science 9

electromagnetic waves. The light waves are electromagnetic waves
because they do not require material medium for their propagation.
Similarly, ultraviolet rays, infrared radiations, X-rays, c-rays,
microwaves, radio waves, etc are electromagnetic waves.

The electromagnetic waves can be categorized into different types
on the basis of their frequency or wavelength. The classification of
electromagnetic waves according to their frequency or wavelength
is known as electromagnetic spectrum. Gamma rays (c- rays) are
the electromagnetic waves of shortest wavelength while the radio
waves are the electromagnetic waves of longest wave length

Electromagnetic spectrum with wave length and frequency

Electromagnetic spectrum Wave length (l) in Frequency (f) in hertz
meters

Gamma rays 10-13−10-11 1019 (and above)

X-rays 10-11−10-9 1017−1019

Ultraviolet radiation 10-9−10-7 1015−1017

Visible light 10-7−10-6 1014−1015

Infrared radiation 10-6−10-3 1011−1014

Microwaves 10-3−10-1 109−1011

Radio-wave 101−104 103−109

Properties of electromagnetic waves

The electromagnetic waves possess the following common properties:
1. The electromagnetic waves are the transverse waves.
2. They do not require material medium for their propagation.
3. They travel with the speed of 3×108m/s in vacuum.
4. Most of the electromagnetic waves are blocked by metal plates except
X-rays and c-rays. The x-rays and c-rays can pass through the thin
plates of metals also.
5. These waves can show the property of reflection, refraction,
interference, diffraction and polarization.

Some part of electromagnetic spectrum is visible to us. The part of
electromagnetic spectrum which is visible to human eyes is called
visible spectrum.

Application of electromagnetic waves

The electromagnetic waves are very useful in our daily life. But
some of the waves are extremely harmful on long term exposure.

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1. X - rays

X-rays have wavelength of 0.01 to 10mm. The x-rays can easily
pass through skin and muscles of body. Butthey cannot pass
through the bones. The x-rays have very common medical
application. They are used to detect fractures of bones or other
conditions inside our body. They are also used in radio therapy.
They are used to study the arrangement of atoms in a crystal.
They are also used in secrity check in airport, custom, etc.
However, the exacessive exposure to x-rays can cause cancer. -

2. Gamma rays

The gamma rays also have medicinal applications. They are
used to sterilize the surgical instruments because they kill
harmful bacteria. Doctors can use gamma rays to kill cancer
cells inside a patient’s body.

3. Microwaves

Microwaves are used in microwave oven. The microwave ovens
produce the microwaves which are used to cook foods.

4. Radio waves

Radio waves are used to transmit radio and television
programmes.

5. Ultraviolet radiations

The ultraviolet radiations have the wavelength of 10-400nm.
They are the harmful radiations and cause the cancer of skin
and cateract of eyes. But their mild or lesser exposure is
somehow useful. These rays are absorbed by our skin to make
vitamin D. The other of UV radiations are:
a. They are used to test the purity of ghee, eggs, ornaments, etc.
b. They are used to sterilize the medical instruments.
c. They are used to praduce vitamin D in the food of animals and
plants.

6. Infrared radiations

These radiations carry heat and are called heat rays. These
rays can be used for heating and cooking purposes.

The X-rays and gamma rays are very harmful on the long term
exposure. They have carcinogenic effect, i.e. they can cause
cancer. A pregnant woman must avoid the exposure to such

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rays because these rays can severely harm the foetus.
Learn and Write

1. An air bubble in a jar of water shines brightly. Why?

When light rays entering from water to the air bubbles make
incident angle more the critical angle, the light rays undergo
total internal reflection. For the observers, the light rays seem
coming from the bubbles. Thus, the air bubbles shine.

2. A stick partially dipped in water seems bent. Why?

When a stick is partially dipped, the light rays coming to the
viewer’s eyes from the immersed part of the stick from water
to the air bend away from the normal. Thus, the dipped part
of the stick seems raised up. But the light rays coming from
the undipped part to the viewer’s eyes do not bend. Hence, the
stick seems bent when partially immersed in water.

3. A rectangular glass slab does not disperse white light.
Why?

A rectangular glass slab is the combination of two prisms of
same size, shape and made up of same materials. When a
white ray of light enters the glass slab, the ray splits into its
constituent colours inside the glass slab. When these different
coloured rays come out to the air, they together to give white
ray. Thus, a rectangular glass slab does not disperse white
light.

4. Radio waves are not harmful. Why?

Radio waves are the electromagnetic waves having less
frequency and more wavelength. Due to this, they do not have
more penetrating capacity. Hence, they are not harmful.

5. What is the cause of refraction of light?

Light has different velocities in different media. It velocity
changes if it travels from one optical medium to the next.
The change in velocity leads to the bending of path of light at
the interface of the two optical media. This process is called
refraction. Hence, the change in velocity of light on going from
one optical medium to another is the cause of refraction of
light.

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6. A fisherman sees a fish in the pond and tries to thrust a
spear in to it. Will he succeed in killing the fish? Explain
with reasons.

He will not succeed in killing the fish. It is because the position
of the fish in the pond is not same as seen by the person. It is
due to the refraction of light. When the rays of light come from
denser (fish in water) to the rarer (air) medium, they bend
away from the normal. Hence, an apparent position of the fish
is seen by fisherman from outside.

7. The sun seems to be red during sunrise and sunset.
Why?

The rays from the sun travel greater distance through the
atmosphere during sunrise and sunset. In this situation, the
white beam of light is dispersed (scattered) by the atmosphere.
The green, blue and other colours of the dispersed light are
scattered away and they cannot reach our eyes. The red light,
which gets scattered the least, reaches our eyes. Hence, the
sun as well as its nearby sky seems red during sunrise and
sunset

Main points to remember

1. The process of bending of light when it passes from one medium to
another is called refraction of light.

2. The change in speed of light when it passes from one optical medium to
another is the cause of refraction.

3. The refraction of light obeys the following laws:

a. The incident ray, refracted ray and normal at the point of incidence
all lie at the same plane.

b. The ratio of sine of angle of incidence to the sine of angle of refrac-tion
for the given pair of media is constant. This law is also known as Snell’s
law. It can be represented as:
Sini

= Constant (m)
Sinr

4. The ratio of velocity of light in vacuum or air to the velocity of light in
a given medium is called refractive index of the given medium.

5. The actual depth of an object from the surface of water is called real
depth.

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6. The virtual depth at which an object appears due to the refraction of
light is called virtual depth.

7. The value of angle of incidence in the denser medium for which the
corresponding value of angle of refraction in rarer medium is 90° is
called critical angle.

8. The process of returning of light into the original denser medium, when
a ray of light passes from a denser to rarer medium with the angle of
incidence greater than critical angle is called total internal reflection.

9. A prism is a wedge shaped block of glass having three rectangular faces
and two triangular faces. A prism causes dispersion of light.

10. The process of splitting of a white ray of light into its constituent seven
colours is called dispersion of light.

11. The waves which do not require any material medium for their
propagation are called electromagnetic waves.

12. The classification of electromagnetic waves on the basis of their frequency
or wavelength is called electromagnetic spectrum.

Exercise

1. Choose the best alternative in each case.

a. If a pair of optical media is set up by water and glass, then

i. Water is denser and glass is a rarer medium.

ii. Glass is denser and water is a rarer medium.

iii. Both of these act as the same medium.

iv. All of the above.

b. The critical angle at the glass-air interface is

i. 90° ii. 24° iii. 42° iv. 48°

c. The sparkling of diamond is due to

i. Small critical angle ii. Total internal reflection

iii. Refraction iv. Both i and ii

d. Which of the following is the most energetic electromagnetic

radiation?

i. X-rays ii. Ultraviolet rays

iii. Microwaves iv. Gamma rays

e. The colour of light which is the most deviated in the spectrum while

passing a white light through a prism is

i. Violet ii. Blue iii. Green iv. Red

2. Answer these questions in very short.
a. What is the velocity of light in water?

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b. What is refractive index?
c. How is refractive index related to the critical angle?
d. Write down the refractive index and critical angle of

diamond.
e. How is optical fibre made?
f. What is endoscopy?
g. How is the speed of light wave related to its frequency and

wavelength?
h. What is a totally reflecting prism?
i. Name the electromagnetic wave which has the shortest

wavelength.
j. Name the electromagnetic wave which has the longest

wavelength.

3. Define:

a. Light b. Rarer medium c. Denser medium

d. Spectrum of light e. Light pipe f. Mirage

4. Differentiate between:
a. Real depth and apparent depth
b. Reflection and refraction of light
c. Refraction and dispersion of light

5. Give reasons:
a. A coin placed in a water filled glass appears to be at less
depth.
b. A stick half immersed in water appears to be bent.
c. Mirage is observed in hot desert roads.
d. Diamond shines brightly but the pieces of glass do not.
e. A glass slab cannot cause the dispersion of light.
f. Stars twinkle at night.
g. A totally reflecting prism is used instead of plane mirror
in binocu lars.

6. Answer these questions.
a. What is refraction of light? Write down its laws.
b. State and explain Snell’s law with suitable diagrams.
c. What is optical fibre? Write down its importance.
d. How is mirage observed? Explain with a diagram.
e. What is dispersion of light? Show with a diagram.
f. Define electromagnetic waves with examples.
g. Write down the uses of x-rays and O-rays.
h. How are ultraviolet rays useful?

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i. Write down the common properties of electromagnetic
waves.

j. What is the cause of dispersion of light? Explain.

7. Diagrammatic questions:

a.

i. What process is shown in the figure?
ii. Copy the diagram and complete it.
iii. Why can’t we observe such process through a glass slab?
iv. Which colour of ray has deviated the most? Why?
v. Which colour of ray has deviated the least? Why?
b. Complete the following diagrams:

45°

45° 90°

8. Solve the following numerical problems.

a. Find the critical angle of diamond if its refractive index is

2.24.

b. Calculate the critical angle of water if the velocity of

light in water is 2.2×108m/s. (Given, velocity of light in air =

3×108m/s).

c. Calculate the apparent depth of a stone inside water if its

real depth is 30m. (Given, refractive index of water is 1.33).

Answers 8. a. 24° b. 49° c. 22.55m

Project Work

1. Take a Newton’s Wheel and turn it rapidly. What do you observe? Why is
such observation? Write down the process, observation and conclusion.

2. Take a water trough and half-fill with water. Place the trough in
sunshine in a slanted position so that the sunlight reflected by water
falls on to the wall. What colour of light did you see on the wall? Write
with reasons.

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Chapter

6 Sound
Heinrich Rudolf Hertz
He is known for the discovery of electromagnetic
radiation, photoelectric effect, Hertz's principle,
etc. Estimated Periods :6

Objectives

At the end of the lesson, students will be able to:
• identify infrasound, audible sound, ultrasound waves and their sources;

• explain nature of sound wave;

• explain the meaning of intensity and pitch of sound;

• explain the factors affecting speed of sound in air;

• describe refraction and reflection of sound with examples;

• explain the causes, effects and preventive measures of sound pollution.

Mind Openers

• What is sound? Can sound pass through vacuum?

• What are the sources of infra, audible and ultrasound?

• Whose sound has more pitch either baby’s or old man’s sound? Discuss.

Introduction

Sound is a form of energy which is produced due to the vibration of
an object.

When a string of a guitar is plucked, we observe
that the string starts vi-brating and sound is
produced. When you observe a ringing bell, you
will see the vibration of the bell. When you touch
it, you can feel the vibrations of the bell. When
you catch it, production of the sound stops after
sometime. It is because the bell stops vibrating.
When you touch a speaker in the front part when
it is producing sound, you can feel the vibrations.
When you put your fingers against your throat
while speaking, you can feel vibrations.

Thus, any object which vibrates is a source of sound. When vibration
of the object stops, the production of the sound also stops.

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Guitar Madal Television Radio

Wave

Wave is a disturbance in a medium which carries energy from one
point to another. It is produced due to vibration of molecules of the
medium.

For example, when sound is produced, it causes the vibration of air
molecules. Sound waves, light waves, radio waves, etc are examples
of waves. Wave is of two types:

(a) Transverse wave (b) Longitudinal wave

a. Transverse wave

Transverse wave is the wave in which particles of the medium move
up and down perpendicular to the direction of energy.

Crest

AB

Trough

Transverse wave

In the figure, the energy passes from point A to B and particles of
the medium move up and down perpendicular to the direction of
energy.

The upward raised part of the wave is called crest and the downward
lower most part is called trough.

Examples: Light wave, radio wave, wave produced in water when
a stone is thrown on it, etc.

Longitudinal wave

Longitudinal wave is the wave in which the particles of the medium
vibrate to and fro in the same direction as the direction of energy.

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Compression

AB

Rarefaction
Longitudinal wave

In the figure, the energy passes from point A to B and the particles
of the medium move to and fro in the same direction. Sound wave,
wave produced in spring, etc are examples of longitudinal wave.
When sound is produced, the air molecules vibrate to and fro in
the same direction as the direction of sound. In this process, air
molecules remain tightly packed in some regions called compression
and remain loosely packed in some regions called rarefaction.

Propagation of sound and sound wave

Activity 6 .1 To study the origin of waves.

Materials required:

Tuning fork, a bowl with water, etc.

Procedure:

1. Take a bowl with water and put it on a table.

2. Take a tuning fork and hit its forks on a rubber pad.

3. Bring the prongs of vibrating tuning fork close to the water and touch
the water .

What do you see?

You can see ripples produced in water surface spreading out from the point
where you touched the water with the prong. These ripples are called waves.
They carry sound from the source to other places

As in water, sound wave is produced in the air also. When an object
vibrates in air, the molecules of the air move in to and fro motion.
Due to this, the air molecules are tightly packed in some regions
and loosely arranged in other regions. The region where the air
molecules are tightly packed is called compression and the region
where the air molecules are loosely arranged is called rarefaction.

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The compressions and rarefactions are formed alternately in the
air. The compressions and rarefactions collectively form wave. This
wave carries sound energy from the source to other places.

AB

Longitudinal wave

Sound wave is a longitudinal wave. A longitudinal wave is wave in
which the particles of the medium move in to and fro motion in the
direction of the wave. In sound wave the particles of the medium
move in to and fro motion forming compressions and rarefactions.

Sound energy is transferred from the source to the other places in
the form of wave. The process of transmission of sound from one
place to the another is called propagation of sound.

Sound wave travels through material medium only. It cannot travel
in vacuum. Therefore, it is called a mechanical wave.

Differences between transverse wave and longitudinal wave

Transverse wave Longitudinal wave

1.The wave in which the particles 1. The wave in which the

of the medium vibrate up and down particles of the medium vibrate

perpendicular to the direction of to and fro in the same direction

energy is transverse wave. as the direction of energy is

called longitudinal wave.

2. It forms crest and trough during the 2. It forms compression and

course of movement. rarefaction during the course of

movement.

3. Light wave, radio wave, etc are 3. Sound wave, wave produced

examples of transverse wave. in a spring, etc are examples of

longitudinal wave.

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Activity 6 .2 To show longitudinal wave Compression

Materials required: Rarefaction
Slinky spring
Procedure :

1. Take a slinky spring and fix one end of it to a wall.

2. Stretch the spring from another end and vibrate 'to' and 'fro'. What do
you observe ?

Observation:

Two types of regions are distinctly formed in the spring. The region where the
rings are tightly packed is compression and the region where the rings are
loosely arranged is rarefaction.

Some terms related to wave

Crest and trough

In transverse wave, particles of medium move up and down from the
mean position. The portion of the wave which lies at the maximum
height above the mean position is crest. Similarly, the portion of the
wave which lies at the lowest depth from the mean position is called
trough.

Crest amplitude

AB

Trough

Transverse wave

Amplitude(a)

The maximum displacement of particles from mean position during
propagation of wave is called amplitude. It is calculated as the
height of crest or trough. It is denoted by ’a’.

Compressions and rarefactions

As already discussed, the region of a longitudinal wave in which
particles are tightly packed is called compression. It has higher
density. The region of a longitudinal wave in which particles are
loosely arranged is called rarefaction. It has lower density.

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Compression

AB

Rarefaction
Longitudinal wave

Complete wave

The wave which contains one crest and one trough in a transverse
wave or one compression and one rarefaction in a longitudinal wave
is called complete wave.

Wave length (l)

The distance between two consecutive troughs or crests in a transverse
wave or the distance between two consecutive compressions or
rarefactions in a longitudinal wave is called wavelength. Length of
a complete wave is also called wavelength. Its SI unit is metre (m).
It is denoted by lambda (l).

Frequency (f)

The number of complete cycles or waves produced in one second is
called frequency. The SI unit of frequency is hertz (Hz). The bigger
units of frequency are kilohertz (KHz), megahertz (MHz), etc. In the
above figure, two complete waves are produced in one second, thus
the frequency is 2 Hertz.

Time period (T)

The time taken by the wave to complete one cycle or one wave is
called time period. Its SI unit is second.

For example: If a wave takes 2 seconds to complete a wave, its time
period is 2 seconds.

Wave speed (v)

The distance travelled by a wave in unit time is called speed. The SI
unit of speed is m/s.

Relation between wavelength, frequency and speed

Suppose, frequency of a wave is 40 λ
Hertz, it means 40 complete waves
are produced in each second. In other A B

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words, this wave travels the distance equal to the length of 40
complete waves in 1 second. If wave length of the wave is 2m, then
the distance travelled by it in 1 second = 40 × 2 = 80 m.

Thus, distance travelled by the wave in 1 second = Wave length × frequency

or, speed = wavelength × frequency

∴v=l×f

Solved Numerical Problem 6.1

Wavelength of a wave is 3.3 m and its frequency is 100 Hz. Find its speed.
Solution,

Wavelength (l) = 3.3 m
Frequency (f) = 100 Hz
Speed (v) = ?
We have,
Speed = Wave length × frequency

= 3.3 × 100
= 330 m/s
∴The speed of the wave is 330 m/s.

Solved Numerical Problem 6.2

The speed of a sound is 330 m/s, its frequency 50 Hz. Find its wavelength.

Solution:

Speed (v) = 330 m/s , Frequency (f) = 50 Hz
Wavelength (l) = ?

We have,
v=f×l

330 = 50 × l
or, l = 6.6 m

∴ Wavelength of the wave is 6.6 m

Sound and its speed in different media

Sound is a form of energy which is produced by any vibrating object.
It produces sensation of hearing in ear. It has following features:

i. It is produced by a vibrating body.

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ii. It needs material medium for transmission. Therefore,
sound wave is called a mechanical wave.

iii. It transmits from one place to another in the form of
longitudinal wave.

iv. It reflects or refracts like other waves.
v. Its speed differs from medium to medium.
vi. It has the highest speed in solid and the lowest speed in gas.

In solids, molecules are tightly arranged. The force or shock exerted
by the sound to one molecule is immediately transferred to other
molecules. Thus, sound travels faster in solid. But molecules are
not tightly packed in liquid and gas. Moreover, molecules are far
apart in gas medium. Thus, force or shock exerted on one molecule
takes long time to transfer to other molecules. This makes the speed
of sound in gas low.

Speed of sound in different media

SN Medium Speed (m/s) SN Medium Speed (m/s)
1. Granite 6000 5. Water 1498
2. Steel 5200 6. Hydrogen 1270
3. Aluminium 5100 7. Air 330
4. Brick 5000 8. Carbon dioxide 258

Factors affecting speed of sound in gas

Following factors affect the speed of a sound in gas medium.

a. Density b. Temperature

c. Humidity d. Direction of air flow

a. Density

The speed of sound in a gas medium is inversely proportional to the
square root of the density of the gas. i.e.

1
Speed a

√Density

Density of oxygen is 16 times greater than that of hydrogen. But,
the velocity of sound in hydrogen is 4 times greater than that in
oxygen. It means speed of sound decreases when density increases.

b. Temperature

The speed of sound is directly proportional to the square root of

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absolute temperature of the gas.

Speed of sound× √Temperature

When temperature of a gas increases, its molecules spread apart
from each other so, its density decreases thereby increasing the
speed.

c. Humidity

The water vapour present in air is called humidity. Water vapour
decreases the density of air. Humid air has less density than dry
air. Since the speed of sound increases with decrease in density of
the gas, the speed of sound is more in humid air.

d. Direction of airflow

The speed of sound is more in the direction of airflow. But, the speed
of sound is less in the direction opposite to the air flow.

But, the following factors have no any effect for the speed of sound

in a gas medium: More Ultra sound
a. Change in frequency

b. Change in wave length 20KHz
c. Change in loudness, pitch, etc Audible sound
d. Change in pressure
20Hz
Spectrum of sound waves Less Infrasonic sound

Frequency of sound wave differs according to its source. Some
sounds have less frequency and some have more frequency. The
frequency ranges from 1Hz to 108 Hz. The graphical representation
of sound waves of various frequencies is called spectrum of sound
wave.

The sound waves are classified into three categories. They are:
infrasonic sound, audible sound and ultrasonic sound.

Infrasonic sound

The sound wave with frequency less than 20Hz is called infrasonic
sound. It is also called sub-sonic or infra sound. It cannot be heard
by human beings. But it can be felt by touching the source. Such
sound is produced by very big masses. The seismic waves produced
by an earthquake are example of infrasonic sound Animals like
elephant use infrasonic sound.

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Audible sound

The sound wave having the frequency between 20Hz to 20KHz is

called audible sound. This type of wave can be heard by human

being. Different instruments like flute, guitar, piano, etc produce

audible sounds.

Ultrasonic sound

The sound wave having the frequency more than 20 kilohertz is

called ultra-sonic sound or ultrasound. Such sound cannot be heard

by human beings. But some animals like bats, whales, dolphin,

etc produce and detect ultrasound. Dogs and cats also can hear

ultrasounds but cannot produce.

Practical uses of ultrasound

1. It is used by doctors to identify the diseases of inner parts and

location of the tumors.

2. Strong beam of ultrasound kills bacteria. Thus, it is used for

sterilization purpose.

3. It is used in bloodless surgery.

It is used to destroy kidney

stones and brain tumors.

4. It can be used to detect cracks Surface of sea

and flaws in metals. Source of sound Sound detector

5. It is used by animals like bats,

whales, etc for the detection of Incident sound Depth of sea
their preys and obstacles of the Reflected sound

path.

6. It is used to find the depth of Bottom of sea
ocean or sea. In this method,

waves of ultrasound are sent towards the bed of sea or ocean by

a fathometer. The waves get reflected from the bed and received

by the hydrophone at the surface of the sea. The time interval

between the emission and receipt of the wave is noted. This

time is used to calculate the depth of sea by using the following

formula.

Depth= Speed of sound × time
2

v×t [∴Depth = Distance]
Or, d = 2

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Solved Numerical Problem 6.3

A hydrophone receives the waves 3 seconds after it is transmitted. Find
the depth of the sea. (Given, speed of water is 1500 m/s).

Solution,

Time (t) = 3 seconds.

Speed of sound in water (v) = 1500m/s

Depth of Sea (d) = ?

We have,

v×t = 1500 × 3 = 2250m
d=
22

∴Depth of the sea is 2250m

Characteristics of Sounds

Sound produced from various instruments or animals differs from
each other. One sound differs from another sound due to four
aspects. They are:

i. pitch ii. intensity iii. amplitude and iv. tone

Pitch

Shrillness or sharpness of sound is called pitch. A sharp sound has
high pitch whereas a hoarse sound has low pitch. Pitch of a sound
depends upon frequency. The higher the frequency, the higher is
the pitch. The sounds of babies and girls have higher pitch and are
sharper.

The frequency and pitch are not some. Frequency can be measured
and has unit. But pitch cannot be measured and cannot be shown in
diagram. But pitch of a sound increases with increase in frequency.
The increase in frequency reduces wavelength. Thus, the sound of
high pitch has less wavelength.

Higher frequency Lower frequency
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Following factors affect the pitch of sound:
Frequency
The greater the frequency, the higher is the pitch.
Length of wire
Short wires produce the sound having hither pitch than the long
wires.
Thickness of wire
Thin wires produce the sound having higher pitch whereas the thick
wires produce sound having lower pitch.
Tension
The higher the tension of the string, the higher is the pitch of sound.

Activity 6 .3 To study the factors affecting frequency

Materials needed :

A guitar, or a sonometer

Procedure :

1. Take a guitar and pluck a string of it.
Listen to the sound produced.

2. Tighten the same string and pluck it
again. Listen to the sound produced again.

3. Make tension of a thick and a thin string
equal. Pluck both of them with an equal force. Listen to the sounds
produced by them.

4. Pluck a certain wire and listen to the sound produced. Reduce
its length by using bridge and pluck it again with the same force as
previous. Listen to the sound thus produced.

Observation :

a. When string is made tight, sharp sound is produced.

b. Sound produced from thin wire is sharper.

c. Sound produced from short wire is sharper.

Conclusion :

Tension, thinness and decrease in length of the wires increase
frequency of a wave.

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Amplitude

Amplitude of a wave is defined as the maximum displacement of
particles of a medium from their mean position. Nature of amplitude
can be better understood in a transverse wave.

Crest

a B
A

Trough

Let AB is the position of water molecules when it is not disturbed.
When we drop a stone on the water, transverse wave is produced
in which some water molecules are raised above surface level and
some water molecules sink below the surface level. The raised water
molecules create the crest whereas the dipped water molecules
create the trough.

The height of the crest or depth of the trough is called amplitude.

When larger stone or the stone of the same size with more force,
is thrown on the water, the waves having higher amplitude are
produced. It is because the wave with more energy has more
amplitude.

Intensity of sound

Intensity of a sound wave is the amount of sound energy per unit

time flowing across unit area perpendicular to the direction of the

wave. E Joule

Intensity of Sound = Sound energy or, I = t × A = Second × m2
(Unit)
Time × Area

[ [ Watt
= Joule
∴ watt=
m2
Second

∴ The SI unit of intensity of Sound is Watt/m2.

But, unit decibel (Db) is more in use.

One decibel intensity is the intensity of sound in which a sound
carries energy of 10-12 Watt/m2. It means

1 decibel = 10-12 Watt/m2

Times' Crucial Science Book - 9 116

For every increase in 10 decibel, the intensity of sound increases by
ten times. For example, the sound with intensity 30 decibel has 10
times more energy than the sound with intensity 20 decibel. In the
same way, the sound with intensity 40 decibel has 100 time more
energy than the sound with intensity 20 decibel.

Intensity of some sounds

S.N. Sound Intensity (in decibel)

1. Threshold of hearing 0

2. Rustle of leaves 10

3. Whispering 20

4. Normal conversation 50

5. Loud conversation 70

6. Ringing of telephone 80

7. Running motorcycle 90

8. Vehicle horn 100

9. Flying jet plane (during take off) 140

Sound with intensity more than 80 dB is harmful to ears. Exposure
to the sound with intensity more than 140Db causes permanent
damage to hearing capacity.

The above table shows that the loudness of sound increases with
increase in intensity. Following factors affect the intensity of sound:

1. Amplitude of vibration

Intensity of sound is directly proportional to the square of amplitude,
i.e.

I  a2

It means sound with more amplitude has high intensity.

2. Distance from the source

When distance from the source increases, the intensity decreases. It
means when the source is nearer, the sound becomes louder.

3. Size of sounding body

Larger the size of sounding body, the louder is the sound produced.
It means intensity of the sound produced from a larger body is more.

117 Times' Crucial Science Book - 9

4. The direction of wind

The loudness of sound is greater in the direction of wind than in the
opposite direction.

Quality or tone
The tone is the property of a sound by which two sounds of same
pitch and intensity from different sources can be distinguished from
each other.
If a harmonium and a piano are played with same pitch and
intensity, their sounds can be distinguished due to difference in
their tones.
Reflection of sound
Like other forms of energy, sound also returns back to the previous
medium when it strikes to any obstacle. The phenomenon of
returning back of sound to the same previous medium when it strikes
to a solid obstacle is called reflection of sound.

Activity 6 .4 To demonstrate the reflection of sound

Materials required: Cardboard tube

Two plastic pipes, a plane mirror, a Cardboard tube
clock, a wooden board, a table, etc.

Procedure :
1. Place a plane mirror in a vertical Analog clock

position on a table.
2. Place a wooden board in vertical position perpendicular to the

plane mirror as shown in figure.
3. Keep two plastic pipes on the table one in each side of the

wooden board so that each pipe makes about 30° angle

with the wooden board.
4. Place a clock at the end of one pipe.
5. Keep your ear at the end of another pipe.

What will you feel ?

Observation :

You can hear the ’tic’ ’tic’ sound of the clock. This is because the
sound produced by the clock is reflected from the plane mirror. And it
reached to the ear.

Conclusion :

Sound is reflected when it strikes a solid obstacle (or surface).

Times' Crucial Science Book - 9 118

Echo

If we shout loudly in front of a hill or in front of a large reflecting
object, we will hear our own sound after sometime. This sound
which we hear later is echo. It is the reflected form of original sound
by hill or reflecting substance. The reflected sound is called echo.
Following conditions are required for the echo to occur.

1. The distance between the source and reflecting surface must be at least
17m away.

2. The reflecting surface should be rigid and large.
3. The sound must have sufficient energy.

Reverberation

When you speak in an empty big hall, you feel that your sound
becomes louder. Here, reflecting surfaces, i.e. walls are closer from
the source of sound. So, reflected sound gets mixed to the original
sound and the original sound becomes longer. This effect is called
reverberation. Reverberation is the prolongation of original sound
due to mixing of reflected sound to the original sound. For the
reverberation to occur the distance between the source of sound and
reflecting surface should be less than 17m.

In auditorium or cinema hall, the excessive reverberation is not
desirable. To reduce reverberation, the roofs and walls of the
auditorium or cinema hall are covered with sound absorbing
materials. Sound can be absorbed by curtain, chair, clothes, beds,
etc. So, reverberation does not occur in well furnished rooms.

Little reverberation is beneficial in musical sounds. It makes the
sound lively.

Condition for reverberation
1. The distance between the source of sound and reflecting surface
should be less than 17m.
2. There should not be any sound absorbing materials in the way of sound.

Differences between echo and reverberation

Echo Reverberation

1. The reflected sound is called 1. Prolongation of original

echo. sound due to mixing of

reflected sound to the original

sound is called reverberation.

119 Times' Crucial Science Book - 9

2. For distinct echo, the reflecting 2.For reverberation, the
surface should be more than 17m reflecting surface should be
away from the source of sound. nearer than 17m from the

source of sound.

3. The intensity of echo is less 3. The intensity of

than that of original sound. reverberation is more than

that of original sound.

Refraction of sound
Sound wave also shows refraction and follows laws of refraction
as that of light. When sound travels in a medium having uniform
density, its velocity remains constant and travels in a straight line.
However, difference in the temperature of the air of different layers
causes the difference in the density of air of respective layers. The
air with higher temperature has less density. The speed of sound is
higher in such air. Similarly, the air with lower temperature has
more density. The speed of air is less in such air.
At day time, the ground is hotter and hence the air nearer the
ground. The temperature of the air diminishes in upward direction.
Thus, the air of the upper layers behaves like denser medium.
When sound is produced on the ground, it gradually moves upwards
bending towards normal (according to the laws of refraction).
Thus, the sound gets refracted upwards and less sound reaches to
the listener. Hence, sound is not heard clearly at day time.

Denser Rarer

Rarer At Day At Night Denser

At night time, ground is colder. Due to this, the air adjacent to
the ground is cooler than the air of upper levels. Thus, the air
becomes rarer when we move upwards. When sound is produced
on the ground, it refracts continuously away from the normal.
Finally, the sound waves undergo total internal reflection and bend
towards downwards. Thus, sound can travel farther and reach to
the listener. Hence, sound can be heard up to long distance clearly
at night time.

Times' Crucial Science Book - 9 120

Furthermore, the environment is calm and silent during the night
time. There is no chirping of birds, heavy movement of vehicles
and sound of human beings at night. All these factors help to make
sound clear and louder at night.

Differences between sound wave and light wave

Sound Wave Light Wave

1. Sound waves are mechanical 1. Light waves are

waves. Therefore, they need material electromagnetic waves.

medium to propagate. Therefore, they do not need

material medium to propagate.

2. They are longitudinal wave. 2. They are transverse wave.

3. Speed of sound wave is 330m/s in 3. Speed of light wave is
air. 3×108m/s in air or vacuum.

4. Sound waves bring sensation of 4. Light waves bring sensation

hearing in ears. of vision in eyes.

Effect of sound

Sound is essential for human beings. It is the means for
communication. We can express our feelings, wants, loves,
knowledge, ideas, etc with the help of sound. Musical sound produced
from musical instruments provides pleasure and relaxation to the
humankind. Besides human beings, other animals also express
their feelings of happiness, sadness, danger, etc through sound.

Sometimes, the sound that we receive may be unpleasant and
disturbing. Such sound is called noise. The sound produced from
factories, industries, vehicles, loud speaker, crowd, etc are such
unpleasant sound or noise.

Noise pollution

The sound which is unpleasant, loud and disturbing is called
noise. The sounds produced from factories, industries, vehicles,
loudspeakers, crowd, etc are examples of noises. The noise adversely
affects the environment. The sound with intensity more than 80 Db
creates discomfort to our ears. Sound above 100Db can make us
deaf. Sounds with 140 Db can tear the eardrum.

Other more effects of noise are as follows:
a. It disturbs normal conversation of people.
b. It disturbs concentration of people towards work.

121 Times' Crucial Science Book - 9

c. It makes people angry, excited and tired.
d. It causes the increase in blood pressure.
e. Exposure to the sound above 100 decibel causes the damage of ears.
f. It disturbs sleeping of people.

Ways to reduce sound pollution

The following steps are to be followed to control sound pollution:
a. Airports, bus parks, etc should be made away from human settlement
area.
b. Cinema hall, saw-mills, and other noise producing factories should be
made away from human settlement area.
c. Machines should be maintained or repaired properly. Grease and oil
should be used in movable parts of the machines.
d. While fixing machines, wooden or rubber pads should be placed at the
base to reduce sound.
e. Ear plugs or ear muffs should be provided to the workers who work in
noisy places for long duration.
f. Silencer should be used in the chimneys of vehicles.
g. Unnecessary blow of the horns should be avoided.
h. While listening to the music and songs, the sound level of radio,
cassettes, television, etc should be kept low. In cinema hall and
auditorium, sound absorbents should be fixed on the walls and roof.
i. Explosion of crackers in the festivals should be avoided.

Learn and Write

1. Sounds having different frequencies may have equal
speed. Why?

Speed of a sound wave is calculated by using the formula, speed
= frequency × wavelength. The sound with more frequency
may have less wavelength and vice versa. So, the product
of frequency and wavelength remains constant. Hence, the
sounds with different frequencies may have equal speed.

2. The sound produced by a person in a big empty room is
heard louder. Why?

In an empty room, there are no any matters to absorb sound.
When a sound is produced, it gets reflected from the wall and
gets mixed to the original sound. Thus, the original sound
becomes longer and is heard louder.

3. Two astronauts cannot talk on the moon as they do on

Times' Crucial Science Book - 9 122

the earth. Why?

Sound needs material medium for its propagation. Since, there
is no air on the moon, the astronauts cannot talk as they do on
the earth.

4. The walls and ceilings of cinema hall are covered with
sound absorbing materials. Why?

When walls and ceilings of a cinema hall are covered with
sound absorbing materials, the sound produced from the
cinema is absorbed by the ab-sorbers preventing from echo and
reverberation. This makes the sound clear.

5. Thunder of lightning is heard some moments after the
flash of the light is seen. Why?

Velocity of sound in air is 330m/s whereas the velocity of
light in air is 3×108m/s. Both the light and sound has to pass
through equal distance to reach to a person from the place of
thunderbolt. Since, the light travels faster than the sound,
flash of the light is seen faster than the sound is heard.

6. The frequency of a sound wave is 50 Hz. What does it
mean?

Hertz (Hz) is the unit for measuring the frequency of waves.
If the frequency of a sound wave is 50 Hz, it means that the
sound makes 50 complete waves in every one second.

7. Bats are able to fly around although they cannot see.
How?

Bats can emit and hear ultrasonic waves. While flying, they
send high frequency ultrasonic waves which spread in all
directions. If there are objects on the way, they reflect the
ultrasonic waves toward the bat. The bats analyze the reflected
sound and detect the shape, size and position of the objects.

8. The sounds of different wavelengths have the same
speed in a medium. Why?

The sound wave with a higher frequency has a shorter
wavelength whereas the sound wave of lower frequency has
a longer wavelength. Hence, the product of wavelength and
frequency, i.e. speed is always constant. Thus, the sounds of
different wavelengths have the same speed in a medium.

123 Times' Crucial Science Book - 9

Main points to remember

1. Sound is a form of energy which is produced by a vibrating object.
2. Sound propagates from one place to another place in the form of

longitudinal wave.
3. Transverse wave is the wave in which particles of the medium move

perpendicular to the direction of energy.
4. Longitudinal wave is the wave in which particles of the medium move

to and fro in the same direction as the direction of energy.
5. One crest and one trough in a transverse wave or one compression and

one rarefaction in a longitudinal wave make a complete wave.
6. Maximum displacement of the particles of a medium from the mean

position is called amplitude.
7. The number of complete waves produced in a second is called frequency.
8. The distance between two consecutive crests or troughs in a transverse

wave or two consecutive compressions or rarefactions in a longitudinal
wave is called wavelength.
9. The product of wavelength and frequency of a wave is its speed.
10. Speed of a sound is the highest in solid and the lowest in gas.
11. Density, temperature, humidity and direction of air flow are the factors
affecting speed of sound in a gas medium.
12. The sound wave having frequency less than 20 Hertz is called
infrasound.
13. The sound having frequency between 20Hz to 20KHz is called audible
sound.
14. The sound having frequency more than 20 KHz is called ultrasound.
15. Shrillness or sharpness of a sound is called pitch.
16. Intensity of a sound wave is the amount of sound energy per unit time
flowing across unit area perpendicular to the direction of wave.
17. Reflected sound is called echo.
18. Prolongation of original sound due to mixing of the reflected sound to
the original sound is called reverberation.
19. The process of bending of sound when it passes from one medium to
another medium is called refraction of sound.

20. The sound which is unpleasant, loud and disturbing is called noise.

Exercise

1. Choose the best alternative in each case.
a. What kind of wave is sound?
i. Transverse wave ii. Longitudinal wave
iii. Parallel wave iv. None

Times' Crucial Science Book - 9 124

b. For the echo to be heard, the minimum distance between the source

of sound and the reflector is

i. 17 m ii. 18 m iii. 20 m iv. 50 m

c. If the amplitude of the sound wave is reduced, its

i. Loudness decreases ii. Loudness increases

iii. Pitch increases iv. Pitch decreases

d. The process of finding the depth of a sea is based on

i. Echo ii. Reverberation

iii. Infrasonic sound iv. All of these

e. If two sounds of same loudness and same pitch are produced by two

different instruments, these sounds differ in their

i. Frequency ii. Amplitude iii. Wave forms iv. None of these

2. Answer these questions in brief.

a. What is sound? How is it produced?

b. Define following terms:

i. Crest ii. Trough iii. Compression

iv. Rarefaction v. Wavelength vi. Frequency

c. What is meant by speed of a wave ? How are speed, wavelength

and frequency related?

d. Differentiate between:

i. Transverse wave and longitudinal wave

ii. Echo and reverberation

iii. Sound wave and light wave

iv. Mechanical wave and electromagnetic wave

e. What are the various factors which affect speed of sound in

a gas medium? Explain.

f. What is spectrum of sound wave? What are the various types

of sound wave?

g. What is ultra sound? What are its practical uses?

h. What is pitch? What are the factors which affect pitch of a

sound?

i. What is intensity of a sound? What are the factors which

affect intensity of a sound?

j. What happens to the wavelength of a sound, when its pitch

increases?

k. What is noise? Mention its effects.

l. Write any four ways to reduce sound pollution.

m. Velocity of a sound in three different gases A, B and C is

given in the table.

125 Times' Crucial Science Book - 9

3. Answer the following questions. Medium Velocity (m/s)
a. Which gas has the highest A 400
density? Why? B 900
b. Which gas has the lowest C 700
density? Why?

c. If frequency of sound in all these

media is same, in which medium does the sound have

longest wavelength?

4. Draw:
i. Two waves having equal wavelength but different
amplitudes
ii. Two waves having different wavelengths but equal
amplitude

5. Give reasons:
i. Size of temple bells are made big.
ii. Echo is experienced at base of hills.
iii. Bats can detect distance, size and direction of obstacles
without seeing at night.
iv. Sounds are heard louder at night than at day.
v. Speed of sound wave is maximum in solid and minimum in
gas medium.
vi. Sound of a baby is sharper than that of an adult.

6. Numerical Problems
a. The velocity of a sound in air at ordinary conditions is 330m/s.
Calculate its wavelength if its frequency is 50,000Hz.
b. A sound wave of 15KHz has a wavelength of 0.22m. Calculate
the speed of the sound wave.
c. Calculate the depth of the sea if the echo is heard after 6
Seconds. (Given: Speed of the sound in water is 1500m/s)
d. Calculate the distance of thunder flash from the observer if
the sound is heard 3 seconds after the flash. (Given: Speed
of sound in air 330m/s).
e. A source produces 20 crests and 20 troughs in 4 seconds.
The second crest is 3cm away from the first crest. Calculate:
i. wave length ii. frequency iii. speed of wave.
f. A boy standing 100m apart from the foot of a high wall
claps his hands loudly and the echo reaches him 0.5s later.
Calculate the speed of sound in air using these data.

Times' Crucial Science Book - 9 126

Answers 6. a. 0.0066 m b. 3300 m/s c. 4500m d. 990 m

e. i. 0.03 m ii. 5 Hz iii. 0.15 m/s f. 400 m/s

Project Work

1. Make a list of different kinds of musical instruments. Which part of
them produces music? Discuss with your friends in the classroom
and write your findings.

2. Take a long rope and tie its one end to a hook or any other rigid
support. Give a jerk to the rope and observe the kind of wave
produced. Draw its diagram and explain your observation and
findings

Glossary
Collectively: together
Alternately: in an alternating sequence, skipping one step forward
Propagation: transmission, travel
Seismic: of earthquake, related to the earthquake
Fathometer: depth finder
Obstacle: resistance, something that blocks the motion of sound
Prolongation: occurring for the long time
Auditorium: an area of theater or a big hall
Hydrophone: an underground microphone

127 Times' Crucial Science Book - 9

Chapter

7 Current Electricity
and Magnetism
Alessandro Volta

He is known for the discovery of Methane and
invention of electric cell.

Estimated Periods :10

Objectives

At the end of the lesson, students will be able to:
• interpret the unit of electricity and use it in measurement;

• demonstrate Ohm’s law and show the relation of Ampere, Volt and Ohm;

• explain factors affecting resistance;

• explain magnetic field and magnetic lines of force. Discuss.

Mind Openers

• What is electricity?
• Why is electricity important?
• What is magnetic field and magnetic lines of force? Discuss.

Introduction

The current electricity has become the integral part of modern
civilization and the advancement in technologies. It has been used
for countless purposes from lighting up rooms to the operation
of communication media and large industries. Electricity is very
useful because it can be converted into any other forms of energy
and can be generated by using mechanical, chemical, heat and
magnetic energy.

Sources of electricity

A device which continuously generates electricity is known as a source
of electricity. The different forms of energy such as mechanical,
chemical, heat, light, atomic energy, etc can be converted into
electrical energy using suitable devices. Some of the devices are
introduced below.

Cell

A cell is a device which converts chemical energy into electrical
energy and produces direct current. The combination of two or more
cells to produce required voltage and current is called battery.

Times' Crucial Science Book - 9 128

Sometimes, a single cell can perform as a battery.

The electric cells can be categorized into two types according to
their reuse. They are primary cells and secondary cells. An electric
cell which cannot be recharged after its use is called primary cell.
Simple cell, dry cell, etc are primary cells.

The dry cells are used to supply direct current in small scale. They
are small in size and portable. They are used in torch, calculator,
radio, camera, watches, clocks, etc.

+

Brass cap

Carbon rod

Copper + – Zinc Carbon dust + MnO2
Muslin bag

Water with Paste of NH4Cl

dilute H2SO4 — Paper cover
Simple cell Zinc container
Dry cell

An electric cell which can be recharged and used for long time is

called secondary cell. These cells are also known as lead-acid cells.

They are commonly used in vehicles. They are also used in home for

lighting purpose and also in computer labs for short term.

A photo cell is a source of electricity which converts light energy
into electrical energy. It is more useful source of energy because it
doesn’t pollute the environment. It just converts solar energy into
electrical energy without producing harmful side effects.

Generator and dynamo

Generator and dynamo are the

devices that convert mechanical Bicycle tyre
Axle
energy into electrical energy.
Coil of wire
They work on the principle Dynamo cap

of electromagnetic induction. Magnet
These devices produce

alternating current and are

used extensively in electric

power stations.

129 Times' Crucial Science Book - 9

The electricity produced by generator using the mechanical energy
of water is called hydroelectricity. It is the best renewable source of
energy because it can be produced continuously in large scale and it
does not pollute the environment. Although its construction cost is
high in the beginning, it is cheaper in the long run use.

Electric circuit

A path for the flow of electric charge which consists of source, load
and switch with conducting wires is called electric circuit. It consists
of source of electricity, connecting wires and electrical devices (load).

An electrical device is an equipment that works by using electricity.
For example, bulb, fluorescent lamp, television, electric heater, rice
cooker, etc.

The electric circuit can be categorized into two types-open circuit

and closed circuit.

Battery Battery
+
+

Conducting Conducting
wire wire

Switch Switch
Open circuit Closed circuit

a. Open circuit

An electric circuit in which electricity cannot flow through the load
is called open circuit. In an open circuit, the switch is off and the
electrical device cannot work.

b. Closed circuit

An electric circuit in which electricity can flow continuously through
the load is called closed circuit. In a closed circuit, the switch is on
and the electrical device functions.

Short circuit Battery

An electric circuit is said to +
be short circuit when load is
replaced by a conducting wire. Conducting
The short circuit results in wire
heavy current flow due to the
negligible resistance. So, it heats Extra wire

Load Switch

Times' Crucial Science Book - 9 130

the circuit (wire) rapidly and may cause fire.
Circuit diagram

The schematic diagram of an electric circuit is called circuit diagram.
Various electrical appliances such as cells or batteries, connecting
wire, switch, resistance, etc are used in an electric circuit. These
appliances are denoted by particular symbols in the electric circuit.
Some of the symbols are given below:

S.N. Component Symbol
1. One Cell Battery
+−
2. Two Cells Battery
+−

3. Many Cells Battery + −

4. Switch Off or,
5. Switch On
6. Bulb A

7. Conducting Wire
8. Ammeter

9. Voltmeter V

Direction of electric current in a circuit

Electric current is the flow of charges from one point to another.
Before the discovery of electron by J. J. Thomson (1897), positive
charge was supposed to flow in an electric circuit and the direction
of current was assumed to be from positive terminal to the negative
terminal. The flow of positive charges or current from positive to
negative terminal of a source is called conventional direction of
current.

batte ry

+

Conventional Actual Cond ucting
direction direction wire

Switch

Actually, the current is the flow of electrons and always flows from
negative terminal to positive terminal. It is the direction opposite

131 Times' Crucial Science Book - 9

to the conventional direction and is the real direction of current
flow. Still, we use the conventional direction of flow of electric
current because many definitions and explanations are based on
conventional direction of electric current.

Electric current

The flow of charge from higher potential region to the lower potential
region per unit time is called electric current.

Mathematically, Q
Charge ∴I=

Electric Current= t
Time taken

Since the SI unit of electric charge is Coulomb and that of time is

second, the SI unit of current is Ampere (A). One Coulomb of charge

is equal to the electronic charge carried by 6.25×1018 electrons.

The charge of one electron is 1.6×10-19 Coulombs. From the above

relation,

Q 1C
I= or, IA=

t 1s

Thus, electric current is said to be one ampere if an electric charge of
one coulomb flows through a conductor in one second.

The sub-multiple units of Ampere are milliampere, microampere,
etc.

1 Ampere = 103 milliamperes
1 Ampere = 106 micro amperes

Measurement of electric current

The magnitude of current ranges from very small to large. The very
small magnitude of current is detected by a galvanometer whereas
the current of larger magnitude is measured by ammeter.

Galvanometer

A device which is used to detect the presence of
electric current in an electric circuit is called
galvanometer. The galvanometer detects very
small magnitude of current and it may be
damaged by the current of larger magnitude. It
is represented by the symbol in a circuit. It
shows the direction of the current also.

Times' Crucial Science Book - 9 132

Ammeter
An ammeter is an electrical device which is used to measure the
magnitude of the electric current flowing in a circuit. It is connected
in series in a circuit so that all the electric current flows through it.

A
– Ammeter

Load
+

Switch

An ammeter has a low resistance and it does not affect the total
resistance of the circuit even through it is connected in series.

Precautions to be taken while measuring current
1. The positive terminal of an ammeter should be connected to the positive
terminal of the source of current while its negative terminal to the
negative terminal of the source.
2. The capacity of the ammeter should be greater than the amount of
current flowing in the circuit.

3. The ammeter should be connected in series with load.

When an ammeter is connected in parallel in a circuit, it will
decrease the total resistance of the circuit. Hence, the current
flowing in the circuit will increase in magnitude. In such case, an
ammeter measures the current flowing through it and not the total
current of the circuit.

Potential difference (Pd)

Let us take two metallic spheres X and Y.

The sphere X is positively charged and the X Electrons Y
sphere Y is negatively charged. Both of them

are put on insulated stand. When these two

spheres are connected by a conducting wire,

the electrons flow from sphere Y to the

sphere X. Since positively charged body has

higher potential and negatively charged

body has lower potential, the electrons flow

from lower potential to the higher potential until the potentials of

both bodies in contact become equal.

133 Times' Crucial Science Book - 9

The difference in the potential between any two points in an
electrical circuit is called potential difference (Pd). It can also be
defined as the amount of work done in moving a unit charge from
one point to another.

AB

Mathematically,

Potential difference(Pd) = Work done(W) ∴ Pd= W

Charge(Q) Q

The SI unit of potential difference is volt (V) or Joule per Coulomb (JC-1).

1J
∴ 1V=

1C

One volt is the potential difference that exists between two points
in an electric circuit when 1 Joule of work is done while moving 1
coulomb of charge from one point to another.

The potential difference is a scalar quantity. It is measured by
using a voltmeter in a closed circuit.

Voltmeter

A voltmeter is an instrument which is used A ba ttery
to measure potential difference across any load
two points in an electric circuit. It is always V
connected in parallel to the device whose
potential difference is to be measured. It has
high resistance so that a very small current
flows through it and the potential difference
across the device is not affected.

While connecting a voltmeter in a circuit, its positive and negative
terminals are connected to the positive and negative terminals of
the source of current respectively.

A voltmeter is not connected in series. If a voltmeter is connected
in series in a circuit, the resistance of the circuit becomes very high
and the current in the circuit is reduced to a very small magnitude.
Thus, the voltmeter does not measure the actual potential difference.

Electromotive force (emf)

The difference in potential between the two terminals of a cell

Times' Crucial Science Book - 9 134

or any source in an open circuit is known as electromotive force
(emf). It drives the current through the entire electric circuit. The

electromotive force is equal to the amount of work done by the

source in taking a unit positive charge once round the circuit. In

other words, it is the energy supplied by the cell from low potential

to high potential in order to move unit positive charge. The emf of a

cell can be calculated by using the formula,

Electromotive force= Amount of work done ∴E= W
Charged particle that moves Q

The SI unit of emf is volt. The emf is equal to the sum of the potential
difference across all the components of the circuit including the
potential difference required to send current through the cell itself.
Hence, emf is always greater than potential difference.

Differences between electromotive force (emf) and potential
difference

Electromotive Force (emf) Potential Difference (pd)

1. It is the amount of energy supplied 1. It is the amount of work done in

by a source to move a unit positive moving unit positive charge from one

charge throughout a circuit. point to another.

2. It is the cause of potential 2. It is the effect of electromotive

difference. force.

3. It is equal to the sum of potential 3. Every component of circuit has its

difference across all the components own potential difference.

of the circuit including the p.d. needed

to send current through the source.

4. It is always greater than potential 4. It is always less than electromotive

difference. force

5. It is measured in open circuit. 5. It is measured in closed circuit.

6. It is independent of external 6. It depends upon the resistance in

resistance in the circuit. the circuit and is proportional to it

Ohm’s law

Ohm’s law shows the relationship between potential difference,
current and the resistance of a conductor under the constant
physical conditions. The law was formulated by a German physicist
George Simon Ohm in 1826AD.
The law states that the current flowing through a conducting wire is
directly proportional to the potential difference across its two ends
at constant physical conditions. The constant physical conditions

135 Times' Crucial Science Book - 9

means there is no change in temperature, length, cross-sectional
area and the nature of the material. Mathematically,

V  I (at constant temperature) ……….(i)
or, V = IR ………..(ii)
Where R is a proportionality constant and is known as resistance
of the conductor.

Experimental verification of Ohm's law

Activity 7 .1 To verify Ohm's law experimentally

Materials required:

Dry cells, torch light bulb, voltmeter, ammeter, Current
copper wire, switch, etc.

Proceduse: Voltage
1. Connect a dry cell, a torch light bulb(reistance)

switch, voltmer and ammeter as shown in the figure.

2. Connect a single dry cell in the electric circuit and measure voltage
and current in the circuit,

3. Measure the voltage and current each time by using two cells, three
cells and four cells in the circuit.

Observation:

Enter your readings in the observation table and calculate the
reistance in each case.

S.N voltage(V) current(A) Reistance(R=V/I) Conclusion
1
2
3
4

Plot a graph putting voltage an X-axis and current in Y-axis. You
will get the curve as given below:
Result:
The voltage is directly proportional to current.
Conclusion:
This experiment verifies the ohm’s law.

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Resistance

The property of a conductor to oppose the flow of current through it is
called resistance. It is represented by R and its SI unit is Ohm (W).

Different materials have different resistance. The metals such as
silver, copper, aluminium, etc have low resistance because these
metals contain large number of free electrons which help to conduct
electricity. Hence, these metals are used in electric circuits for the
conduction of current. Insulators and even some metals have high
resistance. The materials having high resistance are used in heating
and lighting purposes. It is because such materials produce heat or
light while opposing the flow of current through them.

Factors affecting resistance

The resistance of a conductor depends upon the following factors:
1. Length of conductor (l)

The resistance of any conducting wire is directly proportional
to its length. It can be represented as

R  l (Where, R= resistance, l = length of conductor)
2. Area of cross-section (A) or thickness

The resistance of a conductor is inversely proportional to the
cross-sectional area of the conductor. It can be represented as:

R1
A

3. Temperature
The resistance of a conductor is directly proportional to its
temperature in Kelvin scale (T).
RT

4. Nature of material
The nature of material from which the conductor is made also
affects its resistance. For example, the resistance of aluminum
is more than that of copper but less than that of nichrome.

A coiled wire is used in a filament bulb to increase the length of
the wire. It is because the resistance of a conductor increases with
the increase in its length. More the resistance, the more will be the
brightness of the bulb.

Resistivity and conductivity

Let us suppose that R be the resistance, l be the length and A be the
area of cross-section of the conductor.

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We know that, the resistance of a conductor is directly proportional
to its length and inversely proportional to the area of cross-section
i.e.

R  l .........................................................(i)

R  1 .........................................................(ii)
A

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

Rl Or, R= q l .........................................(iii)
A A

Where q (rho) is a proportionality constant and is known as

resistivity or specific resistance of the material of conductor. The

resistivity is given by

q=R×A Resistance × Cross − seetion area
l =

Length of conductor

If l = 1m and A = 1m2, then q = R.

Hence, the resistivity of a conductor can be defined as the resistance
of a conductor of length 1m having the cross-sectional area 1m2.

Unit of q

If l = 1m, A = 1m2 and R = 1 W, then

∴The SI unit of resistivity is Ohm metre (W m). One Ohm metre is
the resistivity of a conducting wire of unit length, unit cross sectional
area and the resistance of 1 ohm.

The reciprocal of resistivity of a conductor is called its conductivity.
It is denoted by r (sigma).

Conductivity(r)= 1 ∴ r = 1
Resistivity(q) q

The SI unit of conductivity is W-1m-1.
Differences between resistance and resistivity

Resistance Resistivity

1. The property of a conductor to 1. The resistance of a conductor

oppose the flow of current is called its of length 1m and cross-

resistance. section area 1m2 is called its

resistivity.

Times' Crucial Science Book - 9 138

2. Its unit is Ohm(W). 2. Its unit is Ohm metre (W m).

3. Its value changes with the change 3. It is a specific value for a

in length and cross-section area. particular conductor

Solved Numerical Problem 7.1

If a current of 5A flows through a conductor having the resistance 3 W,

what will be the potential difference?

Solution: +

Given,

Current (I) = 5 A A
Resistance (R) = 3 V
Voltage (V) = ?

Now, using Ohm’s law, we have

V = IR = 5 × 3

= 15V

∴ The potential difference is 15V

Solved Numerical Problem 7.2

An electric circuit has a 9V battery and resistance 4 W. What will be the
current flowing through the circuit?
Solution:

Given,
Potential difference (V) = 9V
Resistance (R) = 4 W
Electric current (I) = ?
Using Ohm’s law,

V = IR
or, I = V

R
= 9 = 2. 25A.

4

139 Times' Crucial Science Book - 9

Solved Numerical Problem 7.3

Calculate the resistance if the potential difference between two points of
a wire carrying a current of 15A is 220 Volt.

Solution:

Given,

Potential difference (V) = 220V.

Electric current (I) = 15A

Resistance (R) = ?

Using Ohm’s law, we have

V=IR or, R = V = 220 = 14.67 W.
I W15
Thus, the resistance of the conductor is 14.67

Combination of resistors

The devices such as bulbs, fluorescent lamps, heaters, etc connected
in an electric circuit are known as resistors or simply loads. The
resistors can be connected in two ways:
1. Series Combination
2. Parallel Combination

Series combination of resistors

The combination of resistors in which one resistor is connected next
to the other and so on is called series combination of resistors.

R1 R2 R3

V1 V2 V3

I+

Battery Switch

If I is the current flowing through the circuit, R is the total resistance
across the circuit and V is the potential difference supplied by the
source, then

V = IR

Times' Crucial Science Book - 9 140

Also, V = V1 + V2 + V3

or, IR = IR1 + IR2 + IR3 or, IR = I(R1 + R2 + R3)

∴R = R1 + R2 + R3

Hence, the total resistance is the algebraic Battery

sum of resistance of all resistors.

WWW Switch

If the bulbs are connected in series as shown

in the figure, it is called series combination of bulbs (resistors). In

such combination, the same current flows throughout the circuit. If

one of the bulbs is removed, the brightness of other bulbs increases.

On the other hand, if one more bulb is added in the circuit the

brightness of the bulbs decreases accordingly.

Parallel combination of resistors

A combination of resistors in which all positive terminals are
connected at one point and all negative terminals are connected at
another point is called parallel combination of resistors.

R1

R2

R3

Sw itch
V

If R is the total resistance, then

V = IR or, I = V
R
Also, I = I1 + I2 + I3
or, V = V + V + V ∴ 1 = 1 + 1 + 1
R R1 R2 R3
R R1 R2 R3

Thus, in parallel combination, the total resistance decreases with

the increase in the number of resistors Ω

If one of the three bulbs in parallel Ω

combination is blown off, the circuit is still Ω

complete and the remaining bulb keeps V

glowing with same brightness. This type

combination is used in domestic wiring.

141 Times' Crucial Science Book - 9

Magnetism
The property of a magnet to attract magnetic substances such as
iron, cobalt, nickel, etc is known as magnetism. Magnets have the
property of magnetism. A magnet may be natural (e.g. lodestone)
or artificial (e.g, bar magnet, U- shaped magnet, magnetic compass,
etc).
The properties of a magnet are as follows:

a. A magnet attracts magnetic substances such as iron, cobalt, nickel, etc.
b. A freely suspended magnet comes to rest showing north-south

directions.
c. Like poles of magnets repel while unlike poles attract each other.
d. A magnet is strong at the poles but weak in the middle.
e. The magnetic poles can never be separated.
f. The magnetic properties can be transformed from magnet to magnetic

materials by induction.
Neutral point
A neutral point is a particular point in the magnetic field of a
magnet at which the horizontal component of earth’s magnetic field
is equal and opposite to the magnetic field of the magnet. The needle
of a magnetic compass does not show any fixed direction and no
magnetic line of force is obtained at the neutral point. At this point,
the net magnetic field is zero.
The magnetic lines of force do not pass through the neutral point
because the resultant force of two magnetic fields is zero. In the
figure the points marked as (×) near the bar magnets are neutral
points.
If the N-pole of a bar magnet points geographical north, the neutral
points are obtained on the east and west. Similarly, if the N-pole
of a bar magnet points geographical south, the neutral points are
obtained towards north and south.

x

NN

xx

SS

x

Terrestrial magnetism
It has been proved from several experiments and researches that
the earth has a magnetic property. The magnetic property of the
earth is called terrestrial magnetism.

Times' Crucial Science Book - 9 142

It has been imagined that there is a huge magnet inside the earth
whose north pole lies towards the geographical south pole and
the south pole lies towards the geographical north pole. But the

geographical poles and the magnetic poles do not lie at the same
place.

South pole of 1800 Km
the earth’s Geographical North
magnet
Magnetic
Magnetic 17° equator
meridian
Geographical
equator

North pole of
the earth’s
Geographical South magnet

The terrestrial magnetism is supported by the following

evidences:
1. A freely suspended magnet comes to rest pointing N-S direction because
its S-pole is attracted by the geographical south pole and the N-pole is

attracted by the geographical north pole.

2. A freely suspended magnet does not show exact geographical poles at

rest. It is due to the fact that the magnetic poles and geographical poles
of the earth do not coincide. The poles of suspended magnet are attracted

by the magnetic poles, not by the geographical poles.

3. A freely suspended magnet leans at rest when it is taken towards the

poles because one pole of the suspended magnet lies closer and the

another pole lies farther from a particular magnetic pole of the earth’s

magnet.

4. A magnetic substance buried inside the earth pointing N-S direction

gets magnetized after some time.

Magnetic and geographic meridian

The imaginary vertical plane that passes
through the axis of a freely suspended Geographical north
magnet at a place is called magnetic N (Magnetic)

meridian. The magnetic meridian lies

between the magnetic north and magnetic

south poles of the earth.

The imaginary vertical plane that passes Angle of
through the geographical axis is called declination
geographical meridian. The magnetic
meridian and geographical meridian do S (Magnetic)
not coincide with each other.
Geographical south

143 Times' Crucial Science Book - 9

Angle of declination
The angle between the magnetic meridian and
geographical meridian at a place is called
angle of declination of that place. Since the
magnetic poles and geographical poles lie
at different places, the magnetic axis and
geographical axis cross at a point. Thus,
the angle made by the intersection of the
lines joining the magnetic north-south and
geographical north-south is the angle of declination of a particular
place. This angle is always an acute angle. For example, the value
of angle of declination at equator is 17°. Its value differs from place
to place.
The angle of declination plays an important role in flying aeroplanes
during night, in cloudy atmosphere or sailing ships on long voyage,
etc.
Angle of dip or inclination
A magnetic needle suspended at its centre of gravity on a horizontal
axis which is capable to rotate freely in a vertical plane is called dip
needle. A dip needle is shown in the figure below.
The needle of a dip needle always rests in a particular direction
due to the earth’s magnetic field. So, the needle makes a particular
angle with the horizontal axis which is called angle of dip. Thus, the
angle between the direction of earth’s magnetic field at a place and
horizontal line is called angle of dip at that place. The angle of dip
gives the direction of earth’s magnetic field at a place.

Variation of angle of dip

The angle of dip varies according to the place.
a. The dip needle remains parallel to the earth’s surface at the equator. So,
the angle of dip is zero.
b. At poles: At the extreme pole of the earth, the needle rests in a vertical
position. Hence, the angle of dip is 90°. At Kathmandu valley, the
value of angle of dip is 42°. It means that the dip needle makes an angle
of 42° with the horizontal axis in Kathmandu. The angle of dip is used
by pilots, navigators, voyagers, etc to find the position on the earth’s
surface.

Learn and Write

1. How is electric charge or static electricity produced?
When an object is rubbed with another object, it may lose or

Times' Crucial Science Book - 9 144

gain electrons on its surface. Such gain or loss of electrons
produces electric charge or static electricity in the rubbed
bodies. If electrons are removed from a body, it is said to be
positively charged. If electrons are accumulated on the surface
of a body, it is said to be negatively charged.
2. Even though the actual direction of current is from negative to the
positive terminal of a cell, the conventional direction is in use. Why?
Before Thomson, the direction of current was believed to flow
from positive to negative terminal of a cell. But, it was proved
by Thomson that the actual direction of the current is from
negative to positive terminal of a cell. But many definitions
and explanations were based on the conventional direction.
Therefore, conventional direction is in use yet.
3. Ammeter is connected in series with the load. Why?
Ammeter has low resistance. When it is connected in series,
it does not affect the resistance of the circuit and allows the
current flow easily. Then, it measures the value of whole current
flowing through the circuit. If it is connected in parallel to the
load, it does not measure the whole current flowing through
the circuit. Therefore, ammeter is connected in series to the
load.
4. Nichrome wire kept inside the filament lamp is coiled. Why?
When the nichrome wire kept in the filament lamp is coiled,
its length increases. The increase in the length increases the
resistance. More the resistance, the more will be the brightness
of the bulb. Therefore, the nichrome wire kept inside the
filament lamp is coiled.
5. Angle of dip is 90° at the magnetic pole. Why?
At magnetic north of the terrestrial magnet, there will be
immense effect of this pole to the dip needle. But the effect
of the south pole of the terrestrial magnet to the dip needle is
negligible. Due to this, the needle of the dip needle remains in
vertical position pointing the south pole towards the magnetic
north. Similarly, needle of dip needle remains in the vertical
po-sition at the magnetic south pole. Hence, angle of dip is 90°
at the magnetic poles.
6. Voltmeter is always connected in parallel with a load. Why?
A voltmeter has a very high resistance. If it is connected in
parallel with the load, the net resistance of the circuit does not
change. Due to this, the current in the circuit as well as the

145 Times' Crucial Science Book - 9

potential difference across the load are not affected. Hence,
the voltmeter gives the measure of actual potential difference
across the load. On the other hand, if the voltmeter is connected
in series with the load, it alters and sometimes even stops the
flow of current in the circuit. It is the reason why a voltmeter
is always connected in parallel with the load.
7. The compass needle does not show the actual directions at the
neutral points. Why?
The magnetic field of the magnet and the magnetic field of the
earth are equal and in opposite direction at the neutral point.
Hence, the resultant magnetic field at this point is zero. So,
the magnetic compass does not show any definite direction at
this point.
8. Why does the dip needle remain parallel to the horizontal line
at magnetic equator?
The north and south poles of the terrestrial magnet have
equal and opposite influence at all parts lying in the magnetic
equator. When a dip needle is placed at the magnetic equator,
its poles are equally and oppositely by the poles of terrestrial
magnet. As a result, the dip needle remains parallel to the
horizontal axis (making 0° angle with the horizontal line).

Main points to remember

1. Adevicewhichcontinuouslygenerateselectricityisknownassourceofelectricity.
2. A cell is a device which converts chemical energy into electrical energy

and produces current.
3. The flow of charge from higher potential to lower potential per unit time

is called electric current.
4. A device which is used to detect the presence of electric current in an

electric circuit is called galvanometer.
5. An ammeter is an electrical device which is used to measure the

magnitude of electric current flowing in the circuit.
6. The potential difference between two points in an electric field is defined as the

amount of work done in moving a unit charge from one point to another.
7. The difference in potential between two terminals of a cell or any source

in an open circuit is known as electromotive force.
8. Ohm’s law states that the current flowing through a conducting wire

is directly proportional to the potential difference across its two ends at
constant temperature.
9. A neutral point is a particular point in the magnetic field of a magnet

Times' Crucial Science Book - 9 146