Class 9 Science_clone

colours inside the glass slab. When these different coloured rays
come out to the air, they combine together to give white ray.
7KXV D UHFWDQJXODU JODVV VODE GRHV QRW GLVSHUVH ZKLWH OLJKW

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. Its velocity
changes if it travels from one optical medium to the next.
7KH FKDQJH LQ YHORFLW\ OHDGV WR WKH EHQGLQJ RI SDWK RI OLJKW DW
WKH LQWHUIDFH RI WKH WZR RSWLFDO PHGLD 7KLV SURFHVV LV FDOOHG
refraction. Hence, the change in velocity of light on going from
one optical medium to another is the cause of refraction of
light.
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?
7KH UD\V IURP WKH VXQ WUDYHO JUHDWHU GLVWDQFH WKURXJK WKH
atmosphere during sunrise and sunset. In this situation, the
white beam of light is dispersed (scattered) by the atmosphere.
7KH JUHHQ EOXH DQG RWKHU FRORXUV RI WKH GLVSHUVHG OLJKW DUH
VFDWWHUHG DZD\ DQG WKH\ FDQQRW UHDFK RXU H\HV 7KH UHG OLJKW
which gets scattered the least, reaches our eyes. Hence, the sun
as well as its nearby sky seems red during sunrise and sunset

Glossary an area where there is no any material
a medium having less density
Vacuum a medium having more density
Rarer
Denser

97 Times' Crucial Science Book - 9

7ZLQNOH shine with a gleam that changes constantly from
Illusion bright to faint
Sparkle
Binocular wrong or misinterpreted perception
Diffraction shine brightly with flash of light
Polarization an optical instrument with lens for each eye which

is used for viewing distant objects.
spreading of waves from an obstacle
action of restricting the vibrations of a transverse

wave to one direction.

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 refraction
for the given pair of media is constant. This law is also known as
Snell’s law. It can be represented as:

sini = Constant (P)
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.

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
LQFLGHQFH JUHDWHU WKDQ FULWLFDO DQJOH LV FDOOHG WRWDO LQWHUQDO UHÁHFWLRQ

Times' Crucial Science Book - 9 98

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.

7KH FODVVLÀFDWLRQ RI HOHFWURPDJQHWLF ZDYHV RQ WKH EDVLV RI WKHLU
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

L 6PDOO FULWLFDO DQJOH LL 7RWDO LQWHUQDO UHÀHFWLRQ

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?
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.

99 Times' Crucial Science Book - 9

H +RZ LV RSWLFDO ÀEUH PDGH"
f. What is endoscopy?
g. How is the speed of light wave related to its frequency and

wavelength?
K :KDW LV D WRWDOO\ UHÁHFWLQJ SULVP"
i. Name the electromagnetic wave which has the shortest

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

wavelength.

'HILQH c. Denser medium
a. Light b. Rarer medium f. Mirage
d. Spectrum of light e. Light pipe

'LIIHUHQWLDWH EHWZHHQ
a. Real depth and apparent depth
E 5HÁHFWLRQ DQG UHIUDFWLRQ RI OLJKW
c. Refraction and dispersion of light

*LYH UHDVRQV
D $ FRLQ SODFHG LQ D ZDWHU ÀOOHG JODVV DSSHDUV WR EH DW OHVV
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.
J $ WRWDOO\ UHÁHFWLQJ SULVP LV XVHG LQVWHDG RI SODQH PLUURU
in binoculars.

6. Answer these questions.
a. What is refraction of light? Write down its laws.
b. State and explain Snell’s law with suitable diagrams.
F :KDW LV RSWLFDO ÀEUH" :ULWH GRZQ LWV LPSRUWDQFH
d. How is mirage observed? Explain with a diagram.
e. What is dispersion of light? Show with a diagram.
I 'HÀQH HOHFWURPDJQHWLF ZDYHV ZLWK H[DPSOHV
g. Write down the uses of x-rays and gamma-rays.
h. How are ultraviolet rays useful?

Times' Crucial Science Book - 9 100

i. Write down the common properties of electromagnetic waves.
j. What is the cause of dispersion of light? Explain.

'LDJUDPPDWLF TXHVWLRQV
a.

L :KDW SURFHVV LV VKRZQ LQ WKH ÀJXUH"
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?
E &RPSOHWH WKH IROORZLQJ GLDJUDPV

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 2.2×108P V Given, velocity of light in air = 3×108P V).
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. 26.51° b. 47.16° c. 22.55m

Project Work

7DNH D 1HZWRQ·V :KHHO DQG WXUQ LW UDSLGO\ :KDW GR \RX REVHUYH" :K\ LV
such observation? Write down the process, observation and conclusion.

7DNH D ZDWHU WURXJK DQG KDOI ILOO ZLWK ZDWHU 3ODFH WKH WURXJK LQ
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.

101 Times' Crucial Science Book - 9

Chapter 6RXQG

6 Heinrich Rudolf Hertz

Objectives +H LV NQRZQ IRU WKH GLVFRYHU\ RI HOHFWURPDJQHWLF
UDGLDWLRQ SKRWRHOHFWULF HIIHFW +HUW]
V SULQFLSOH
etc.

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ ϲ

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗

• LGHQWLI\ LQIUDVRXQG DXGLEOH VRXQG XOWUDVRXQG ZDYHV DQG WKHLU VRXUFHV

• H[SODLQ QDWXUH RI VRXQG ZDYH

• H[SODLQ WKH PHDQLQJ RI LQWHQVLW\ DQG SLWFK RI VRXQG

• H[SODLQ WKH IDFWRUV DIIHFWLQJ VSHHG RI VRXQG LQ DLU

• GHVFULEH UHIUDFWLRQ DQG UHÁHFWLRQ RI VRXQG ZLWK H[DPSOHV

• H[SODLQ WKH FDXVHV HIIHFWV DQG SUHYHQWLYH PHDVXUHV RI VRXQG SROOXWLRQ

Mind Openers

• :KDW LV VRXQG" &DQ VRXQG SDVV WKURXJK YDFXXP"
• What are the sources of infra, audible and ultrasound?
• :KRVH VRXQG KDV PRUH SLWFK HLWKHU EDE\·V RU ROG PDQ·V VRXQG" 'LVFXVV

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 vibrating 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.
:KHQ \RX SXW \RXU ÀQJHUV DJDLQVW \RXU WKURDW
while speaking, you can feel vibrations.
7KXV, any object which vibrates is a source of sound. When vibration
of the object stops, the production of the sound also stops.

Times' Crucial Science Book - 9 102

*XLWDU 0DGDO 7HOHYLVLRQ 5DGLR

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
RI ZDYHV :DYH LV RI WZR W\SHV
D 7UDQVYHUVH ZDYH E /RQJLWXGLQDO ZDYH
a. Transverse wave
7UDQVYHUVH ZDYH LV WKH ZDYH LQ ZKLFK SDUWLFOHV RI WKH PHGLXP PRYH
up and down perpendicular to the direction of energy.

Crest

AB

7URXJK

7UDQVYHUVH ZDYH

,Q WKH ÀJXUH WKH HQHUJ\ SDVVHV IURP SRLQW A to B and particles of
the medium move up and down perpendicular to the direction of
energy.

7KH XSZDUG UDLVHG SDUW RI WKH ZDYH LV FDOOHG FUHVW DQG WKH GRZQZDUG
lower most part is called trough.

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

103 Times' Crucial Science Book - 9

Compression

AB

Rarefaction
Longitudinal wave

,Q WKH ÀJXUH WKH HQHUJ\ SDVVHV IURP SRLQW 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.

Fact Reason

Why is sound wave called mechanical wave?
Sound wave is called mechanical wave because it needs material
medium for propagation.

Propagation of sound and sound wave

Activity 6 .1 7R VWXG\ WKH RULJLQ RI ZDYHV

0DWHULDOV UHTXLUHG
7XQLQJ IRUN D ERZO ZLWK ZDWHU HWF
Procedure:
7DNH D ERZO ZLWK ZDWHU DQG SXW LW

on a table.
7DNH D WXQLQJ IRUN DQG KLW LWV IRUNV RQ D UXEEHU SDG
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
ZKHUH \RX WRXFKHG WKH ZDWHU ZLWK WKH SURQJ 7KHVH ULSSOHV DUH FDOOHG ZDYHV
7KH\ FDUU\ VRXQG IURP WKH VRXUFH WR RWKHU SODFHV

Times' Crucial Science Book - 9 104

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
DQG ORRVHO\ DUUDQJHG LQ RWKHU UHJLRQV 7KH UHJLRQ ZKHUH WKH DLU
molecules are tightly packed is called compression and the region
where the air molecules are loosely arranged is called rarefaction.
7KH FRPSUHVVLRQV DQG UDUHIDFWLRQV DUH IRUPHG DOWHUQDWHO\ LQ WKH
DLU 7KH FRPSUHVVLRQV DQG UDUHIDFWLRQV FROOHFWLYHO\ IRUP ZDYH 7KLV
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. 7KH SURFHVV RI WUDQVPLVVLRQ RI VRXQG IURP RQH
place to the another is called propagation of sound.

Sound wave travels through material medium only. It cannot travel
LQ YDFXXP 7KHUHIRUH LW LV FDOOHG D mechanical wave.

Differences between transverse wave and longitudinal wave

Transverse wave Longitudinal wave

1. 7KH ZDYH LQ ZKLFK WKH SDUWLFOHV RI 1. 7KH ZDYH LQ ZKLFK WKH
the medium vibrate up and down particles of the medium
perpendicular to the direction of vibrate to and fro in the same
energy is transverse wave. direction as the direction of
energy is called longitudinal
wave.

105 Times' Crucial Science Book - 9

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

Activity 6 .2 7R VKRZ ORQJLWXGLQDO ZDYH

0DWHULDOV UHTXLUHG Compression

Slinky spring Rarefaction
Procedure :

7DNH D VOLQN\ VSULQJ DQG À[ RQH HQG RI LW WR D ZDOO

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

2EVHUYDWLRQ

7ZR W\SHV RI UHJLRQV DUH GLVWLQFWO\ IRUPHG LQ WKH VSULQJ 7KH UHJLRQ ZKHUH WKH
rings are tightly packed is compression and the region where the rings are
loosely arranged is rarefaction.

Fact Reason

We cannot hear sound wave in vacuum, why?
Sound wave needs material medium for propagation as it is
mechanical wave. So, we cannot hear sound wave in vacuum.

Some terms related to wave

Crest and trough

In transverse wave, particles of medium move up and down from the
mean position. 7KH SRUWLRQ RI WKH ZDYH ZKLFK OLHV DW WKH PD[LPXP
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

7URXJK

Times' Crucial Science Book - 9 7UDQVYHUVH ZDYH

106

Amplitude(a)
7KH PD[LPXP GLVSODFHPHQW RI SDUWLFOHV IURP PHDQ SRVLWLRQ GXULQJ
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. 7KH UHJLRQ RI D ORQJLWXGLQDO ZDYH LQ ZKLFK SDUWLFOHV DUH
loosely arranged is called rarefaction. It has lower density.

Compression

AB

Rarefaction
Longitudinal wave
Complete wave
7KH ZDYH ZKLFK FRQWDLQV RQH FUHVW DQG RQH WURXJK LQ D WUDQVYHUVH
wave or one compression and one rarefaction in a longitudinal wave
is called complete wave.

Wavelength (O)
7KH GLVWDQFH EHWZHHQ WZR FRQVHFXWLYH WURXJKV RU FUHVWV LQ D WUDQVYHUVH
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 (O).
Frequency (f)
7KH QXPEHU RI FRPSOHWH F\FOHV RU ZDYHV SURGXFHG LQ RQH VHFRQG LV
called frequency. 7KH 6, XQLW RI IUHTXHQF\ LV KHUW] (Hz) 7KH ELJJHU
units of frequency are kilohertz (KHz), megahertz (MHz), etc. In the
DERYH ÀJXUH WZR FRPSOHWH ZDYHV DUH SURGXFHG LQ RQH VHFRQG WKXV
the frequency is 2 Hertz.
Time period (T)
7KH WLPH WDNHQ E\ WKH ZDYH WR FRPSOHWH RQH F\FOH RU RQH ZDYH LV
called time period. Its SI unit is second.
)RU H[DPSOH ,I D ZDYH WDNHV VHFRQGV WR FRPSOHWH D ZDYH LWV WLPH
period is 2 seconds.

107 Times' Crucial Science Book - 9

Wave speed (v)

7KH GLVWDQFH WUDYHOOHG E\ D ZDYH LQ XQLW WLPH LV FDOOHG VSHHG 7KH
6, XQLW RI VSHHG LV P V

Relation between wavelength, frequency and speed

Suppose, frequency of a wave is 40 O

Hertz, it means 40 complete waves are
produced in each second. In other words, A
B

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

7KXV GLVWDQFH WUDYHOOHG E\ WKH ZDYH LQ VHFRQG :DYH OHQJWK ð IUHTXHQF\

or, speed = wavelength × frequency
?v=O×f

Solved Numerical Problem 6.1

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

Wavelength (O) = 3.3 m
Frequency (f) = 100 Hz
Speed (v) = ?
We have,
v =O×f

= 3.3 × 100
P V
? 7KH VSHHG RI WKH ZDYH LV P V

Solved Numerical Problem 6.2

7KH VSHHG RI D VRXQG LV P V LWV IUHTXHQF\ +] )LQG LWV ZDYHOHQJWK.
Given,
6SHHG Y P V

Frequency (f) = 50 Hz
Wavelength (O) = ?
We have,
v =O×f
330 = 50 × O
or, O = 6.6 m
? Wavelength of the wave is 6.6 m

Times' Crucial Science Book - 9 108

Sound and its speed in different media
Sound is a form of energy which is produced by any vibrating object.
,W SURGXFHV VHQVDWLRQ RI KHDULQJ LQ HDU ,W KDV IROORZLQJ IHDWXUHV

L ,W LV SURGXFHG E\ D YLEUDWLQJ ERG\
ii. It needs material medium for transmission. Therefore, sound

ZDYH LV FDOOHG D PHFKDQLFDO ZDYH
iii. It transmits from one place to another in the form of

ORQJLWXGLQDO ZDYH
LY ,W UHIOHFWV RU UHIUDFWV OLNH RWKHU ZDYHV
Y ,WV VSHHG GLIIHUV IURP PHGLXP WR PHGLXP
YL ,W KDV WKH KLJKHVW VSHHG LQ VROLG DQG WKH ORZHVW VSHHG LQ JDV

,Q VROLGV PROHFXOHV DUH WLJKWO\ DUUDQJHG 7KH IRUFH RU VKRFN H[HUWHG
by the sound to one molecule is immediately transferred to other
PROHFXOHV 7KXV VRXQG WUDYHOV IDVWHU LQ VROLG %XW PROHFXOHV DUH
not tightly packed in liquid and gas. Moreover, molecules are far
DSDUW LQ JDV PHGLXP 7KXV IRUFH RU VKRFN H[HUWHG RQ RQH PROHFXOH
WDNHV ORQJ WLPH WR WUDQVIHU WR RWKHU PROHFXOHV 7KLV PDNHV WKH VSHHG
of sound in gas low.

Speed of sound in different media

SN Medium 6SHHG P V SN Medium 6SHHG P V
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
F +XPLGLW\ G 'LUHFWLRQ RI DLU ÁRZ

a. Density

7KH VSHHG RI VRXQG LQ D JDV PHGLXP LV LQYHUVHO\ SURSRUWLRQDO WR WKH
square root of the density of the gas. i.e.

Speed D 1
¥'HQVLW\

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.

109 Times' Crucial Science Book - 9

b. Temperature

7KH VSHHG RI VRXQG LV GLUHFWO\ SURSRUWLRQDO WR WKH VTXDUH URRW RI
absolute temperature of the gas.

Speed of sound v ¥7HPSHUDWXUH

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

c. Humidity

7KH ZDWHU YDSRXU SUHVHQW LQ DLU LV FDOOHG KXPLGLW\ :DWHU YDSRXU
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.

G 'LUHFWLRQ RI DLU ÁRZ

7KH VSHHG RI VRXQG LV PRUH LQ WKH GLUHFWLRQ RI DLU ÁRZ %XW WKH
VSHHG RI VRXQG LV OHVV LQ WKH GLUHFWLRQ RSSRVLWH WR WKH DLU ÁRZ

But, the following factors have no any effect for the speed of sound
LQ D JDV PHGLXP
a. Change in frequency b. Change in wave length
c. Change in loudness, pitch, etc d. Change in pressure

Spectrum of sound waves

Frequency of sound wave differs according to its source. Some
sounds have less frequency and some have more frequency. 7KH
graphical representation of sound waves of various frequencies is
called spectrum of sound wave.

7KH VRXQG ZDYHV DUH FODVVLÀHG LQWR WKUHH FDWHJRULHV 7KH\ DUH
infrasonic sound, audible sound and ultrasonic sound.

Infrasonic sound

7KH VRXQG ZDYH ZLWK IUHTXHQF\ More than 20 KHz Ultra sound
less than 20Hz is called
infrasonic sound. It is also 20 Hz – 20 KHz Audible sound
called sub-sonic or infra sound. Less than 20 Hz Infrasonic sound

It cannot be heard by human beings. But it can be felt by touching
WKH VRXUFH 6XFK VRXQG LV SURGXFHG E\ YHU\ ELJ PDVVHV 7KH VHLVPLF
waves produced by an earthquake are example of infrasonic sound.
Animals like elephant use infrasonic sound.

Times' Crucial Science Book - 9 110

Audible sound
7KH VRXQG ZDYH KDYLQJ WKH IUHTXHQF\ EHWZHHQ +] WR .+] LV FDOOHG
audible sound. 7KLV W\SH RI ZDYH FDQ EH KHDUG E\ KXPDQ EHLQJ 'LIIHUHQW
LQVWUXPHQWV OLNH ÁXWH JXLWDU SLDQR HWF SURGXFH DXGLEOH VRXQGV
Ultrasonic sound
7KH VRXQG ZDYH KDYLQJ WKH IUHTXHQF\ PRUH WKDQ NLORKHUW] LV
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.

Fact Reason

Why cannot we hear the sound of bat?
Bat produce ultrasounds whose frequency is above the human
audible range i.e. above 20 KHz. So, we cannot hear the sound of bat.

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 and
Surface of sea

ÁDZV LQ PHWDOV Source of sound Sound detector

5. It is used by animals like bats,
ZKDOHV HWF IRU WKH GHWHFWLRQ RI WKHLU
preys and obstacles of the path. Incident sound Depth of sea
5HÁHFWHG VRXQG
6. ,W LV XVHG WR ÀQG WKH GHSWK RI RFHDQ
RU VHD ,Q WKLV PHWKRG ZDYHV RI
XOWUDVRXQG DUH VHQW WRZDUGV WKH EHG Bottom of sea

RI VHD RU RFHDQ E\ D IDWKRPHWHU 7KH ZDYHV JHW UHÁHFWHG IURP WKH EHG
and received by the hydrophone at the surface of the sea. The time
LQWHUYDO EHWZHHQ WKH HPLVVLRQ DQG UHFHLSW RI WKH ZDYH LV QRWHG 7KLV
WLPH LV XVHG WR FDOFXODWH WKH GHSWK RI VHD E\ XVLQJ WKH IROORZLQJ IRUPXOD

Depth = Speed of sound × time
2
v×t
Or, d = 2 [ Depth = Distance]

111 Times' Crucial Science Book - 9

Solved Numerical Problem 6.3

A hydrophone receives the waves 3 seconds after it is transmitted. Find
WKH GHSWK RI WKH VHD *LYHQ VSHHG RI ZDWHU LV P V

Given,

7LPH W VHFRQGV

6SHHG RI VRXQG LQ ZDWHU Y P V

Depth of Sea (d) = ?

We have,

v×t = 1500 × 3 = 2250m
d= 2 2

7KHUHIRUH GHSWK RI WKH VHD LV P

Characteristics of Sounds

Sound produced from various instruments or animals differs from
each other. One sound differs from another sound due to four
DVSHFWV 7KH\ DUH

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
XSRQ IUHTXHQF\ 7KH KLJKHU WKH IUHTXHQF\ WKH KLJKHU LV WKH SLWFK 7KH
sounds of babies and girls have higher pitch and are sharper.

Fact Reason

7KH VRXQG SURGXFHG E\ JLUO LV VKDUSHU WKDQ ER\V ZK\"
7KH SLWFK RI WKH VRXQG LV PRUH LQ WKH VRXQG RI WKH JLUO EHFDXVH RI
higher frequency which increases the sharpness and shrillness of
the sound. Hence, the sound produced by girl is sharper than boys.

7KH IUHTXHQF\ DQG SLWFK DUH QRW VDPH )UHTXHQF\ FDQ EH PHDVXUHG
and has unit. But pitch cannot be measured and cannot be shown in
diagram. But pitch of a sound increases with increase in frequency.
7KH LQFUHDVH LQ IUHTXHQF\ UHGXFHV ZDYHOHQJWK 7KXV WKH VRXQG RI
high pitch has less wavelength.

Times' Crucial Science Book - 9 112

Higher frequency Lower frequency

)ROORZLQJ IDFWRUV DIIHFW WKH SLWFK RI VRXQG

Frequency
7KH JUHDWHU WKH IUHTXHQF\ WKH KLJKHU LV WKH SLWFK

Length of wire
Short wires produce the sound having higher pitch than the long wires.

Thickness of wire
7KLQ ZLUHV SURGXFH WKH VRXQG KDYLQJ KLJKHU SLWFK ZKHUHDV WKH WKLFN
wires produce sound having lower pitch.

Tension
7KH KLJKHU WKH WHQVLRQ RI WKH VWULQJ WKH KLJKHU LV WKH SLWFK RI VRXQG

Activity 6 .3 7R VWXG\ WKH IDFWRUV DIIHFWLQJ IUHTXHQF\

Materials needed :

A guitar or a sonometer

Procedure :
7DNH D JXLWDU DQG SOXFN D VWULQJ RI LW

Listen to the sound produced.
7LJKWHQ WKH VDPH VWULQJ DQG SOXFN LW DJDLQ

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.

2EVHUYDWLRQ
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 :
7HQVLRQ WKLQQHVV DQG GHFUHDVH LQ OHQJWK RI WKH ZLUHV LQFUHDVH
frequency of a wave.

113 Times' Crucial Science Book - 9

Amplitude

$PSOLWXGH RI D ZDYH LV GHÀQHG DV WKH PD[LPXP GLVSODFHPHQW RI
particles of a medium from their mean position. Nature of amplitude
can be better understood in a transverse wave.

Crest

a B
A

7URXJK

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
VRPH ZDWHU PROHFXOHV VLQN EHORZ WKH VXUIDFH OHYHO 7KH UDLVHG ZDWHU
molecules create the crest whereas the dipped water molecules
create the trough.

7KH KHLJKW RI WKH FUHVW RU GHSWK RI WKH WURXJK LV FDOOHG DPSOLWXGH

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
WLPH ÁRZLQJ DFURVV XQLW DUHD SHUSHQGLFXODU WR WKH GLUHFWLRQ RI WKH
wave.
Sound energy E Joule
Intensity of Sound = 7LPH ð $UHD or, I = t×A = Second × m2
(Unit)
[ [
= Watt Joule
m2 ? Watt= Second

7KHUHIRUH WKH 6, XQLW RI LQWHQVLW\ RI 6RXQG LV :DWW P2.

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 :DWW P2.

1 decibel = 10-12 :DWW P2

Times' Crucial Science Book - 9 114

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. 7KUHVKROG RI KHDULQJ 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.

7KH DERYH WDEOH VKRZV WKDW WKH ORXGQHVV RI VRXQG LQFUHDVHV ZLWK
LQFUHDVH LQ LQWHQVLW\ )ROORZLQJ IDFWRUV DIIHFW WKH LQWHQVLW\ RI VRXQG
1. Amplitude of vibration
Intensity of sound is directly proportional to the square of amplitude,
i.e.

, Ĵ D2
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.

115 Times' Crucial Science Book - 9

4. The direction of wind

7KH ORXGQHVV RI VRXQG LV JUHDWHU LQ WKH GLUHFWLRQ RI ZLQG WKDQ LQ WKH
opposite direction.

Quality or tone
7KH WRQH LV WKH SURSHUW\ RI D VRXQG E\ ZKLFK WZR VRXQGV RI VDPH
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.
5HÁHFWLRQ RI VRXQG
Like other forms of energy, sound also returns back to the previous
medium when it strikes to any obstacle. 7KH SKHQRPHQRQ RI
returning back of sound to the same previous medium when it
VWULNHV WR D VROLG REVWDFOH LV FDOOHG UHÁHFWLRQ RI VRXQG

Activity 6 .4 7R GHPRQVWUDWH WKH UHIOHFWLRQ RI VRXQG

0DWHULDOV UHTXLUHG Cardboard tube
7ZR SODVWLF SLSHV D SODQH PLUURU D 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
SODQH PLUURU DV VKRZQ LQ ÀJXUH
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 ?

2EVHUYDWLRQ

<RX FDQ KHDU WKH ·WLF· ·WLF· VRXQG RI WKH FORFN 7KLV LV EHFDXVH WKH
VRXQG SURGXFHG E\ WKH FORFN LV UHÁHFWHG IURP WKH SODQH PLUURU $QG LW
reached to the ear.

Conclusion :

6RXQG LV UHÁHFWHG ZKHQ LW VWULNHV D VROLG REVWDFOH RU VXUIDFH

Times' Crucial Science Book - 9 116

Echo

,I ZH VKRXW ORXGO\ LQ IURQW RI D KLOO RU LQ IURQW RI D ODUJH UHÁHFWLQJ
REMHFW ZH ZLOO KHDU RXU RZQ VRXQG DIWHU VRPHWLPH 7KLV VRXQG
ZKLFK ZH KHDU ODWHU LV HFKR ,W LV WKH UHÁHFWHG IRUP RI RULJLQDO VRXQG
E\ KLOO RU UHÁHFWLQJ VXEVWDQFH 7KH UHÁHFWHG VRXQG LV FDOOHG HFKR
Following conditions are required for the echo to occur.

7KH GLVWDQFH EHWZHHQ WKH VRXUFH DQG UHÁHFWLQJ VXUIDFH PXVW EH DW OHDVW
17m away.

7KH UHÁHFWLQJ VXUIDFH VKRXOG EH ULJLG DQG ODUJH
7KH VRXQG PXVW KDYH VXIÀFLHQW HQHUJ\

Reverberation

When you speak in an empty big hall, you feel that your sound
EHFRPHV ORXGHU +HUH UHÁHFWLQJ VXUIDFHV L H ZDOOV DUH FORVHU IURP
WKH VRXUFH RI VRXQG 6R UHÁHFWHG VRXQG JHWV PL[HG WR WKH RULJLQDO
VRXQG DQG WKH RULJLQDO VRXQG EHFRPHV ORQJHU 7KLV HIIHFW LV FDOOHG
reverberation. Reverberation is the prolongation of original sound
GXH WR PL[LQJ RI UHÁHFWHG VRXQG WR WKH RULJLQDO VRXQG For the
reverberation to occur the distance between the source of sound and
UHÁHFWLQJ VXUIDFH VKRXOG EH OHVV WKDQ P

In auditorium or cinema hall, the excessive reverberation is not
GHVLUDEOH 7R UHGXFH UHYHUEHUDWLRQ WKH URRIV DQG ZDOOV RI WKH
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.

/LWWOH UHYHUEHUDWLRQ LV EHQHÀFLDO LQ PXVLFDO VRXQGV ,W PDNHV WKH
sound lively.

Condition for reverberation
7KH GLVWDQFH EHWZHHQ WKH VRXUFH RI VRXQG DQG UHÁHFWLQJ VXUIDFH
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. 7KH UHÁHFWHG VRXQG LV FDOOHG 1. Prolongation of original sound
echo. GXH WR PL[LQJ RI UHÁHFWHG
sound to the original sound is
called reverberation.

117 Times' Crucial Science Book - 9

2. For distinct echo, the 2. For reverberation, the
UHÁHFWLQJ VXUIDFH VKRXOG EH UHÁHFWLQJ VXUIDFH VKRXOG EH
more than 17m away from nearer than 17m from the
the source of sound. source of sound.

3. 7KH LQWHQVLW\ RI HFKR LV OHVV 3. 7KH LQWHQVLW\ RI UHYHUEHUDWLRQ
than that of original sound. is more than that of original

sound.

Fact Reason

Why are the ceilings of cinema hall curved?
7KH FHLOLQJV RI FLQHPD KDOO DUH FXUYHG WR SUHYHQW WKH HFKR DQG
UHYHUEHUDWLRQ FDXVHG E\ WKH UHÁHFWLRQ RI VRXQG

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. 7KH
air with higher temperature has less density. 7KH VSHHG RI VRXQG
is higher in such air. Similarly, the air with lower temperature has
PRUH GHQVLW\ 7KH VSHHG RI DLU LV OHVV LQ VXFK DLU
At day time, the ground is hotter and hence the air nearer the
JURXQG 7KH WHPSHUDWXUH RI WKH DLU diminishes in upward direction.
7KXV WKH DLU RI WKH XSSHU OD\HUV EHKDYHV OLNH GHQVHU PHGLXP
When sound is produced on the ground, it gradually moves upwards
bending towards normal (according to the laws of refraction).
7KXV WKH VRXQG JHWV UHIUDFWHG XSZDUGV DQG OHVV VRXQG UHDFKHV WR
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
JURXQG LV FRROHU WKDQ WKH DLU RI XSSHU OHYHOV 7KXV WKH DLU EHFRPHV
rarer when we move upwards. When sound is produced on the

Times' Crucial Science Book - 9 118

ground, it refracts continuously away from the normal. Finally, the
sound waves undergo WRWDO LQWHUQDO UHÁHFWLRQ and bend towards
GRZQZDUGV 7KXV VRXQG FDQ WUDYHO IDUWKHU DQG UHDFK WR WKH OLVWHQHU
Hence, sound can be heard up to long distance clearly at night time.
Furthermore, the environment is calm and silent during the night
WLPH 7KHUH LV QR FKLUSLQJ RI ELUGV KHDY\ PRYHPHQW RI YHKLFOHV
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 electromagnetic
ZDYHV 7KHUHIRUH WKH\ QHHG ZDYHV 7KHUHIRUH WKH\ GR QRW QHHG
material medium to propagate. material medium to propagate.

2. 7KH\ DUH ORQJLWXGLQDO ZDYH 2. 7KH\ DUH WUDQVYHUVH ZDYH

3. 6SHHG RI VRXQG ZDYH LV P V 3. Speed of light wave is 3×108P V
in air. in air or vacuum.

4. Sound waves bring sensation 4. Light waves bring sensation of
of hearing in ears. 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 6XFK VRXQG LV FDOOHG QRLVH 7KH VRXQG SURGXFHG IURP
factories, industries, vehicles, loud speaker, crowd, etc are such
unpleasant sound or noise.

Noise pollution

7KH VRXQG ZKLFK LV XQSOHDVDQW ORXG DQG GLVWXUELQJ LV FDOOHG
noise. 7KH VRXQGV SURGXFHG IURP IDFWRULHV LQGXVWULHV YHKLFOHV
ORXGVSHDNHUV FURZG HWF DUH H[DPSOHV RI QRLVHV 7KH QRLVH DGYHUVHO\
DIIHFWV WKH HQYLURQPHQW 7KH VRXQG ZLWK LQWHQVLW\ PRUH WKDQ 'E
creates discomfort to our ears. Sound above 100Db can make us
deaf. Sounds with 140 Db can tear the eardrum.

119 Times' Crucial Science Book - 9

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

7KH IROORZLQJ VWHSV DUH WR EH IROORZHG WR FRQWURO VRXQG SROOXWLRQ
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.
G :KLOH À[LQJ PDFKLQHV ZRRGHQ RU UXEEHU SDGV VKRXOG EH SODFHG DW WKH

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
DXGLWRULXP VRXQG DEVRUEHQWV VKRXOG EH À[HG RQ WKH ZDOOV DQG URRI
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
IUHTXHQF\ ð ZDYHOHQJWK 7KH VRXQG ZLWK PRUH IUHTXHQF\
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.

Times' Crucial Science Book - 9 120

When a sound is produced, it gets reflected from the wall and
JHWV PL[HG WR WKH RULJLQDO VRXQG 7KXV WKH RULJLQDO VRXQG
becomes longer and is heard louder.
3. Two astronauts cannot talk on the moon as they do on
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 absorbers preventing from echo and
UHYHUEHUDWLRQ 7KLV PDNHV WKH VRXQG FOHDU
5. Thunder of lightning is heard some moments after the
flash of the light is seen. Why?
9HORFLW\ RI VRXQG LQ DLU LV P V ZKHUHDV WKH YHORFLW\ RI
light in air is 3×108P V %RWK WKH OLJKW DQG VRXQG KDV WR SDVV
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
XOWUDVRQLF ZDYHV WRZDUG WKH EDW 7KH EDWV DQDO\]H WKH UHIOHFWHG
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?
7KH VRXQG ZDYH ZLWK D KLJKHU IUHTXHQF\ KDV D VKRUWHU

121 Times' Crucial Science Book - 9

wavelength whereas the sound wave of lower frequency has
a longer wavelength. Hence, the product of wavelength and
IUHTXHQF\ L H VSHHG LV DOZD\V FRQVWDQW 7KXV WKH VRXQGV RI
different wavelengths have the same speed in a medium.

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

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.
'HQVLW\ WHPSHUDWXUH KXPLGLW\ DQG GLUHFWLRQ RI DLU ÁRZ DUH WKH IDFWRUV
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.

Times' Crucial Science Book - 9 122

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

ÁRZLQJ DFURVV XQLW DUHD SHUSHQGLFXODU WR WKH GLUHFWLRQ RI ZDYH
5HÁHFWHG VRXQG LV FDOOHG HFKR
3URORQJDWLRQ RI RULJLQDO VRXQG GXH WR PL[LQJ RI WKH UHÁHFWHG VRXQG WR

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

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

RI VRXQG DQG WKH UHÀHFWRU LV

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

G 7KH SURFHVV RI ¿QGLQJ WKH GHSWK RI D VHD LV EDVHG RQ

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

GL൵HUHQW LQVWUXPHQWV WKHVH VRXQGV GL൵HU LQ WKHLU

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?
E 'HILQH IROORZLQJ WHUPV
L &UHVW LL 7URXJK LLL &RPSUHVVLRQ
iv. Rarefaction v. Wavelength vi. Frequency
c. What is meant by speed of a wave ? How are speed,
wavelength and frequency related?
G 'LIIHUHQWLDWH EHWZHHQ
L 7UDQVYHUVH ZDYH DQG ORQJLWXGLQDO ZDYH
ii. Echo and reverberation
iii. Sound wave and light wave
iv. Mechanical wave and electromagnetic wave

123 Times' Crucial Science Book - 9

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.

3. Answer the following questions. Medium 9HORFLW\ P V
a. Which gas has the highest
density? Why? A 400
b. W h i c h g a s h a s t h e l o w e s t
B 900
density? Why? C 700

c. If frequency of sound in all these media is same, in which
medium does the sound have longest wavelength?

'UDZ
L 7ZR ZDYHV KDYLQJ HTXDO ZDYHOHQJWK EXW GLIIHUHQW DPSOLWXGHV
LL 7ZR ZDYHV KDYLQJ GLIIHUHQW ZDYHOHQJWKV EXW HTXDO DPSOLWXGH

*LYH UHDVRQV
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
D 7KH YHORFLW\ RI D VRXQG LQ DLU DW RUGLQDU\ FRQGLWLRQV LV P V
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.

Times' Crucial Science Book - 9 124

c. Calculate the depth of the sea if the echo is heard after 6
6HFRQGV *LYHQ 6SHHG RI WKH VRXQG LQ ZDWHU LV P V

d. Calculate the distance of thunder flash from the observer if
WKH VRXQG LV KHDUG VHFRQGV DIWHU WKH IODVK *LYHQ 6SHHG
RI VRXQG LQ DLU P V

H $ VRXUFH SURGXFHV FUHVWV DQG WURXJKV LQ VHFRQGV 7KH
VHFRQG FUHVW LV FP DZD\ IURP WKH ILUVW FUHVW &DOFXODWH
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.

Answers D P E P V F P G P
H L P LL +] LLL P V I P V

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.

7DNH D ORQJ URSH DQG WLH LWV RQH HQG WR D KRRN RU DQ\ RWKHU ULJLG
support. Give a jerk to the rope and observe the kind of wave
produced. Draw its diagram and explain your observation and
findings

125 Times' Crucial Science Book - 9

Chapter
7 &XUUHQW
(OHFWULFLW\ DQG
0DJQHWLVP
Alessandro Volta

+H LV NQRZQ IRU WKH GLVFRYHU\ RI 0HWKDQH DQG
LQYHQWLRQ RI HOHFWULF FHOO

ƐƟŵĂƚĞĚ WĞƌŝŽĚƐ ͗ϭϬ

Objectives

ƚ ƚŚĞ ĞŶĚ ŽĨ ƚŚĞ ůĞƐƐŽŶ͕ ƐƚƵĚĞŶƚƐ ǁŝůů ďĞ ĂďůĞ ƚŽ͗

• LQWHUSUHW WKH XQLW RI HOHFWULFLW\ DQG XVH LW LQ PHDVXUHPHQW
• GHPRQVWUDWH 2KP·V ODZ DQG VKRZ WKH UHODWLRQ RI $PSHUH 9ROW DQG 2KP

• H[SODLQ IDFWRUV DIIHFWLQJ UHVLVWDQFH
• H[SODLQ PDJQHWLF ÀHOG DQG PDJQHWLF OLQHV RI IRUFH

Mind Openers

• What is electricity?
• Why is electricity important?
• :KDW LV PDJQHWLF ÀHOG DQG PDJQHWLF OLQHV RI IRUFH" 'LVFXVV

Introduction
7KH 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. 7KH GLIIHUHQW IRUPV RI HQHUJ\ VXFK DV
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. 7KH FRPELQDWLRQ RI WZR RU
more cells to produce required voltage and current is called battery.

Times' Crucial Science Book - 9 126

Sometimes, a single cell can perform as a battery.
7KH HOHFWULF FHOOV FDQ EH FDWHJRUL]HG LQWR WZR W\SHV DFFRUGLQJ WR
WKHLU UHXVH 7KH\ 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.
7KH GU\ FHOOV DUH XVHG WR VXSSO\ GLUHFW FXUUHQW LQ VPDOO VFDOH 7KH\
DUH VPDOO LQ VL]H DQG SRUWDEOH 7KH\ DUH XVHG LQ WRUFK FDOFXODWRU
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
Zinc container
Simple cell Dry cell

An electric cell which can be recharged and used for long time is
called secondary cell. 7KHVH FHOOV DUH DOVR NQRZQ DV OHDG DFLG FHOOV
7KH\ DUH FRPPRQO\ XVHG LQ YHKLFOHV 7KH\ DUH DOVR XVHG LQ KRPH IRU
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 tŚĞĞů ƌƵďƐ ĂŐĂŝŶƐƚ ƚŚĞ ƚLJƌĞ
devices that convert mechanical ƚŽ ŵĂŬĞ ƚŚĞ ŵĂŐŶĞƚ ƚƵƌŶ
energy into electrical energy.
7KH\ ZRUN RQ WKH SULQFLSOH RI LJŶĂŵŽ
HOHFWURPDJQHWLF LQGXFWLRQ 7KHVH Magnet
devices produce alternating current ŽŝůƐ ŽĨ ǁŝƌĞ
and are used extensively in electric tŝƌĞ ŐŽŝŶŐ ƚŽ ůĂŵƉ
power stations.

127 Dynamo in bicycle
Times' Crucial Science Book - 9

7KH HOHFWULFLW\ SURGXFHG E\ JHQHUDWRU XVLQJ WKH PHFKDQLFDO HQHUJ\
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

$ SDWK IRU WKH ÁRZ RI HOHFWULF FKDUJH ZKLFK FRQVLVWV RI VRXUFH ORDG
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.
)RU H[DPSOH EXOE ÁXRUHVFHQW ODPS WHOHYLVLRQ HOHFWULF KHDWHU ULFH
cooker, etc.

7KH HOHFWULF FLUFXLW FDQ EH FDWHJRUL]HG LQWR WZR W\SHV RSHQ FLUFXLW
and closed circuit.

Battery Battery
+ +

Conducting Conducting
wire wire

Switch Switch
Open circuit Closed circuit

a. Open circuit

$Q HOHFWULF FLUFXLW LQ ZKLFK HOHFWULFLW\ FDQQRW ÁRZ WKURXJK WKH ORDG
is called open circuit. In an open circuit, the switch is off and the
electrical device cannot work.

b. Closed circuit

$Q HOHFWULF FLUFXLW LQ ZKLFK HOHFWULFLW\ FDQ ÁRZ FRQWLQXRXVO\ WKURXJK
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 Conducting
replaced by a conducting wire. wire

7KH VKRUW FLUFXLW UHVXOWV LQ Extra wire
KHDY\ FXUUHQW ÁRZ GXH WR WKH
negligible resistance. So, it heats Load Switch

WKH FLUFXLW ZLUH UDSLGO\ DQG PD\ FDXVH ÀUH

Times' Crucial Science Book - 9 128

Circuit diagram

7KH VFKHPDWLF GLDJUDP RI DQ HOHFWULF FLUFXLW LV FDOOHG FLUFXLW GLDJUDP
Various electrical appliances such as cells or batteries, connecting
ZLUH VZLWFK UHVLVWDQFH HWF DUH XVHG LQ DQ HOHFWULF FLUFXLW 7KHVH
appliances are denoted by particular symbols in the electric circuit.
6RPH RI WKH V\PEROV DUH JLYHQ EHORZ

S.N. Component Symbol
1. One Cell Battery
+ï
2. 7ZR &HOOV %DWWHU\
+ï

3. Many Cells Battery + ï

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

(OHFWULF FXUUHQW LV WKH ÁRZ RI FKDUJHV IURP RQH SRLQW WR DQRWKHU
Before the discovery of electron by - - 7KRPVRQ , positive
FKDUJH ZDV VXSSRVHG WR ÁRZ LQ DQ HOHFWULF FLUFXLW DQG WKH GLUHFWLRQ
of current was assumed to be from positive terminal to the negative
terminal. 7KH ÁRZ RI SRVLWLYH FKDUJHV RU FXUUHQW IURP SRVLWLYH WR
negative terminal of a source is called conventional direction of
current.

batte ry

+

Conventional Actual Cond ucting
direction direction wire

Switch

Actually, WKH FXUUHQW LV WKH ÁRZ RI HOHFWURQV DQG DOZD\V ÁRZV IURP
negative terminal to positive terminal. It is the direction opposite
to the conventional direction and is the real direction of current

129 Times' Crucial Science Book - 9

ÁRZ 6WLOO ZH XVH WKH FRQYHQWLRQDO GLUHFWLRQ RI ÁRZ RI HOHFWULF
FXUUHQW EHFDXVH PDQ\ GHÀQLWLRQV DQG H[SODQDWLRQV DUH EDVHG RQ
conventional direction of electric current.

Electric current

7KH ÁRZ RI FKDUJH IURP KLJKHU SRWHQWLDO UHJLRQ WR WKH ORZHU SRWHQWLDO
region per unit time is called electric current.

Mathematically, ?I= Q
Electric Current= Charge t
7LPH WDNHQ

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.
7KH FKDUJH RI RQH HOHFWURQ LV ð -19 Coulombs. From the above
relation,
I= Q
t or, 1A= 1C
1s

7KXV electric current is said to be one ampere if electric charge of
RQH FRXORPE ÁRZV WKURXJK D FRQGXFWRU LQ RQH VHFRQG

7KH VXE PXOWLSOH XQLWV RI $PSHUH DUH PLOOLDPSHUH PLFURDPSHUH
etc.

1 Ampere = 103 milliamperes
1 Ampere = 106 micro amperes
Measurement of electric current
7KH PDJQLWXGH RI FXUUHQW UDQJHV IURP YHU\ VPDOO WR ODUJH 7KH YHU\
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. 7KH JDOYDQRPHWHU
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 - 9 130

Ammeter
An ammeter is an electrical device which is used to measure the
PDJQLWXGH RI WKH HOHFWULF FXUUHQW ÁRZLQJ LQ D FLUFXLW It is connected
LQ VHULHV LQ D FLUFXLW VR WKDW DOO WKH HOHFWULF FXUUHQW ÁRZV WKURXJK LW

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.

Fact Reason

Ammeter is connected in series, why?
Ammeter is a device which is used to measure the current in a
circuit. It has very less resistance and does not affect the total
amount of current through a circuit. So, ammeter 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
FXUUHQW ÁRZLQJ LQ WKH FLUFXLW
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
ÁRZLQJ LQ WKH FLUFXLW ZLOO LQFUHDVH LQ PDJQLWXGH ,Q VXFK FDVH DQ
DPPHWHU PHDVXUHV WKH FXUUHQW ÁRZLQJ WKURXJK LW DQG QRW WKH WRWDO
current of the circuit.

Potential difference (Pd)

Let us take two metallic spheres X and Y 7KH VSKHUH X is positively
charged and the sphere Y is negatively charged. Both of them are
put on insulated stand. When these two spheres are connected by a
FRQGXFWLQJ ZLUH WKH HOHFWURQV ÁRZ IURP VSKHUH Y to the sphere X.
Since positively charged body has higher potential and negatively

131 Times' Crucial Science Book - 9

charged body has lower potential, WKH HOHFWURQV ÁRZ IURP lower
potential to the higher potential until the
potentials of both bodies in contact become
equal. X Electrons Y

7KH GLIIHUHQFH LQ WKH SRWHQWLDO EHWZHHQ
any two points in an electrical circuit is
called potential difference (Pd). It can also
EH GHÀQHG DV WKH DPRXQW RI ZRUN GRQH LQ
moving a unit charge from one point to
another.

AB

Mathematically,

Potential difference(Pd) = Work done(W) ? Pd= W
Charge(Q) Q

7KH 6, XQLW RI SRWHQWLDO GLIIHUHQFH LV YROW 9 RU -RXOH SHU &RXORPE -&-1).

? 1V= 1J
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.

7KH SRWHQWLDO GLIIHUHQFH LV D VFDODU TXDQWLW\ ,W LV PHDVXUHG E\
using a voltmeter in a closed circuit.

Voltmeter

A voltmeter is an instrument which is used 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 A
potential difference is to be measured. It has
high resistance so that a very small current
ÁRZV WKURXJK LW DQG WKH SRWHQWLDO GLIIHUHQFH
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

Times' Crucial Science Book - 9 132

and the current in the circuit is reduced to a very small magnitude.
7KXV WKH YROWPHWHU GRHV QRW PHDVXUH WKH DFWXDO SRWHQWLDO GLIIHUHQFH

Electromotive force (emf)

7KH GLIIHUHQFH LQ SRWHQWLDO EHWZHHQ WKH WZR WHUPLQDOV RI D FHOO
or any source in an open circuit is known as electromotive force
HPI ,W GULYHV WKH FXUUHQW WKURXJK WKH HQWLUH HOHFWULF FLUFXLW 7KH
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
WR KLJK SRWHQWLDO LQ RUGHU WR PRYH XQLW SRVLWLYH FKDUJH 7KH HPI RI D
cell can be calculated by using the formula,

Electromotive force= Amount of work done ?E= W

Charged particle that moves Q
7KH 6, XQLW RI HPI LV volt 7KH HPI LV HTXDO WR WKH VXP RI WKH 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
charge throughout a circuit. one 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

133 Times' Crucial Science Book - 9

SK\VLFDO FRQGLWLRQV 7KH ODZ ZDV IRUPXODWHG E\ D *HUPDQ SK\VLFLVW
George Simon Ohm in 1826AD.
7KH ODZ VWDWHV WKDW WKH FXUUHQW ÁRZLQJ WKURXJK D FRQGXFWLQJ ZLUH LV
directly proportional to the potential difference across its two ends
at constant physical conditions. 7KH FRQVWDQW SK\VLFDO FRQGLWLRQV
means there is no change in temperature, length, cross-sectional
area and the nature of the material. Mathematically,

9 Ĵ , DW FRQVWDQW WHPSHUDWXUH ««« L
or, V = IR ………..(ii)
Where R is a proportionality constant and is known as resistance
of the conductor.

([SHULPHQWDO YHULÀFDWLRQ RI 2KP
V ODZ

Activity 7 .1 7R YHULI\ 2KP
V ODZ H[SHULPHQWDOO\

0DWHULDOV UHTXLUHG
Dry cells, torch light bulb, voltmeter, ammeter, copper wire, switch, etc.
Proceduse:
1. Connect a dry cell, a torch light bulb (resistance) switch, voltmer
DQG DPPHWHU DV VKRZQ LQ WKH ÀJXUH
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.
2EVHUYDWLRQ
Enter your readings in the observation table and calculate the
resistance in each case.

S.N voltage(V) current(A) 5HVLVWDQFH 5 9 , Conclusion

1

2

3

4

Plot a graph putting voltage on X-axis and current on Y-axis. You
ZLOO JHW WKH FXUYH DV JLYHQ EHORZ

Result:

7KH YROWDJH LV GLUHFWO\ SURSRUWLRQDO WR FXUUHQW Current

Conclusion:

7KLV H[SHULPHQW YHULÀHV WKH RKP·V ODZ Voltage

Times' Crucial Science Book - 9 134

Resistance
7KH SURSHUW\ RI D FRQGXFWRU WR RSSRVH WKH ÁRZ RI FXUUHQW WKURXJK LW LV
called resistance. It is represented by R and its SI unit is Ohm (
).
'LIIHUHQW PDWHULDOV KDYH GLIIHUHQW UHVLVWDQFH 7KH PHWDOV VXFK DV
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
UHVLVWDQFH 7KH PDWHULDOV KDYLQJ KLJK UHVLVWDQFH DUH XVHG LQ KHDWLQJ
and lighting purposes. It is because such materials produce heat or
OLJKW ZKLOH RSSRVLQJ WKH ÁRZ RI FXUUHQW WKURXJK WKHP

Factors affecting resistance
7KH UHVLVWDQFH RI D FRQGXFWRU GHSHQGV XSRQ WKH IROORZLQJ IDFWRUV
1. Length of conductor (l)
7KH UHVLVWDQFH RI DQ\ FRQGXFWLQJ ZLUH LV GLUHFWO\ SURSRUWLRQDO

to its length. It can be represented as
5 Ĵ l (Where, R= resistance, l = length of conductor)
2. Area of cross-section (A) or thickness
7KH UHVLVWDQFH RI D FRQGXFWRU LV LQYHUVHO\ SURSRUWLRQDO WR WKH

FURVV VHFWLRQDO DUHD RI WKH FRQGXFWRU ,W FDQ EH UHSUHVHQWHG DV
5 Ĵ 1
A

7HPSHUDWXUH
7KH UHVLVWDQFH RI D FRQGXFWRU LV GLUHFWO\ SURSRUWLRQDO WR LWV

WHPSHUDWXUH LQ .HOYLQ VFDOH 7
5 Ĵ 7

4. Nature of material
7KH QDWXUH RI PDWHULDO IURP ZKLFK WKH FRQGXFWRU LV PDGH DOVR

affects its resistance. For example, the resistance of aluminum
is more than that of copper but less than that of nichrome.
$ FRLOHG ZLUH LV XVHG LQ D ÀODPHQW EXOE WR LQFUHDVH WKH OHQJWK RI
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.

Fact Reason

:K\ LV YHU\ WKLQ ZLUH XVH LQ ÀODPHQW RI D EXOE"
Resistance is inversely proportional to the cross section area
RI WKH PDWHULDO 6R WKLQ ÀODPHQW ZLUH LV XVHG WR LQFUHDVH WKH
resistance, so that it becomes hot and produce light.

135 Times' Crucial Science Book - 9

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.
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.

5 Ĵ l .........................................................(i)
5 Ĵ 1 .........................................................(ii)

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

5 Ĵ l Or, R= ! l .........................................(iii)
A A

Where ! (rho) is a proportionality constant and is known as
UHVLVWLYLW\ RU VSHFLÀF UHVLVWDQFH RI WKH PDWHULDO RI FRQGXFWRU 7KH
resistivity is given by

R×A = Resistance × Cross-section area
! = l Length of conductor

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

Hence, WKH UHVLVWLYLW\ RI D FRQGXFWRU FDQ EH GHÀQHG DV WKH UHVLVWDQFH
of a conductor of length 1m having the cross-sectional area 1m2.

Unit of !

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

?7KH 6, XQLW RI UHVLVWLYLW\ LV 2KP PHWUH
m). One Ohm metre
is the resistivity of a conducting wire of unit length, unit cross
sectional area and the resistance of 1 ohm.

7KH UHFLSURFDO RI UHVLVWLYLW\ RI D FRQGXFWRU LV FDOOHG LWV FRQGXFWLYLW\
It is denoted by " (sigma).

Conductivity(") = 1 ? " = 1
Resistivity(!) !

7KH 6, XQLW RI FRQGXFWLYLW\ LV
-1m-1.

Times' Crucial Science Book - 9 136

Differences between resistance and resistivity

Resistance Resistivity

1. 7KH SURSHUW\ RI D FRQGXFWRU WR 1. 7KH UHVLVWDQFH RI D FRQGXFWRU
RSSRVH WKH ÁRZ RI FXUUHQW LV FDOOHG of length 1m and cross-section
its resistance. area 1m2 is called its resistivity.

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

3. Its value changes with the change 3. ,W LV D VSHFLÀF YDOXH IRU D
in length and cross-section area. particular conductor

Solved Numerical Problem 7.1

,I D FXUUHQW RI $ ÁRZV WKURXJK D FRQGXFWRU KDYLQJ WKH UHVLVWDQFH
,
what will be the potential difference?

Given, +

Current (I) = 5 A

Resistance (R) = 3

Voltage (V) = ? A

Now, using Ohm’s law, we have V
V = IR = 5 × 3

= 15V

7KHUHIRUH WKH SRWHQWLDO GLIIHUHQFH LV 9

Solved Numerical Problem 7.2

An electric circuit has a 9V battery and resistance 4
. What will be the
FXUUHQW ÁRZLQJ WKURXJK WKH FLUFXLW"

Given,

Potential difference (V) = 9V

Resistance (R) = 4


Electric current (I) = ?

Using Ohm’s law,

or, I = V V = IR
R
9
= 4 = 2. 25A.

7KH FXUUHQW ÁRZLQJ WKURXJK WKH FLUFXLW LV $

137 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.

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
.
7KXV WKH UHVLVWDQFH RI WKH FRQI GXFWRU LV1 5


Combination of resistors

7KH GHYLFHV VXFK DV EXOEV ÁXRUHVFHQW ODPSV KHDWHUV HWF FRQQHFWHG
in an electric circuit are known as resistors RU VLPSO\ ORDGV 7KH
UHVLVWRUV FDQ EH FRQQHFWHG LQ WZR ZD\V
1. Series Combination
2. Parallel Combination

Series combination of resistors

7KH FRPELQDWLRQ RI UHVLVWRUV LQ ZKLFK RQH UHVLVWRU LV FRQQHFWHG QH[W
to the other and so on is called series combination of resistors.

R1 R2 R3

V1 V2 V3

I+

Battery Switch

If I LV WKH FXUUHQW ÁRZLQJ WKURXJK WKH FLUFXLW R is the total resistance
across the circuit and V is the potential difference supplied by the
source, then

V = IR

Also, V = V1 + V2 + V3 or, IR = I(R1 + R2 + R3)
or, IR = IR1 + IR2 + IR3
R = R1 + R2 + R3
?

Times' Crucial Science Book - 9 138

Hence, the total resistance is the algebraic sum of resistance of all
resistors.

If the bulbs are connected in series as shown Battery
LQ WKH ÀJXUH LW LV FDOOHG VHULHV FRPELQDWLRQ
of bulbs (resistors). In such combination, the
VDPH FXUUHQW ÁRZV WKURXJKRXW WKH FLUFXLW ,I
one of the bulbs is removed, the brightness


Switch

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

7KXV LQ SDUDOOHO FRPELQDWLRQ WKH WRWDO UHVLVWDQFH GHFUHDVHV ZLWK
the increase in the number of resistors
If one of the three bulbs in parallel
combination is blown off, the circuit is still Ω
complete and the remaining bulb keeps Ω
Ω

JORZLQJ ZLWK VDPH EULJKWQHVV 7KLV W\SH V
combination is used in domestic wiring.

Magnetism
7KH SURSHUW\ RI D PDJQHW WR DWWUDFW PDJQHWLF VXEVWDQFHV VXFK DV LURQ
cobalt, nickel, etc is known as magnetism. Magnets have the property

139 Times' Crucial Science Book - 9

of magnetism. A magnet may be natural (e.g. lodestone) RU DUWLÀFLDO
(e.g, bar magnet, U- shaped magnet, magnetic compass, etc).
7KH SURSHUWLHV RI D PDJQHW DUH DV IROORZV

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
$ QHXWUDO SRLQW LV D SDUWLFXODU SRLQW LQ WKH PDJQHWLF ÀHOG RI D
PDJQHW DW ZKLFK WKH KRUL]RQWDO FRPSRQHQW RI HDUWK·V PDJQHWLF ÀHOG
LV HTXDO DQG RSSRVLWH WR WKH PDJQHWLF ÀHOG RI WKH PDJQHW 7KH QHHGOH
RI D PDJQHWLF FRPSDVV GRHV QRW VKRZ DQ\ À[HG GLUHFWLRQ DQG QR
magnetic line of force is obtained at the neutral point. At this point,
WKH QHW PDJQHWLF ÀHOG LV ]HUR
7KH PDJQHWLF OLQHV RI IRUFH GR
not pass through the neutral
x

point because the resultant S N
IRUFH RI WZR PDJQHWLF ÀHOGV LV N xx
]HUR ,Q WKH ÀJXUH WKH SRLQWV
marked as (×) near the bar x S

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.

Terrestrial magnetism 1800 Km
Geographical North
It has been proved from several South pole of Magnetic
experiments and researches the earth’s
magnet 17° equator
Geographical
that the earth has a magnetic Magnetic equator
property. 7KH PDJQHWLF meridian
property of the earth is called
terrestrial magnetism. North pole of
It has been imagined that the earth’s
Geographical South magnet

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.

Times' Crucial Science Book - 9 140

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 sometime.

Fact Reason

A freely suspended magnet shows N-S direction, why?
7KH QRUWK SROH RI WKH IUHHO\ VXVSHQGHG PDJQHW LV DWWUDFWHG E\
the south pole of the earths magnet which is in geographical
north. Similarly, the south pole of the freely suspended magnet
is attracted by the north pole of the earth's magnet which is in
geographical south. Hence, a freely suspended magnet shows
N-S direction.

Magnetic and geographic meridian Ge ographical north
7KH LPDJLQDU\ YHUWLFDO SODQH WKDW SDVVHV (Magnetic)
N

through the axis of a freely suspended
magnet at a place is called magnetic
meridian. 7KH PDJQHWLF PHULGLDQ OLHV
between the magnetic north and magnetic
south poles of the earth.
Angle of
declination

7KH LPDJLQDU\ YHUWLFDO SODQH WKDW SDVVHV S (Magnetic)

through the geographical axis is called Geographical south
geographical meridian. 7KH PDJQHWLF PHULGLDQ DQG JHRJUDSKLFDO
meridian do not coincide with each other.

Angle of declination
7KH DQJOH EHWZHHQ WKH PDJQHWLF PHULGLDQ
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.

141 Times' Crucial Science Book - 9

7KXV 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. 7KLV DQJOH LV DOZD\V DQ DFXWH
angle. For example, the value of angle of declination at equator is
17°. Its value differs from place to place.
7KH DQJOH RI GHFOLQDWLRQ SOD\V DQ LPSRUWDQW UROH LQ Á\LQJ DHURSODQHV
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
GLS QHHGOH LV VKRZQ LQ WKH ÀJXUH EHORZ
7KH QHHGOH RI D GLS QHHGOH DOZD\V UHVWV
in a particular direction due to the
HDUWK·V PDJQHWLF ÀHOG 6R WKH QHHGOH
makes a particular angle with the horizontal axis which is called angle
RI GLS 7KXV WKH DQJOH EHWZHHQ WKH GLUHFWLRQ RI HDUWK·V PDJQHWLF ÀHOG
at a place and horizontal line is called angle of dip at that place. 7KH
DQJOH RI GLS JLYHV WKH GLUHFWLRQ RI HDUWK·V PDJQHWLF ÀHOG DW D SODFH

Variation of angle of dip

7KH DQJOH RI GLS YDULHV DFFRUGLQJ WR WKH SODFH
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,
QDYLJDWRUV YR\DJHUV HWF WR ÀQG WKH SRVLWLRQ RQ WKH HDUWK·V VXUIDFH.

Fact Reason

Angle of dip is 90 degrees at poles, why?
At poles, the dip needle is solely pulled by the poles of the earths
magnet due to which it remains perpendicular to the surface.
So, angle of dip is 90 degrees at poles.

Learn and Write
1. How is electric charge or static electricity produced?

When an object is rubbed with another object, it may lose or
gain electrons on its surface. Such gain or loss of electrons
produces electric charge or static electricity in the rubbed

Times' Crucial Science Book - 9 142

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?
%HIRUH 7KRPVRQ WKH GLUHFWLRQ RI FXUUHQW ZDV EHOLHYHG WR IORZ
from positive to negative terminal of a cell. But, it was proved
E\ 7KRPVRQ WKDW WKH DFWXDO GLUHFWLRQ RI WKH FXUUHQW LV IURP
negative to positive terminal of a cell. But many definitions
and explanations were based on the conventional direction.
7KHUHIRUH FRQYHQWLRQDO GLUHFWLRQ LV LQ XVH \HW
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
IORZ HDVLO\ 7KHQ LW PHDVXUHV WKH YDOXH RI ZKROH FXUUHQW IORZLQJ
through the circuit. If it is connected in parallel to the load, it
does not measure the whole current flowing through the circuit.
7KHUHIRUH DPPHWHU LV FRQQHFWHG LQ VHULHV WR WKH ORDG
7XQJVWHQ ZLUH NHSW LQVLGH WKH ILODPHQW ODPS LV FRLOHG :K\"
When the tungsten wire kept in the filament lamp is coiled,
LWV OHQJWK LQFUHDVHV 7KH LQFUHDVH LQ WKH OHQJWK LQFUHDVHV WKH
resistance. More the resistance, the more will be the brightness
RI WKH EXOE 7KHUHIRUH WKH WXQJVWHQ ZLUH NHSW LQVLGH WKH
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 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.

143 Times' Crucial Science Book - 9

7KH FRPSDVV QHHGOH GRHV QRW VKRZ WKH DFWXDO GLUHFWLRQV DW WKH
neutral points. Why?

7KH PDJQHWLF ILHOG RI WKH PDJQHW DQG WKH PDJQHWLF ILHOG RI WKH
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?

7KH QRUWK DQG VRXWK SROHV RI WKH WHUUHVWULDO PDJQHW KDYH
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).

Glossary

Negligible very small amount
Potential a quantity determining energy of a charge
Insulator a substance which does not allow current cannot pass
through it
7HUUHVWULDO related to the earth
Cross sectional area thickness

Main points to remember

1. A device which continuously generates electricity is known as source of electricity.
2. A cell is a device which converts chemical energy into electrical energy

and produces current.
7KH ÁRZ RI FKDUJH IURP KLJKHU SRWHQWLDO WR ORZHU SRWHQWLDO SHU XQLW WLPH

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

PDJQLWXGH RI HOHFWULF FXUUHQW ÁRZLQJ LQ WKH FLUFXLW
7KH SRWHQWLDO GLIIHUHQFH EHWZHHQ WZR SRLQWV LQ DQ HOHFWULF ÀHOG LV GHÀQHG DV WKH

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.
2KP·V ODZ VWDWHV WKDW WKH FXUUHQW ÁRZLQJ WKURXJK D FRQGXFWLQJ ZLUH

is directly proportional to the potential difference across its two ends at
constant temperature.

Times' Crucial Science Book - 9 144

$ QHXWUDO SRLQW LV D SDUWLFXODU SRLQW LQ WKH PDJQHWLF ÀHOG RI D PDJQHW
DW ZKLFK WKH KRUL]RQWDO FRPSRQHQW RI HDUWK·V PDJQHWLF ÀHOG LV HTXDO DQG
RSSRVLWH WR WKH PDJQHWLF ÀHOG RI WKH PDJQHW

10. The angle between the magnetic meridian and geographical meridian at
a place is called angle of declination of that place.

7KH DQJOH EHWZHHQ WKH GLUHFWLRQ RI HDUWK·V PDJQHWLF ÀHOG DW D SODFH DQG
the horizontal line is called the angle of dip at that place.

Exercise

1. Choose the best alternative in each case.

a. The presence of electric current in an electric circuit is detected by

i. Ammeter ii. Voltmeter iii. Galvanometer iv. Potentiometer

E :KDW LV WKH 6, XQLW RI SRWHQWLDO GL൵HUHQFH"

i. Ohm ii. Volt iii. Ampere iv. Ohm x m

c. A battery of 9 volts and a bulb of 6 Watts have been used in a torch

light. What will be the amount of current in the circuit?

i. 1.5A ii. 2.0A iii. 2.25A iv. 0.67A

d. The value of angle of dip at the magnetic pole is

i. 0° ii. 90° iii. 45° iv. 72.5°

e. The value of the angle of dip at the Kathmandu valley is

i. 90° ii. 50° iii. 42° iv. 35°
2. Answer these questions in very short.
a. What is a cell?
b. Write down differences between a cell and a generator.
c. Draw the symbols for battery and galvanometer.
d. Why do we use conventional direction of current although
it is not correct?
e. What is magnetism?
f. Write down any two advantages of hydroelectricity.
'HILQH
a. Galvanometer b. Ammeter
c. Voltmeter d. Neutral points
e. Magnetic meridian f. Geographic meridian
'LIIHUHQWLDWH EHWZHHQ
a. Potential difference and electromotive force
b. Angle of declination and angle of inclination
c. Resistance and resistivity
d. Ammeter and voltmeter

*LYH UHDVRQV
a. An ammeter is always connected in series in an electric circuit.

145 Times' Crucial Science Book - 9

b. A voltmeter is connected parallel to load in an electric circuit.
c. A bulb contains coiled wire.
G 7KH YDOXH RI HPI LV PRUH WKDQ WKH YDOXH RI 3G
e. Bulbs are connected in parallel combination.
f. A freely suspended magnet rests pointing N-S direction.
g. Angle of dip is 90° at magnetic poles.
h. A magnetic compass does not show any particular direction

at neutral point.

6. Solve the following numerical problems.
a. How much charge flows through a wire carrying 3A of
current in 10 minutes?
b. What is the resistance of a material that draws 200mA of
current at 20V potential difference?
F &DOFXODWH
7RWDO UHVLVWDQFH IURP WKH JLYHQ GLDJUDP.
)LJ D )LJ E 6


4
6


6


12v 12v

d. What is the potential difference across a resistor having
resistance of 4
which draws a current 3A.

e. What is the total resistance when two resistors having
resistance 6
and 9
are connected (i) in series and (ii)
parallel combination?

Answers 6. a. 1800 Coulomb b. 100
F L )LJ D
,
)LJ E
d. 12V e. 15
, 3.6


Project Work

7DNH D FRQGXFWLQJ ZLUH KDYLQJ IHHW OHQJWK D
WRUFKOLJKW EXOE WZR GU\ FHOOV HWF 7KHQ FRQQHFW
two cells in series combination. Now join the cells
to each end of the bulb with conducting wires. Put
a switch in the conducting wire. Now the electric
circuit is ready. Observe the brightness of bulb
by changing the number of cells. Again connect the cells in parallel
and observe brightness. Write your observations with reasons.

Times' Crucial Science Book - 9 146