A).
One ampere current
The current is said to be one ampere if 1 coulomb of charge flows through a conductor in 1
second. There are 6 × 1018 electrons in 1 coulomb charge. Each electron contains the charge
1.6 × 10– 19C.
Symbols used in an electric circuit
S.N. Devices Symbols
1. Cell
2. Battery
3. Switch Open (off) G
A
Closed (on) V
4. Resistance
5. Rheostat (variable resistance)
6. Bulb
7. Galvanometer
8. Ammeter
9. Voltmeter
10. Wires Straight
Cross
Joint
resistance /rɪˈzɪst(ə)ns/ - the property of a conductor by virtue of which it opposes the flow of electric current
coulomb /ˈkuːlɒm/ - the SI unit for measuring electric charge
116 Oasis School Science - 9 PHYSICS
7.7 Ammeter
The device which is used to measure the total current
flowing through an electric circuit is called ammeter. It
helps to measure the current in ampere or milliampere
or micro ampere depending on the calibration on the
measuring device.
Ammeter has low resistance in it. So there is no change in Fig. 7.5 Ammeter
the total resistance as a whole. Due to this reason, ammeter
is connected in series in an electric circuit.
The following points are to be remembered when ammeter
is connected to the electric circuit:
i. The positive terminal of an ammeter should be connected to the positive terminal of a cell.
ii. Ammeter should always be connected in a series in an electric circuit.
7.8 Electromotive force (emf) and Potential difference (pd)
The valence electrons of metal are free to move. So they are called free electrons. Current
can flow due to the presence of free electrons. But it is to be noted that free electrons move
randomly. They cannot flow from one point of a conductor to another along its length unless
an external source of energy is used. A cell or battery connected across the wire provides
necessary force for electrons to move them in a particular direction and while moving electrons
some work is done.
Free electron atom
+
Random motion of free electrons Motion of free electrons in the same
when no source is applied direction when source is applied
Fig 7.6 Condition of free electrons in a conductor
The amount of energy supplied to carry 1 coulomb charge AB
from one terminal of a source to another terminal of the
same source through the external circuit is called
electromotive force (emf). The emf creates the potential
difference across the end of wires A and B as shown in the
figure alongside.
Due to the creation of the potential difference, an electric Fig. 7.7
current will flow from positive pole of a source (cell or
battery) to the negative pole. It is supposed that positive pole
has higher potential and negative pole has lower potential. So, current always flows from
higher potential to the lower potential. Thus, potential difference between two points can
be defined as the amount of work done in bringing a unit positive charge from one point to
PHYSICS Oasis School Science - 9 117
another of a conductor. The SI unit of potential difference is volt (V).
Thus, potential difference between A and B is given by
VAB = W = Work done
Q Charge moved from A to B
One volt potential difference
The potential difference between two points equals to 1 volt if the work done in moving 1
coulomb of charge from one point to another is 1 joule.
∴ 1 volt = 1 joule
1 coulomb
Differences between emf and p.d.
emf p.d.
1. The amount of enery supplied to carry one 1. The amount of work done in bringing one
coulomb charge throughout the whole coulomb charge from one point to another
circuit is emf. point is p.d.
2. It is always more than p.d. 2. It is less than emf.
3. It is measured in open circuit. 3. It is measured in closed circuit.
4. It is cause of p.d. 4. It is effect of emf.
7.9 Voltmeter
Voltmeter is a device which is used to measure the potential
difference between any two points of an electric circuit or source.
A voltmeter is always connected parallel to a device. It has high
resistance. When the voltmeter is connected in a series, the effective
resistance will be increased. Due to this reason, voltmeter is always
connected parallel to the device or source to find the difference of
the potential between any two points.
Fig. 7.8 Voltmeter
Reasonable fact-1
Write any two differences between Ammeter and Voltmeter.
1. Ammeter is used to measure the mag- 1. Voltmeter is used to measure the po-
nitude of electric current flowing tential difference.
through a circuit.
It is always connected in parallel with
2. It is always connected in series with 2. a resistance.
the circuit.
118 Oasis School Science - 9 PHYSICS
Reasonable fact-2
Ammeter is connected in the series while a voltmeter is connected in parallel.
Give reason.
Ammeter is a device used to measure the electric current. So, total current flowing through
the circuit must pass through the ammeter. Therefore, it is connected in series. While
voltmeter is a device used to measure the potential difference between two ends. Therefore,
voltmeter is connected in parallel to measure the potential difference across two ends.
7.10 Resistance
When electrons flow from one part of a conductor to another, they collide with other electrons
present in the conductor. Due to the collision, there is some obstruction in the flow of electric
current through the conductor. So, the property of a conductor by virtue of which it opposes
the flow of electric current through it is called resistance. The SI unit of resistance is ohm (Ω).
(a) (b)
Fig. 7.9
In figure (a) a single bulb is connected to the cell. When the switch is turned on, the bulb
glows with certain brightness (depending on its power). In figure (b), two bulbs are connected
in series. When the switch is turned on, both the bulbs do not glow as much as they were
glowing individually. This is because filaments of the bulbs offer some resistance to the flow
of current. As the bulbs are connected in series, the resistance offered has increased and the
current passing through each bulb decreases resulting in reduction in brightness of bulbs.
Reasonable fact-3
The filament of the bulb glows bright when current flows through it but other
wires connected to the bulb do not glow. Why?
The filament of the bulb is made up of the material having high resistance and high melting
point. So, it converts the electrical energy into heat and light energy. But the wire connected
with filament has low resistance, so, it does not glow.
Combination of Resistance
There are two types of combination of resistances:
a. Series combination of resistances
b. Parallel combination of resistances
a. Series combination of resistances
The combination of resistances is said to be series combination if one resistance is con-
nected after another one as shown in the following figure. In such a combination, the
same current flows through all the resistances.
PHYSICS Oasis School Science - 9 119
+–
R1 R2 R3
Fig. 7.10 Series combination of resistances
Let us suppose that three resistances R1, R2 and R3 are connected in series. Then the total
resistance is equal to the sum of individual resistance.
Total resistance (R) = Sum of individual resistance, i.e.
R = R1 + R2 + R3
Main features of series combination of resistances
1. In a series combination, total resistance increases with the increase in the number
of resistances.
2. The current in series resistance decreases with the increase in the number of
resistances.
3. In the series combination, if one bulb gets fused, another bulb stops glowing as the
circuit is opened.
Activity 1
• Connect two dry cells in a series with one electric bulb. Observe the brightness
of the bulb. Similarly, connect two electric bulbs in a series. Observe the
brightness. What do you notice? What can you conclude from this activity?
b. Parallel combination of resistances R1
R2
Two or more than two resistances are said to be R3
connected in parallel if they are individually connected
to the two terminals of the source (cell or battery). In V
such a combination, the potential difference across each Fig. 7.11 Parallel
resistance remains the same. So, the current flowing combination of resistances
through each resistance also remains the same.
When the resistance R1, R2 and R3 are connected in
parallel, the total resistance is equal to the sum of
reciprocal of individual resistances.
Thus, 1 = 1 1+ 1
R R1+ R2 R3
Let us take the example of two electric bulbs in parallel as shown in the figure.
These bulbs are said to be in parallel combination since each bulb is individually
connected to the terminals A and B of the cell. The current from the cell gets divided
120 Oasis School Science - 9 PHYSICS
into two parts. Some current flows through bulb B1 and the rest flows through B2. On
switching, both bulbs light up, each having the source of brightness as before. Thus, the
combined brightness is equal to the sum of individual brightness of the bulbs.
B– + A
B1
B2
Fig. 7.12
The combined brightness of the bulbs increases due to the increase in current which
shows that the resistance for the flow of current has decreased. Thus, the combined
resistance in parallel combination is less than that of either series resistance.
Main features of parallel combination of resistances
1. In parallel combination, the total resistance decreases with an increase in the number of
resistances.
2. The current flowing in any resistance in parallel will be inversely proportional to the
resistance.
3. In parallel combination, if one bulb gets fused other bulbs still continue to glow as there
is unbroken circuit through the other bulb.
In domestic electric circuits, all appliances are connected in parallel so that if one
appliance goes off due to any reason, the others may still keep on working.
7.11 Ohm's law
On the basis of his experiments, a German physicist Georg Simon Ohm, in 1826 AD, established
a relationship between potential difference (V) and electric current (I) in an electric circuit.
This relationship is known as Ohm's law.
Ohm's law states that, "The electric current (I) flowing through a conductor is directly
proportional to the potential difference (V) across its ends, provided that the physical
conditions like temperature, pressure, etc. do not change, i.e.
I ∝ V
Or, V ∝ I
Or, V = IR [where 'R' is a constant called resistance of the conductor.]
V
The above equation can also be written as R = I . From the above relation, it can be concluded
that the ratio of potential difference across the ends of a conductor (V) and the current (I)
PHYSICS Oasis School Science - 9 121
flowing through it is a constant quantity called resistance (R).
The resistance is said to be one ohm (1Ω) if one ampere (1A) current is flowing with the applied
voltage of one volt. (1V).
Worked out Numerical 1
Calculate the resistance of a conductor if 4A current flows through it when it is connected
to 6V battery.
Solution:
Voltage (V) = 6 V Remember
Current (I) = 4 A Physical quantity SI unit
Resistance (R) = ? Voltage (p.d.) V
We have, Current A
V
Resistance Ω
R = I
6 R= V
I
= 4
= 1.5 Ω
∴ The resistance of the conductor is 1.5 Ω.
Worked out Numerical 2
Calculate the emf required to flow 3 A current through an electric bulb of resistance 5 Ω.
Solution:
Current (I) = 3 A
Resistance (R) = 5Ω
Voltage(V) = ?
We have, V = IR
= 3×5
= 15 V
∴ The required emf is 15V.
122 Oasis School Science - 9 PHYSICS
7.12 Factors Affecting the Resistance of a Conductor
The resistance of a conductor or a wire depends on various factors which are discussed below:
i. Length of the wire
It has been found by an experiment that on increasing the length of a wire, its
resistance increases and on decreasing the length of the wire its resistance
decreases.
Resistance of a conductor increases with the increase in the length of the conductor.
So, resistance of a conductor is directly proportional to its length, i.e.
Resistance (R) ∝ length (l) of the wire
∴ R∝I Short wire Long wire
Low resistance High resistance
It means that if the length of the wire is doubled, the resistance is also doubled and
if the wire is halved, the resistance caused by it is also halved.
ii. Area of cross-section of wire (A)
The resistance of a wire is inversely proportional to its area of cross-section.
i.e. 1 where 'R' is the resistance and 'A' is the area of cross-section of
R∝A
the wire.
Small cross-section Large cross-section
High resistance Less resistance
It means that if the area of cross-section of a conductor is doubled, the resistance is
halved. Similarly, if the area of cross-section of a conductor is halved, the resistance
gets doubled.
iii. Temperature
The resistance of a conductor is directly proportional to the temperature of the
wire or conductor, i.e.
R ∝ T where, 'R' is the resistance and 'T' is the temperature of the conductor.
iv. Nature of the materials in circuit
Some materials have low resistance whereas some others have much higher
resistance. Metals like copper, silver, aluminum, etc. have very low resistance. But
nichrome, tungesten, constantan, etc. have high resistance. That is why nichrome
is used as a heating element of the electric heater, toaster, etc. and copper,
aluminium, etc. are used to make conducting wires.
v. Shape of the wire in circuit
The resistance of the coiled wire is more than that of the straight wire.
PHYSICS Oasis School Science - 9 123
7.13 Resistivity and Conductivity
We know that the resistance of a conductor is directly proportional to its length and
inversely proportional to its thickness.
Let, 'R' be the resistance, 'l' and 'A' be the length and the thickness of the wire respectively.
Then,
R ∝ l …………………… (i)
1
R ∝ A …………………… (ii)
Combining (i) and (ii)
l
R ∝ A
l
R= A ρ
Where ρ is called resistivity of the material of a conductor.
Resistivity is defined as the resistance of a unit length (1m) and unit cross-sectional area
(1 m²).
l
Since R = A ρ
Taking A = l m2, l = 1m, we get,
1
∴ R = 1 ρ
R = ρ
SI unit of resistivity: Ohm – meter (Ω m).
The reciprocal of resistivity is called conductivity. It gives the idea whether a material is
good conductor or bad conductor of electricity, i.e.
1
Conductivity = resistivity
124 Oasis School Science - 9 PHYSICS
B. Magnetism
7.14 Introduction
In the early days, the Greeks noticed that a naturally occurring substance called lodestone
had the property of attracting iron pieces. 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 is a substance that attracts magnetic substances like iron, nickel, etc. and rests in
north-south direction when suspended freely. 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.
Horse shoe-shaped magnet Bar magnet U-shaped magnet
Fig. 7.13
When two magnets are kept together, they exert forces on each other. In fact, a magnet creates
a magnetic field around it and when another magnet is placed in this field, the field exerts a
force on it.
7.15 Magnetic Compass
A magnetic compass is a simple device which has a magnetic needle Fig. 7.14 Magnetic Compass
which is free to rotate on a pivot at the centre of a round box. In a
compass, the needle which is free to rotate always rests in the north-
south direction.
Uses
• Magnetic compass is used to find the direction of a magnetic field.
• It is used to test the polarity of a magnet.
• It is used to find the magnetic north-south direction.
magnetism / ˈmaɡnɪtɪzəm/ - the phenomenon related to attractive and directive properties of a magnet
PHYSICS Oasis School Science - 9 125
If a magnet is taken close to the compass needle, the needle deflects. This is because the magnet
produces a magnetic field and this field exerts a force on the needle to deflect it. If the magnet
is strong, the direction of the needle from its south-north pole changes to the direction of the
magnetic field produced by the magnet.
Activity 2
• Take a bar magnet and suspend it freely as shown N S
in the figure or float it on a block of wood in Fig. 7.15
water. The magnet comes to rest after some time.
Disturb it a little from its position of rest and see
what happens. You will find that the magnet
always comes to rest in the north-south direction.
7.16 Process of Locating the Pole of a Magnet
The imaginary line passing through the poles of a magnet is called magnetic axis. The imaginary
line bisecting the effective length is called magnetic equator. The distance between the two
poles of a magnet is called its effective length or magnetic length. The distance between the
two ends of a magnet is called actual or geometric length.
Fix a white paper on a drawing board. Put a bar magnet in the middle of the paper. Draw
an outline with a pencil. Place a compass needle near one end of the bar magnet. Mark two
dots A and B at the ends of the compass needle as shown in the figure. Change the position of
the compass needle. Mark the two dots A' and B' at the ends of the compass needle. Find the
intersection point of AB and A'B'. The intersection point is the pole.
Actual length BN
Effective length A S
Compass needle
Magnetic north pole Magnetic south pole A'
Magnetic axis N B'
S
Fig. 7.16
7.17 Magnetic Field and Magnetic Lines of Force Fig. 7.17 Magnetic Field
When a magnetic compass is placed on any surface, it can be
observed that its needle rests roughly in the north-south direction.
But, if the compass is placed at different positions around a magnet,
it is observed that the compass needle points in different directions.
The space around a magnet where the magnetic influence can
be felt is called magnetic field. So, any magnetic material kept in
the magnetic field will be attracted. The effect of magnetic field
decreases as we move away from the magnet.
126 Oasis School Science - 9 PHYSICS
Activity 3
• Take a bar magnet and place a steel pin at some distance from it. Nothing happens.
Now, bring the steel pin near the pole of the bar magnet. The steel pin sticks to the
magnet.
Above activity shows that the magnetic substances experience a magnetic force from a magnet.
The magnetic field can be represented by drawing a set of lines called magnetic lines of force
or magnetic field lines. These are the imaginary lines but drawn to visualize the magnetic
field. If a unit north pole is placed in a magnetic field near the north pole of a magnet, the north
pole of the magnet repels it and south pole of the magnet attracts it.
Activity 4
• Take a drawing board and fix a paper on it. Sprinkle fine powder of iron on it such
that it is spread evenly on the paper. Now place a bar magnet in the middle of the
paper. Tap the drawing board gently. The iron filings rearrange themselves in the
form of curves.
These curves represent the magnetic lines of force. Such arrangement of iron powder
also gives us a rough picture of the pattern of the magnetic field produced by a bar
magnet.
The path along which a unit north pole moves in a magnetic field is called magnetic line of
force. In our practice, we cannot have a unit north pole. So, the magnetic lines of force are
traced by a small magnetic compass.
Properties of magnetic lines of force
1. Magnetic lines of force are closed and continuous curves.
2. They always start from the north pole of a magnet and end at south pole (outside
the magnet).
3. The tangent to a line of force at any point gives the direction of magnetic field at
that particular point.
4. These lines are crowded near the poles of the magnet as magnetic field is stronger
at the poles than in the middle of the magnet.
i) North pole pointing north ii) North pole pointing south
Fig. 7.18 Magnetic lines of force
PHYSICS Oasis School Science - 9 127
Activity 5
• Take a drawing board and fix a white sheet of pa-
per on it. Place a magnet on the paper and draw
its boundary. Now place a small compass needle
close to the N-pole of the magnet and mark two
pencil dots exactly at the two ends of the needles.
Mark the points as 1 and 2. Lift the compass and
place it in such a way that one end of the needle
points towards point 2. Now mark the new end Fig. 7.19 Tracing magnetic
as 3. Repeat the process of moving the compass lines of force of a bar magnet
and marking the dot up to the other end of the
bar magnet. Join the points 1, 2, 3, 4, 5, …….. etc. to get a continuous curve. Thus,
one magnetic line of force is traced. Repeat the process from the north pole of the
magnet starting from a different point and trace another magnetic line of force.
7.18 Neutral Points
When a bar magnet is placed on any surface, the magnetic field around the magnet is not only
due to the magnet alone, but in fact, the field is the combination of the magnetic fields of the
magnet and the earth, i.e. horizontal component of the earth's magnetic field.
Close to a magnet, the magnetic field of the magnet is strong but the earth's magnetic field is
negligible. As we move away from the magnet, the magnetic field of the magnet is almost zero
and the earth's magnetic field is dominating. Within this region, there are two points where
the magnetic field of the magnet is exactly equal and opposite to that of the earth's magnetic
field. These points are called neutral points (or null points).
Neutral points are those points where the resultant magnetic field intensity due to earth's field
and magnet is zero. Neutral points are the points near a magnet where the magnetic field due
to the magnet is completely neutralized by the horizontal components of the earth's magnetic
field. Since the magnetic field due to the earth is fixed, we shall locate the position of neutral
points depending upon the position of the bar magnet in the following two ways.
a. When the N-pole of a magnet points towards north and the magnet is kept in
magnetic meridian, the neutral points are on the particular bisector of the magnetic
axis [fig. 7.18(i)].
b. When the N-pole of the magnet points towards south and the magnet is kept in magnetic
meridian, the neutral points are on the axis of the magnet [fig. 7.18 (ii)].
It should be noted that at the neutral points, the magnetic lines of force due to the magnet and
the earth's magnet act opposite to each other. If a compass needle is placed at any one of the
neutral points, it is not affected at all and, therefore, it points in any direction. It is to be noted
that there are always two neutral points in a horizontal plane.
128 Oasis School Science - 9 PHYSICS
7.19 Terrestrial Magnetism Rotation axis North
Geographical pole
Dr. William Gillbert suggested that the earth itself is a
huge magnet. The earth shows the magnetic properties. Magnetic field lines
The earth can be considered as a huge magnet called a
terrestrial magnet. The following are the facts of existence
that indicate existence of magnetism in the earth.
1. A freely suspended magnet always rests in north Magnetic axis
and south direction.
Fig. 7.20
2. The iron ore has magnetic property.
3. There is the existence of angle of dip and angle of
declination.
4. Neutral points are formed in the magnetic field.
7.20 Angle of Dip
A dip needle is a magnetic needle which is Magnetic needle
free to rotate in the vertical plane. The angle
between the dip needle and the horizontal Horizontal
line at a certain place is called angle of dip. plane H
The instrument which is used to measure Angle of dip
the angle of dip is called dip circle. The dip M
needle remains parallel to the magnetic
lines of force of the earth. The value of angle Magnetic axis
of dip varies from place to place. In the
Kathmandu valley, it is about 420. It means
the dip needle makes 420 with horizontal
line. The value of angle of dip at magnetic Fig. 7.21 Angle of dip
poles is 900 because the needle remains
vertically at the poles. Similarly, the dip needle remains parallel to the ground at magnetic
equator. So, its value is zero because the effect of earth's magnetic north pole and south pole
are equal on the dip needle. Due to this, the dip needle remains parallel to the horizon. As we
move towards the northern hemisphere, the earth's magnetic south pole becomes nearer. So,
the value of angle of dip increases. A similar case happens when we go towards the southern
hemisphere. The effect of the magnetic north pole of the earth is more on the south pole of the
compass. So, the south pole of the dip needle points towards the ground. The magnetic south
pole is situated at northern Canada and north pole is situated at the corner of Antarctica.
Uses of angle of dip
It is used by cave-searchers, navigators, pilots, travellers to find the position on the earth's
surface.
terrestrial /təˈrestrɪəl/ - connected with the planet earth Oasis School Science - 9 129
PHYSICS
7.21 Angle of Declination Geographic north
Geographic meridian
The geographical poles and the magnetic poles
of the earth do not coincide. So, there are two N Angle of declination
planes. The vertical plane passing through the S Magnetic meridian
true geographic north and geographic south is
called geographical meridian and the vertical
plane passing through magnetic south and
magnetic north is called magnetic meridian.
The angle between the geographical meridian Fig. 7.22 Angle of declination
and magnetic meridian at a place is called angle
of declination. The value of angle of declination
differs from place to place. To find the true geographical direction, the angle of declination of
that place should be known.
Uses of angle of declination
It is used by cave-searchers, navigators, pilots, travellers to find the position on the earth's
surface.
Reasonable fact-3
At any given place to know the actual direction, one should know the angle of
declination. Give reason.
The angle made by the intersection of the line joining the magnetic north, south and the line
joining the geographical north-south at a place is called angle of declination. There is single
value of angle of declination at a fixed place. Therefore, the value of angle of declination
gives the position of that place. So, at any given place to know the actual direction, one
should know the angle of declination.
SUMMARY
• The electricity produced due to the continuous flow of electrons is called current
electricity.
• Those substances which conduct electric current through them are called conduc-
tors. Examples: Copper, aluminium, silver, etc.
• Those substances which do not conduct electric current through them are called
insulators. Examples: Rubber, plastic, dry wood, etc.
• Current is defined as the rate of flow of charge. Its SI unit is ampere (A).
• The device which is used to measure the total current flowing through an electric
circuit is called an ammeter.
• Voltmeter is a device which is used to measure the potential difference between
meridian /amnəyˈrɪtdwɪəon/po-intosneooffathne elinlesctthraitciscdirracwunitfroomr sthoeuNrocreth. pole to the South pole on the map of the world
130 Oasis School Science - 9 PHYSICS
SUMMARY
• The property of a conductor by virtue of which it opposes the flow of electric cur-
rent through it is called resistance.
• Ohm's law states that, "The current flowing through a conductor is directly pro-
portional to the potential difference across the ends of the conductor."
• The factors that affect the resistance of a conductor are length of the wire, area of
cross-section of the wire, temperature, nature of materials used, and shape of the
wire used in the circuit.
• Ohm's law states that, "The electric current (I) flowing through a conductor is
directly proportional to the potential difference (V) across its ends, provided that
the physical conditions like temperature, pressure, etc. do not change".
• Resistance of a conductor increases with the increase in the length of the conductor.
• The resistance of a wire is inversely proportional to its area of cross-section.
• The resistance of a conductor is directly proportional to the temperature of the
wire or conductor.
• The reciprocal of resistivity is called conductivity. It gives the idea whether a ma-
terial is good conductor or bad conductor of electricity.
• A magnet is a substance that attracts iron, cobalt, nickel, etc. and has two poles,
viz. north pole and south pole.
• A freely suspended bar magnet always rests in north-south direction.
• The space around a magnet where the magnetic influence can be felt is called
magnetic field.
• The path along which a unit north pole moves in a magnetic field is called mag-
netic line of force.
• The point where the magnetic field due to magnet is completely neutralized by
the horizontal component of the earth's magnetic field is called neutral point.
• The vertical plane passing through the geographic north and geographic south is
called geographical meridian.
• The vertical plane passing through the magnetic south and magnetic north is
called magnetic meridian.
• The angle between the geographical meridian and magnetic meridian is called
angle of declination.
• The angle between the dip needle and the horizontal line at a certain place is
called angle of dip.
• Angle of dip and angle of declination are used by cave searchers, navigators, pi-
lots, travellers, etc. to find the position on the earth's surface.
PHYSICS Oasis School Science - 9 131
Exercise
Group-A
1. What is current electricity?
2. What is source of electricity?
2. Define electric circuit.
3. Define cell and battery.
4. What is hydroelectricity?
5. What types of electric circuit are called open circuit and closed circuit?
6. What is an electric load? Write with an example.
7. What is the actual direction of current flow?
8. Define conductor and insulator with any one example of each.
9. What is ammeter? How is it connected in an electric circuit?
10. What is potential difference?
11. Define electromotive force.
12. What is voltmeter? How is it connected in an electric circuit?
13. What is electric resistance? Write down its SI unit.
14. Define one ohm resistance.
15. What is constantan?
16. Define semiconductor.
17. What are the factors that affect the resistance of a conductor?
18. What is the relation among electric current, potential difference and resistance?
19. What is magnetism?
20. What is angle of declination?
21. What is angle of dip? What is its magnitude at the equator?
22. Define magnetic line of force.
23. Write down the use of angle of declination.
24. Define neutral point.
25. What is the magnitude of angle of dip in North Canada and Southern hemisphere of the
earth?
26. What is the magnitude of angle of dip at the magnetic equator?
132 Oasis School Science - 9 PHYSICS
Group-B
1. Write any two differences between open electric circuit and closed circuit.
2. Differentiate between ammeter and voltmeter.
3. The use of dry cells is very important in our daily life. Give reason.
4. Hydroelectricity is called a pollution-less source of energy, why?
5. Copper is called conductor and silicon is called semi-conductor, why?
6. Ammeter is connected in series and voltmeter is connected parallel in an electric circuit,
why?
7. Write any two differences between angle of declination and angle of dip.
8. Write any two differences between potential difference and electromotive force.
9. The earth itself can be considered as a huge magnet. Give reason.
10. A freely suspended magnet always rests itself pointing the north and south direction,
why?
11. The value of angle of declination changes from place to place on the earth, why?
12. The magnitude of angle of dip in the magnetic equator of the earth is zero, why?
13. The angle of dip in the magnetic south pole of the earth is 900. Give reason.
14. We should know the angle of declination to know the actual geographical direction of a
certain place. Justify this statement.
Group-C
1. Write any three evidences that prove the magnetic property of the earth.
2. Define neutral point and write any two properties of magnetic lines of force.
3. How is galvanometer connected in an electric circuit? Write down the utility of angle of
declination.
4. Define series combination and parallel combination of cells. In which condition are cells
connected in series?
5. How is the angle of dip at a certain place measured? Describe in brief. Where do the
earth's magnetic south pole and magnetic north pole lie? Write.
6. Write any three uses of magnetic compass.
PHYSICS Oasis School Science - 9 133
Group-D
1. Study the given diagrams and calculate the total current flowing through each circuit.
2Ω [Ans: 6A, 1.5 A]
2Ω 2Ω
2Ω
6V 6V
(a) (b)
2. 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 calculated resistance of the bulb. Differentiate between
semiconductor and insulator.
3. 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.
4. 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.
5. Which one of the following is the diagram for the magnetic lines of force pointing
magnetic N-pole towards south.
Fig (a) Fig (b)
134 Oasis School Science - 9 PHYSICS
Compulsory Science
Part 2 : Chemistry
Scope and sequence of the subject matter
Area Unit Syllabus
Change of Matter 1. Classification of • Element and compound
• Atomic structure
Elements • Electronic configuration
• Valence electrons and valency
2. Chemical Reaction • Chemical bond
• Molecular formula
• Radical
• Ion
• Chemical equation
Matter Around Us 3. Solubility • Saturated and unsaturated solution
• Supersaturated solution
4. Some Gases • Solubility
5. Metal • Relation between solubility and
6. Carbon and Its
temperature
Compounds • Crystallization
• Preparation, properties and uses of
hydrogen, oxygen and nitrogen gas
• General properties of metals
• Differences between metal and non-metal
• Role of metals in organisms
• Carbon in simple substance
• Physical and chemical properties of carbon
• Organic and inorganic compounds
7. Water • Water, sources of water, properties, uses
and types
• Methods of removing hardness of water
Chemicals Used in Daily 8. Chemical Fertilizers • Types and utilities of nitrogen, phosphorus
Life Used in Agriculture and potassium fertilizers
CHEMISTRY Oasis School Science - 9 135
8UNIT Estimated teaching periods
Theory 8
Practical 2
Classification
of elements John Dalton
Objectives
After completing the study of this unit, students will be able to:
• describe and demonstrate the atomic structure of elements and their electronic
configuration.
• define valency in terms of combining capacity of elements and write molecular
formulae of some common compounds.
• describe radicals and ions with examples.
• write molecular formulae of some simple compounds.
8.1 Introduction
The branch of science which deals with the study of matter, its composition and properties
is called Chemistry. Anything that occupies space and has mass is called matter, e.g. stone,
water, air, brick, milk, oxygen, etc. All the substances are made of matter.
There are two types of matter. They are pure matter and impure matter. Pure matter includes
elements and compounds whereas impure matter includes homogeneous and heterogeneous
mixtures. Matter is made of tiny particles called atoms.
8.2 Element
An element is the simplest pure form of a substance which cannot be split up into other
simpler substances by any chemical method. An element is made of similar type of atoms.
Hydrogen, carbon, oxygen, sodium, gold, etc. are some examples of elements. Altogether 118
elements have been discovered so far but only 92 elements occur in the nature while the rest of
the elements are prepared in the laboratory by means of nuclear reactions. All the substances
which are present in our surroundings are either these elements or the combination of these
elements. For example, copper, silver, gold, iron, etc. are the elements whereas protein, fat,
carbon dioxide, water, ammonia, etc. are the combined forms of elements.
8.3 Atom
The smallest particle of an element that can take part in chemical reaction without division is
called atom. All the atoms of an element are the same whereas atoms of different elements are
element /ˈelɪm(ə)nt/ - a pure form of a matter made of similar atoms
homogeneous /ˌhɒmə(ʊ)ˈdʒiːnɪəs/ - consisting of things of the same type
136 Oasis School Science - 9 CHEMISTRY
different. For example, all the atoms of gold in a gold ring are the same but an atom of gold is
different from an atom of silver.
11 p+
12nº
1 p+
a. Hydrogen atom b. Sodium atom
Fig. 8.1
8.4 Compound
Compound is a chemical substance formed by the combination of two or more elements
in a fixed proportion by weight. Examples: Water (H2O), Sodium chloride (NaCl), Calcium
carbonate (CaCO3), etc. Formation of a compound is a chemical change, so the properties of
a compound differ from its constituent elements. For example, sodium is a toxic metal and
chlorine is a toxic gas but when they combine together they give edible salt (NaCl). In each
compound, the elements are combined by a fixed ratio of their weight. For example, in water
(H2O) molecule, the ratio of weight of hydrogen and oxygen is 1:8.
8.5 Molecule
The smallest particle of an element or a compound which is capable of independent existence
is called molecule. Molecule represents the smallest stable condition of matter but it is so small
that it cannot be seen with our naked eyes. In a molecule, there may be one or more than one
atom. These atoms may be similar or dissimilar. For example, helium (He) molecule has one
atom, oxygen molecule (O2) has two similar atoms whereas carbon dioxide (CO2) molecule has
one carbon and two oxygen atoms.
The molecule having only one atom is called monoatomic molecule, e.g. Helium molecule
(He), Neon molecule (Ne), Argon molecule (Ar), etc. The molecule having only two atoms
of the same element is called diatomic molecule, e.g. H2, N2 O2, Cl2, Br2, I2, etc. The molecule
having two or more atoms of different elements is called polyatomic molecule, e.g. NaCl, H2O,
CaCO3, MgSO4, Ca(NO3)2, etc.
8.6 Symbol
A symbol is the abbreviation of full name of an element which is represented by one or two
English letters. It is used to make the study easy and fast. If the symbol of an element has one
letter, it is written in capital letter. If it has two letters, the first letter is written in capital and
the second letter in small.
edible /ˈedəbl/ - fit or suitable to be eaten
constituent /kənˈstɪtjʊənt/ - one of the parts of sth that combine to form whole
CHEMISTRY Oasis School Science - 9 137
Some examples of elements which are represented by single letter are given below:
Atomic number Elements Symbols Atomic number Elements Symbols
1 Hydrogen P
5 Boron H 15 Phosphorus S
6 Carbon V
7 Nitrogen B 16 Sulphur I
8 Oxygen U
9 Fluroine C 23 Vanadium
N 53 Iodine
O 92 Uranium
F
There are some cases where the names of elements start with the same letter. For example,
hydrogen and helium begin with 'H'. Carbon, chlorine, chromium, cobalt, cadmium, and
calcium start with 'C'. In such conditions, we take the first letter and another significant letter
to symbolize these elements.
Atomic number Elements Symbols Atomic number Elements Symbols
2 Helium He 12 Magnesium Mg
3 Lithium Li 25 Manganese Mn
17 Chlorine Cl 30 Zinc Zn
20 Calcium Ca 40 Zirconium Zr
24 Chromium Cr 22 Titanium Ti
27 Cobalt Co 73 Tantalum Ta
48 Cadmium Cd
There are certain elements whose names are written in English but symbols are used from the
Latin or German name.
English names Latin/German names Symbols
Iron Ferrum (Latin name) Fe
Copper Cuprum (Latin name) Cu
Gold Aurum (Latin name) Au
Silver Argentum (Latin name) Ag
Mercury Hydrargyrum (Latin name) Hg
Sodium Natrium (Latin name) Na
Potassium Kalium (Latin name) K
Tungesten Wolfram (German name) W
138 Oasis School Science - 9 CHEMISTRY
8.7 Structure of an Atom 6 protons
+6 neutrons
Atom is the smallest particle of an element. Its
diameter is approximately 10–10m. In each atom, electron
there are three fundamental particles. They are proton
electron, proton and neutron. Protons and neutrons neutron
are present at the nucleus of an atom whereas
electrons are present outside the nucleus in different Carbon atom
shells. Electrons revolve around the nucleus in fixed
orbits or shells. Fig 8.2 Structure of an atom
Structure of a sodium atom
Symbol – Na
Atomic number – 11
No. of protons – 11 11 p+
No. of neutrons – 12 12nº
No. of electrons – 11
Atomic mass – 23 Fig. 8.3 Structure of sodium atom
Summary of fundamental or subatomic particles of an atom
S.N. Name of the fundamental Symbol Position Mass Charge (in e.s.u)
particles
(in a.m.u)
1. Electron e- Shell 1/1837 amu -1 e.s.u
2. Proton p+ Nucleus 1 amu +1 e.s.u
3. Neutron n° Nucleus 1 amu 0 e.s.u
Fact File-1
1 amu = 6 × 10–23 g [amu = atomic mass unit]
1 coulomb = 6.25 × 1018 esu [esu = electrostatic unit]
Note: Nucleus of an atom is positively charged because it has positively charged protons.
Similarly, an atom is a neutral particle because it has the same number of protons and
electrons having equal but opposite charges.
8.8 Atomic Number
The total number of protons present in the nucleus of an atom is called atomic number. It is
denoted by Z. Chemical properties of an atom depend upon atomic number of that atom.
Atomic number of an atom is also equal to the number of electrons present in a neutral atom.
Atomic number = No. of protons = No. of electrons in a neutral atom
CHEMISTRY Oasis School Science - 9 139
8.9 Atomic Mass
The sum of the number of protons and the number of neutrons present in the nucleus of an
atom is called atomic mass. It is denoted by 'A'. Atomic mass of an atom is calculated by the
given formula.
Atomic mass = No. of protons + No. of neutrons
Worked out Example
The atomic number and atomic mass of potassium atom are 19 and 39 respectively. Find out
the number of electrons, protons and neutrons in that atom.
Here, Atomic number (Z) = 19
39
Atomic mass (A) = ?
?
No. of protons (P+) = ?
No. of electrons (e–) = No. of protons = No. of electrons
19
No. of neutrons (n°) = 19
Atomic mass – Atomic number
Now, 39 – 19
20
Atomic number =
19
So, No. of protons = 19
No. of electrons =
Again, No. of neutrons =
=
=
Hence, In a potassium atom,
No. of protons =
No. of electrons =
No. of neutrons = 20
8.10 Electronic Configuration
The systematic distribution of electrons in different shells of an atom is called electronic
configuration. The number of shells in an atom ranges from 1 to 7, i.e. K, L, M, N, O, P and
Q. The shell K is the nearest to the nucleus whereas shell Q is the farthest. To explain the
systematic distribution of electrons in different shells, Bohr and Bury proposed a law which
is given below:
1. The maximum number of electrons in each shell is determined by 2n² formula where 'n'
denotes the number of shell in an atom.
For example,
configuration /kənˌfɪɡəˈreɪʃ(ə)n/ - an arrangement of the part of sth
140 Oasis School Science - 9 CHEMISTRY
The maximum number of electrons present in shell K,
2n2 = 2 × 12 [∴ In shell K, n = 1]
= 2 × 1 × 1
= 2
The maximum number of electrons present in shell L,
2n2 = 2 × 22 [∴ In shell L, n = 2]
= 2 × 2 × 2
= 8
Similarly, the maximum number of electrons present in shell M,
2n2 = 2 × 32 [In shell M, n = 3]
= 2 × 3 × 3
= 18
This rule is not applicable to 5th, 6th and 7th shells where maximum number of electrons
is 32,18 and 8 respectively.
2. The maximum number of electrons is not more than 8 and 18 in the outermost shell and
second last shell respectively.
3. It is not necessary to fill the electrons according to the 2n² formula only but a new shell
can be started when there are 8 electrons in second last shell.
4. The shells which are nearer to the nucleus have less energy and the shells far from the
nucleus have more energy.
7
6
5
4
3 Shells or orbits
2
1
Nucleus 2 8 18 32 32 18 8
P+ n0
K
L
M
N
O
P
Q
Fig 8.4
CHEMISTRY Oasis School Science - 9 141
According to Bohr and Bury scheme, the electronic configurations of the first 20 elements are
given below:
S.N. Elements Symbol Atomic number Electronic configuration
K L MN
1. Hydrogen H1 1
2. Helium He 2 2
3. Lithium Li 3 21
4. Beryllium Be 4 22
5. Boron B5 23
6. Carbon C6 24
7. Nitrogen N7 25
8. Oxygen O8 26
9. Fluorine F9 27
10. Neon Ne 10 28
11. Sodium Na 11 281
12. Magnesium Mg 12 282
13. Aluminium Al 13 283
14. Silicon Si 14 284
15. Phosphorus P 15 285
16. Sulphur S 16 286
17. Chlorine Cl 17 287
18. Argon Ar 18 288
19. Potassium K 19 2881
20. Calcium Ca 20 2882
The atomic structures of the first 20 elements are given below:
1p+ 2p+ 3p+ 4p+ 5p+
0n0 2n0 4n0 5n0 6n0
Hydrogen Helium Lithium Beryllium Boron
6p+ 7p+ 8p+ 9p+ 10p+
6n0 7n0 8n0 10n0 10n0
Carbon Nitrogen Oxygen Fluorine Neon
142 Oasis School Science - 9 CHEMISTRY
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