Blooming science-8 part-1- press

Chapter Sound

8

Learning Outcomes
Estimated Periods: 2+1

On the completion of this unit, the students will be able to:
 introduce some terms related to sound.(velocity, frequency, wavelength)
 introduce and tell effect of echo and reverberation.
 differentiate echo and reverberation.

We hear various kinds of sound every day. Animals and birds can produce sound of
their own. We can produce sound. But sound of every individual is different from
other. So we can identify our friends merely by listening their sound. Sound takes
time to travel but it travels very fast. Producing and listening sound is one of the
important human activities. Sound is also a kind of energy. It is a mechanical form of
energy because it needs material medium for its propagation.

You will study about source, nature, medium of transmission, reflection, absorbtion
etc. of sound here in this unit.

Source of Sound

Vibration of any substance solid, liquid or gas can produce sound. So, a source of
sound is a vibrating object. When a bell is struck with a gong, its brim will vibrate.
This vibration set up waves in air, which travel far and wide in all directions. These
waves strike the ear drum in our ear producing sensation and thus we hear sound.

Various instruments, the sources of sound
Blooming Science & Environment Book 8 101

Vibration is to and fro motion of a body or its parts or particles. When these vibrations
reach our ear, we hear the sound. Human ears cannot hear all the vibrations. If the
frequency of vibrations is less than 20 Hz, our ears cannot hear them. Such vibrations
are called infrasonic waves or infrasound. If the frequency of vibrations is between
about 20 Hz to 20,000 Hz, our ears can perceive them and we hear the sound. Such
vibrations are called audible sound. The higher the frequency, the more shriller (or
sharper) is the sound wave. If the frequency of vibrations is higher than 20,000 Hz,
our ears cannot hear them. Such vibrations are called ultrasonic waves or ultrasound.
Animals like dog, bat, dolphin, etc. can hear these sounds.

When a mosquito flies, the frequency of the vibration of its wings are 120 Hz, so it
produces audible wave (sound). A sound with frequency above 20,000 Hz is inaudible.

The vibration with frequency more than 20 Hz and having enough energy produces
audible sound.

Wave

A wave is a disturbance in a medium which carries energy from one point to another
without there being a contact between the two points. A wave is produced by the
vibrations of the particles of the medium through which it passes. e.g. water waves,
light waves, sound waves, radio waves, etc.

R CR C RC
C RC

When a bell rings, sound waves are produced which move away from the bell
through the air and reach to our ears, here, only the air molecules vibrate and the
sound energy travels through the vibrations of the molecules of air but the air does
not moves. So, a wave moves through the medium without the actual movement of
the medium.

There are two types of wave. They are:

(i) Transverse wave (ii) Longitudinal wave

(i) Transverse Wave Crest l Crest
The wave in which the particles of the
medium vibrate up and down at right angles l
to the direction of propagation of the wave
Trough Trough
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is called a transverse wave. Transverse wave is possible only in solids and liquids.
In a transverse wave, when the vibrating particles move upward, a hump is formed.
This hump is called crest. When the vibrating particles move downwards a hollow is
formed. This hollow is called trough.

The examples of transverse wave are water wave, waves produced on a rope, light
waves and radio waves, etc.

(ii) Longitudinal Wave

The wave in which the particles of the

medium vibrate to and fro (back and R CR C
forth) in the same direction in which the

wave propagates is called a longitudinal wave.

In a longitudinal wave the vibrations causing the wave are in the same direction as
the wave itself.

Longitudinal wave is possible in all three mediums i.e. in solid, liquids and gasses.
Longitudinal wave travels in the form of compressions and rarefactions. Compression
is the part of the longitudinal wave in which particles of the medium coming closer
together and rarefaction is the part of the longitudinal wave in which the particles
of the medium getting further apart. In the figure, C represents the position of the
compression and R represents the position of the rarefaction.

The examples of longitudinal wave are sound wave, the wave produced in a spring,
etc.

Nature of Sound Wave

Sound is transmitted in the form of a longitudinal wave.

The wave, in which the direction of vibration of particles of medium are same to the
direction travel by sound, is called longitudinal wave.

Wave Length
The distance between two successive compressions or rarefactions is called wave
length. Its symbol is l (lamda). The unit of wave length is metre.

Frequency
It is the number of waves produced per second. Its unit is hertz (Hz). The biger units
of frequency are kilohertz (KHz) and megahertz (MHz).
1000 Hz = 1KHz
1000 KHz = 1MHz

Blooming Science & Environment Book 8 103

Speed of Sound

Speed of sound is the product of wave length and its frequency.

i.e. Speed (v) = frequency (f) x wave length (l)
or, v = f x l

Solved Numerical Problems

1. If a sound produced by a baby has wave length 0.5m, what is its frequency?

The speed of sound is 330 m/s in the air.

Using the equation v = f x l or, f = v = 330 = 660 Hz.
l 0.5

\ The frequency of the sound produced is 660 Hz.

2. What is the velocity of the medium wave 400m and 792 KHz?

Solution: = 792Khz = 792 x 1000 Hz = 792000 Hz
Frequency (f) = 400 m
Wavelength (l) = f x l = 792000 x 400 = 316800000 m/s
Velocity (v) = 316800 Km/second


3. In 30 seconds the waves travel from the centre of a pond to its edges. The

length of the pond is 15m and a piece of wood goes up and down two times

in a second. What is the length of the wave?

Solution:

Here, frequency (f)= 2Hz 15
2
Length between the origin and edge of the pond = = 7.5m

Velocity = distance
time

= 73.05 = 0.25 m/sec
v
Wavelength = f = 0.25 = 0.125 m = 12.5 cm
2

Reflection of Sound

When a sound wave strikes a wall, cliff or the hard surface of a rock or any other hard
material, it returns back like other kinds of wave.

You should have experienced or heard the reflected sound in long tunnel or cave, dip
well, in between the cliffs, at night or in dip forest. If the reflected sound is distinct
then it is called echo. Generally, the sound reflected from the distance greater than 17
metres from the source, produces an echo effect. When the reflected sound doesn’t
merge into the sound produced from the source, echo is heard.

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When the reflected sound merges into the sound produced from source, the prolonged
sound is heard and this is called reverberation. when the distance between source and
reflector is less than 17m, then the reflected sound is mixed with original sound and

it is heard extended for longer period of time and cause reverberation.

Reverberation is necessary to some extend to make the sound lively but too much
reverberation makes speech indistinct and noisy. That is the reason that the walls of
a theater and a cinema hall are covered with sound absorbing material to reduce the
reverberation effect.

The walls, ceiling and floor of radio studios, are covered well with sound absorbing
materials to minimize the reverberation effect. Porous and soft materials especially
non-metals are sound absorbing materials. Sound waves are also absorbed by
house hold materials like curtain, desk, chair, tables, beds, etc. to some extent. So,
reverberation is not heard in the room which is used.

We can determine the distance of reflecting medium using the following relation.

Velocity of sound = distance travelled by sound Scan for practical experiment
time

v = 2d
t

\ d = vt
2

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Solved Numerical Problem

A person can hear the reflected sound after 0.1 seconds. What is the distance
between the person and the reflecting surface? (v = 332 m/s)

Solution:

Given,

t = 0.1 s

v = 332 m/s

d=?

We have,

d = vt = 332 × 0.1 = 33.2
2 2 2

= 16.6 m

The distance between the person and the reflecting surface is 16.6 m.

Blooming Science & Environment Book 8 105

Differences between Echo and Reverberation

Echo Reverberation

1. An echo is the repetition of 1. A reverberation is a long drawn
original sound heard after the impression of sound which dies away
sound is reflected from a far off gradually.
surface.

2. For an echo to occur, the reflecting 2. For reverberation two close surfaces

surface must be at least 17m away must be there (distance between the

from the source of sound. source of sound and the reflecting

surface is too short i.e., less than 17m).

3. Sound is reflected only once. It 3. The sound is reflected several times by

occurs in the hills, big halls and two or more close surfaces, because

conference rooms. of which we hear the first sound

prolonged. It occurs in empty ordinary

rooms.

Sound Absorption

In theaters and cinema halls the reverberation is there to some extent only because
large extent of reverberation may lead to disturbance to the musical sequential
tone. Generally, the walls and ceiling of cinema halls are made by sound absorbing
materials. Soft materials, foam, cushion, etc. absorb the sound.

Refraction of Sound

Sound has the properties such as reflection and refraction. It also exhibits diffraction
that is a phenomenon exhibited by wave by passing on the edge of an opaque
body which causes redistribution of the energy. Sound also exhibits interference.
Interference is the process in which two or more light, sound or electromagnetic
waves of the same frequency combine to reinforce or cancel each other in which
the amplitude of the resulting wave being equal to the sum of the amplitudes of the
combining waves also. Due to such properties sound is known as a form a energy.
We can hear a sound which has 1 cm to 15 m long wave length.

At night the sound of sirens, trains, trucks, etc., is heard louder and more clear .This
is because air layers near the ground cools more than those above .Sound travels
more slowly in cooler air than warmer air. As sound travels in different density of air,
it bends while moving. This bending of sound waves is known as refraction of sound.
We hear louder sound at night as they tend to be refract downwards to the earth rather
than being lost in upper layers.

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Main Points to Remember

1. Any disturbance in a material medium produces sound.

2. Sound wave is longitudinal wave. Longitudinal waves are those in which the
vibration of particles of the medium moves to and fro in the direction of wave.

3. The vibrations which are too large to hear for human ear are called ultrasonic
vibration.

4. The vibrations of frequency less than 20 vibration per second are infrasonic
vibrations.

5. The velocity, frequency and wavelength can be related as: Velocity (m/s) =

frequency (Hz) x wavelength (m)

6. We can hear the sound wave of frequency 20Hz to 20KHz which is called
audible sound.

7. Sound travels faster in solid medium.

8. The repetition of sound produced due to reflection by a distant extended surface
like a cliff, hill, well, building, etc. is called an echo. The reflecting medium in
this case must be more than 17m.

9. The ceiling of cinema halls, theaters, etc. are made up of sound absorbing
materials to prevent reverberation and echo.

10. When the reflected sound merges into the sound produced from source, the
prolonged sound is heard and this is called reverberation.

11. Material medium is necessary for the transmission of sound.

12. Sound possesses the properties such as reflection and refraction.

13. Velocity of sound = distance travelled by sound
time

PRO J ECTWORK
Produce a loud sound nearby your school or house or jungle or better in ridge of
a hill. Can you hear echo ? What time does it take to hear echo after you produce
sond. Find the time and calculate the distance.

Exercise

1. Answer the following questions.
(a) What kind of wave is sound wave?
(b) How do you produce sound?
(c) What is longitudinal wave?
(d) Why do we hear louder sound in a newly constructed room than in a
furnished room?

Blooming Science & Environment Book 8 107

(e) What is the frequency of the wave we can hear?
(f) What is the velocity of sound in the air at NTP?
(g) Show the relationship between velocity, frequency and wavelength.
(h) What is the name of wavelength of the wave that we hear?
(l) Why does an astronaut have to use electrical medium to talk in the space?
(j) Why does the sound stop as a ringing bell is held tightly?
(k) How can the depth of the ocean be found out with the help of the

transmission of sound? Explain.
(l) Distinguish between:
(i) Longitudinal wave and Transverse wave
(ii) Echo and Reverberation.

2. Describe following terms.

(a) Echo (b) Reverberation (c) Wavelength

(d) Frequency (e) Velocity of wave

3. Solve the following numerical problems.

(a) The light from a lightening reaches us in 5 seconds. Considering the

time taken for light to reach us is 0 second, find out the distance of the

lightening. (v = 3 x 108 m/s) (7.5 × 108 m)

(b) A spectator can hear the sound of football after 3 seconds of its bouncing.

What is the distance of the ball from the spectator? (498 m)

(c) If the velocity of the sound is 330 m/s what is the wavelength of the wave

of frequency 100Hz ? (3.3 m)

(d) What is the wavelength of a wave whose frequency is 100Hz and travels

with a velocity 340 m/s? (3.4 m)

Glossary

Sound transmission : the phenomenon due to which the sound travels from one to
another place.

Speed of sound : the distance covered by sound in unit time in a medium

Echo : the repetition of a sound due to the reflection of sound waves

Reverberation : the prolongation of sound due to mixing of the reflected sound
with original sound

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Chapter Magnetism

9

Learning outcomes Estimated Periods: 2

On the completion of this unit, the students will be able to:

 describe molecular theory of magnetism.
 introduce, describe and demonstrate the magnetic induction.
 describe demagnetisation with causes and methods of prevention.

In modern science a magnet is known as an essential component of studies to

mankind. It has wide range of application in the modern technology to mankind. The

history of magnetism (the property of magnet) dates back to 2000 B.C. The world

magnetism has been derived from an iron ore-magnetite (Fe3O4) found in Magnesia
in ancient Asia Minor. The Chinese knew the property in 2000 B.C. and they invented

magnetic compass in 1100 A.D. In Hindus and Greek philosophy also, magnetism

has been explained. In writing of Norwegian (1068), magnetite has been named as

load-stone (meaning-leading stone). Gilbert in 1600 wrote a book De Magnet which

dealt with magnetism. Scan for practical experiment

The load-stone is a natural magnet. Artificial magnets can be

prepared from an iron or a steel bar by the following methods:

(1) By rubbing the iron bar with a magnet.

(2) By passing the current through a wire wound round an
iron piece.

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Faraday showed that all substances are affected by a magnet. Substances like iron,
nickel and cobalt are strongly attracted by a magnet and are known as ferromagnetic
substances. Substances like manganese, platinum and oxygen show feeble attraction
and are known as paramagnetic substances. Substances like bismuth, antimony,
phosphorous, zinc, mercury, lead, tin, water and copper are repelled by magnets and
are known as diamagnetic substances.

In modern sense, magnetism is the property of a magnet or it is a form of energy, and
magnet is a substance which stores energy in the form called magnetism.

Blooming Science & Environment Book 8 109

Molecular Theory of Magnetism
The theory was first propounded by Weber in 1852AD and was later developed
by Ewing in 1890AD. Due to this fact it is also known as Weber-Ewing Theory.
According to this theory, every magnetic substance consists of very tiny metals
known as molecular magnets. In general it is said that not only the molecules of a
magnet but even the molecules of any magnetic substance are complete magnets in
themselves.

In an unmagnetised piece of iron, molecules are present in all possible directions and
form (closed chains), whereas in the case of a magnet the molecules are regularly
arranged with their north poles pointing one direction and the south poles pointing
in the opposite direction.
The comparative study of molecular arrangement in a magnet and magnetic material
is called molecular theory of magnetism.

Evidences in support of molecular theory of magnetism
1. A magnet can be broken into two or more small parts, each of them is a complete

magnet.

2. Two poles of a magnet cannot be separated. They exist in pair side by side.
3. The two poles of a magnet are of equal strength. This is because the number

of free north poles at one end is equal to the number of free south poles at the
other end.
4. Heating, hammering, rough handling or dropping of magnet on the floor
causes demagnetization. In all these processes, the orderly arranged molecular
magnets of the magnet will be disturbed. The free poles of the molecular
magnets are not present at the ends of the demagnetized magnet.
Magnetic Field
Bring a compass needle near a magnet, it gets deflected. (A compass needle always
shows north-south direction.) Move the compass needle above, below and to the side
of the magnet, it gets deflected showing thereby magnetic influence all around the
magnet. When you take the magnetic needle farther and farther away, the magnetic

110 Blooming Science & Environment Book 8

influence becomes weaker and weaker. The compass needle is not influenced at all
when it reaches certain distance away from the magnet.

“The space around the magnet up to which its magnetic influence can be felt is called
the magnetic field.” It is strongest near the magnet and decreases with distance.

Magnetic Induction
If a small piece of iron is brought near a magnet, it will be
attracted. The iron piece will be attached with the magnet. Let
us suppose that the iron bar is attached at the south-pole of the
bar magnet. Now, if you bring another piece of iron near the
previous iron, it also will be magnetized. The first iron piece
attached at south pole will act as a magnet.
Thus temporary magnetism can be produced called magnetic
induction. As you detach the first iron piece the other will also
fall since they lose their magnetic property.
In the experiment, instead of iron piece, you can use a steel nib (pen-nib) or a steel
piece. That will bear magnetic property for some time even after detaching from the
bar magnet.

The property of a magnet by virtue of which it is transferred to another body which
is attracted by it for a while is called magnetic induction.

Demagnetization

The process of losing magnetic property of a magnet is called demagnetization. The
main cause of demagnetization is the disturbance in the chain of molecular magnets
in a magnet. When the molecular magnets in a magnet are rearranged in the form of
closed chains, the magnet demagnetizes. A magnet demagnetizes by the following
activities:
1. By dropping or by striking the magnets.
2. By heating the magnets.
3. By storing the magnets without using keepers.
4. By rubbing similar poles of the magnets.
5. By keeping magnets pointing similar poles to same direction for long time.

Ways of Saving Magnetic Property

The magnetic property of magnet can be protected by the following ways:
1. Avoiding the dropping or hammering ( striking ) the magnets.
2. Avoiding the heating of magnetis .
3. Storing the magnets by using keepers so that like poles are not kept together.
4. Preventing from rusting of the magnets.

Blooming Science & Environment Book 8 111

Main Points to Remember

1. Those bodies, which can affect magnetic substances, are called magnets and
the property of a magnet is called magnetism.

2. Magnets attracts magnetic bodies. They lie in north-south direction at rest when they
are suspended freely to move. They have more force at poles than at their middle.

3. Magnetic poles cannot exist freely. Like poles repel and unlike poles attrat.

4. Molecular theory of magnetism states that when the molecular magnets are
found in the form of closed chain in a body, the body is damagnetized and
when they are found in the form of an open chain, the body is magnetized.

5. The region around a magnet upto which the magnet can affect is called magnetic
field.

6. Magnetic field is represented by lines fo force.

7. The path followed by a compass needle from North to South pole of a magnet
is called magnetic line of force.

8. The process of losing magnetic property is called demagnetization.

9. The property of a magnet by virtue of which its magnetic property is transferred
to a magnetic material for a while is called magnetic induction.

PRO J ECTWORK

Take a soft iron plate or nail or a blade and a permanent magnet. Magnetise the
taken magnetic material by stroking method. Observe magnetic induction in thus
prepared magnet. Show it to your teacher and discuss in your class.

Exercise

1. Fill in the blanks.
a. Magnet can affect on.................... substance.
b. The area around a magnet upto which the magnet can affects is
called...................
c. The process of getting magnetic property is called ...................

d. Like poles....................... and unlike poles..........................

2. Match the following. natural magnet
Area around a magnet magnetic N and S poles
Ring form (closed chain) magnetic field
Open chain demagnetization
Magnetic meridian magnetization
Load stone

112 Blooming Science & Environment Book 8

3. Answer the following questions.

a. What are properties of a magnet?

b. What is a magnetic field?

c. What is magnetic induction?

d. Classify the following matters as magnetic and non-magnetic substances. Brass,
steel, Copper, Cobalt, Zinc, Silver, Nickel, Aluminium, Manganese, Iron.

e. What is meant by molecular magnet? Though magnetic substances are
formed by molecular magnets, they do not show the magnetic behavior,
why?

f. Explain the difference between a magnet and an ordinary piece of iron on
the basis of the molecular theory of magnetism.

g. Magnetic properties remains in each piece when a bar magnet is broken
down into pieces. Give reason.

h. Molecular magnets when arranged gives magnet. Why do magnetic
substance not exhibit the property of magnet?

i. Why are the poles of a magnet, when broken down into pieces, not
separated from each other?

4. Give scientific reasons.

a. Poles of magnet cannot be separated.

b. A copper piece cannot be made a magnet.

c. A magnet gets demagnetized when it is heated or hammerred.

d. A magnet has more force at poles than at its middle.

5. Draw the diagrams to show.

a. the arrangement of molecules in a magnet and magnetic substances.

b. the arrangement of molecules in a bar magnet.

Glossary

Directive property : the property of showing direction

Navigators : one who guide or steer a ship, aircraft, etc.

Repel : to push away

Scatter : sprinkle or to throw about in various places

Keepers : a magnetic piece used to avoid the demagnetization

Hemisphere : a half sphere formed by a plane passing through the centre of
the sphere

Meridian : A great circle drawn from any point on the earth’s surface and
passing through both poles

Coincide : to have the same position in space

Blooming Science & Environment Book 8 113

Chapter Electricity

10

Learning Outcomes Estimated Periods: 3+1

On the completion of this unit, the students will be able to:
 describe and demonstrate the structure and uses of simple cell and dry cell.
 introduce simple domestic electric circuit and its devices.
 tell the introduction of fuse and MCB.

Electrical Cell

A cell is a device which maintains a potential difference between the positive and
negative terminals. It can therefore, generate a conventional current, which flows
from the positive terminal to the negative terminal when they are connected externally
through a wire.

Cells convert chemical energy into electrical energy. The chemical reactions which cause
electrons to flow are not readily reversible and the cells cannot be easily charged.

Primary Cell and Secondary Cell

The cells which cannot be recharged (voltaic or simple cell and dry cell) are primary
cells. Once they have been used to emf falls and cannot be used again because they
cannot be recharged. The cells which can be recharged and used again are called
accumulators or storage cells. The cells which can be used in this way again and
again are called secondary cells. Thus, accumulators or storage cells are secondary
cells.

CELLS

PRIMARY CELLS SECONDARY CELLS

Eg. Simple cell, Dry cell Eg. Accumulator
(Cannot be recharged) (Can be recharged)
Simple Cell

A simple cell is called voltaic cell. It is used to get small current. It consists of a vessel
containing dilute sulphuric acid with a copper plate and a zinc plate dipped in it. The

114 Blooming Science & Environment Book 8

two metals act differently in the presence of the acid, and a potential difference of
about 1 volt is developed. The metal plates are called electrodes and the dilute acid is
called the electrolyte. The copper plate is at higher potential and is called the positive
electrode. The zinc plate is at low potential and is called the negative electrode.
When two plates are connected by wire, the bulb connected in it glows. The bulb
does not glow for long time because it has two defects.

Copper plate

Hydrogen
bubbles

Dilute sulphuric acid Zinc plate
(electrolyte)

Fig: Simple cell

Polarization

During chemical reaction within the cell, hydrogen gas bubbles gather around the
copper plate. These bubbles behave as insulator and prevent the flow of current.
Thus, the bulb fades away gradually and ultimately goes out. This effect is called
polarization. The polarization of a cell weakens the main current flow and after
sometime, stops the current flow.

The mechanical method of removing this defect is to take out the copper plate and
brush it several times to remove the layer of hydrogen.

The chemical method is to add a depolarizer such as potassium dichromate (K2Cr2O7)
which oxidizes hydrogen to water.

Local Action

Zinc plate used in the simple cell is commercial zinc. It contains many impurities
such as carbon, lead, iron etc. Since any two different metals in an electrolyte would
form a cell, therefore, zinc and carbon, zinc and lead, zinc and iron in contact with
acid form cells on the zinc plate. These tiny cells cause tiny current to circulate
continuously within the zinc plate itself, and zinc goes on dissolving even when the
cell is not in use. The defect is called the local action. Local action reduces the life of
the cell. The defect is removed by covering the zinc plate with a thin layer of mercury
(amalgamated zinc).

Blooming Science & Environment Book 8 115

Disadvantages of a Simple Cell

1. Current cannot be produced for a long time.

2. It is not easy to carry from one place to other because liquid electrolyte (dilute
sulphuric acid) is used in it.

Dry Cell
A dry cell is a portable cell. Instead of an acid, ammonium chloride is used in dry cell
in Jelly form as an electrolyte in it. So electrolyte cannot spill and should not be kept
upright. This makes it easy to carry from one place to another i.e., portable.

Brass cap

Carbon rod
Ammonium
chloride paste

Mixture of
manganese dioxide

Plastic covering

Fig: Dry cell

A dry cell consists of a cylindrical zinc container with a paste of ammonium chlo-
ride in its inner wall. A carbon rod surrounded by a compressed mixture of carbon
powder and manganese dioxide is placed in a cloth bag. This cloth bag is placed
in the zinc container. The open end of the zinc container is sealed with wax, tar or
plastic etc. to prevent the drying up of the electrolyte.

The zinc vessel acts as a negative terminal and the carbon rod will brass cap acts as
a positive terminal. Ammonium chloride is the electrolyte and manganese dioxide
is the depolarizer. So polarization is prevented. But local action takes place even
though the cell is not in use and reduces its life.

Advantages of Dry Cells Scan for practical experiment
1. They are compact and portable.
2. They do not contain liquid which otherwise could spill. visit: csp.codes/c08e14

So it is easy to carry.
3. They are cheaper to manufacture.
4. They are found in variable shapes and sizes.

116 Blooming Science & Environment Book 8

Differences between Simple Cell and Dry Cell

Simple Cell Dry Cell

1. Dilute sulphuric acid is used as 1. Ammonium chloride paste is used as

electrolyte. an electrolyte

2. Zinc plate is used as negative electrode 2. Zinc container is used as an electrode

and copper as positive electrode. and carbon rod as positive electrode.

3. The potential difference 1.0 V. 3. The potential difference 1.5 V.

4. Two defects or simple cell are local 4. Polarization is prevented by using

action and polarization. depolarizer but local action exists.

5. It is not compact and portable. 5. It is compact and portable.

Household Electrification (Domestic Circuit and its Connection)

The circuit connected in places like industries, factories, houses etc. is called domestic
circuit. Domestic circuit is A.C. circuit.

The electric power of electricity is generated at places far away from where it is to
be consumed. This electric power is transmitted over long distance at high voltage
through cables, which is gradually decreased by step-down transformer to 220V and
supplied to the consumer’s residence.

The cable which brings L

electricity supply to a N S
E 5A 5A

house contains two wires, KW meter 5A S

the live or phase (L) and ELN F Fan
the neutral (N). These
Main fuse & Main switch 5A S
wires enter the meter

box. The fuse of 30A is

connected to the live wire F

before it is connected to the Power circuit

meter. This fuse is called 30A
electricity authority’s fuse.
S
F 30A

The meter records the units Power circuit

(kilowatt hour) of electric Fig: House wiring

power consumed. The wires coming out of the meter are connected to main switch.

A separate fuse called consumer’s fuse of 15A is connected in the live wire after

main switch. The main switch box is made up of iron. It is connected to the earth

wire. Electric current is supplied throughout the house only when the main switch is

turned on. Several pairs of the neutral and live wires are taken out of the main switch

board to the distribution board. From distribution board electricity is supplied to all

parts of the house.

Blooming Science & Environment Book 8 117

During the wiring, all of the electric appliances are connected in parallel. Each and
every appliance has a separate switch. Figure shows a domestic circuit connection
with earthing.

The connection from live wire is taken to one terminal of the appliance through a
switch. The other terminal is connected to the neutral wire to complete the circuit.
The earth wire also goes along with the live and neutral wires. The earth wire is for
safety and carries current only when there is a fault. All the appliances are connected
in parallel so that they work on the same voltage.

Colour Coding in Wires

Electrical appliances such as TV, electric iron, etc. have three core flexible cable,
which have different colour insulation. This is known as the colour coding of wires.

If all the wires have same colour of insulation, it will be difficult to locate the two
ends of any one wire. By convention, the green wire (of the three-wire cable used for
connecting electrical appliances to three pin-sockets) is always made the earth wire.
It is this green wire that is connected to the metal casing of the electrical appliance
on the other side. Colour coding has the following advantages:

i. It makes easy to identify the type of wire in underground wiring.

ii. It makes easy to identify the phase (live), neutral and earth wire.

Wire Old Convention New International Convention
Live Red Brown
Neutral Black Light blue
Earth Green Green and Yellow

Fuse

A wire cannot pass any amount of current. Fuse wire and its circuit symbol
The maximum current that can be passed
through the wire with safety is called its
current rating. If the current through the
wire exceeds its current rating, the wire
may burn. The excess current in an electric
circuit is due to two reasons: They are
overloading and short-circuiting.

Suppose a connecting wire in an electric circuit can carry 5A current with safety.
If electrical appliances used in the circuit draw large current than 5A, the wire may
burn or damage. This is known as overloading.

118 Blooming Science & Environment Book 8

On the other hand positive and negative terminals of an electrical source in an electric
circuit may come in direct contact due to bad insulation or some defects. The total
resistance in the circuit becomes low and a very large current flows in the circuit.
This is known as short-circuiting.

Fuse is a circuit breaker made up of lead or tin with low melting point and high
resistivity. It is connected in live wire. When a fuse wire blows, it should be replaced
by a fuse wire of the same diameter (gauge). Commercially available fuses are 3A,
5A, 10A, 13A, 15A, 30A for domestic purposes. Nowadays, cartridge and MCB
(main/miniature circuit breaker) types of fuse are also in use. Cartridge type fuse is
used in volt guards and other electrical appliances and MCB is used in meter box and
at other points of household circuit.

Differences between Fuse Wire and Simple Wire.

Fuse Wire Simple Wire

1. It is a short piece of wire having 1. It is a long piece of wire having high

low melting point. melting point.

2. It is a thin wire of an alloy of tin 2. It is a wire available in variable size

and lead. and is made of copper, aluminum,

silver etc.

3. It is a safe device and melts itself 3. It is an agent through which current

when excess current flows in the flows from one point to another

circuit. through the circuit.

Miniature circuit Breaker ( MCB) MCB
A miniature circuit breaker ( MCB) is an automatically operated
electronics switch. It is the developed from of fuse and prevents
us from electrical accidents by breaking the circuits when current
overflows in it. It switches off electrical circuit during in overload
condition and faulty condition. It is based on the principle of elec-
tromagnetic induction. It does not need any wire like in fuse and
can be reset and reused again after its functions.

Main Points to Remember

1. Electricity is the energy which is possessed by a body due to the change in the
number of electrons or flow of the electrons.

2. Cell and dynamo are two main sources of current electricity.
3. Cells change chemical energy into electrical energy and dynamo changes

mechanical energy into electrical energy.

Blooming Science & Environment Book 8 119

4. Simple cell has defects like local action and polarization.
5. MnO2 is the depolarizer used in a dry cell. It removes polarization.
6. Dynamo is used as sources of current in bicycle, scooter, etc.
7. Combinations of cells are of three types; they are series, parallel and mixed.
8. Fuse is a circuit breaker with low m.p and high resistivity.
9. MCB (miniature circuit breaker) is a developed form of fuse which disconnects

the electric circuit itself in abnormal conditions.

PRO J ECTWORK

1. Collect the names of different equipment that use electricity in your
house and school. Mention the source from which they get electricity.

2. Cut a dry cell vertically, expose its inner parts and observe.

Exercise

1. Fill in the blanks.
a. ......................charge is produced due to more concentration of electrons.
b. Emf of a dry cell is ................... volt.
c. If we have to light a lamp for a long time, we should use ...............
combination of cells.
d. The source of current which converts chemical energy into electrical
energy is ...............
e. The source of current which converts mechanical energy into electrical
energy is ..............

2. Answer the following questions.
a. What is current?
b. What is an electric cell?
c. Name the positive electrode, negative electrode and the electrolyte in a
simple cell.
d. What materials could be used for the positive and negative plates of a
simple cell and in what liquid are they placed?
e. What materials act as the positive and negative plates of a dry cell?
f. Name the requirements for the preparation of a simple cell. What do you
mean by local action in this cell?

120 Blooming Science & Environment Book 8

g. What is polarization? How can it be removed?

h. How is dry cell made? Explain.

i. Mention two causes due to which dry cell is widely used?

j. There is no polarization in a dry cell. Explain.

k. Write the differences between a simple cell and a dry cell.

l. What is fuse?

3. Write the differences between.

a. polarization and local action

b. simple and dry cell

c. fuse and mcb

4. Sketch the diagrams of.

a. simple cell b. dry cell

5. Study the given figure and answer the following questions.

a. What is shown in the figure?

b. What chemical is used as ‘c’?

Copper plate

c. Which defect is found at the plate

‘Zn’? How can you remove it? Hydrogen
bubbles

d. What defect is found at the plate c Zinc plate
‘Cu’? How can you remove it?

e. Write the chemical reactions that take place at ‘Cu’ and ‘Zn’?

6. A dry cell is shown in the figure. Answer the following questions on the
basis of it.

a. What are A,B,C and D? A
b. Write functions of A, B, C and D. B
c. How does the cell differ from a simple cell? C
d. Write the chemical reaction that takes place at ‘A’. D
e. Write the chemical reaction that takes place at ‘D’.

Blooming Science & Environment Book 8 121

Glossary

Electric charge : the properties of some subatomic particles like protons and
electrons which causes them to exert a force on each other

Positive charge : charge produced by protons

Negative charge : charge produced by electrons

Cell : a device which causes the flow of charge in a circuit or unit of
life

Local action : defect which occurs at zinc plate of simple cell due to impurities
in it.

Polarization : defect which occurs at copper plate due to formation of
hydrogen layer on it.

122 Blooming Science & Environment Book 8