Modern Concept Science and Environment – 8 93
Characteristics of the image formed by a plane mirror
i) The image formed by a plane mirror is laterally
inverted.
ii) Image is virtual.
iii) It is erect.
iv) Its size is equal to the object. Image formed by a plane mirror
v) The distance between image and mirror is equal to the distance between object
and mirror. Memory Tips
Applications of a plane mirror The phenomenon due to which the
i) A plane mirror is commonly used as a looking image of an object turns through
glass in our daily life. an angle of 180° about a vertical
axis is called lateral inversion. Due
ii) Plane mirrors are used as reflectors in optical to lateral inversion, the left side of
instruments like periscope, kaleidoscope, etc. an object looks right in the image
and vice versa.
2. Spherical Mirror AA
Two curved pieces are obtained when Concave Memory Tips
a hollow glass sphere is cut as shown Convex
in the given figure. If the silver is PC P The mirror which is a part of a
polished on the outer side then the Hollow sphere glass sphere is called spherical
reflective surface is towards the hollow mirror.
side and acts like a spherical mirror. Reflecting B
surface
B
Spherical Mirrors
Similarly, if the silver is polished on the inner side then the reflective surface is towards the
outer side and acts like a spherical mirror. The nature of image formed by these spherical
mirrors is different. Thus, the curved mirror made by polishing either the inner curved side or
the outer curved side of a cut portion of the hollow glass sphere is called a spherical mirror.
Types of spherical mirrors
Spherical mirrors are of two types. They are concave mirror and convex mirror.
i) Concave mirror : In a concave mirror, the polishing or silvering Resfluercftaincge Polished surface
is done on the outer surface of the cut part of a hollow sphere.
In this mirror, the inner surface is reflective surface. It is also C P
called a converging mirror.
Concave Mirrors
FACT WITH REASON
Concave mirror is also called a converging mirror, why?
After reflection, the reflected light rays from a concave mirror meet at a point. So, the concave mirror
is also called a converging mirror.
ACTIVITY 1
Take a concave mirror. Hold it facing the sun. Focus the reflected light on the paper. Adjust the distance
between the paper and the mirror until you get a sharp bright spot.
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ii) Convex mirror : In convex mirror, the polishing or silvering Psoulirsfhaecde Reflecting surface
is done on the inner surface of the cut part of a hollow
sphere. In this mirror, the outer surface is a reflective C P
surface. It is also called a diverging mirror.
FACT WITH REASON Convex Mirrors
Convex mirror is also called a diverging mirror, why?
After reflection, the reflected light rays from a convex mirror diverse in different direction. So, the
convex mirror is also called a diverging mirror.
7.9 General Terms Used in Spherical Mirrors
Pole
The geometric center of a spherical mirror is called pole. It is denoted by the letter 'P'.
Center of curvature
The center of the hollow glass sphere of which the spherical mirror is a part is called center of
curvature. It is denoted by the letter 'C'. Memory Tips
Radius of curvature The point on the principal axis
The radius of a hollow glass sphere of which the spherical at which all rays parallel to the
mirror is a part is called radius of curvature. It is denoted by principal axis meet or appear to
the letter 'R'.
meet after reflection from a mirror
Principal axis is called principal focus.
The line passing through the center of curvature and pole of the mirror is called principal axis.
Principal Focus
a) Principal focus of a concave mirror
A point on the principal axis of a concave mirror at which a C P
parallel beam of light with principal axis meet after reflection
F
is called the principal focus of a concave mirror. It is denoted
by the letter 'F'. Principal focus of a concave mirror
b) Principal focus of a convex mirror
A point on the principal axis of a convex mirror from which a
parallel beam of light with principal axis appear to diverge is F
called principal focus of a convex mirror. It is denoted by the P
letter 'F'.
Focal length Principal focus of a convex mirror
The distance between pole and principal focus of a spherical mirror
is called focal length. It is denoted by 'f'.
Relation between radius of curvature and focal length of Memory Tips
a spherical mirror
Focal length in mirror = half of its
Focal length (f) = Radius of curvature = R radius of curvature
22
Modern Concept Science and Environment – 8 95
7.10 Image Formation by the Spherical Mirrors
Image formation by a concave mirror
ACTIVITY 2
Take a stainless steel spoon. Look your image in the inner side of the spoon. Do you find your
image inverted?
ACTIVITY 3
Fix a concave mirror on a stand and place it on a table. Keep a lighted candle on the
table in front of the mirror. Try to obtain the image of the flame on a paper as shown
in the given figure. Move the position of the candle towards and away from the mirror
and obtain the corresponding images. Observe the nature of image in each case.
Rules to draw the image formed by a concave mirror
a) A ray of light which is parallel to the principal axis of a concave mirror passes through
its focus after reflection.
b) A ray of light passing through the center of curvature of a concave mirror is reflected
back along the same path.
c) A ray of light passing through the focus of a concave mirror becomes parallel to the
principal axis after reflection.
F PF P F P
C C C
i ii iii
Rules to draw the image fromed by a concave mirror
Images formed by a concave mirror
a) When the object is at infinity (distant object)
i) The image is formed at F
ii) It is real
iii) It is inverted
iv) It is highly diminished
b) When the object is beyond centre of curvature
i) The image is formed between F and C
ii) It is real
iii) It inverted
iv) It is diminished
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c) When the object is at centre of curvature Object
i) The image is formed at C Image
ii) It is real
iii) It is inverted Object
iv) It has same size to the object Image
d) When object is between F and C Object
i) The image is formed beyond C
ii) It is real Image
iii) It is inverted
iv) It is magnified Object Image
e) When object is at F
i) The image is formed at infinity
ii) It is real
iii) It is inverted
iv) It is highly magnified
f) When the object is in between P and F
i) The image is formed behind the mirror
ii) It is virtual
iv) It is erect
v) It is magnified
Summary of the images formed by a concave mirror
SN. Position of an object Position of the image Nature of the image formed
1. At infinity At F Real, inverted, diminished
2. Beyond C Between F and C Real, inverted, diminished
3. At C At C Real, inverted and of the same size to
the object
4. Between C and F Beyond C Real, inverted and magnified
5. At F At infinity Real, inverted, enlarged
6. Between F and P Behind the mirror Virtual, erect, enlarged
7.11 Image Formation by a Convex Mirror
ACTIVITY 4
Take a stainless steel spoon. Bring its outer face near to you and look into it. Do you see your image on
it? Is it different from what you see in a plane mirror? Observe the natures of the image thus formed.
OBSERVATION:
The image formed by a convex mirror is virtual, diminished, erect and behind the mirror.
Modern Concept Science and Environment – 8 97
Rules to draw the image formed by a convex mirror
a) A ray of light parallel to the principal axis appears to diverge from the focus.
b) A ray of light passing through centre of curvature is reflected back along the same path.
c) A ray of light from an object which strikes the pole of the mirror at a certain angle,
reflects at the same angle.
P FC P FC 450 FC
450 P
i ii iii
Rules to draw the image fromed by a convex mirror
Images formed by a convex mirror
a) When the object is at infinity
i) The image is formed at focus 'F' behind the mirror
ii) It is highly diminished
iii) It is virtual
iv) It is erect
b) When the object is between infinity and the pole
i) The image is formed between P and F behind the mirror
ii) It is diminished
iii) It is virtual Object
iv) It is erect
Applications of the spherical mirrors
a) Applications of a concave mirror
i) Doctors use a concave mirror to focus the parallel beam of light on the patient's
organ like throat, ear, etc. for their close examination.
ii) Dentist use a special concave mirror to see back of the teeth by placing it behind
the tooth.
FACT WITH REASON
Dentist use a special concave mirror to see back of the teeth, why?
A concave mirror forms an enlarged and erect image of an object when it is
in between the pole and focus of the mirror. When a doctor places a concave
mirror behind the teeth, it forms a virtual, erect and enlarged image of the
teeth. So, dentist use a special concave mirror to see the back of the teeth.
iii) It is used as a shaving mirror or make-up mirror.
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FACT WITH REASON
Concave mirror is used as a shaving mirror, why?
When our face is in between principal focus and pole of a concave mirror, an erect and magnified
image is formed. So, a concave mirror is used as a shaving mirror or make-up mirror.
iv) It is used as a reflector in making reflecting telescopes.
v) Concave mirrors are used as reflector in the headlights of automobiles, such as
cars, trucks, motor bike, etc.
vi) It is also used as a reflector in search lights, torch light, etc.
FACT WITH REASON
A concave mirror is used as a reflector in headlight or in torch light, why?
Light rays from the focus of a concave mirror form a parallel beam after
reflection. When the bulb in a torch light or in headlights of the vehicles is placed
at principal focus of a concave reflector, then a powerful parallel beam of light
is formed. So, a concave mirror is used as reflector in headlight or in torch light.
b) Applications of Convex Mirror
i) Convex mirror is used in the street light as a reflector.
FACT WITH REASON
Convex mirror is used in the street light, why?
A convex mirror is used in the street light to diverge the light rays over a wide range.
ii) It is used as a security mirror in the shop, gates, etc.
iii) It is used as a rear view mirror in the automobiles.
FACT WITH REASON
A convex mirror is used in vehicles as a rear view, why?
Convex mirror has a wide range of view. It forms a small, erect and diminished
image from wide range. Hence, a convex mirror is used in vehicles to look a rear
view like the traffic behind him.
7.12 Optical Medium
The substance through which light propagates or tends to propagate is called a medium. For
example, glass, water, air, etc. Light travels at high speed in the form of a wave. In vacuum, it
travels at the rate of 3 lakh kilometers per second. Speed of light changes in different medium.
On the basis of the speed of light in a medium, there are two types of media. They are denser
medium and rarer medium.
Modern Concept Science and Environment – 8 99
Denser medium and rarer medium
The speed of light in vacuum is 3×108 m/s. It is considerably less in glass and water. The value
of speed of light in water is 2.25×108 m/s and that in glass is 2×108 m/s. The medium in which
light travels faster is called optically rarer medium and the medium in which light travels
relatively slower is called optically denser medium.
7.13 Refraction of Light
According to ray model, light travels in a straight line. When light Refraction of light
enters obliquely from one transparent medium to another, the
direction of light changes in second medium. This phenomenon
is called refraction. Thus, the phenomenon of bending of light
as it passes obliquely from one optical medium to another is
called refraction of light.
Cause of refraction
Light travels more slowly when it enters into an optically denser medium from an optically
rarer medium. On the other hand, when light travels from an optically denser to an optically
rarer medium its speed becomes more. Thus, the change in speed of a light wave when it
passes from one optical medium to another optical medium is the main cause of refraction.
General terms used in refraction of light
a) Incident ray: The ray which strikes the surface of
separation of two optical media is known as incident
ray. In the given figure, IO is the incident ray.
b) Refracted ray: The ray which travels in the second optical
medium with changed direction is called refracted ray.
In the given figure, OR is the refracted ray.
c) Normal: A perpendicular line drawn at the point of Refraction of light through a glass slab
incidence is called normal. In the given figure, MN
and M'N' are the two normals.
d) Angle of incidence: The angle which is made by incident ray with a normal at the
point of incidence is known as angle of incidence. In Memory Tips
the given figure 'i' is the angle of incidence. The ratio of sine of angle of
e) Angle of refraction: The angle which is made by the incidence to the sine of angle
refracted ray with a normal at the point of incidence of refraction, for a given pair of
is known as the angle of refraction. In the given figure media, is constant. This constant
'r' is the angle of refraction. is called refractive index, i.e.
f) Lateral shift: The perpendicular distance between Refractive index (µ) = Sini/Sin r
incident ray and emergent ray is called lateral shift or This law is popularly called Snell’s
lateral displacement. law.
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Laws of refraction
a) The incident ray, refracted ray and normal, all lie on the same plane at the point of
incidence.
b) When a ray of light travels from an optically rarer medium to an optically denser
medium, it bends towards the normal at the interface of two media.(figure a)
c) When a ray of light travels from an optically denser medium to an optically rarer
medium, it bends away from the normal at the interface of two media. (figure b)
d) When a ray of light strikes the interface of two media at an angle of 900, it does not bend
from its path. (figure c)
Air Normal
Glass
Examples of refraction of light
FACT WITH REASON
A pencil immersed in water appears to be bent at the air water boundary, why?
When the rays of light from the immersed part of the pencil pass from water to air, they bend away
from the normal. Hence, the pencil appears to be bent at the air water boundary.
a) A stick immersed obliquely in water appears to be bent Apparent bend
When the rays of light from the immersed part of the stick pass Apparent position
from water (denser medium) to air (rarer medium), they bend of stick
Surface
away from the normal. Hence, the stick appears to be bent at the Water
air water interface.
b) A coin kept at the bottom in a glass of water seems raised up
When the rays of light from the immersed coin pass from water
(denser medium) to air (rarer medium), they bend away from
the normal. Hence, a coin kept at the bottom in a glass of water
seems raised up. Apparent position Water
of the coin Coin (real position)
c) A swimming pool seems shallower than its actual depth
When the rays of light from the bottom of the swimming pool pass from water (denser
medium) to air (rarer medium), they bend away from the normal. Hence, a swimming
pool seems shallower than its actual depth.
d) When a thick glass slab is placed over a page of a book, the letters appear raised
while viewing through the glass slab from the sides.
The direction of light reaching to our eyes from the letters under the glass slab through
glass (optically denser medium) to air (optically rarer medium) bends away from air
glass interface. Due to this the letters seem above their actual position under the slab.
Modern Concept Science and Environment – 8 101
FACT WITH REASON
When a thick glass slab is kept over a page of a book, the letters appear raised while viewing through
the glass slab from the sides. why?
When the rays of light from the book pass from glass (denser medium) to air (rarer medium), they
bend away from the normal. Hence, the letters in the book seem raised up.
ANSWER WRITING SKILL
1. What is light? Write down the speed of light in air, water and glass.
Ans: Light is a form of energy which makes objects visible to us. Light travels at a speed of 3 × 108 m/s in
air, 2.25×108 m/s in water and 2×108 m/s in glass.
2. What are the sources of light? Write any three examples.
Ans: Those bodies which emit light are called sources of light. For example, the sun, electric bulb, lamp, etc.
3. What are rays and beam of light?
Ans: The narrow path of the light which is represented by a straight line with an arrow is called a ray of
light. Similarly, the collection of rays of light in a certain pattern is called a beam of light.
4. What is a spherical mirror? Write down its types.
Ans: The curved mirror made by polishing either the inner curved side or the outer curved side of a
cut portion of the hollow glass sphere is called spherical mirror. There are two types of spherical
mirror. They are concave mirror and convex mirror.
5. Concave mirror is used as a make-up mirror. Why?
Ans: When our face is in between principal focus and pole of a concave mirror, an erect and magnified
image is formed. So, concave mirror is used as a make-up mirror.
6. Concave mirror can be used in the solar cooker?
Ans: Concave mirror converges parallel rays of light at the focus. As a result, accumulation of heat takes
place to cook food.
7. Write any two differences between concave mirror and convex mirror.
Ans: Differences between concave mirror and convex mirror are:
S.N. Convex mirror S.N. Concave mirror
1 In this mirror, the outer surface is reflective 1 In this mirror, the inner surface is reflective
surface and inner surface is polished. surface and the outer surface is polished.
2 It is also called a diverging mirror. 2 It is also called a converging mirror.
8. Write any two differences between reflection and refraction of light.
Ans: Differences between reflection and refraction of light are:
S.N. Reflection of light S.N. Refraction of light
1 In reflection, bouncing of light takes 1 In refraction, bending of light takes place
place in the same medium. when light passes from one medium to
another medium.
2 It occurs in mirror and smooth surfaces. 2 It occurs in glass slab, lens, etc.
102 Light
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) The center of a spherical mirror is called …… .
b) A …… mirror has a real focus.
c) The image which can be obtained on a screen is called ……. image.
d) The focal length of a convex mirror is half of its……. of curvature.
e) ……. mirror is used as a shaving mirror.
f) A ray of light travelling from a rarer medium to a denser medium bends …….
the normal.
2. Write True for the correct and False for the incorrect statements.
a) A concave mirror is also called a diverging mirror.
b) A virtual image cannot be obtained on the screen.
c) Convex mirror is used as a reflector in headlights of a car.
d) Convex mirror has wider range of view as compared to that of concave mirror.
e) The angle of incidence is equal to the angle of refraction.
f) Light rays passing from air to glass bend towards the normal.
g) When light travels from air to water, its speed increases.
STEP 2
3. Answer the following questions in one word.
a) What is the angle of incidence when the angle between the incident ray and the
reflected ray is 60o?
b) Which type of mirror always form an image equal to the size of object?
c) Which type of mirror used in vehicle to see the traffic at the back side?
d) Which mirror is used to see an enlarged image of our face?
e) What is called to a perpendicular line drawn at the point of incidence?
4. Write any two differences between:
a) Plane mirror and spherical mirror b) Real image and virtual image
c) Concave mirror and convex mirror ` d) Denser medium and rarer medium
5. Give reasons.
a) A plane mirror is used to look our face.
b) A concave mirror is also called a converging mirror.
c) Dentist use a special concave mirror to see the back of the teeth.
d) Concave mirror is used as a reflector in the headlights of automobiles.
Modern Concept Science and Environment – 8 103
e) Concave mirror is used as a reflector in a solar cooker.
f) Convex mirror is used in street light.
g) A stick immersed obliquely in water appears to be bent.
h) A swimming pool seems shallower than its actual depth.
6. Answer the questions with the help of the given figure.
a) Identify the medium 1 and medium 2 as denser medium and rarer
medium.
b) Copy and complete the ray diagram to show the image formation.
Also, write the nature of the image formed.
CF P CF P FC
(a) (b) (c)
STEP 3
7. Answer the following questions
a) Define:
i. Light energy ii. Center of curvature
iii. Principal axis iv. Principal focus
v. Reflection of light vi. Refraction of light
b) Write down the laws of reflection of light.
c) What is an image?
d) What is a plane mirror?
e) Write the characteristics of image formed by plane mirror.
f) Write the applications of a plane mirror.
g) Write the applications of a concave mirror and convex mirror.
h) What is an optical medium? Write down their types.
i) Write down the cause of refraction of light.
j) Write down the laws of refraction.
k) Explain any two effects of refraction of light.
8. Draw the diagram.
a) to show the image formed by a plane mirror. Also write down the characteristics
of the image formed.
b) to show the principal focus of a concave mirror and convex mirror.
c) to show the image formed by a concave mirror when the object is placed
i) beyond 2F ii) at 2F (or C)
iii) in between F and 2F iv) at F
Also, write the nature of the image formed.
d) to show the image formed by a convex mirror.
e) to show the refraction of light through a glass slab.
f) to show the bending of a pencil partially and obliquely dipped in a beaker of water.
104 Sound Estimated teaching periods Theory Practical
UNIT 2 1
8 Sound
Syllabus issued by CDC Temple bell
Introduction to sound
Sound wave, wavelength, frequency
Speed of sound
Reflection of sound
Echo, reverberation
Simple numerical problems based on echo
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
define terminology related to sound: speed, frequency and wavelength.
introduce echo and reverberation and differentiate between them.
explain the effects of echo and reverberation.
Key terms and terminologies of the unit
1. Sound : Sound is a form of energy which causes a sensation of hearing through the
vibration of the ear drum.
2. Vibration : A rapid to and fro or up and down movement about a mean position is known as
vibration.
3. Sources of sound : Those objects which produce sound due to their vibration are called sources of sound.
4. Wave : Wave is a periodic disturbance in a medium which carries energy from one place
to another without transferring matter.
5. Sound waves : Sound travels through a medium in the form of waves, called sound waves.
6. Longitudinal wave : The wave in which the particles of the medium vibrate back and forth in the
direction of the wave is called a longitudinal wave.
7. Compressions : In a longitudinal wave, compressions are the regions where density as well as
pressure is high.
8. Rarefactions : In a longitudinal wave, rarefactions are the regions of low pressure where particles
are spread apart.
9. Amplitude : The size of a compression or a rarefaction is called amplitude.
10. Wavelength : The distance between two consecutive compressions (C) or two consecutive
rarefactions (R) is called the wavelength.
11. Frequency : The number of waves produced per second is called frequency.
12. Propagation of sound : The transmission of sound from one place to another place is called propagation
of sound.
Modern Concept Science and Environment – 8 105
13. Speed of sound : The distance travelled per unit time by a sound wave is called speed of sound.
14. Reflection of sound : The bouncing back of sound when it strikes a hard surface is called reflection of
sound.
15. Echo : The repetition of sound because of reflection of sound waves is called echo.
16. Echolocation : Echolocation is the process of locating an object in the surrounding with the help of
reflected sound.
17. Reverberation : The phenomenon of prolongation of sound due to series of reflections is called
reverberation.
18. Medium : Any material through which wave propagates is called a medium.
8.1 Introduction
Every day we hear different kinds of sounds such as sound from humans, birds, bells,
machines, vehicles, televisions, radios, etc. Sound is a form of energy which causes a sensation
of hearing through the vibration of the ear drum. It is an important aspect for the existence of
many living beings. Animals use sound for different purposes such as searching, informing,
enjoying, sharing of feeling, etc. Sound causes different effects. Some sounds are irritating and
some are pleasant. For example, the sound of a siren makes us attentive, the noise makes us
irritation, while the sound of music makes us relax. Sound is produced due to the vibration in
the body and moves in the form of waves. Sound travels through the medium in the form of
waves, called sound waves.
Sources of sound
8.2 Production of Sound
Have you ever touched a ringing bell? We feel
vibrations in the ringing bell when we touch it.
A rapid to and fro or up and down movement
about a mean position is known as vibration.
Vibration in a body is the cause of sound. Sound Producing sound from a tunning fork
is produced due to vibration in solid, liquid or
gaseous substance. Examples: a vibrating tuning fork, vibrating ruler, vibrating air column
in a flute, vibrating vocal cord, etc. produce sound. The vibrating body causes the medium
like water, air, etc. around it to vibrate. Such vibrations in a medium are called waves. These
waves carry energy from one place to another.
FACT WITH REASON
The sound from a ringing bell stops when it is caught by our hand, why?
Vibration in a bell is the cause of sound. When a ringing bell is caught by our hand, the vibrations in it
stop and the sound gets stopped.
106 Sound
ACTIVITY 1
1. Take a drum and beat with the help of a drum stick.
2. Do you hear the sound of the drum?
3. Touch the membrane of the drum. What do you feel?
4. After sometimes, the sound stops. Touch the membrane of the drum again.
Do you feel vibrations on the membrane of the drum?
CONCLUSION:
It can be concluded that the vibrating membrane of the drum produces sound.
Sources of sound
Those objects which produce sound due to their vibration are called sources of sound. For
example, sitar, guitar string, flute, tabala, bell, madal, etc. There are different methods of
producing sound from different sources. They are plucking, rubbing, blowing, striking, etc.
8.3 Wave
When a piece of stone is thrown into the still water, circular
ripples are formed. They spread in all directions on the
surface of water. These ripples carry energy from stone to
all parts without actual movement of the water molecules
from one point to another. To check wheather water moves
along with the ripples or not, we can observe a floating Waves in water
object like a cork or a leaf kept on the surface of water. As
the ripples move in all possible direction on the water surface, the leaf on the water surface
vibrates up and down but does not move from one place to another. Thus, a wave is a periodic
disturbance in a medium which carries energy from one place to another without transferring
matter. It is a fine path through which light or sound transmits from one place to another.
Longitudinal wave
The wave in which the particles of the medium vibrate back Memory Tips
and forth in the direction of the wave is called a longitudinal In a longitudinal wave, the
wave. For example, sound wave, waves in spring, etc. In direction of the vibrating particle
a longitudinal wave, the motion of the particles is back- of a medium is parallel to the
and-forth. When a stretched spring is released then waves direction of the wave propagation.
propagate through it in the form of compression and
rarefaction.
Compressions and rarefactions
Modern Concept Science and Environment – 8 107
Terms used in a longitudinal wave
a) Compressions : In a longitudinal wave, Tuning Greater Less Greater Less Greater Less
compressions are the regions where fork density density density density density density
density as well as pressure is high.
They are represented by the letter "C". C = Compression
R = Rarefaction
b) Rarefactions : In a longitudinal
wave, rarefactions are the regions of low pressure where particles are spread apart.
They are represented by the letter "R".
FACT WITH REASON
The sound waves are longitudinal waves, why?
Sound waves move in air making area of high pressure called compressions and area of low pressure
called rarefactions. So, the sound waves are called longitudinal waves.
c) Amplitude : The size of a compression or a rarefaction is called amplitude.
d) Wavelength : The distance between two consecutive compressions (C) or two consecutive
rarefactions (R) is called the wavelength. Memory Tips
e) Frequency : The number of waves produced per
second is called frequency. It is represented by ' f '. Its If a wave has a frequency of 50
SI unit is hertz (Hz). A wave with one complete cycle Hz, this means that 50 complete
in one second has frequency of 1 Hz. The bigger units vibrations are made in 1 second.
of hertz are: = 103 hertz (Hz) Memory Tips
1 kilohertz (kHz) = 106 hertz (Hz)
Time period is reciprocal of the
1 Megahertz (MHz) frequency.
f) Time period : The time required to complete one full i.e. T = 1
f
cycle of a wave is called time period. It is represented
by 'T'. Its SI unit is second.
8.4 Sound Wave
Sound is produced by a vibrating body. It travels in all directions from the source. Sound waves
are mechanical waves because they need a material medium to Compressions
propagate. Sound waves are longitudinal wave because they
move in the direction of wave propagation making compression
and rarefaction. Sound can propagate through solids, liquids
and gases. But it cannot propagate through vacuum. The sound
waves produced from a vibrating body propagate through air Rarefaction
and reach to our ears. As a result, we hear the sound.
Wavelength
Propagation of sound
Sound wave
The transmission of sound from one place to another place is called propagation of sound. For
example, sound propagates through air and reaches up to our ears.
108 Sound
a) Medium : Any material through which wave propagates is called a medium. For
example, solid, liquid, and gas. A material medium is essential for propagation of sound
waves. Sound cannot propagate without a medium in vacuum.
b) Transmission of sound in different media : In a medium, when the molecules of solid,
liquid and gas vibrate then one molecule pushes the nearby molecules. This makes
compressions and rarefactions and sound wave propagates from one point to another.
Due to lack of atmosphere (vacuum), communication on the surface of the moon is not possible
as on the earth. Electronic devices are used for communication on the surface of the moon.
FACT WITH REASON
Sound does not propagate on the surface of the moon, why?
Sound is a mechanical wave. It needs a material medium for propagation. So, sound does not propagate
on the surface of the moon due to the lack of air (vacuum).
c) Speed of sound : The distance travelled per unit time by a sound wave is called speed
of sound. Sound propagates with different speed in different media. The speed of sound
in air at 20oC is 343 m/s. The speed of sound is equal to the product of wavelength (λ)
and frequency (f).
Mathematically, v = f × λ
Solved Numerical 8.1
A female vocalist can sing as high as 1000 Hz. If the speed of sound is 344 m/s, calculate the
wavelength of the sound waves.
Solution:
Frequency of sound wave (f) = 1000 Hz
Speed of sound (v) = 344 m/s
Wavelength (λ) = ? λ = v = 344 = 0.344 m
f 1000
According to the formula, wavelength of the sound wave,
Thus, the wavelength of the sound wave is 0.344 m.
Speed of sound in different media
S.N. Medium Examples Speed of sound
1. Gases Air (0°C) 332 m/s
343 m/s
Air (20°C) 316 m/s
1284 m/s
Oxygen (25°C)
1210 m/s
Hydrogen (25°C) 1325 m/s
1450 m/s
2. Liquids Alcohol (25°C) 1498 m/s
1531 m/s
Turpentine (25°C)
Mercury (25°C)
Distilled water (25°C)
Sea water (25°C)
Modern Concept Science and Environment – 8 109
3. Solids Copper (25°C) 3560 m/s
5100 m/s
Aluminium (25°C) 5130 m/s
5640 m/s
Iron (25°C) 5960 m/s
Pyrex Glass (25°C)
Steel (25°C)
The speed of sound varies in different media. The property Memory Tips
of a medium determines the speed of sound in it. In a
compact medium, the speed of sound is faster. Temperature Out of three media (gas, liquid
also affects the speed of sound. and solid), sound waves travel
the slowest through gases, faster
Molecules are tightly packed in solids. So, waves transmit through liquids and the fastest
faster in solids. This is a reason why the propagation of through solids. Vgas < V liquid < V solid
sound in solid is faster than that in gas and liquid. In case
of liquids, the molecules are loosely packed than solid. So, it Memory Tips
takes more time for the transmission of waves in liquid. This
is why, the propagation of sound in liquid is faster than that Iron has a density of about 7,800 kg/
in gas but slower than in solid. Molecules in a gas are very m3, while mercury has 13,600 kg/m3,
loosely packed. So, it takes more time for the propagation of but the speed of sound is 1,450 m/s
sound waves in a gas than that in solid and liquid. in mercury and 5,130 m/s in iron. So,
mercury has a higher density but
FACT WITH REASON sound travels slower in it.
When a firework explodes, the light is perceived before the sound, why?
Sound travels more slowly than light. So, when a firework explodes, the light is perceived before the
sound energy.
8.5 Reflection of Sound
When a sound propagating in a medium strikes a surface, it returns to the same medium like
that in case of light. This process is called reflection of sound. Thus, the bouncing back of
sound when it strikes a hard surface is called reflection of sound. A smooth and hard surfaces
like hills, walls, etc. reflects sound waves. The sound waves that get reflected at the surface of
a solid follow the same laws of reflection of light.
ACTIVITY 2
1. Mount a hard and smooth surface AB vertically over a horizontal board. A
2. Clamp two tubes P and Q pointing towards the surface AB. P B
Q
3. Pipes are adjusted in order to listen the sound from a source inside the
tube 'P' through Q.
When the angle of tubes are measured.
a. Angle of incidence is equal to the angle of reflection
b. The incident wave and the reflected wave are on the same plane.
110 Sound
Echo
Echo is an example of reflection of sound. We can hear echo near the walls, hills, etc. One can
hear the echo by shouting loudly at a place surrounded by big buildings or by hills. After
shouting loudly, the same sound reaches to the ears after reflecting from the surface of the
walls. Thus, the repetition of sound because of reflection of sound waves is called echo. Echo
can be heard from almost all large surfaces such as a large wall, building, cliff, etc.
FACT WITH REASON
One can hear the repeated sound of shouting at a place near the big buildings or by hills, why?
Similar to the light waves, sound waves also reflect from a hard surface. After shouting, the same
sound reaches the ears after reflection by the surface of the walls. So, one can hear the repeated sound
by shouting at a place near the big buildings or by hills.
Conditions to listen echo Memory Tips
a) Reflection of sound must reach to the brain after an Reflection of sound must reach to
interval of 0.1 second or the reflector of sound must the brain after an interval of 0.1
be at least at 17 m, second or the reflector of sound
b) The reflecting surface must be relatively flat, must be at least at 17 m but not
so far away because the echo
perpendicular to the source of the sound.
becomes fainter.
c) The loudness of sound must be sufficient.
Echolocation or sound navigation and ranging (SONAR)
Echolocation is the process of locating an object in the surrounding with the help of reflected
sound. The reflection property of sound is used in SONAR (sound navigation and ranging) to
measure the depth of a sea and to locate the position of objects in a sea.
Echolocation in animals
Animals like dolphins, bats, etc. use echolocation to hunt food and
to find objects in their path.
Calculation of depth of the ocean bed
A fathometer on a ship generates ultrasound waves. Let the
downward distance travelled by the ultrasound up to the bed of the ocean is 'd'.
When these waves reflect back from the bed of the ocean and received by the fathometer, the
upward distance travelled is also 'd'. Hence, the total distance travelled is equal to '2d'. If 'v'
is the velocity of ultrasound in water and 't' is the total time taken for the waves to reach back
to the fathometer then,
Speed (v) = Distance travelled = 2d
Time taken t
∴ d = v × t
2
This formula is used to calculate the depth of the ocean bed.
Modern Concept Science and Environment – 8 111
Solved Numerical 8.2
An ultrasound is sent from a fathometer to the bottom of the ocean. If the fathometer receives
the reflected ultrasound from the bed of the ocean in 6s and the speed of the ultrasound in
sea water at 25°C is 1531 m/s. Calculate the depth of the ocean.
Solution:
Speed of ultrasound in water at 25oC (v) = 1531 m/s
Time taken to receive the reflected sound (t) = 6 s
The depth of the ocean (d) =?
From the formula, the depth of the ocean is given by d = v × t = 1531× 6 = 4593
22
Thus, the depth of the ocean is 4593 m.
Solved Numerical 8.3
If you shout facing inside a well with the water surface at a depth of 122.5m from its top,
then how soon after you will hear echo? (Speed of the sound is 345 m/s)
Solution:
Speed of ultrasound (v) = 345 m/s
Depth of the well (d) = 122.5 m
Time taken to receive the reflected sound (t) =?
From the formula, v = 2d
t
or, t = 2 × 122.5 = 0.55 s
345
Thus, the echo will be heard after 0.55 seconds.
Reverberation
In a small closed room, when a source produces sound, the observer hears the sound that
travels not only directly from the source, but also indirectly from a series of reflections taking
place on the walls of the room. Due to multiple reflections, the observer can hear sound
even after the source has stopped producing it. It is called prolongation of sound. Thus, the
phenomenon of prolongation of sound due to series of reflections is called reverberation.
Sound becomes too unclear to be heard in the auditorium because of reverberation. This
can often lead to irritation. To overcome this problem, sound absorbing materials, such as
curtains, plant fibre, compressed fireboard, carpets, etc. are used in the auditorium. These
materials absorb undesired reflected sound and reduce reverberation.
a) Conditions for reverberation
i) The distance between the source of sound and the reflector should be less than 17 m.
ii) There should not be sound absorber in the room or hall.
112 Sound
b) Methods to reduce reverberation
In an auditorium or a big hall, excessive reverberation is highly unwanted. Following measures
are taken to reduce reverberation:
i) The walls and roofs are covered with sound absorbing materials like a rough plaster.
ii) Sound absorbing material such as carpet is used on the floor.
iii) Heavy curtains are used at the entrance, exit, door, windows, etc.
iv) Sound absorbing boards are kept near the stage.
FACT WITH REASON
In an auditorium or a big hall, the walls and ceilings are covered with sound absorbing materials, why?
In an auditorium or a big hall due to multiple reflection of sound, the observer can hear sound even
after the source has stopped producing it. Sound becomes too unclear to be heard. So, in an auditorium
or a big hall, the walls and roofs are covered with sound absorbing materials.
Differences between echo and reverberation
S.N. Echo S.N. Reverberation
1 The repetition of sound due to 1 The phenomenon of prolongation of
reflection of sound waves is called sound due to a series of reflections is
echo. called reverberation.
2 In echo, the distance between the 2 In reverberation, the distance between
source of sound and the reflector the source of sound and the reflector
should be more than 17 m. should be less than 17 m.
Applications of reflection of sound
a) The velocity of sound in air can be calculated with the help of echo.
b) Reflection of sound is used in SONAR to find the depth of the ocean bed.
c) Bats can move easily in the dark with the help of reflected ultrasound.
d) Reflection of sound is used in whispering galleries.
e) Stethoscope is used to hear the sound of internal organs. It works on the laws of reflection of sound.
f) Sound board is used to send the sound towards audience in big hall or auditorium.
FACT WITH REASON
The ceilings of the concert halls are curved, why?
A curved ceiling acts as a sound board. The sound after reflection from the curved ceiling spreads
evenly across the width of the hall. So, the ceilings of the concert halls are curved.
ANSWER WRITING SKILL
1. What is sound? Write down any four sources of sound.
Ans: Sound is a form of energy which causes a sensation of hearing through the vibration of the ear drum.
The sources of sound are sitar, guitar string, flute, tabala, bell, madal, etc.
Modern Concept Science and Environment – 8 113
2. What are longitudinal waves? Write down their examples.
Ans: If the direction of the vibrating particle of the medium is parallel to the direction of the wave
propagation, the wave is called a longitudinal wave. For example, sound wave, waves in a
stretched spring, etc.
3. What is wavelength? Write down its SI unit.
Ans: The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is
called the wavelength. Its SI unit is hertz (Hz).
4. Write down the increasing order of sound propagation in solid, liquid and gas.
Ans: Out of three media, i.e. gas, liquid and solid, the sound waves travel the slowest through gases,
faster through liquids and the fastest through solids. The increasing order is vgas < vliquid < v .solid
5. What is the minimum distance for an echo and maximum distance for reverberation?
Ans: The minimum distance for an echo is 17 m and the maximum distance for reverberation is 17 m.
6. What should be the minimum time to hear an echo?
Ans: The minimum time to hear an echo is 0.1 second.
7. What is SONAR? Write its use.
Ans: SONAR (sound navigation and ranging) is a method of finding the depth of sea and ocean or to
locate the position of objects in the sea. The reflection property of sound is used in SONAR.
8. What are the applications of reflection of sound?
Ans: The applications of reflection of sound are:
i) Reflection of sound is used in SONAR to find the depth of the ocean bed.
ii) Bats can move easily in the dark with the help of reflected ultrasound.
iii) Reflection of sound is used in whispering galleries.
iv) Stethoscope is used to hear the sound of internal organs.
9. Write down the effects of reverberation?
Ans: Reverberation causes disturbance in the classroom, auditorium, meeting hall, etc. to hear the
sound clearly. It causes disturbance in reading and talking. It irritates us.
10. A longitudinal wave produced on a toy slinky travels at a speed of 30 m/s with a frequency of 20 Hz.
Find the minimum separation between the consecutive compressions of the slinky.
Solution: Frequency of sound wave (f) = 20 Hz Speed of sound (v) = 30 m/s
Wavelength of the sound wave (λ) = ?
According to the formula, wavelength of the sound wave, λ = v = 30 = 1.5 m
f 20
Thus, the separation between the consecutive compressions of the slinky is 1.5 m.
11. A boy scouts from the surface of a deep well and hears the echo after 0.4 seconds. If the speed of
sound is 340 m/s, find the depth of the water level in the well.
Solution:
Speed of sound (v) = 340 m/s Time taken to receive the reflected sound (t) = 0.4s
Depth of the well (d) = ?
114 Sound
From the formula, v = 2d
t
or, d = vt = 340 × 0.4 = 136 = 68 m
2 2 2
Thus, the depth of the water level in the well is 68m.
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) Sound is produced by the ………..bodies.
b) Sound is carried by …..….. waves
c) The maximum displacement of a wave from its mean position is called …….
d) The distance between any two successive compressions is called …….
e) The full form of SONAR is……………..
2. Write True for the correct and False for the incorrect statements.
a) Longitudinal waves are formed on the water surface.
b) In the wave motion, medium particles also move.
c) The distance between two successive compressions is called wavelength.
d) Sound cannot travel through vacuum.
e) A non-vibrating body cannot produce sound.
f) The minimum time for echo to hear is 1/10 seconds.
STEP 2
3. Answer the following questions in one word.
a) What is the name of a fine path which carries energy?
b) In which medium does sound travel the fastest?
c) What is the minimum distance in meter which is required to hear an echo?
d) Write the name of a device which works on the basis of reflection of sound.
e) What kind of wave is sound?
4. Write any two differences between: echo and reverberation
a) compressions and rarefactions b)
c) wavelength and frequency
5. Give reasons.
a) The sound from a bell ceases when we touch it.
b) The sound waves are longitudinal waves.
c) Sound does not propagate on the surface of the moon.
Modern Concept Science and Environment – 8 115
d) When a firework explodes, the light is perceived before the sound.
e) Lightning is seen earlier and thunder is heard later.
f) One can hear the repeated sound near a big building or by hill.
g) In an auditorium the walls and ceilings are covered with sound absorbing materials.
h) The ceilings of a concert halls are curved.
i) The speed of sound varies in different media.
STEP 3
6. Answer the following questions.
a) Define:
i) longitudinal wave ii) wavelength iii) frequency
iv) speed of sound v) reflection of sound
vi) echo vii) reverberation
b) How is sound produced?
c) Frequency of a sound wave is 70 Hz, what does it mean?
d) What is propagation of sound?
e) Will the sound be audible if the string is set into vibrations on the surface of the moon?
f) Write down the conditions required for echo.
g) Write down the conditions required for reverberation.
h) Write any two effects of reverberation.
i) What is echolocation?
j) Write any two examples of echolocation in animals.
k) Write down the methods to reduce reverberation.
l) Write applications of reflection of sound.
7. Numerical problems.
a) A longitudinal wave has 20 compressions and 20 rarefactions in 0.1s. Find its
frequency. (Ans: 200 Hz)
b) When a stone is dropped in water, 10 crests and 10 troughs are formed in 2 s. Find
the frequency of the wave. [Ans: 5 Hz]
c) A longitudinal wave produced on a toy slinky travels at a speed of 30 m/s with
a frequency of 20 Hz. Find the minimum separation between the consecutive
compressions of the slinky. (Ans: 1.5 m)
d) The speed of sound in air is 332 m/s. Find the wavelength of a wave of frequency
10 Hz. [Ans: 33.2 m]
e) A radio station transmits a musical program at 220 m wavelength and 1200 Hz
frequency. Calculate speed of the radio waves. [Ans: 240000 m/s]
f) The wavelength of a wave of frequency 100 Hz is 3.3m. Find its speed.[Ans: 330 m/s]
g) A boy shouts inside a deep well and hears the echo 0.4 s after shouting. If the
speed of sound is 340 m/s, find the depth of the water level in the well. ( Ans: 68 m)
h) A man blows whistle in front of a cliff and listens echo in 0.2s. Find the distance
of the cliff from the man. ( Speed of sound = 332 m/s) [Ans: 33.2 m]
i) A submarine on the sea surface sends a SONAR sound waves travelling at 1500
m/s. The reflected sound is received after 4 s. Find its depth? [Ans:3000m]
116 M a g n e t iEssmtimated teaching periods Theory Practical
UNIT 2 0
9 Magnetism
Syllabus issued by CDC Magnetic compass
Introduction to magnetism
Molecular theory of magnetism
Magnetic induction
Demagnetization
Methods of conserving magnetism
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
explain molecular theory of magnetism.
define magnetic induction and demonstrate it with explanation.
state causes of demagnetization.
state the methods of conserving magnetism.
Key terms and terminologies of the unit
1. Magnet : The substance which attracts magnetic substances like iron, nickel, cobalt,
etc. is called a magnet.
2. Magnetic substances : Those substances which get attracted towards a magnet are called magnetic
substances.
3. Non-magnetic substances : Substances which are not attracted by a magnet are called non-magnetic
substances.
4. Magnetism : The various properties of a magnet are called magnetism.
5. Molecular magnet : All magnets and magnetic substances are composed of tiny molecules called
molecular magnets.
6. Magnetic field : The space around a magnet within which magnetic effect can be experienced
is called the magnetic field.
7. Magnetic line of force : The curve path around a magnet along which a unit north pole moves is
called magnetic line of force.
8. Magnetization : The process of making magnet from the soft magnetic materials is called
magnetization.
9. Magnetic induction : Magnetic induction is a process by which a magnetic material develops
magnetic properties in presence of a magnet.
10. Demagnetization : The process by which a magnet loses its properties is called demagnetization.
Modern Concept Science and Environment – 8 117
9.1 Introduction
The substance which attracts magnetic substances like iron, nickel, cobalt, etc. is called a
magnet. Magnet always rests in the north-south direction when freely suspended. It attracts
only magnetic materials like iron, nickel, cobalt, etc. Thus, those substances which get attracted
towards a magnet are called magnetic substances. Magnet also attracts some alloys which
contain magnetic substances. Two such alloys are steel and alnico. Steel is an alloy of iron
and carbon and alnico is an alloy of aluminium, nickel and cobalt. Substances which are not
attracted by a magnet are called non-magnetic substances. For example, wood, plastic, brass,
aluminium, copper, etc. Magnetic materials can be used to make magnet but non-magnetic
materials cannot.
9.2 Discovery of a Magnet
The discovery of magnets is credited to the shepherd boy,
Manganus. There does not have exact date or year when he
discovered the magnet. Manganus used to live in a town
of Magnesia in Asia Minor (now Turkey). One day while
roaming on Mt. Ida, he found that his iron-strapped sandals Lodestone
got stuck to the mountain and created difficult for walking. Magnetite
This was due to a black stone containing iron in it. This stone has a power to attract Iron. He
also observed that the long thin stone taken from the mountain always pointing in a certain
direction when it is freely suspended. Due to this direction indicating property of the stone, it
was named lodestone which means 'leading stone'.
9.3 Magnetism
The various properties of a magnet are called magnetism. For example, a magnet attracts
magnetic substances, it has two opposite poles, i.e. North Pole and South Pole, like poles repel
and opposite poles attract, and magnet has more magnetic power at the ends, etc.
Properties of a magnet
a) Attracting property: A magnet attracts magnetic substances.
b) Property of indicating direction: A magnet when suspended freely with a thread comes
to rest in the north-south direction.
c) The magnetic force is more at the ends of a bar magnet.
d) The two unlike poles of the magnets attract and like poles repel.
e) The poles of a magnet cannot be separated.
Bar magnet Horse-shoe shaped magnet Magnetic compass U-shaped magnet
118 Magnetism
9.4 Molecular Theory of Magnetism
Each molecule of a magnet behaves like a magnet. It shows all the properties of a magnet.
According to the molecular theory of magnetism, all magnets and magnetic substances are
composed of tiny molecular magnets. In a magnetic substance, the molecular magnets are
randomly scattered in the form of a closed chain and in a magnet they are lined up in one
direction in the form of a open chain.
In the magnetic substances like iron, nickel, cobalt, the molecular magnets are scattered in
the form of a closed chain. This pattern of molecular magnets in the form of closed chain
neutralizes the adjacent molecular magnets and magnetic effect is removed.
Arrangement of molecular magnets in a magnet Arrangement of molecular magnets in a magnetic substance
FACT WITH REASON
An iron bar has molecular magnets but it does not attract another iron bar, why?
According to the molecular theory of magnetism, in a magnetic substance like iron bar, the molecular
magnets are scattered in the form of a closed chain. This pattern of molecular magnets in the form of a
closed chain neutralizes the adjacent molecular magnets. So, the magnetic effect is removed.
On the other hand, a magnet has its molecular magnets in the straight line direction. In such
arrangement, the north pole of each molecular magnet points in one direction and the South
Pole faces to the opposite direction. This arrangement of the molecular magnets in the form
of open chain produces a strong effective north pole at one end and a strong effective south
pole at the other end.
FACT WITH REASON
A bar magnet has an effective north pole at one end and an effective south pole on the other end, why?
In a bar magnet, the north pole of each molecular magnet points in one direction and the south pole
points in opposite direction. The opposite poles of the molecular magnets in the middle of a bar magnet
neutralize each other. So, a bar magnet has an effective north pole at one end and an effective south
pole at the other end.
Evidences of the molecular theory of the magnetism
Following properties of a magnet support the molecular theory of magnetism.
a) The poles of a magnet cannot be separated.
b) Only the magnetic substances can be magnetized.
c) A magnet gets demagnetized when it is hammered or heated.
d) A bar magnet has more force at its poles than that at its middle.
Modern Concept Science and Environment – 8 119
FACT WITH REASON
The strength of a bar magnet is more at its poles, why?
A bar magnet has its molecular magnets lined up in a particular direction. In such arrangement, the
north pole of each molecular magnet points in one direction and the south pole point in opposite
direction. However, the opposite poles of the molecular magnets in the middle of the bar magnet
neutralize each other. So, the strength of a bar magnet is more at its poles.
Memory Tips
Main points of the molecular theory of the magnetism
1. All magnets and magnetic substances are composed of tiny and independent molecular magnets.
2. In a magnet, molecular magnets are arranged in a straight chain.
3. In a magnetic substance, molecular magnets are arranged in a closed ring.
4. The poles of a magnet cannot be separated.
9.5 Magnetic Field
The space around a magnet within which magnetic effect can be experienced is called a
magnetic field. A strong magnet has a large magnetic field and small magnet has a small
magnetic field. Magnetic field can be represented with the help of imaginary magnetic lines
of force drawn around a magnet.
Magnetic lines of force
If a unit north pole is placed in a magnetic field of a bar magnet, Magnetic lines of force
the north pole of the bar magnet repels the unit north pole and the
south pole of the bar magnet attracts the unit north pole. So, a unit
north pole moves from the north pole of the bar magnet to the south
pole along a curved path called magnetic lines of force. The curved
path around a magnet along which a unit north pole moves is called
magnetic line of force. Magnetic lines of force can be traced by using
a small magnetic compass. The pattern of magnetic lines of force is
different in different magnets.
ACTIVITY 1
1. Take a drawing board and fix a card board paper with the help of pins.
2. Sprinkle fine iron powder (iron filings) evenly on the paper.
3. Place a bar magnet in the middle of the card board paper and beat the paper
gently. Does the iron filings rearrange in the form of curves? These curves
represent the magnetic lines of force.
9.6 Magnetic Induction : A Method of Magnetization
The process of making a magnet from a soft magnetic materials is called magnetization. The
magnetic materials like iron, nickel, cobalt, etc. can be made magnet using different methods.
This process is also called magnetic induction. Thus, magnetic induction is a process by
120 Magnetism
which a magnetic material develops magnetic properties in presence of a magnet. In magnetic
induction, an ordinary piece of a magnetic substance acquires temporary magnetic properties.
Bar magnet
Stand Stand Steel bar behaves
Unmagnetised as a magnet
steel bar Iron nails
Iron nails
(a) (b)
Magnetic induction
For example, take an iron or steel bar and attach to the clamp connection as shown in the
figure (a). Put some iron nails near to it. In this condition, the iron or steel bar does not attract
iron nails. Again, take a bar magnet near to the iron bar as shown in the figure (b). Now, the
iron or steel bar attracts iron nails. It shows that in presence of a magnet, the steel bar also
develops magnetic properties.
ACTIVITY 2
1. Take a bar magnet and keep on a table slightly facing outside.
2. Touch the pins with a bar magnet one by one as shown in the given figure.
3. You will see a long chain of pins.
4. You will find that the pins are attracted one another and with the magnet. This is
due to the fact that the iron pins also become magnet if we touch them with a magnet.
9.7 Demagnetization
The magnetic properties can get lost in mishandling of an artificial magnet. According to the
molecular theory of magnetism, if a bar magnet is broken into many pieces, each small piece
becomes a complete magnet. When the arrangement of the molecular magnets in a magnet is
disturbed, the magnet gets demagnetized. Thus, the process by which a magnet loses its
properties is called demagnetization. A magnet can be demagnetized in various ways. Some
common ways are listed below: Memory Tips
a) By heating: When a magnet is heated, its molecular The disturbance in the chain
magnets gain thermal energy and their kinetic energy of molecular magnets in a
increases. The alignment of molecular magnets in an magnet is the main cause of
open chain gets changed in a closed chain and the demagnetization.
magnet is demagnetized.
b) By dropping or hammering: When a magnet is dropped frequently or heated regularly,
it changes the alignment of molecular magnets into a closed chain.
Modern Concept Science and Environment – 8 121
iii) By rubbing: A magnet loses its magnetic properties by rubbing the like poles of the
magnets.
iv) Haphazard storing: A magnet loses its magnetic properties by storing magnets
haphazardly without using keepers. When a magnet is kept nearer to another magnet of
similar strength with like poles facing each other, then both of them get demagnetized
due to induction in few days.
v) Electrical method: A magnet loses its magnetic properties when we pass a high voltage
alternating current through a coil wrapped around it.
FACT WITH REASON
Heating and hammering results demagnetization, why?
When a magnet is heated or hammered, the molecular magnets start vibrating vigorously. It breaks
the straight chain arrangement of the molecular magnets. So, heating and hammering a magnet results
demagnetization.
9.8 Conservation of the Magnetism
Magnet becomes weaker if their poles are left free for long time. This is called self-
demagnetization. Magnets should be stored in pairs, with opposite poles placed side by side
after their use. A soft iron piece is placed across the poles of the magnets. These soft iron
pieces are known as magnet keepers. They help the magnets to retain their magnetic properties.
Some common methods for conserving magnetism are given below:
i. We should not heat or hammer a magnet.
ii. We should not rub the like poles of two magnets.
iii. We should not drop a magnet. Keeper of U-shaped magnet
soft iron
iv. We should store magnets by using keepers with Bar magnet
opposite poles placed side by side. piece
ANSWER WRITING SKILL
1. What is molecular theory of magnetism?
Ans: According to the molecular theory of magnetism, all magnets and magnetic substances are
composed of tiny molecular magnets. In a magnetic substance, the molecular magnets are arranged
in a closed chain and in a magnet they are lined up in one direction in the form of an open chain.
2. What is the main difference between magnet and magnetic substance based on molecular theory?
Ans: In a magnetic substance, the molecular magnets are randomly scattered in the form of a closed
chain and in a magnet molecular magnets are lined up in one direction in the form of an open chain.
3. Define molecular magnet?
Ans: All magnets and magnetic substances are composed of tiny molecules which show the properties
of a magnet are called molecular magnets.
122 Magnetism
4. What is magnetic induction?
Ans: Magnetic induction is a process by which a magnetic material develops magnetic properties in
presence of a magnet.
5. What is demagnetization? Write down any four methods of demagnetization.
Ans: The process by which a magnet loses its properties is called demagnetization. Demagnetization is
done by heating, hammering, dropping, rubbing the same poles, etc.
6. How can we save a magnet? Write down any four methods of conservation of a magnet.
Ans: We can conserve a magnet by the following ways:
i. We should not hammer a magnet.
ii. We should not heat a magnet.
iii. We should not rub the like poles of two magnets.
iv. We should not drop a magnet.
7. Write down any two differences between magnet and magnetic substances.
Ans: Differences between magnet and magnetic substances are:
S.N. Magnet S.N. Magnetic substance
1. The substance which attracts 1. Those substances which get
magnetic substances like iron, attracted towards a magnet are
nickel, cobalt, etc. is called a magnet. called magnetic substances.
2. In a magnet, molecular magnets are 2. In magnetic substances, the
lined up in one direction in the form of molecular magnets are scattered in
an open chain. the form of a closed chain.
8. In the given figures identify magnet and magnetic substances. Also write down the reason.
Ans: The figure (a) is of a magnet. This is because molecular magnets are in a straight chain. The figure
(b) is of a magnetic substance. This is because the molecular magnets are in a closed ring.
a. b.
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) The discovery of magnet is credited to the shepherd boy ……..
b) Magnet is the term derived from ……. .
c) The property of a magnet is called …….
d) Hammering a magnet violently destroys its ……..
e) A freely suspended bar magnet rests in………direction.
2. Write True for the correct and False for the incorrect statements.
a) The maximum strength of a bar magnet lies at the ends.
b) Cobalt is a magnetic metal.
Modern Concept Science and Environment – 8 123
c) Lodestone is an example of an artificial magnet.
d) Permanent magnet loses its magnetism after falling on the ground.
e) Magnetic poles cannot be separated.
STEP 2
3. Answer the following questions in one word.
a) What is called to the alloy made from aluminium, nickel and cobalt?
b) What is called to the process of developing magnetic property?
c) What is called to the process in which a magnet loses its property?
d) What is called to the process in which the magnets become weaker after some
time if their poles are left free?
4. Write any two differences between:
a) Magnet and magnetic substances
b) Magnetization and demagnetization
5. Give reasons.
a) A piece of soft iron is attracted by a magnet.
b) A plastic pen is not attracted by a magnet.
c) Most of the iron filing cling near the ends of a bar magnet.
d) When a magnet is heated strongly, it gets demagnetized.
e) Poles of a magnet cannot be separated.
6. Answer the questions with the help of the given figure. figure a
figure b
a) Which figure represents the magnet and magnetic
substance, why?
b) Which theory is explained by the given diagrams?
STEP 3
7. Answer the following questions
a) What is a magnet? Write any three properties of a magnet.
b) What is a molecular magnet?
c) State the molecular theory of magnetism.
d) Explain the arrangement of molecular magnets in a magnet and in a magnetic
substance with the help of a diagram.
e) Write the evidences of molecular theory of magnetism.
f) Define magnetic field and magnetic lines of force.
g) What do you understand by magnetic induction?
h) Explain an experiment to show the method of magnetization.
i) Write down any four activities which cause demagnetization.
j) Write any four methods of safe handling of magnets.
k) Explain the role of magnet keepers.
124 El e c t r i cEisttyimated teaching periods Theory Practical
UNIT 3 1
10 Electricity
Syllabus issued by CDC Electric heater
Simple cell and dry cell
Structure and uses of simple cell and dry cell
Household wiring system
Some domestic electric appliances (electric lamp, heater, electric bell, radio, television,
telephone, mobile and computer)
Fuse and MCB
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
describe the structure and uses of simple cell and dry cell.
introduce household electrification (wiring system) and domestic electric appliances.
introduce fuse and MCB.
Key terms and terminologies of the unit
1. Electricity : Electricity is a form of energy which is produced due to the continuous flow of
electrons through a conductor.
2. Sources of electricity : The devices which produce electricity are called sources of electricity.
3. Cell : A cell is a device which converts chemical energy into electrical energy.
4. Primary cell : The cell which converts chemical energy into electrical energy and cannot be
5. Polarization recharged is called a primary cell.
: The thin film of hydrogen around the copper plate in a simple cell acts as a
resistance. It reduces the e.m.f. of the cell and the cell soon becomes inactive.
This effect is known as polarization.
6. Depolarizer : A chemical substance which removes the hydrogen formed around the copper
electrode in a simple cell to remove its polarization defect is called a depolarizer.
7. Local action : The defect of local current which reduces the output voltage of a simple cell due
to the presence of impurities in a zinc electrode is called local action.
8. Secondary cell : The cell which can be used again and again after recharging is called secondary cell.
9. Domestic electric circuit : An electric circuit made in industries, factories, houses, etc. is called a domestic
electric circuit.
10. Fuse : Fuse is a thin wire made of tin (63%) and lead (37%) having a low melting point
and high resistance.
11. MCB : Miniature Circuit Breaker (MCB) is the advanced form of a fuse. It is used to
protect the household wiring from overloading or short circuit.
Modern Concept Science and Environment – 8 125
12. Electric lamp : The electrical device which converts electrical energy into light and heat energy
13. Filament lamp is called an electric lamp.
14. Fluorescent lamp
15. Heater : The electrical device which converts electrical energy into light and heat energy
16. Electric motor with the help of a tungsten filament is called filament lamp.
17. Electric bell
: A discharge lamp coated with fluorescent material from inside is called a
fluorescent lamp.
: Heater is an electrical appliance that converts electrical energy into heat energy.
: An electric motor is a device that converts electrical energy into mechanical
energy.
: The electrical appliance that converts electrical energy into sound energy is
called electric bell.
10.1 Introduction
Electricity is an important source of energy in the present world. It is the most useful and
convenient form of energy for mankind. It is used in our home for various purposes like
lighting, heating, cooking, etc. Electricity can be converted into different forms of energy
easily. It can be stored and transmitted over a long distance.
Study of electricity involves the study of charges. Charges can be produced on the surface
of a body and can flow through the wires. The charge developed on a body produces static
electricity and the flow of charges through a wire produces current electricity. Thus, electricity
is a form of energy which is produced due to the continuous flow of electrons through a
conductor. Current electricity is very useful to us. In this unit, we will discuss about the
sources of electric current, domestic electrification, some electric appliance, fuse, MCB, etc.
10.2 Source of Electricity
We use a large number of electrical and electronic devices in our daily life. For example,
computer, television, radio, camera, washing machine, refrigerator, calculator, watch, etc. We
need electrical energy to operate these appliances. This electrical energy is derived from the
various sources. Thus, the devices which produce
electricity are called sources of electricity. For example,
dry cell, photo cell, dynamo, generator, etc.
Cell Dry cell Photo cell Generator
A cell is a device which converts chemical energy into electrical energy. The combination of cells
is called a battery. Generally, cells are of two types. They are primary cell and secondary cell.
a) Primary cell : Primary cell gets discharged when the chemicals used in it are consumed.
It cannot be recharged. So, the cell which converts Memory Tips
chemical energy into electrical energy and cannot be Primary cell cannot be recharged
recharged is called a primary cell. Simple Voltaic cell, and cannot be used again and
Leclanche cell, Daniel cell, dry cell, etc. are some of again.
the examples of primary cell.
126 Electricity
b) Secondary cell : The cell which can be used again and again after recharging is called a
secondary cell. For example, lead acid cell, nickel-cadmium cell, etc. In a secondary cell,
electrical energy is stored in the form of chemical Memory Tips
energy and when required, the chemical energy is Secondary cell can be recharged
changed into electrical energy. and used again and again.
Differences between a primary cell and a secondary cell
S.N. Primary cell S.N. Secondary cell
1. A primary cell cannot be recharged. 1. A secondary cell can be recharged.
2. 2.
The chemical reaction in a primary The chemical reaction in a secondary
3. cell cannot be reversed. 3. cell can be reversed.
4. 4.
It gives a small amount of current. It gives a large amount of current.
Primary cell is generally quite cheap. Secondary cell is expensive.
10.3 Simple Cell
Simple cell is also called a Voltaic Copper wire Bulb
cell. It consists of a glass container in Zinc plate
which a copper and zinc plates are Copper plate
immersed in dilute sulphuric acid Beaker
(i.e. H2SO4 + H2O). Sulphuric acid Dil.H2SO4
is called an 'electrolyte'. Similarly,
copper plate and zinc plate are called
electrodes.
Working mechanism Simple cell
The molecules of sulphuric acid
dissociate into hydrogen ion (H+) and sulphate ion (SO4--).
H2SO4 2H+ + SO4– –
Reaction at zinc plate
The dissociated sulphate ions (SO4- -) reach to the zinc plate. Each atom of zinc leaves two
electrons and forms zinc ion (Zn++ ) in the electrolyte. The zinc ions derived from the zinc plate
combine with sulphate ions and make zinc sulphate. As a result, the zinc electrode acts as the
negative terminal (i.e. cathode).
Zn Zn++ + 2 e-
Zn++ + SO4- - ZnSO4
Reaction at copper electrode
The dissociated hydrogen ions (H+) from the sulphuric acid reach to the copper plate. The
hydrogen ions absorb two electrons from the copper plate and form hydrogen gas. Now,
hydrogen gas is released at the copper plate.
Modern Concept Science and Environment – 8 127
2H+ + 2e - H2
As a result of the above reaction, the copper plate acts as positive electrode (cathode) and the
zinc plate acts as negative electrode (anode).
Direction of current flow
The positive charges flow from 'anode' to 'cathode' in the external conducting wire. It is called
conventional electric current. Actually, electrons move from cathode to anode in the external
conducting wire. It is called actual electric current. So, the conventional current and actual
electric current are opposite to each other.
Defects of a simple cell
Simple cell is not a perfect cell. A simple cell suffers from two defects. They are polarization
and local action.
a) Polarization : When we produce current from a simple cell, small bubbles of hydrogen
gas are formed around the copper plate. The thin film of hydrogen around the copper
plate acts as resistance. This layer stops the reaction between acid and the copper plate.
As a result, current does not flow through the circuit. This effect is known as polarization.
FACT WITH REASON
A simple cell cannot provide electric current for a long time, why?
In a simple cell, the formation of insulating layer of hydrogen around the copper plate stops the flow of
current. So, a simple cell cannot provide electric current for a long time.
Remedy Memory Tips
i) Polarization can be removed by rubbing the A chemical substance which
copper plate with a wire brush. removes the layer of hydrogen
around the copper electrode in a
ii) We can use some chemical substances such as simple cell is called a depolarizer.
potassium dichromate (K2Cr2O7), manganese For example, potassium
dioxide (MnO2), copper sulphate (CuSO4), etc. dichromate (K2Cr2O7), manganese
as depolarizer. dioxide (MnO2), copper sulphate
(CuSO4), etc.
b) Local action : A commercial zinc usually contains
particles of iron, copper, tin, etc. When a commercial Memory Tips
zinc plate is used as an electrode, small cells are
formed between these impurities and zinc plate. Thus, The defect which reduces the
local currents are circulated between impurities and external voltage of the simple cell
zinc plate. This defect reduces the external voltage of due to the presence of impurities in
the simple cell. the zinc plate is called local action.
Remedy
i) To reduce local action, the zinc plate is coated with mercury amalgam. This
process is called amalgamation.
ii) To reduce local action, the impure zinc plate is replaced by a pure one.
128 Electricity
Disadvantages of a simple cell
i) It is not portable.
ii) It cannot supply current continuously for a long time.
10.4 Dry Cell + Positive terminal
A dry cell was invented in 1865 by Leclanche Air space
George. It is a portable cell. It consists of a vessel
made up of thick zinc plate. The vessel is covered Paste of NH4Cl
by an insulator from its outside. The zinc vessel is Graphite rod (anode)
filled with an electrolyte of ammonium chloride
in the form of paste. The paste of electrolyte – Zinc can (cathode)
prevents spilling. A cylindrical cavity is left in the Metal jacket
middle of the ammonium chloride. It is separated Dry cell
by a muslin bag. A carbon rod along with a brass Insulator
cap is present at the middle of the muslin bag. A Negative terminal
mixture of carbon and manganese dioxide is filled
in the muslin bag. The top of the cell is sealed by
a porous seal.
Working mechanism
When a conductor connects the metal cap of the carbon rod and the zinc vessel, a chemical
reaction takes place in the cell. The molecules of ammonium chloride are dissociated into
ammonium ions (NH4+) and chloride ions (Cl -) as shown in the given reaction.
NH4Cl NH4+ + Cl –
Reaction at the zinc vessel
The dissociated chloride ion (Cl -) from the ammonium chloride reaches to the zinc vessel.
Each atom of zinc leaves two electrons and form zinc ions (Zn++ ) in the electrolyte. So, the
zinc vessel acts as negative terminal (i.e. cathode). The zinc ion derived from the zinc vessel
combines with chloride ion to form zinc chloride.
Zn Zn++ + 2 e-
Zn++ + 2 Cl - ZnCl2
Reaction at carbon rod
The ammonium ion (NH4+) derived from the ammonium chloride reaches to the carbon rod.
At the carbon rod, ammonium ion produces hydrogen gas and ammonia gas.
2 NH4+ + 2e- 2NH3 + H2
Role of MnO2 in a dry cell
Manganese dioxide (MnO2) used in muslin bag acts as a depolarizer. It reacts with hydrogen
gas to remove the polarization effect in the dry cell.
Modern Concept Science and Environment – 8 129
FACT WITH REASON
There is no polarization in a dry cell, why?
There is manganese dioxide around the carbon rod in a dry cell. It reacts with the hydrogen gas to
remove the polarization effect. So, there is no polarization defect in a dry cell.
Advantages of a dry cell
i) Dry cell is compact and portable.
ii) It can be made in variable shape and size.
iii) The manufacturing cost of a dry cell is very low and it is cheaper.
Differences between a simple cell and dry cell
S.N. Simple cell S.N. Dry cell
1. In a simple cell, sulphuric acid is 1. In a dry cell, paste of ammonium
used as an electrolyte. chloride is used as an electrolyte.
2. Zinc plate acts as a negative electrode 2. Zinc vessel acts as a negative
and the copper plate acts as a positive terminal and the carbon rod acts as
electrode. a positive electrode.
3. It has two defects, namely local action 3. The polarization defect is prevented
and polarization. in a dry cell by using a depolarizer.
4. It is not a portable cell. 4. It is compact and a portable cell.
ACTIVITY 1
Take an exhausted dry cell and cut it longitudinally. Observe the internal structure of the cell. Now,
draw a neat and labelled diagram of the dry cell showing all parts.
10.5 House Wiring System
An electric circuit made in industries, factories, houses, etc. is called a domestic electric circuit.
It is a circuit for alternating current (A.C.). In domestic circuit, all kinds of loads are connected
in parallel. The electricity generated in a hydropower station is brought to our houses through
overhead wires on the poles. The two wires from the pole and a third wire connected to the
earth are distributed on the walls of our house to connect electric appliances. These three
wires are live wire (L), neutral wire (N) and the earth wire (E).
Corporation Consumer's fuse (F2) Socket
Fuse(F1) L
L N
E
N Bulb
Fan
E
Meter Main Main
fuse
box switch box(S)
Distribution board
(kWh meter- M)
House wiring system
130 Electricity
Before distributing the electricity in our house, a kilowatt hour kWh meter (M) and a fuse wire
are connected in series with the live wire. This fuse is called corporation fuse or electricity
authority's fuse (F1). The kilowatt hour meter (M) measures the electric energy consumed by
the various electric appliance in our home. From the kilowatt hour meter, both live and neutral
wires are connected to the main switch box (S). After the main switch, there is a consumer fuse
(F2) to control the electric current flowing in our house.
FACT WITH REASON
In domestic electric circuit, if one appliance stops functioning, the other keeps on working, why?
In domestic electric circuit, all electric loads are connected in parallel. There is an individual circuit
and switch for every load. All loads are supplied with equal potential difference. So, in a domestic
electric circuit, if one appliance stops functioning, the other keeps on working.
Wires connected in domestic electric circuit
Two wires from a pole and a third wire connected to the Neutral wire Earthing wire
earth are distributed on the walls of our house to connect Live wire
electrical appliances. These three wires are live wire (L),
neutral wire (N) and the earth wire (E). Wire
a) The live wire (phase wire) :Live wire or phase wire
is connected to the high potential of 220 V. It brings
current. If accidentally, we come in direct contact
with this wire, we may get an electric shock.
b) The neutral wire : Neutral wire is kept at zero potential by connecting it to the earth.
It provides return path for the current. A live wire is at the potential of 220 V and the
neutral wire is at the ground potential of zero volt. The potential difference between the
live wire and the neutral wire is 220- 0 = 220 V.
c) Earth wire (E) : The third wire which carries the leakage current from a faulty or live
appliance to the ground is called earth wire. One end of the earth wire is connected to
the copper plate buried deep under the earth. It conducts leakage current to the ground.
The earth wire protects a person from the possible electric shocks.
Colour code in domestic wiring
In domestic wiring, three wires are distributed on the walls. The electrical appliances with
metal case like refrigerator, electric iron, etc. have three core of flexible cable. These wires have
different colours. The colour present in the live wire, neutral wire and the earth wire is called
colour code of the wire.
Wire Old Convention New International Convention
Live Red Brown
Neutral Black
Earth Green Light blue
Green and yellow
Modern Concept Science and Environment – 8 131
Advantages of using colour code for domestic wiring
i) Colour code makes easy to identify the live, neutral and earth wire.
ii) Repairing in underground domestic wiring becomes easier by using colour code.
FACT WITH REASON
Colour code is used for wires in domestic wiring, why?
There are three different types of wires in a domestic circuit. They are live wire, neutral wire and earth
wire. In order to identify them, the wires are coated with specific coloured insulator. It helps to repair
the circuit.
10.6 Some Devices in a Domestic Circuit
Fuse
Fuse is a safety device. It avoids the flow of unwanted current Fuse wire melts and breaks the circuit
in the circuit. Thus, fuse is a thin wire made of tin (63%) and
lead (37%) having a low melting point and high resistance. Fuse
Fuse limits the value of current that can pass through a circuit. For example, a fuse of 5A is so
manufactured that this fuse melts and breaks the circuit, if a current greater than 5 ampere (A)
flows through it. If any fault arises in a circuit, its fuse wire melts without affecting the other
circuit. It protects the circuit from fire and avoids fire hazards.
FACT WITH REASON
A fuse wire is of high resistance and low melting point, why?
Fuse is a safety device of the circuit. During excessive heating of an electric circuit, the fuse wire melts
and opens the circuit. As a result, it protects the other circuit components from damage and fire. The
high resistance of the fuse wire helps in heating and low melting point helps to burn fast. So, the fuse
wire is of high resistance and low melting point.
Corporate fuse or electricity authority's fuse (F1)
Before the live and the neutral wires enter into the kilowatt hour
kWh meter (M), a fuse wire is connected in series with the live wire.
It is called corporate fuse (or electricity authority's fuse).
Kilowatt hour meter (M)
It is a meter which measures the electric energy consumed by Corporate fuse
various electric appliances in kilowatt hour or unit. It is the commercial unit of electric energy
consumed.
132 Electricity
FACT WITH REASON
The kilowatt hour (kWh) meter is connected before the main switch, why?
The kilowatt hour (kWh) meter is connected before the main switch so that the total current drawn by
all the appliances can be calculated easily.
Main switch box (S)
From the kWh meter, both live and neutral wires are connected to a
main switch box (S). It is used to switch off the supply to the entire
house whenever it is necessary to repair any fault in the wiring.
Consumer's fuse (F2)
After the main switch, there is a consumer fuse to control the
electric current flowing in the house. It protect the electric circuit connected in our hose.
Miniature circuit breaker (MCB)
MCB is a circuit breaker. It is used in place of a fuse to open a circuit
automatically for protection against overcurrent. Thus, miniature circuit
breaker' (MCB) is the advanced form of a fuse. It is used to protect the
household wiring from overloading or short circuit.
A MCB allows current flow as long as the current in the circuit does not MCB
exceed the current rating of the fuse. If excess current flows, then the
MCB switches off making the circuit open.
Advantage of a MCB over fuse: A fuse melts during overloading. It must be replaced to use
the circuit again. But a MCB can be reset after the fault has been corrected.
FACT WITH REASON
MCB is better than fuse, why?
A fuse melts during overloading or short circuit. It must be replaced to use the circuit again. But MCB
trips when there is a fault in the circuit and current exceeds the safe limit. It can be reset after the fault
has been corrected. MCBs are safer, easier, and offer more protection. So, MCB is better than fuse.
Differences between corporation fuse and consumer's fuse
S.N. Corporation fuse or electricity S.N. Consumer's fuse
1. authority's fuse 1. An electrician fits the consumer's
2. 2. fuse in the house.
3. The electricity authority fits the main 3. It is connected after a main switch.
fuse in a house.
4. 4. It is used for protection of the
It is connected before a kilowatt hour domestic circuit.
(kWh) meter.
It is not sealed.
It is used for protection of the meter
as well as for protection of the
domestic circuit.
It is sealed by the electricity authority.
Modern Concept Science and Environment – 8 133
10.7 Electric Appliances
Electric lamps
The electrical device which converts electrical energy into light and heat energy is called an electric
lamp. Filament lamp, fluorescent lamp, LED bulb, etc. are different types of electric lamps.
a) Filament lamp : The electrical device which converts electrical
energy into light and heat energy with the help of tungsten
filament is called filament lamp. A filament lamp is also called
electric bulb. It has a hollow glass ball with two wires joined
by a tungsten filament. Air is taken out from the bulb and
filled with a mixture of nitrogen and argon gases. When Filament bulb
electric current is passed through the filament, it gets Memory Tips
heated. It becomes white and emits light. It converts Nowadays, CFL (Compact
only about 10 % of the energy supply into light energy. Fluorescent Lamp) and LED
(Light Emitting Diode) bulb are
Rest of the energy is transferred into heat. used instead of filament lamp
b) Fluorescent lamp : A discharge lamp coated with and tube lights. They consume
fluorescent material from inside is called fluorescent less electric energy.
lamp. The fluorescent lamp is commonly known as tube Starter
light. It consists of a tube coated with fluorescent power
from inside and filled with mercury vapour. There are Fluorescent power
coils at the ends of the tubes to supply power. Tube light
It converts only about 30 % of the electrical energy into Mercury
light energy. Rest of the energy is transferred into heat.
Electrode vapour
Electric heater
Choke coil A.C. 220v
Fluorescent lamp
Heater is an electrical appliance that converts electrical energy into heat energy. It consists of a
heating element (nichrome) with high melting point and high resistance. Heaters are used for
different purpose like cooking food, making the room warm, ironing clothes, heating water, etc.
Electric heater Immersion rod Electric iron Electric kettle
Electric heaters
134 Electricity
Electric motors
An electric motor is a device that converts electrical energy into mechanical
energy. We use electric motor in several electrical appliances like fan,
water lifting pump, mixture, etc.
Electric bell Electric motor
The electrical appliance that converts electrical energy into Hammer
sound energy is called electric bell. It works on the principle of Gong
magnetic effect produced by electric current. In an electric bell,
there is an electromagnet to attract a soft-iron bar to ring the Electromagnet
bell. It is used at the gate, school, office, vehicles, etc.
Electric bell
Radio and television
Radio and television are the means of one way communication. Radio and television
They run by using electrical energy. Radio is used to listen
sound whereas television is used to listen sound as well as to
watch the visual display on its screen.
Telephone and mobile phone
Telephone and mobile phone are the means of two ways
communication. They are run by using electrical energy.
Computer Telephone and mobile phone
Computer
A computer is an electronic device that is able to take information
(input), and process it to make new information (output). It is used
to store a large number of information in the form of data. People in
different parts of the world use computers to communicate through
internet. Nowadays, there are computers in e-library to read books
and find information through internet.
ANSWER WRITING SKILL
1. What are cell and battery? Write down the types of cells.
Ans: A cell is a device which converts chemical energy into electrical energy. The combination of cells
is called a battery.
Generally, cells are of two types. They are primary cell and secondary cell.
2. What are the defects of a simple cell?
Ans: Simple cell is not a perfect cell. A simple cell suffers from two defects. They are polarization and
local action.
3. There is no polarization in a dry cell, why?
Ans: There is manganese dioxide around the carbon rod in a dry cell. It reacts with the hydrogen gas to
remove the polarization effect. So, there is no polarization defect in a dry cell.
4. Which materials are responsible for local action in a simple cell?
Ans: The impurities like iron, copper, tin, etc. present in a zinc plate are responsible for the local action
in a simple cell.
Modern Concept Science and Environment – 8 135
5. What is the main difference between a fuse and MCB?
Ans: A fuse melts during overloading. It must be replaced to use the circuit again. But a MCB can be reset
after the fault has been corrected.
6. Which materials are present in a fuse so that it has low melting point and high resistance?
Ans: Fuse is a thin wire made of tin (63%) and lead (37%). Due to these materials, a fuse has low melting
point and high resistance.
7. What is an electric bell? Where do we use it?
Ans: The electrical appliance that converts electrical energy into sound energy is called an electric bell.
It is widely used in houses, offices, schools, vehicles, etc.
8. Why is fuse called a safety device?
Ans: A fuse protects overloading and overflow of current. As a result, it prevents various electrical and
electronic devices in our home. So, fuse is called a safety device.
STEPS EXERCISE
STEP 1
1. Fill in the blanks with appropriate words.
a) A cell converts ……. energy into ……. energy.
b) In a simple cell zinc plate works as……………. terminal.
c) Fuse is made up of……….and………….
d) …………removes polarization in a dry cell.
e) Miniature circuit breaker can also be used in place of ……. .
2. Write True for the correct and False for the incorrect statements.
a) A dry cell converts electrical energy into chemical energy.
b) Ammonium chloride is used in the dry cell as an electrolyte.
c) A fuse is a safety device in an electric circuit.
d) A fuse is made of copper and iron.
e) Electric bulb has a tungsten filament which has very high resistance.
f) The working of an electric bell is based on the magnetic effect of electricity.
STEP 2
3. Answer the following questions in one word.
a) What is called to the electrode present at positive terminal?
b) Which plate is at the positive terminal of a simple cell?
c) Which device converts electrical energy into mechanical energy?
d) Which defect is caused due to hydrogen around the copper plate in a simple cell?
e) Which metal is used to make the cathode of a dry cell?
f) What is used as a safety device in an electric circuit?
g) Which metal is used to make filament of a filament bulb?
4. Write any two differences between:
a) Primary cell and secondary cell b) Dry cell and simple cell
136 Electricity
c) Fuse and MCB d) Filament lamp and fluorescent lamp
5. Give reasons.
a) There is no polarization defect in a dry cell.
b) Fuse is used in a domestic circuit.
c) Fuse wire has a high resistance and low melting point.
d) A filament lamp is filled with nitrogen or inert gas.
e) MCB is better than fuse.
f) Colour code is used for different wires in a domestic circuit.
6. Answer the questions with the help of the given figure.
a) Name each of the metal plate which acts as a
positive terminal and negative terminal.
b) What is the electrolyte kept inside the beaker?
c) Around which plate is the formation of bubbles of
hydrogen?
7. Observe the figure and answer the following questions.
a) Label the parts of the dry cell.
b) Name its positive terminal and negative terminal
STEP 3
8. Answer the following questions
a) Define electricity.
b) What are electric sources? Write with examples.
c) What is an electric cell? Write its types.
d) What are simple cell and dry cell?
e) Explain the following defects in a simple cell:
i) Polarization ii) Local action
f) Write the remedy of the defects in a simple cell.
g) What is the role of MnO2 in a dry cell?
h) Write the advantages of the dry cell over a simple cell.
i) What is a domestic circuit?
j) What is the role of a fuse in an electric circuit?
k) Define live, neutral and earth wires used in a domestic circuit.
l) Write the colour code of the different wires used in a domestic circuit.
m) What is a fuse? Explain the working method of a fuse.
n) What is a miniature circuit breaker (MCB)? Write advantages of MCB over a
normal fuse.
o) Name four electric devices which are used in a domestic circuit.
UNIT Estimated teaching periods Theory MProadcetrincCaol ncept Science and Environment – 8 137
9 1
11
Matter
Syllabus issued by CDC Moseley
Structure of an atom (electron, proton and neutron)
Electronic configuration of first 20 elements and their valency
Molecular formula
Simple introduction to periodic table
Atomic number and atomic weight
Molecular weight
Simple chemical reactions (word equation and formula equation)
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
describe the structure of an atom and state the properties of proton, neutron and electron.
write the molecular formula.
introduce Mendeleev's periodic table.
define valency and find out the valency of the first 20 elements on the basis of their electronic
configuration.
define atomic number and atomic weight and find out the number of protons, electrons and
neutrons on the basis of atomic number.
define molecular weight and calculate the molecular weight of some common compounds.
define chemical reaction and equation, express some simple chemical changes into word
equation and formula equation.
Key terms and terminologies of the unit
1. Matter : Anything that has mass and occupies volume is called matter.
2. Element : The pure form of a substance that cannot be further simplified into other simpler
substances is called an element.
3. Atoms : Atoms are the smallest particles of elements that take part in chemical reactions
without division.
4. Compounds : The pure substances which are formed by the combination of two or more elements
in definite proportion by their weight are called compounds.
5. Molecule : T he smallest particle of a compound or an element that can exist freely in nature is
called molecule.
6. Sub-atomic particles : Each and every atom is made up of tiny particles (electrons, protons and neutrons)
called subatomic particles or elementary particles.
7. Protons : Protons are the positively charged sub-atomic particles of an atom.
8. Electrons : Electrons are the negatively charged sub-atomic particles of an atom.
9. Neutrons : Neutrons are the chargeless sub-atomic particles of an atom.
138 Matter
10. Atomic mass unit : Atomic mass unit is a unit that is used to express the weight of elementary
particles like protons, neutrons and electrons.
11. Atomic number : Atomic number of an atom is the number of protons present in the nucleus
of that atom. It is represented by ‘Z’.
12. Atomic mass : The sum of numbers of protons and neutrons present in the nucleus of an
atom is called atomic mass. It is represented by ‘A’.
13. Electronic configuration : The systematic arrangement of electrons of an atom in its shells and sub-
shells is called electronic configuration of an atom.
14. Symbol : The abbreviation of full name of an element is called a symbol.
15. Valency : The combining capacity of an atom or radical with other atom or radical to
form a new molecule (compound) is called valency.
16. Duplet state : The state in which an atom contains 2 electrons in its outermost K- shell if
atom has only one shell is called duplet state.
17. Octet state : The state in which an atom contains 8 electrons in its outermost shell is
called octet state.
18. Radical : Charged atom or group of atoms that behaves as a single unit during
chemical reaction is called a radical.
19. Electropositive radical : Positively charged atoms or group of atoms are called electropositive
radicals.
20. Electro negative radical : Negatively charged atoms or group of atoms are called electronegative
radicals.
21. Molecular formula : The symbolic representation of a substance that shows the actual number
and kinds of atoms present in it is called a molecular formula.
22. Molecular weight : The sum of mass of the atoms present in a molecule is called molecular
weight (molecular mass).
23. Periodic table : The table in which elements are arranged on the basis of their similar and
dissimilar physical and chemical properties is called periodic table.
24. Mendeleev’s periodic law : According to Mendeleev’s periodic law, the physical and chemical properties
of elements are the periodic function of their atomic mass (weight).
25. Mendeleev’s periodic table : The table in which elements are arranged on the basis of increasing atomic
weight is called Mendeleev’s periodic table.
26. Modern periodic law : According to modern periodic law the physical and chemical properties of
elements are the periodic function of their atomic number.
27. Modern periodic table : The table in which elements are arranged on the basis of their increasing
atomic number is called modern periodic table.
28. Chemical reaction : The process which changes one or more substances into new substances
of different chemical properties is known as chemical reaction (chemical
change).
29. Word equation : When a chemical change is expressed by naming the reactants and products
in words is called word equation.
30. Formula equation : When a chemical change is expressed by writing the reactants and products
in symbols is called formula equation.
Modern Concept Science and Environment – 8 139
31. Unbalanced chemical equation : The chemical equation in which the number of atoms of each element in
reactants and products are unequal is called an unbalanced or a skeleton
chemical equation.
32. Balanced chemical equation : The chemical equation in which the number of atoms of each element in
reactants and products are equal is called a balanced chemical equation.
33. Irreversible reaction : The chemical reaction in which the products cannot be converted into
reactants is called an irreversible reaction.
34. Reversible reaction : The chemical reaction in which the products can be converted into
reactants is called a reversible reaction.
11.1 Introduction
Anything that has mass and occupies volume is called matter. Ice, brick, stone, sugar, alcohol,
glycerine, gold, iron, soil, rock, smoke, cooking gas, water, air, etc. are some examples of
matters. Solid, liquid and gas are three states of matter. Some matters are made up of only
one kind of substance whereas others can be of different substances. The matters which are
composed of the same kind of substances are called pure matters while the matters which
are composed of different substances are called impure matters. Ice is a pure matter as it
contains water only. Smoke is an impure matter as it contains many gaseous substances in it
like oxygen, carbon dioxide, black soot. Mixtures are impure matters.
Elements
The pure form of a substance that cannot be further simplified
into other simpler substances is called an element. Gold,
silver, copper, oxygen, hydrogen, bromine, chlorine, etc.
are some examples of elements. Can we prepare different
substances from the foil of silver? No, we cannot. Every time
we break or try to simplify silver into a number of pieces, we get silver. Silver will never be
decomposed to give other substances like iron, copper, gold, etc. So, silver is an element.
Elements occur naturally but scientists have also synthesized some elements in laboratory.
There are altogether 118 elements, in which 92 are natural and 26 are artificial.
Atoms
Atoms are the smallest particles of an element that take part in Shell or orbit
chemical reactions without division. Atoms of an element are similar Nucleus
in all features while atoms of different elements are different from Proton
each other. For example, all atoms of gold have the same features Neutron
but atoms of gold and atoms of silver are different from each other. Electron
Therefore, there are altogether 118 different types of atoms of 118
different elements. Atom
Compounds
The pure substances which are formed by the combination of two
or more elements in a definite proportion by their weight are called
compounds. Compounds are very useful substances in our daily life.
Water, alcohol, carbon dioxide, sodium chloride, sugar, etc. are some
examples of compounds. The earth contains millions of compounds.
140 Matter
Molecules Memory Tips
The smallest particles of a compound or an element that
can exist freely in nature are called molecules. A molecule 1. Sodium chloride (NaCl) is also
known as table salt.
is formed when two or more atoms of the same or different
2. aWbautnedr an(tH2cOo)mpiosuntdheonmtohset
elements combine with each other. For example, hydrogen earth.
(H) is an element but it exists as H2 which is a molecule 3. O represents an atom of
of hydrogen. H2 is formed by combining two atoms of
hydrogen. Gaseous elements exist in the form of a molecule. oxygen, O2 represents
molecule of oxygen and 2O
For example, O2, N2, Cl2, etc. These gaseous molecules are represents 2 atoms of oxygen.
formed by the combination of two similar atoms. Similarly,
O3is a molecule of ozone which is formed by the combination
of three atoms of oxygen. H2O is a molecule of water which is formed by the combination of
two hydrogen atoms and one oxygen atom.
HH OO Na Cl HH
O
Hydrogen molecule Oxygen molecule Sodium chloride molecule Water molecule
FACT WITH REASON
H2 is a molecule but H is an atom. Give reason.
H2 is a molecule because it is a stable chemical compound that occurs freely in nature but H is an atom
because it quickly undergoes chemical reaction to give a new product.
Atomic structure – – Nucleus
Proton (p+)
Each and every atom is made up of tiny particles called – Neutron (n0)
fundamental particles or sub-atomic particles or elementary
particles. They are protons, neutrons and electrons. These ++ Shell or orbit
particles can neither be created nor be destroyed. Thus, an – ++ Electron (e–)
atom consists of sub-atomic particles.
++
Protons
–
–
Structure of atom
Protons are positively charged sub-atomic particles of an Memory Tips
atom. They are present along with neutrons in the nucleus
of an atom. A proton is represented by p+. The mass of a A positively charged hydrogen ion
proton is equal to the mass of a hydrogen atom which is 1 (H+) is also called proton because
amu (atomic mass unit). it has only one proton.
Neutrons
Neutrons are chargeless sub-atomic particles of an atom. They are present along with the
protons in the nucleus of an atom. A neutron is represented by n0. The mass of a neutron is 1
amu. Its mass is equal to the mass of a proton, or 1837 times more than the mass of an electron.
Modern Concept Science and Environment – 8 141
Electrons
Electrons are negatively charged sub-atomic particles of an atom. They revolve round the
nucleus in an elliptical orbit. While revolving, they do not colloid with each other. An electron
1
is represented by the symbol e-. The mass of an electron is 1837 amu.
Atomic mass unit
We cannot see the sub-atomic particles with our naked eyes. They are extremely small and
their weight is negligible. Due to their importance in energy change, we need to know their
weight. They cannot be weighed using physical balance and their mass cannot be expressed
in kilogram, grams and milligrams.
Atomic mass unit is a unit that is used to express the weight of elementary particles like
protons, neutrons, electrons, etc.
From various mathematical calculations it is known that,
6.023 × 1023 amu is equal to 1 gram.
The mass of one proton is equivalent to 1 amu. Therefore, we need 6.023 × 1023 protons to make
1
1 gram of protons. Also, mass of electron is equal to 1837 mass of protons. Hence we can say
that, the mass of 1proton is equal to the mass of 1 neutron and it is also equal to the mass of
1837 electrons. That is:
the mass of 1p+ = the mass of 1n° = the mass of 1837 e–
Electric charge
We know that protons and electrons have electrical charge on them. Protons are positively
charged and electrons are negatively charged. In SI unit, their charge is measured by a unit
known as coulomb (C).
Mathematically,
1 coulomb = 6.25 × 1018 electrons
It means that one coulomb charge is equivalent to the charge of 6.25 × 1018 electrons.
An atom has equal number of protons and electrons. Equal number of positively charged
protons and negatively charged electrons cancel each other. Therefore, an atom is electrically
neutral in nature.
S.N Sub-atomic particles Symbol Location Mass Charge
1 amu +
1. Proton p+ Nucleus 1 amu
Nil (0)
2. Neutron n0 Nucleus 1 amu
1837 –
3. Electron e– Shell or orbit
FACT WITH REASON
An atom is electrically neutral in nature, why?
Atoms are electrically neutral in nature because in an atom the number of positively charged protons is
exactly equal to the number of negatively charged electrons.
142 Matter
Atomic number
All 118 elements have definite number of protons in their nucleus. The number of protons
gives atomic number. Thus, atomic number of an atom is the number of protons present in the
nucleus of that atom. It is represented by ‘Z’.
Since, an atom has equal number of protons and electrons, so it can be represented as;
Atomic number (Z) = No. of protons (p+) = No. of electrons (e-)
Atomic mass or atomic weight
We know that protons, neutrons and electrons have some mass. The mass of electron is
negligible in comparison to the mass of a proton and neutron. Therefore, atomic mass (weight)
is the sum of numbers of protons and neutrons present in the nucleus of an atom. It is
represented by ‘A’.
Atomic mass is calculated by:
Atomic mass (A) = number of protons (p+) + number of neutrons (n°)
A = p+ + n° Mass number A X Chemical
Z symbol for
On the basis of above formula, the number of neutrons of an the element
atom can be calculated as: Atomic number =
Number of neutrons (n°) = A – p+ number of protons
Number of neutrons (n°) = A – Z 7 Li
3
From the above expression, it is clear that, once we know the atomic number
and atomic mass of an atom, we can find its protons, neutrons and electrons.
EXAMPLE : 1
Calculate the mass of an oxygen atom. (Hint: oxygen atoms have 8 protons and 8 neutrons).
Solution: We have,
No. of protons (p+) = 8
No. of neutrons (n0) = 8
Therefore, atomic mass = no. of p+ + No. of n° = 8 + 8 = 16
EXAMPLE : 2
Calculate the number of neutrons in a sodium atom whose atomic number is 11 and atomic
mass is 23.
Solution: We have,
Atomic number = 11
Atomic mass = 23
We know, atomic number (Z) = no. of protons = 11
Atomic mass = no. of p+ + No. of n°
or, 23 = 11 + n°
or, 23 – 11 = n°
Therefore, n° = 12
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