Modern Graded Science 8

OA and AI represent object distance and image distance respectively. If object distance
(OA) = 10 cm, then image distance (AI) = 10 cm.

4. The image is laterally inverted EƎ
Fig 7.5
The letter E appears as Ǝ. The side way inversion of the image of an
object i.e. the left part appearing right part and right appearing left in
the plane mirror is called lateral inversion.

Uses of plane mirror

The following are some uses of a plane mirror:

a. It is used as a looking glass at home, in shops or in the hair-cutting saloons.
b. It is used on dressing tables.
c. It is used in the construction of periscope and kaleidoscope.
d. It is used in the vehicles in front of the seat of the driver to look behind him/her.
e. It is used for various purposes in laboratories.

B. Spherical mirrors

The mirrors whose reflecting surfaces are curved are called curved mirrors or spherical
mirrors.

Suppose, some parts of glass are cut

from a spherical hollow glass as shown in

the figures. The cut-portion of the hollow

sphere acts as a reflecting surface if one

of its surfaces is made opaque by coating Fig 7.6 (a) concave mirror (b) convex mirror
it with a thin layer of mercury or silver. In

the figures, the shaded portion is the opaque surface. If the outer part of the cut-portion

of the glass sphere is silvered, the reflection takes place from the inner surface and we

call it a concave mirror. On the other hand, if the inner surface of the cut-portion of the

glass sphere is silvered, the reflection takes place from the outer surface and we call it

a convex mirror.

A spherical mirror is a part of the surface of a sphere that obeys the laws of reflection. It
is of two types: concave mirror and convex mirror.

1. Concave mirror: If the reflecting surface is reflecting P AQ

curved inwards and the outside is silvered, it is surface focus F

called a concave mirror. The concave mirror is P O
also known as a converging mirror because it

converges the light rays falling on it. Fig 7.7 concave mirror B

Light 97

2. Convex mirror: If the reflecting surface is reflecting P A
curved outwards and the inside is silvered, surface Fig 7.8 convex mirror Q

it is called a convex mirror. The convex P focus
mirror is also known as a diverging mirror F
O
because it diverges the light rays incident B

on its reflecting surface.

Activity 7.3 concave surface

Take a steel spoon. The middle of the spoon is grooved. spoon
This surface is a concave surface. Then, look at the back convex surface
of the spoon.
Fig 7.9

The surface of the spoon bulges out. This is called a
convex surface.

Reasonable Question

A torchlight bulb is placed at the middle or centre of the concave mirror used in a
torchlight. What is the application of this adjustment and why?

Some terms associated to spherical mirrors

The following terms are studied to explain the formation of images by a spherical mirror.

a. Pole of the mirror (O/P) outer surface if polished inner surface if polished
A
The geometrical centre of a mirror A
(concave or convex) is called the

pole of the mirror. It is the centre Cf O CfO
of the reflecting surface of a reflecting
spherical mirror. All the distances surface (inner)
B B reflecting surface
are measured from this point. It is (outer)
Fig 7.10 (a) concave mirror (b) concave mirror
denoted by O/P.

b. Centre of curvature (C)

The centre of the sphere, of which a mirror forms a part, is called the centre of curvature.
The centre of curvature of a concave mirror is in front of the reflecting surface while the
centre of curvature of a convex mirror is behind the reflecting surface. It is generally
denoted by C.

c. Radius of curvature (R) Do you know?

The radius of the sphere of which a

mirror forms a part is called radius The sunlight that we see here on the earth

of curvature. In other words, it is actually leaves the sun about 10 minutes before.

the distance between the centre of

curvature and the pole of the curved mirror. It is denoted by R.

98 Modern Graded Science and Environment Book 8

d. Aperture
The surface of a mirror from which reflection takes place is called its aperture.

e. Principal axis

The straight line of a mirror passing through the pole O and the centre of curvature C is
called its principal axis.

f. Focus

When a parallel beam to the principal axis is incident on a spherical mirror (concave or
convex), after reflection it passes or appears to pass through a point on the principal
axis. This point is called focus.

In figure (a), and figure (b), F is the focus of the mirror. The reflected rays actually meet
at a point on the principal axis in the case of the concave mirror and hence it has a real
focus. But the reflected rays do not meet actually and only appear to meet at a point in
the case of a convex mirror, so it has a virtual focus.

P A P A
focus Q
Q
FO focus

B F
Fig 7.11 (a) concave mirror O

B
(b) convex mirror

g. Focal length (f)

The distance between focus F and pole O/P of a mirror is called focal length. It is generally
denoted by f and is measured in metre.

Focal length is taken as positive for a concave mirror and negative for a convex mirror. This
is because a concave mirror has a real focus but a convex mirror has a virtual focus. Notice
that the radius of curvature (R) of a spherical mirror is twice of its focal length. That is,

Radius of curvature = 2 x Focal length

Focal length = Radius of curvature
2
R
i.e. f = 2

For example, if a concave mirror has its radius of curvature, R = 20 cm, it must be a part

of the sphere for which the radius is 20 cm. The given equation shows that its focal length

f is R = 20 = 10 cm. Thus, if parallel rays of light fall on the concave mirror, they must
2 2

converge at a distance of 10 cm from its pole i.e. at its focus.

Light 99

Construction of ray-diagrams

The diagrams which show the information of an image are called ray diagrams. The
following rules are used to draw ray diagrams of concave mirrors:

QQ QQ

CFO CFO O O

CF F

P P P P
Fig 7.12 (a) (b) (c) (d)

1. A ray of light from an object going parallel to the principal axis is passed through
the focus F after reflection as shown in figure (a).

2. A ray of light from an object passing through the focus F is reflected parallel to the
principal axis as shown in figure (b).

3. A ray of light from an object passing through the centre of curvature C is reflected
along its own path as shown in figure (c).

4. A ray of light from an object is incident at the pole of the mirror, which is reflected
making the same angle as the angle of incidence as shown in figure (d).

Activity 7.4

To show the ray diagram when an object is placed beyond the centre of curvature of a
concave mirror

Materials required

a compass, a pencil and a cardboard paper

Procedure
1. Draw an arc PQ on the cardboard paper using compass. Mark the point where the

compass needle resides as C. This acts as a concave mirror.

2. Mark the middle of arc PQ as O. Shade the outer side of arc PQ.

3. Draw a straight line that passes through O and C as the principal axis.

4. Measure the actual mid point of OC and name it as F.

5. Place an object AB just beyond the C on the principal axis as shown in the figure.

Thus, PQ serves as aperture of a concave mirror, C as the centre of curvature, O
is the pole of the mirror, the straight line through C and O as the principal axis, F
is the centre of C and O as the focus, the length OF is the focal length (f) and the
length CO as the radius of curvature (R).

6. Now, follow any two of the rules used to draw ray diagrams of a concave mirror.

100 Modern Graded Science and Environment Book 8

That is:

a. Draw a straight line parallel to the principal axis from the point A and D as AD.

b. Draw another line from A to E passing trhought the F as AE.

C. The line AD is an incident ray which reflects and passes through the F which
is a reflected ray. Similarly, the incident ray AE on the arc PQ passes through
F, reflects back and passes parallel to the principal axis as EA'. Now, both
the reflected rays intersect together point A in between the F and C.

d. Draw a perpendicular from A' to B' on Q
the principal axis. A'B' is the image of the A D
object AB. Measure the size and position
of the image.

7. The size of the image formed by it is real, B' F O
smaller than the size of the object and inverted. BC E
It is obtained in between the F and C.
A'

Image formed by a spherical Mirror P
Fig 7.13
A. Position and nature of image formed
by a concave mirror

The size, location and nature of Do you know?
the image formed by a concave
mirror depend on the position of Ultraviolet light (UV rays) are often used by
the object. They are given below forensic scientists to see details which are not
with illustrative diagrams: seen by the naked eyes.

Object beyond C [2F] Q

When an object is placed beyond C of a concave mirror, its image object A
is formed between C and F, which is real, inverted and diminished. BC
O
image F

Object at C [2F] Fig 7.14

When an object is placed at C of a concave mirror, its image is A F O
formed at C which is real, inverted and equal to the size of the object. object B
image C Q
O
Object between C and F Fig 7.15 P

When an object is placed between C and F of a concave mirror, its A
image is formed beyond C which is real, inverted and magnified. object
image C B F

Fig 7.16

Light 101

Object at F Q

When an object is placed at the focus of a concave mirror, its image is object O
formed at infinity which is real, inverted and highly magnified.
CF
image

Fig 7.17

Object at infinity Q

When an object is at infinity, the parallel beam coming from it meets object at image O
at the focus F of the concave mirror after reflection. The formed infinity C F P
image is real, inverted and highly diminished.
Fig 7.18

Object between Pole [O] and Focus [F] Q
object A
When an object is placed between focus F and pole O of a concave
mirror, its image is obtained behind the mirror which is virtual, erect image
and magnified. C FB O

P
Fig 7.19

Image formed by a concave mirror

Position of object Position of image Nature and magnification
At focus (F) Real, inverted, highly
1. At infinity Between focus (F) and diminished
centre of curvature (C) Real, inverted, diminished
2. Beyond centre of At centre of curvature
curvature (C or 2 F) Real, inverted, same size
Beyond centre of curvature
3. At centre of curvature Real, inverted, magnified
(C or 2 F) At infinity
Real, inverted, highly
4. Between centre of Behind the mirror magnified.
curvature (C or 2 F) and Virtual, erect, magnified.
focus (F)

5. At focus (F)

6. Between focus(F) and
pole of the mirror (O)

Activity 7.5

Take a concave mirror and keep it about 40 cm away from you and look at your face. How
does it look like? Then, take the mirror and move it towards yourself. What difference
is seen between the two images? Write it down in your notebook. Bring the mirror even
closer towards you. What further difference do you observe? Is the image smaller or
larger and erect or inverted? What differences in the image do you see as you bring
the mirror closer to you? What can you learn from this experiment? Note down in your

notebook and show it to your teacher.

102 Modern Graded Science and Environment Book 8

Note: A virtual erect image is formed only when an object is placed within the focal
length of a concave mirror. The image is real and inverted for all the positions of the

object at the focus or beyond F of a concave mirror.

Rules to draw ray diagrams in a convex mirror

The principal focus and the centre of curvature are located behind the convex mirror. The
following rules are used to draw the ray diagrams in a convex mirror.

1. A ray of light passing from an object parallel to the O F C

principal axis, appears to pass through the focus F object
as in figure 7.20 after reflection.

Fig 7.20

2. A ray of light coming from an object along the line

through the centre of curvature is reflected back object
along the same path as in figure 7.21.
OF C

3. A ray of light coming from an object strikes on the Fig 7.21
pole of the mirror at a particular angle. It reflects at O FC
the same angle as in the figure 7.22.

By using any two of these rules, we can draw the image Fig 7.22
formed by a convex mirror.

B. Position and nature of the image formed by a convex mirror

1. Object between the infinity and the pole (in front Q A1
of mirror) A B1 F

When an object is placed in front of a convex mirror B O C
at any point, the image is formed behind the mirror convex mirror
which is virtual, erect and diminished.
AB = object P
2. Object at infinity
A1B1 = image fig 7.23

When an object is at infinity, the image is formed at the focus object at image
behind the mirror which is virtual, erect and highly diminished. infinity P FC

Uses of spherical mirrors

Uses of concave mirrors Fig 7.24

Some uses of concave mirrors are as follows:

1. If a person keeps his face between the focus and the pole of a concave
mirror, it forms a virtual image of his face, which is enlarged. This property of
the concave mirror helps him for shaving or doing make-up.

Light 103

2. Doctors use the concave mirror to observe the interior parts of our body such
as ear, nose, mouth and throat.

3. Torches, searchlight and headlight of cars use concave mirrors to focus light.
4. Concave mirrors are used in the construction of astronomical telescopes and

radio telescopes as a reflector.
5. Concave mirrors are used in solar cookers to converge the solar radiation

i.e. sun's rays.

Use of convex mirror

Some uses of convex mirrors are as follows:

1. Convex mirrors are used in street lights to scatter light in the wider regions.
2. Convex mirrors are widely used in automobiles to have a clear view of the

traffic behind.

A convex mirror always forms a virtual, erect and diminished image of an object and it
has also a wide field of view. The given figures show why a convex mirror has a wider
field of view than a plane mirror of the same size.

C plane mirror

convex mirror ir ir
ir ir eye

eye narrow field
wide field of view of view

Fig 7.25

Difference between concave mirror and convex mirror

Concave mirror Convex mirror

1. The inner surface this mirror reflects 1. The outer surface of this mirror reflects

whereas the outer surface is polished. whereas the inner surface is polished.

2. It may form an enlarged, diminished or 2. It always forms a diminished image of

equal-sized image of an object. an object.

3. It forms a real and inverted image of an 3. It always forms an erect and virtual
object except when an object is placed image.
between F and O.

4. It is used for make-up and in many optical 4. It is used in vehicles as side mirrors.
instruments

104 Modern Graded Science and Environment Book 8

Refraction of light

When a ray of light, AO in air is incident on A angle of incidence
a glass slab's surface, it bends along OB in M air
the glass as shown in the figure. MN is the incident ray normal
normal to the glass surface at O. The ray P i
OB then emerges along BC in air. Q
glass O refracted ray
We notice that the light ray AO has bent r

along OB while it is travelling from air to BR
B'
glass. In the absence of the glass medium, N
it would have travelled in a straight line OB' S C
in air. Thus, the bending of the light as it
passes through one optical medium to angle of refraction
another is called refraction of light. In the emergent ray

Fig 7.26

given figure, AO is an incident ray and OB is a refracted ray. The incident ray AO makes

an angle AOM with the normal MN drawn at the point O, which is called the angle of
incidence (i). The refracted ray OB makes an angle NOB with the normal MN, which is
called the angle of refraction (r). The point 'O' is called the point of incidence. OB is the

original direction of the incident ray AO.

The velocity of light in air, water and glass is 3 x 108 m/s, 2.2 x 108 m/s and 2 x 108 m/s

respectively. Light travels more slowly when it enters into a denser medium like glass

or water. Because of change in the velocity of light in different media, it bends when it

travels from one optical medium to another medium.

Medium of light and cause of refraction M

The optical media i.e. the media through which light A normal
travels, are classified as a rarer medium and denser incident ray
medium. The medium through which light passes i angle of incidence
easily or in which the velocity of light is more is called air
a rarer medium. The medium in which the velocity of O glass slab angle of
light is less, is called a denser medium. For example, r refraction

in case of air and water, air is a rarer medium and B refracted
water is a denser medium because the velocity of the N ray
light in water is lesser than that in air. For water and

glass, water is a rarer medium and glass is a denser Fig 7.27

medium. Thus, we cannot list the different media as rarer and denser but we can list any

two media as denser and rarer by comparing the velocity of light in these media. Thus, a

rarer medium and a denser medium are relative terms.

Note: Denser and rarer media cannot be classified on the basis of density. For
example, steam has less density than dry air, therefore, it rises up. But steam is a

denser medium and dry air is a rarer medium of light.

Therefore, the change in speed of light when it passes from one medium to another is
the cause of the refraction of light.

Light 105

Laws of refraction

The following are the laws of refraction of light:

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

In the figure 7.26, the incident ray AO, the refracted ray OB and the normal MN at
the point of incidence O, all lie on the same plane of the glass slab.

2. The ratio of sine of the angle of incidence to the sine of the angle of refraction is
constant for any two media. This is called Snell's law.

Mathematically,

Sine of the angle of incidence = Sin i = µ [Constant]
Sine of the angle of refraction Sin r

In the figure, ∠i is the angle of incidence in a medium (say air) and ∠r is the angle of
refraction in the other medium (say glass) in which the light enters.

The Snell's law can be stated in the following ways:

a. When a light ray travels from a rarer to a denser medium, it bends towards the
normal. It means the angle of incidence is greater than the angle of refraction.

b. When light passes through a denser to a rarer medium, it bends away from the normal.
It means the angle of incidence (∠i) is smaller than the angle of refraction (∠r).

c. When light passes along the normal i.e. at right angle to the surface, it passes
through without its bend.

air (rarer) air (rarer)
i
glass (denser)

i

glass (denser)

r

r air (rarer) glass (denser)

(a) (b) (c)
Fig 7.28

Activity 7.6

To verify the laws of refraction
Materials required

a drawing board, thumb pins, pins, a pencil, a ruler, a white sheet of paper and a glass slab

106 Modern Graded Science and Environment Book 8

Procedure Aa M normal

1. Fix a white sheet of paper on a P air b Q
drawing board with the help of thumb glass slab i lateral
pins. displacement
O
2. Place a glass slab in the middle of the r
paper and draw its outline PQRS.
M1
3. Remove the slab and draw a normal
MN at point O for the face PQ. E R

4. Then, draw an angle ∠MOA, (say S N ec
30°). Press two pins a and b in the line
OA so that the pins a and b lie at least M2 d B1
10 cm apart. refraction through glass slab D

Fig 7.29

5. Put the slab in its outline and fix a pin c so that three pins a, b and c lie in a straight
line while viewing through the face RS.

6. Press another pin d so that all the four pins a,b,c and d lie in a straight line and the
pins c and d lie at least 10 cm apart.

7. Remove the glass slab and then the pins a, b, c and d. Join the points c and d and
produce to meet the face RS at E. Finally, join O and E.

8. Produce line AO to B1 but in the format a dotted line

Here, AO is the incident ray, OE is the refracted ray, ED is the emergent ray and OB1
is the original path of the incident ray. We notice that the ray AO has bent at OE while
passing through the air to the glass. The incident ray AO, the refracted ray OE and the
normal MN at the point of incidence O, all lie on the plane of the paper. Since the sides
PQ and RS of the slab are parallel, the incident ray AO and the emergent ray ED are
parallel to each other.

In this way, the given activity has proved the laws of refraction.

Effects of refraction

Some effects of refraction are as follows:

1. A straight stick appears bent upward when it is partially immersed in water.

In the figure, the portion AB of a stick is eye
in water. The rays of light from the end B
of the stick pass through the water to air air A
while reaching the observer's eye. In the water
process, the rays coming from the end B B'
are bent away from the normal because the B
light has travelled from a denser medium Fig 7.30
(water) to a rarer (air) medium.

Light 107

When the refracted rays in air are produced backwards, they meet at B'. Thus B' is the
image of B. The same reason applies to any point on the immersed AB of the stick, so
that the observer sees an image apparently in the portion AB'.

Note: When an object in a denser medium is observed from a rarer medium, the object
appears to be in less depth due to refraction. Such less depth is called apparent depth

and the actual depth is called real depth. D eye

2. A pond or a swimming pool appears air A C
much shallower than it actually is.

The pond appears shallower than its actual B
depth. This is because the depth of the pond water
observed from air acts as a rarer medium
which is actually apparent depth caused O Fig 7.31
due to refraction of light.

In the figure, from the bottom, two rays OA and OB from O are refracted along AD and
BC respectively. These divergent rays appear to come from I which is the virtual image

of the object O, and it appears shallower than O.

THINGS TO KNOW

1. A ray is a single narrow path of light which is represented by a straight line with an
arrow head.

2. A collection of rays of light is called a beam of light.

3. A mirror is the surface which reflects a large fraction of incident light and forms an image
of an object. In general, mirrors are of two types: plane mirror and spherical mirror.

4. A spherical mirror is the part of a sphere of which either the inner or the outer
surface is polished.

5. An image which can be obtained on a screen is called a real image.

6. An image which cannot be obtained on a screen is called a virtual image.

7. When a beam of light is incident parallel to the principal axis of a spherical mirror,
after reflection, it passes or appears to pass through a point on the principal axis.
This point is called focus.

8. A concave mirror changes a parallel beam of light into a converging beam. Hence,
it is called a converging mirror.

9. A convex mirror changes a parallel beam of light into a diverging beam of light.
Hence, it is called a diverging mirror.

10. The phenomenon of bending of light when it passes through one optical medium
to another is called refraction of light.

11. Laws of refraction state that:
a. The incident ray, refracted ray and the normal at the point of incidence, all lie
on the same plane.

b. The ratio of sine of the angle of incidence to the sine of angle of refraction is
constant for any two given media (Snell's law).

108 Modern Graded Science and Environment Book 8

THINGS TO DO

a. Take a concave mirror with its focal length 15 cm.

b. Look at your face in the concave mirror by placing it about 40 cm away from you.
How do you see your own face? Is your face small or magnified? Is the image of
your face real or virtual?

c. Look at your face in the mirror by placing it about 25 cm away from you. What
difference do you find?

d. Look at your face in the mirror by placing it about 10 cm away from you. What is
the position and nature of the image of your face?

TEST YOURSELF

1. Fill in the blanks.
a. A narrow path of light is called .......... .
b. The image which is formed on the screen is called a ................. image.
c. The distance between the focus and the pole of mirror is called .............. .
d. The angle of incidence is .............. to angle of reflection.
e. The side mirror of a vehicle is a .............. .

2. Match the following:

plane mirror real

concave straight line passes from F and O

focal length virtual

principal axis distance between F and O

convex virtual and erect

3. Tick () the correct answer (MCQs).

a. The mid point of a mirror is called: iii. focus iv. aperture
i. centre of curvature ii. pole

b. Which mirror always forms a virtual, real and diminished image?

i. concave ii. convex iii. plane iv. all of them

c. The phenomenon of bending of light as it passes from one medium to another
medium is called:

i. reflection ii. dispersion iii. refraction iv. diffraction

d. The speed of light in water is:

i. 3×108m/s ii. 2×108m/s iii. 2.2×108m/s iv. 3×108km/s

e. The total reflecting surface of a mirror is called:

i. aperture ii. focus iii. pole iv. principal axis

Light 109

4. Define the following terms.

a. convex mirror b. incident ray c. refracted ray

d. plane mirror e. refraction f. spherical mirror

5. Differentiate between:

a. concave mirror and convex mirror b. real image and virtual image
6. Give reasons.

a. A pencil appears bent when dipped partially in a beaker containing water.

b. A concave mirror is used as a reflector in a torch light.

c. A convex mirror is used in automobiles to have a clear view of the traffic behind.

d. We use a plane mirror to see our face.

e. A pond appears shallower than it actually is.

f. A fish cannot be killed by throwing an arrow into the water.
7. Answer the following questions.

a. What are spherical mirrors?
b. Define lateral inversion.
c. What does the point of incidence mean?
d. What do you mean by focus?
e. State the laws of refraction?
f. A person is standing 5 m away in front of a plane mirror. Where is the image

formed? What is the distance between him and the image?
g. What are the uses of plane mirror and convex mirror?
h. Explain the refraction of light on a glass slab.
i. How is the direction of a ray of light changed when it travels from an optically

denser medium to an optically rarer medium?
j. A 4 feet tall student went for swimming in a pool. He saw the depth of the

water in the pool less than 4 feet. Will he be drowned? Write with a reason.
k. When an object is placed near a concave mirror, at what position does it:

i. form a magnified and inverted image?
ii. form a magnified and erect image?
l. If a man shoots a spear at a place where he sees the fish inside water, is it
possible that it will hit the fish? Write with a reason.

8. Diagrammatic Questions:

a. What type of image is formed by a convex mirror? Write with a diagram.

b. Draw ray diagrams to show the position of the image formed in each of the
cases by a concave mirror. Describe the nature of the image.

i. When an object is placed beyond C.

ii. When an object is placed at the C.

iii. When an object is placed between F and C.

iv. When an object is placed at point F.

110 Modern Graded Science and Environment Book 8

c. Write the characteristics of the image formed by a plane mirror with the help
of a figure.

d. When the angle of incidence of a ray is 45°, the corresponding angle of
refraction is 30°. If the media were air and water, write with a diagram from
which medium the light emerges and which medium it enters.

e. Complete the following ray diagram:

air air glass air
glass 42° glass

(a) (b) (c)
f.
Complete the ray diagram showing the formation of an image.

AA

BO F C C FB O



g. Study the given figure and answer these questions. A Q
i. Name the type of mirror shown in the figure.

ii. Name the points C, F and O. CB F O
iii. Write the relationship between OC and OF.

iv. What is the distance OF called? Write its symbol. P
v. Write the name of the line that passes through O and C.

vi. Complete the given diagram and write the nature of the image formed.

GLOSSARY

Lateral displacement : the perpendicular distance between the incident ray
produced and emergent ray from a glass slab/ lateral shift

Periscope : an instrument which uses mirrors to see things which
are hidden to us

Kaleidoscope : an interesting toy which makes use of multiple reflections
by using three plane mirrors held at 60° inclination to
each other

Search light : a device that is used for throwing off strong beam of
light in any direction

Light 111

8Lesson SOUND

Total Estimated Pds: 4 [Th. 3 + Pr. 1]

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

introduce the terms related to sound such as velocity, frequency and wave
length and interrelate them.

introduce echo and reverberation with their effect and differentiate between
them.

Sound is an essential energy for our activities. It is necessary for communication and
information. It is very difficult to communicate without sound. Sound is used while talking with
each other. We get different information from the media like radio and television. We also
use sound for entertainment. Sound is defined as that type of energy which is possessed
due to vibration of bodies which can be experienced by our ears.

Activity 8.1

To demonstrate vibration is a source of sound

Materials required plastic ruler

a scale, a brick and a table
Procedure

1. Take a 1 foot long plastic ruler and place it on a table. table
2. Let about 10 cm of the ruler be projected out from

the edge of the table.
3. Now press the ruler on the table with your hand or

a brick.
4. Release the other end by pressing it gently down.

What happens? We observe that the plastic ruler Fig 8.1 vibrating ruler
vibrates and a sound is heard. This activity shows that

sound is produced due to the vibration in the ruler. When a body moves to and fro about

its mean position, it is said to vibrate. Sound is produced due to the vibration of a body.

In the figure 8.1, the ruler is in position 'a' before pressing it. When it is pressed downwards,
it will move from 'a' to 'b', When It is released free, it will come back to 'a', then to 'c' and
finally back to 'a'. This process is continued. This motion of the ruler from ‘a’ to ‘b’, ‘b’ to
‘a’, ‘a’ to ‘c’ and back to ‘a’, is called one vibration. Here, position 'a' is called the mean
position and, ‘b’ and ‘c’ are two extreme positions.

Sound 1

Sound wave

We have studied that sound can transmit through solid, liquid and gas media. In these media,
sound is transmitted in a special pattern of particles that is called a longitudinal wave.

In order to understand the motion of particles in such a sound wave, once again study
the following diagram.

spring

Fig 8.2 (a) a spring adjusted fixed in between the points A and B
compressions

rarefactions

(b) the end A is hammered

In the figure, a spring is fixed between point 'A' and point 'B'. Then in figure 'b' the spring
is hammered at its end 'A' by which the adjustment of the spring changes. The number
of compressions and rarefactions are passing towards side 'B'. In fact, it is the energy
applied by the hammer which is transmitted from A to B by forming such compressions
and rarefactions.

In the same way, if the particles of the medium vibrate compressions
to and fro along the path to which the wave travels
through the medium, it is called a longitudinal wave. Fig 8.3 digrammatic presentation of
The wave is also called a sound wave. The wave is compressions and rarefactory
represented with the help of the given diagram.

Some terms related to sound wave

1. Amplitude

The maximum displacement of a compressions

vibrating body or particles from its a

mean position is called its amplitude. λ
It is found that the larger the amplitude,
louder the sound. For example, if a reflection
madal is struck gently, a soft sound is Fig 8.4

produced. This is because the amplitude of vibration is less. But, if it is hit hard, a loud

sound is produced, because the amplitude of the vibration is large. It is denoted by a. Its

SI unit is metre (m). Look at the diagram given.

2 Modern Graded Science and Environment Book 8

2. Frequency
The number of complete vibrations made by a particle of a body in one second is called
its frequency. It is denoted by the letter f. The SI unit of frequency is hertz (Hz).
For example, if a body makes 15 complete vibrations in one second, we say that its
frequency is 15 Hz. The larger unit of frequency are Kilohertz (kHz) and Megahertz (MHz).
1 Kilohertz (1 kHz) = 103Hz
1 Megahertz (1 MHz) =106 Hz
In the above diagram 2.5 complete waves being produced in one second are shown.
Thus, frequency of the wave is 2.5 Hz.

3. Time period
The time taken by a vibrating body to complete one vibration is called time period of that
vibration. It is denoted by the symbol T. For example, the time period of a vibrating body
is 2 seconds; it means, it takes 2 seconds to complete one vibration. The SI unit of the
time period is second (s).

4. Wavelength
The distance between two consecutive compressions or rarefactions of a sound wave
is called wavelength of that wave. It is denoted by l (lambda) and its SI unit is m. For its
clear concept, look at the diagram given above.

5. Speed of sound wave
The distance covered by a sound wave in one second is called the speed of a sound
wave. It depends on the product of wavelength and frequency of the wave.

Let the wavelength (l) of a sound wave be 0.25 m and its frequency (f) 1200 Hz. It
means that in one second 1200 complete waves are produced whose wavelength is
0.25. Now, for the speed of sound:

V = f x l
= 1200 x 0.25 = 300 m/s
Thus, speed of sound is 300 m/s.

Solved numerical problems

1. In one second, 700 complete waves are produced in a sound wave. If the
wavelength of the wave is 0.46 m, calculate the speed of the wave.

Solution:
Here,
Wavelength (l) = 0.46 m
Frequency (f) = 700 Hz
Speed of sound (v) = ?
We have,
V =fxl

Sound 3

= 700 x 0.46
= 322 m/s

Thus, speed of the sound is 322 m/s.

2. The speed of a sound wave in a steel rod is 7000 m/s and its wavelength is
0.16 m. Calculate the frequency of sound.

Given: Here, Do you know?
Speed of sound (v) = 7000 m/s
Wave length (l) = 0.16 m There is no sound in space because
frequency (f) = ? there are no molecules. Sound
We have, cannot travel through space since
v =fxl there are no molecules to travel
7000 = f x 0.16 through. Here on earth, we have air
molecules which vibrates to our ears.

f = 7000/0.16 = 43750 Hz

Thus, the frequency of the wave is 43750 Hz.

Nature of sound

The main characteristics of sound are as given below:

(i) It shows the phenomenon like reflection and refraction as light does.

(ii) It is a form of energy.

(iii) It travels in the form of a wave. A sound wave is a longitudinal wave.

(iv) Sound requires a material medium (solid, liquid or gas) to travel. It cannot travel
in a vacuum. The speed of sound is maximum in solid and minimum in gas.

Speed of sound in different media

The speed of sound in different media is different. It is maximum in solids and minimum
in gases. The speed of sound in some media is as given below:

Medium Speed (approx.)
5000-7000 m/s
Steel 4000-5000 m/s
Wood 5000 m/s
Glass 1400 m/s
Water 1210 m/s
Alcohol 332 m/s
Air 260 m/s
Hydrogen gas

4 Modern Graded Science and Environment Book 8

Can you find the height of the cloud from where lightning takes place?

As the speed of light (which is 3 x 108 m/s) is very high in comparison to the speed

of sound (which is 332 m/s in air), the flash of lightning is seen before the sound of a

thunder. To measure the height of the clouds, we start a stopwatch as soon as we see

the flash of lightning and stop as we hear the sound of the thunder. The time interval

between the flash of light we see and the sound of the thunder we hear gives the time

taken (t) by the sound to travel from the clouds to us by using the formula:

Height = velocity of sound x time [\v = s or, s=vx t]
t

We can find the height of the clouds from where a lightning takes place.

The speed of sound is very less than the speed of light. Speed of light in air or vacuum
is 3 x 108 m/s and that of sound is nearly 332 m/s in air. Because of this, the sound of
a thunder is heard later than the flash of lightning. The time interval between the flash
of lightning, we see and the sound of the thunder we hear depends on the height of the
clouds. Nearer the clouds, less the time interval. This is the reason why thunder is heard
some time after the flash of lightning.

Reflection of sound

Activity 8.1

To prove that sound obeys the laws of reflection of light

Materials required

a plane mirror, two plastic pipes of 1/2 inch diameter, a wooden board and a table clock

Procedure plane mirror

1. Place a plane mirror in a vertical position PQ
on a table and a wooden board in the 30°
middle perpendicular to it.

2. Place pipe ‘Q’ on the table so that it makes
an angle of 30° with the wooden board.

3. Now place a table clock at the end of wooden board clock
pipe ‘Q’.
Fig 8.5 laws of reflection of sound
4. Place another pipe ‘P’ on the other side
of the wooden board.

5. Move pipe ‘p’ in a clockwise or anti-clockwise direction, keeping your ear
at the end of it to get clear 'tick-tick' sound of the clock.

This is because the sound wave produced by the clock is reflected from the mirror and
it reaches our ear. If the angle between the wooden board and pipe P is measured, it is

Sound 5

also found to be 30º. Thus, the angle between pipe Q and the wooden board is equal to
the angle between pipe P and the wooden board. That is, the angle of incidence is equal
to the angle of reflection. This experiment proves that sound obeys the laws of reflection
of light.

Echo

If we shout in front of a hill or in a cave, we may hear our own sound repeated. It is
because the sound we produce is reflected from the hill or the wall of the cave and it is
repeated. The repetition of sound caused due to its reflection is called an echo.

The following conditions are required to occur an echo:

(i) The minimum distance between the source of sound and the reflecting
body should be 17m.

(ii) The loudness of sound should be sufficient so that it can be heard after its
reflection.

(iii) The reflector should have a large area for enough reflection of sound. And
it should be hard too.

We do not hear an echo in a small room because the distance between any two walls of
the room is less than 17m; hence the original and the reflected sound mix up and the
echo is not heard but the sound is elongated.

Reverberation

When it thunders, we hear the sound of a thunder total distance = 2d
for a comparatively long time. This is because the d
sound of the thunder undergoes multiple reflections
in the clouds, cliffs, rocks, etc. Such multiple source of sound d reflector
reflections of sound cause reverberation. Fig 8.6

Reverberation is defined as prolongation of sound produced by a series of reflection of
sound. For reverberation, the distance between the source of sound and reflector must
be less than 17 meters. In this situation, the original sound and the reflected sound mix
with each other. It is more beneficial in musical concerts.

We can use echo method to estimate the distance between the source of sound and
the distant surface from which the sound is being reflected to produce an echo. Let the
distance be d. Obviously, the sound emitted from its source travels the distance of 2d to
hear the echo by an observer staying near the source of sound. If v is the speed of sound
in air and t is the time during which echo is heard,

then v = 2td

6 Modern Graded Science and Environment Book 8

Solved numerical problems

1. A boy shouts loudly in front of a hill and an echo is heard after 0.5 seconds.
Find the distance between the boy and the hill. (Speed of sound in air = 332 m/s)

We have v = 2d
or, 332 t

2d
= 0.5

d = 332 × 0.5 = 83 m
2
Hence, the distance between the boy and the hill is 83 m.

2. A person can hear an echo 0.11 sec. after he shouted near a hill. If the speed of
sound in air is 332 m/s, calculate the distance between the person and the hill.

Solution: Do you know?
Here,
Time (t) = 0.11 sec Whale voices are able to travel a
Speed of sound (v) = 332 m/s whopping 479 miles or km through
Distance (d) = ? the water of the ocean. They have
We have, for echo- the ability to communicate with
v = 2d/t each other for long distance.
or, 2d = vt

or, vt = 332×0.11 = 18.26 m
2 2

Thus, the hill is 18.26 m away from the person.

Absorption of sound

When we talk in an empty large room, it is difficult to understand each other. This is due
to more reverberation inside the room. Thus, the sound absorbing materials are used at
walls of the auditoriums to minimize reverberation. These materials absorb more sound
and reflect only a little; therefore the quality of sound heard by the people becomes clear.
Soft surfaces such as wood, carpets, curtains and clothes are better absorbers whereas

hard surfaces such as the wall of a room, hill, etc. are better reflectors of sound.

THINGS TO KNOW

1. Sound is produced due to vibrations in a body.
2. Sound is transmitted from place to place in the form of a longitudinal wave or

sound wave.

Sound 7

3. Sound requires a material medium to travel from one place to another. Its
speed is highest in solids and lowest in gases.

4. The wave in which the particles of the medium vibrate to and fro in the
direction the wave travels is called a longitudinal wave.

5. Amplitude is the maximum displacement of particles from their mean position
in a wave.

6. The distance between two consecutive compressions or rarefactions of a
longitudinal wave is called the wave length of that wave.

7. The number of complete waves produced in one second is its frequency.
8. The distance covered by a complete wave in one second is called speed of

the wave [v = l x f].
9. Sound follows the laws of reflection of light.
10. The repetition of sound, which is reflected from a hill or wall of a cave is

called an echo.
11. To hear an echo, the minimum distance between the source of sound and

reflecting body should be 17 m.
12. The prolongation of sound caused by multiple reflections is called reverberation.
13. Soft surfaces are better absorbers and hard surfaces are better reflectors of sound.

THINGS TO DO

Make a telephone using local materials.

1. Take two small empty plastic cups.
2. Pass a cotton thread through a small

hole made at the base of the cup.
3. Tie the other end of the thread to the

base of another cup.
4. Hold the cups between you and your

friend so that the thread is kept taut.
5. Produce a soft sound by speaking and ask the friend whether he/she hears the

sound or not.

TEST YOURSELF

1. Fill in the blanks.

a. Prolongation of sound is called ................. .

b. Sound is possessed by ................. .

c. Speed of sound in solids and liquids is ............ than the speed of sound in air.

d. Sound cannot travel through ................. .

e. We can hear an echo only if we stand at least .......... metres away from a
wall and shout towards it.

8 Modern Graded Science and Environment Book 8

2. Tick () the correct answer (MCQs).

a. Sound can pass through:

i. solid ii. liquid iii. gas iv. all of them

b. Frequency is denoted by:

i. F ii. l iii. f iv. a

c. Sound absorbers are used at the wall of auditoriums to minimize:

i. echo ii. noise iii. reverberation iv. refraction

d. Normal talking is not possible on the moon due to lack:

i. gravity ii. atmosphere iii. water iv. sunlight

e. The value of v is equivalent for:

i. λf ii. λf iii. f x l λ
3. Answer the following questions. iv. 2f

a. Define a sound wave. What are compressions and rarefactions?

b. What is an echo? Write the conditions necessary for occurring an echo.

c. Define amplitude and frequency of a wave.

d. Define reverberation. How does it occur?

e. What is vibration? What do you mean by the mean position and extreme
position of particles in vibration?

f. Define a longitudinal wave. What is it made of?

g. What is transmission of sound? What type of medium is required for it?

h. Prove with the help of an activity that sound obeys the laws of reflection of
light.

4. Differentiate between:
a. frequency and wavelength

b. echo and reverberation

5. Give reasons.
a. Echo is not heard in a small room.
b. Big cinema halls are carpeted and their walls are made of some rough
materials.
c. We hear louder sound in a newly built room than in a furnished room.
d. In space, astronauts use an electrical medium to talk to each other.
e. When a ringing bell is touched, no sound is heard.
f. Sound is repeated near hills.

Sound 9

6. Diagrammatic Questions: Medium Velocity of sound

a. Velocity of sound in three different media A 332 m/s
A, B and C is given in the table. Identify
solid, liquid and gas among them. B 1500 m/s
C 5200 m/s
b. Both the waves shown in the diagram

are produced in one second. A

i. Label A, B, C and D. B
C

ii. Which one has more frequency.

iii. Which one has more amplitude. D

7. Numerical Problems:

a. A person shouts loudly at the mouth of a well facing towards the water. The
echo is heard clearly after 0.2 seconds. What is the distance between the
person and the surface of water? [Velocity of sound in air = 332 m/s]

b. The gap between thundering and flashing of light is found to be 10 seconds.
How high may the clouds be in the sky?

c. The distance between a boy and a hill is 250 metres. If the boy shouts, after
how long will the echo be heard?

d. Calculate the speed of sound whose wavelength is 12 m and frequency 200 Hz.

e. The speed of sound in air is 332 m/s. If its wavelength is 33.2 m, calculate its
frequency.

f. If wavelength and speed of a wave are 4 m and 332 m/s respectively, calculate
its frequency.

g. A person heard his echo 3 s after he shouted near a hill. Calculate the
distance of the hill from him if the speed of sound is 332 m/s in air.

Answers

7. a. 33.2 m b. 3320 m c. 1.5 s d. 2400 m/s
e. 10 Hz f. 83 Hz g. 498 m



GLOSSARY

Mean position : the position of zero displacement

Longitudinal wave : a wave that vibrates in the direction it is moving

Auditorium : the part of a theatre, concert hall, etc. in which the audience
sits

Taut : stretched tight

10 Modern Graded Science and Environment Book 8

9Lesson MAGNETISM

Total Estimated Pds : 3 [Th. 2 + Pr. 1]

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

describe molecular theory of magnetism.
define magnetic induction, describe it and demonstrate.
tell the cause of demagnetization and methods of conserving magnetic

strength.

A magnet is a device that can affect magnetic bodies. Its property is called magnetism.
Magnets are very useful in our daily life. They are used for many purposes such as for
separating mixture, for pulling out magnetic substances from the eyes and wounds, in
the field of electronics and medicine etc. Lodestone is a natural magnet having a mineral
called magnetite. Magnets are those bodies which can affect magnetic bodies.

Nowadays, man-made magnets are also used. They are available in many shapes.
Magnets show some particular properties. Such properties are collectively called
magnetism.

Properties of magnets

1. A magnet attracts magnetic bodies. The NS
attracting force is more at its poles (ends)
than at the middle of it. bar magnet

Fig 9.1 a magnet attracting pins

2. A magnet suspended freely adjusts in string
north-south direction when it is at rest.

3. Magnetic poles of a magnet cannot be N S
separated by breaking it into pieces.

Fig 9.2 a magnet in particular directions

N S N SN S N S N S N S NS

Fig 9.3 magnetic poles cannot be separated

Magnetism 1

4. A magnet transfers its magnetic property to bar magnet
other bodies which are in contact with it or NS
near it. The process is called magnetic
induction. pin

Fig 9.4 transfer of magnetic energy

5. Like poles of a magnet repel and unlike
poles attract.

Activity 9.1 SS
N
To prove that magnetic poles cannot be separated
Materials required N

a bar magnet, a razor blade and magnetic compass. Fig 9.5 like poles repel

Procedure N
S
1. Take a half razor blade as shown in the figure
and make it magnet by the stroking method. SN

2. Break the blade and test the magnetic poles by SN S N
using a magnetic compass. Fig 9.6

3. Repeate the process till you can break the blade.

Have you got a piece of blade that has a single
pole? Impossible. It proves that magnetic poles
cannot be separated.

Molecular theory of magnetism

The process of making a magnet Do you know?
is called magnetization and the
process of losing magnetic proporty The people of Asia Minor found a stone-like
is called demagnetization. Molecular substance in Magnesia around 800 BC. It
theory of magnetism explains the was able to attract small pieces of iron and
mechanism of magnetization and showed particular direction when it was
demagnetization of magnetic bodies. suspended to move freely. It was named
lodestone or leading stone because it was
It states that a magnetic body is used by ancient Chinese navigators for
made of magnetic molecules or directive purpose. Since it was discovered
molecular magnets. Each magnetic in Magnesia, it was named magnet.
molecule has S and N poles as in
a magnet. Because of this reason,
magnetic poles of a magnet cannot
be separated.

2 Modern Graded Science and Environment Book 8

SN SN N SN SN SN SN S
S
N N SN SN SN SN S
S N SN SN SN SN S

N
N

S
N

S
S

N
S
N
N
S
N
S
S
N
S
N
NS NS NS

Fig 9.7 (a) closed chains of magnetic molecules (b) open chains of magnetic molecules

When these magnetic molecules are arranged in the form of a closed chain or ring form
in magnetic bodies, they lose their magnetic force by attracting each other. The body
gets demagnetized. But when the magnetic molecules are arranged in the form of an
open chain the body gets magnetized. At the poles of a magnet, magnetic molecules
do not lose their magnetic force by attracting the opposite poles. At the middle, the
magnetic molecules lose their force by attracting each other. Therefore, a magnet has
more magnetic force at the ends than in the middle.

Activity 9.2

To demonstrate molecular theory of magnetism
Materials required

a bar magnet, a test tube, iron filings, a magnetic compass and a cork

Procedure

Sbar magnet
N
test tube

magnetic compass magnetized iron fillings

iron fillings cork

Fig 9.8 magnetization of iron particles

1. Take a test tube half filled with iron filings. Close the test tube with a cork.

2. Stroke the test tube by using a permanent magnet in a single direction about 40
times by keeping the test tube in north-south direction.

3. Place the magnetic compass on a table.

4. Bring the test tube close to the magnetic compass and observe the change.

5. Now, shake the test tube frequently for a bit long time and bring it again close to the
compass. What happens now?

6 . Try to understand the molecular theory of magnetism by these activities.

Magnetism 3

Magnetic field

Activity 9.3

To show magnetic field of a magnet
Materials required

thick paper, a drawing board, iron filings and a bar magnet

Procedure

1. Keep a bar magnet at the middle of a drawing
board.

2. Place a thick paper over the bar magnet which
is placed facing N-S direction.

3. Scatter iron filings over the paper carefully.

4. Now, hit the paper gently and study the Fig 9.9 lines of force
arrangement of the iron filings.

The iron filings show the magnetic field of the magnet.

Magnetic field is defined as the area around a magnet up to which it can affect. Magnetic
field is represented by the lines of force. Direction of magnetic force from N to S is also
shown on the lines of force.

Evidence of molecular theory of magnetism

The following points can be taken as evidence for molecular theory of magnetism:
1. The poles of a magnet cannot be separated.
2. Only magnetic bodies can be changed into magnets.
3. A magnet gets demagnetized when it is hammered or heated.
4. A magnet has more force at its poles than in its middle.

Magnetic Induction

When a magnetic body is kept in contact with or close to a magnet, it shows magnetic
property or it gets magnetized. The process is called magnetic induction. A magnetic
body has magnetic property till it is close to the magnet. The end of the magnetic body
close to the magnetic pole forms an opposite magnetic pole. It means that the end of
the magnetic body close to the north pole of a magnet forms a south pole and vice versa.

4 Modern Graded Science and Environment Book 8

Activity 9.3

To demonstrate magnetic induction

Materials required

a stand with clamps, a bar magnet, a nail and some pins

Procedure
1. Adjust a bar magnet and a nail in a stand as
shown in the diagram.

2. Take some pins close to the nails.

3. Now remove the magnet from the clamp.

Do the pins remain attached with the nail as before? Why?

Till the nail remains close to the magnet, magnetic induction Fig. 9.10 magnetic inductor
takes place in it and the nail works as a magnet. But when
the magnet is taken away from the nail, the nail loses its
magnetic property.

Demagnetization

The process of losing its magnetic property by 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 get demagnetized. A magnet gets demagnetized by the following

activities:

1. By dropping the magnet Do you know?

2. By heating the magnet A collapsed star, known as neutron star, has
the strongest magnetic force of any objects
3. By storing the magnet in the universe.
without using keepers

4. By rubbing similar poles
of the magnet

5. By hammering the magnet

Measures of conserving magnetic strength

We can conserve the magnetic property of a magnet by handling and storing the magnet
properly. The following measures can be applied to conserve the magnetic strength of a
magnet:

1. We should not drop a magnet; we should not hammer it as well.
2. A magnet should not be heated.

3. The similar poles of magnets should not be rubbed together.

Magnetism 5

4. Magnets should be stored by using keepers.

5. We should avoid rubbing magnets.

keepers

S Bar Magnet N N

wood/plastic S

N Bar Magnet S (b) 'U' shaped magnet
with keepers
Fig 9.11 (a) bar magnets
with keepers

THINGS TO KNOW

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

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

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

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

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

6. Magnetic field is represented by the lines of force.

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

8. When a magnetic body is found in contact with a magnet, it also has magnetic
property. The process is called magnetic induction.

z THINGS TO DO

1. Mount a white paper over a cardboard by using thumb pins.

2. Place a bar magnet in N-S direction at the thumb pins
middle of the paper and draw its border
line. bar magnet line of force

3. Now, place a magnetic compass close to

the N-pole of the bar magnet. The S-pole

of the compass is attracted by the N-pole

of the magnet. Now, mark the point

shown by N-pole of the compass by using

a pencil.

4. Replace the compass in such a way that

S-pole is showing the point marked. magnetic compass

6 Modern Graded Science and Environment Book 8

5. Repeat the process again and again till the compass reaches the S-pole of the
magnet. Now, by connecting the points, a line of force is obtained. By the repetition
of the above activities again and again, the number of lines of force can be plotted.
Do the lines of force intersect each other? They are lines of force.

TEST YOURSELF

1. Fill in the blanks.

a. Magnetic property of a magnet is called ................. .

b. Magnets can affect .............. substance.

c. The process of getting magnetic property is called ................ .

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

e. A magnetic body kept close to a magnet shows magnetic property. The
process is called .................... .

2. Tick () the correct answer (MCQs).

a. There is more magnetic force at:

(i) N-pole of magnet (ii) S-pole of magnet

(iii) middle of magnet (iv) (i) and (ii) both

b. When the magnetic molecules are in the closed chain form in a body, the
body is:

(i) a magnet (ii) magnetic body

(iii) non-magnetic body (iv) induced body

c. Opposite poles of a magnet:

(i) attract (ii) repel (iii) no effect (iv) slightly repeled

d. In a magnetized body, the magnetic molecules are in:

(i) open chain (ii) closed chain (iii) ring chain (iv) (ii) and (iii) both

e. We use ..................... to protect a magnet:

(i) protector (ii) keeper (iii) conductor (iv) Non conductor

3. Give reasons.

a. The poles of a magnet cannot be separated.
b. A copper piece cannot be made a magnet.
c. A magnet gets demagnetized when it is heated or hammered.
d. A magnet has more force at poles than at its middle.
e. A magnetic compass is used to determine directions.
f. A nail gets magnetized when kept close to a magnet.
g. Two pins attracted by a bar magnet maintain a distance between their the

other ends.

Magnetism 7

4. Answer the following questions.
a. What is magnetism?
b. What is a molecular magnet?
c. Define magnetic induction?
d. What role do molecular magnets play to magnetize and demagnetize bodies?
e. Why do magnetic bodies lose their magnetic force?
f. What evidence can you give in the support of the molecular theory of magnetism?
g. Demonstrate magnetic induction by an experiment.
h. How can you protect a magnet from demagnetization? Show by diagram also.

5. Diagrammatic questions:
a. Study the given diagram and answer the following questions.

(i) (ii)

i. Which one of them is a magnet?
ii. What is the adjustment of magnetic molecule in figure (ii) called?
iii. Fig (i) and (ii) both do not have magnetic force in the middle. Why ?
iv. What theory is proved by the above diagram?

GLOSSARY

Directive property : the property of showing direction

Navigator : one who guides or steers a ship, aircraft, etc.

Repel : to push away

Scatter : to sprinkle or throw about in various places

Keepers : magnetic pieces used to avoid demagnetization

8 Modern Graded Science and Environment Book 8

10Lesson ELECTRICITY

Total Estimated Pds: 5 [Th. 4 + Pr. 1]

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

describe the structure and uses of a simple cell and a dry cell.
introduce domestic wiring and the appliances used in it.
introduce fuse and MCB and tell their uses.

Electricity is a very important source of energy in present days. We use electricity to do
about all the work of our daily life ranging from very simple work to very complex work.
It is caused by the change in number of electrons or by the flow of the charged particles
called electron.

All the matters are composed of atoms which may be of their own or of their components.
The fundamental particles of atoms are electrons, protons and neutrons. Protons are
positively charged, electrons are negatively charged and neutrons are chargeless.
Protons and neutrons are found in the nucleus of an atom but electrons revolve around
the nucleus (in the shells or orbits). When the electrons increase or decrease or flow, the
body shows electrical property.

Thus, the energy which is possessed by a body due to change in the number of electrons
or due to the flow of electrons in that body is called electrical energy or electricity.

Electric charges

Electrical property of a particle or a body is called electric charge. The fundamental
particles of an atom also have this property. Electric charges are of two types: positive
charge and negative charge.

The charge of protons (p+) is called a positive charge and the charge of electrons
(e-) is called a negative charge. Similar charges repel and dissimilar charges attract.

Method of charging bodies

Naturally, an atom is electrically neutral because it has an equal number of positively
charged particles protons (p+) and negatively charged particles electrons (e-). When
the number of electrons is reduced by any method in that body, the body has a positive
charge due to the excess of protons. When the number of electrons is increased in
the body, the negatively charged particles are found more than the positively charged
particles. Due to this reason, the body becomes negatively charged.

Electricity 1

Types of electricity

When we rub a plastic comb or a tennis ball with wool, it gains the capacity of attracting
small pieces of paper. It is due to the electrical property of the body. This type of electricity
is static electricity.

Fig 10.1 (a) rubbing a comb on hair (b) piece of paper (c) paper being attached towards the comb

Static electricity is that energy which is possessed by the change in the number of electrons
in bodies. It cannot be transferred from one point to another through a conducting wire. It is
possessed in insulators like plastic, glass, etc. This type of electricity is the cause of lightning.

Current electricity is that energy which is possessed due to the flow of charges in bodies.
It can be transferred from one point to another through a conducting wire. This electricity
is used in our daily life for many purposes.

Sources of electricity

We know that current electricity is possessed due to the flow of electrons and it can
be transported from one point to another through a conducting wire. It is the source of
electricity which supplies energy for the flow of electricity in a circuit. The main sources
of current electricity can be classified mainly into two types. They are :

(a) cell (b) generator

a. Cell

A cell is defined as that source of current electricity which converts chemical energy into

electrical energy. There are many types of cells but under this topic, we are going to

study about two types of chemical cells: simple cell and dry cell. switch

galvanometer

Simple cell G
conducting

A simple cell is very simple in structure. To construct wire

it, a beaker with some dilute sulfuric acid (H2SO4) Zn plate
is taken. Two plates, one of copper and the other
of zinc, are placed apart in the acid. When the two

plates are connected by a conducting wire with a Cu plate dilute H2SO4
galvanometer, the galvanometer indicates the flow

of current electricity through the conducting wire. beaker

Fig 10.2 simple cell

2 Modern Graded Science and Environment Book 8

Working method of a simple cell

The main chemical of a simple cell is dilute H2SO4. It is dissociated into hydrogen cations
(H+) and Sulphate anions (SO4– –) in an aqueous solution.

H2SO4 2H+ + SO4– –

Here, hydrogen loses electrons and sulphate gains electrons, hence hydrogen becomes
a positive ion and sulphate becomes a negative ion.

H+ ions are attracted towards the copper plate and SO4– – ions are attracted towards the
zinc plate.

In the copper plate

H+ ions come to the copper Do you know?
plate and form the molecules of
hydrogen by gaining electrons Lightning is a process of discharging
from the copper plate. electricity in the atmosphere. Lightning bolts
can travel at around 130,000 miles per hour
2H+ + 2e- H2 (208,000 km/h) and reach nearly 54000°F
(about 3000°C in temperature).
Since the copper plate loses
electrons, it becomes a positive
terminal.

In the zinc plate

SO4- - ions come to this electrode and form the molecules of ZnSO4 by reacting with the
zinc of the plate and the excess electrons are left into the zinc plate which makes the
plate a negative terminal.

SO4– – + Zn ZnSO4 + 2e–

In Detail:

Zinc of the electrode forms Zn++ by losing electrons inside the plate

Zn Zn++ + 2e-

Zn++ ionsZant+t+ra+ctSSOO44---- to form zinc sulphate.
ZnSO4

In this condition, if two terminals are connected by a conducting wire, electrons flow from
the negative to the positive terminal i.e. from the zinc plate to the copper plate. Its emf
is about 1 volt.

Defects of a simple cell

A simple cell is not a perfect cell as it has some defects. It is not a portable cell. The main
defects of this cell are local action and polarization.

Electricity 3

Local action – Fe (Impurity)

The zinc plate used in a simple cell contains impurities like Fe, + Zn plate
C and Cu. These particles work as the copper plate and local
currents are formed in the cell. The local currents increase the + local current
internal resistance of the cell. This defect is called local action. + C (Impurity)
Cu (Impurity)
For the remedy of the local action, either pure zinc is used or +
the zinc plate is coated with mercury. The process is called
amalgamating. Coated mercury brings the pure zinc on the Fig 10.3 local current
surface by dissolving it and impurity is covered by it. It removes
the defect of the local action.

Polarization Cu plate

Polarization is another defect of a simple cell. It is caused due layer of
to the formation of a hydrogen layer on the copper plate. Since hydrogen
hydrogen gas is an insulator, it resists the flow of electrons and
internal resistance of the cell increases. Due to it the cell does Fig 10.4 polarization
not work longer.

To avoid the polarization, use of a depolarizer is practicable.
Potassium dichromate (K2Cr2O7), copper sulphate (CuSO4) and
manganese dioxide (MnO2) are some examples of depolarizers.

b. Dry cell

A dry cell is the improved form of the leclanche cell. A leclanche cell is an improved form

of the simple cell in which MnO2 is used as the depolarizer. Later on, the leclanche cell is
improved in the form of the dry cell in which paste is kept instead of liquid.

To construct a dry cell, at first, a muslin brass cap
bag is taken with a carbon rod in it. The wax seal
carbon rod has a brass cap at its upper lining of paper
end. The bag is filled with a mixture of zinc vessel
manganese dioxide (MnO2) and carbon paste of NH4cl
powder. A washer (cardboard) is placed at muslin bag
the bottom of the zinc can. Then the bag is
placed in the can and the remaining part of mixture of Mn O2
aand C powder

the can is filled with a paste of ammonium carbon rod

chloride (NH4Cl). The paste is made by cardboard
mixing ammonium chloride with water,
Fig 10.5 a dry cell
flour, plaster of paris, starch, etc. The open

end of the can is sealed with a wax-like

substance. The curved surface of the cell is wrapped in paper, over which it may be

wrapped in polythene or metal.

4 Modern Graded Science and Environment Book 8

Working method of a dry cell

The main chemical of this cell is NH4Cl which dissociates into NH4+ and Cl- ions in the
paste form.
+ Cl-
NH4+ NH4Cl NH4+ carbon rod and Cl- ions move towards the zinc vessel.
ions move towards the

At the carbon rod

NH4+ ions come to the carbon rod and form molecules of NH3 and H2­by using electrons
of the rod, therefore, the rod becomes a positive terminal.

2NH4+ + 2e- 2NH3 + H2

In the zinc can

Cl- ions come to the can and form the molecules of ZnCl2 by reacting with the zinc of the can.
The excess electrons are left in the can. Therefore, the can becomes a negative terminal.

Zn + 2Cl- ZnCl2 + 2e-

In detail, Do you know?

Zn Zn++ + 2e- Benjamin Franklin carried out extensive
electricity research in the 18th century,
Zn++ + 2Cl- ZnCl2 inventing the lightning rod amongst his many
discoveries. In the event of a lightning strike,
In this condition, when these two the lightning rod conducts the strikes through
terminals are connected by a grounded wire, protecting a building.
conducting wire, electrons flow
from the zinc can to the carbon
rod. Its emf is 1.5 volt.

Role of MnO2 in a dry cell
MnO2 works as a depolarizer in a dry cell. It removes polarization by changing the
hydrogen into water. Manganese trioxide is also formed in this reaction.

2MnO2 + H2 Mn2O3 + H2O case of the dynamo

Maganese dioxide Maganese trioxide

Dynamo tyre
head or cap of the
We have seen a dynamo in a bicycle which axle dynamo
is used as a source of electricity to glow the permanent
headlight of the bicycle. magnet
coil
A dynamo consists of a case of metal or
plastic. In the case, a coil is placed in the Fig 10.6 a bicycle dynamo
magnetic field of a permanent magnet. The
magnet is connected to a small wheel with
the help of an axle in such a way that the
magnet can be freely spined.

Electricity 5

While working, the wheel of a dynamo is touched with the wheel of the bicycle to spin
the magnet. When the magnet spins, the lines of force are cut by the closed coil, which
is placed in the magnetic field of the magnet. In this condition, current is induced in the
wire of the coil.

[Note : Dynamo is given for expansion of your knowledge not for final exam]

Domestic wiring

The electricity used in our houses is produced at power stations. From the power stations,
the energy is transported to our houses through conducting wires. A cable wire is drawn
to our houses from the nearby electric poles. The cable has two conducting wires inside
it, which are separated by plastic insulations. The central thick wire is a live wire or phase
wire that brings electricity into our houses. The other net-like wire is neutral wire through
which electric current returns from our houses. The special pattern of connection of
electrical appliances in our houses is called domestic wiring.

The cable from the nearby pole is taken into the meter box; the live wire is connected to a
fuse in it which is called authority's fuse. Then the wire is divided into two, before entering
the main switch box.

light circuit switch
N
lamp two pin
socket

iron

Power circuit

consume's E
fuse LN

authority's
fuse

switch three pin socket three pin

Fig 10.7 dynamo circuit

6 Modern Graded Science and Environment Book 8

The main switch box also has two fuses of different capacities for the protection of light
circuit and power circuit. The fuses are called consumer's fuses. The main switch box
is also connected to a neutral wire as well as an earth wire. One end of the earth wire
is connected to a three-pin socket of the power circuit and the other end is buried in the
ground with a metallic sheet. In a domestic circuit, switches and fuses are connected to
the live wire. The electric loads are connected in such a way in the circuit that they can be
conducted independently by an individual switch. The connection of the loads is called a
parallel combination. In a light circuit, a lamp and a two-pin socket for the connection of
electric loads are connected. The circuit is protected by 5A fuse in the main switch box.

In a power circuit, three-pin sockets are connected for the connection of heavy electric
loads. The earth wire of the power circuit protects the consumers from electric shock by
the touch of metallic part of heavy electric loads.

Some devices of domestic wiring

Many types of electrical appliances are used in a domestic circuit for different purposes.
Some of them are described below:

1. Meter Box

A meter box is an electrical device which records electrical energy Fig 10.8 meter box
consumed by the consumer. On the basis of it, the consumers pay
their bill to the Electric Authority. The box is protected by a fuse called
MCB (Miniature circuit breaker). It is the Authority's fuse. A meter box
is sealed by the Electric Authority and it is not legal to break the seal by
an unauthentic person.

2. Main Switch Box

It is a metallic box having a long lever and two porcelain cases Fig 10.9 main switch box
for fuse wires. It provides protection to the electrical loads of light
circuit and power circuit through the fuses of different capacities.
The fuses are consumer's fuses. The lever of the main switch
helps to disconnect the electric circuit. The metallic box is also
connected with the earth wire.

3. Fuse

A fuse is an electrical device which is used in an electrical circuit as a safeguard. When
current overflows in a circuit by any cause, the fuse wire disconnects the circuit. In this
condition, electrical appliances are protected from their damage and the houses are
protected from electric fire. Fuse is found of different capacities and it is used as its
requirement. It is used in the meter box, the main switch box and other places in an
electric circuit. Fuse is used in the form of wire or in the form of MCB.

Electricity 7

porcelain case fuse wire glass tube porcelain or hard
plastic

fuse wire

Fig 10.10 (a) fuse wire (b) cartridge type fuse (c) MCB

Fuse wire is made of an alloy of lead and tin and it has a low melting point. When current
overflows in a circuit, it melts to disconnect the circuit. It is used in a porcelain case or
inside a small glass tube called a cartridge.

MCB is an improved fuse. It is convenient to use. When current overflows in the circuit,
its switch turns off to disconnect the circuit. We have to switch on for the re-connection
of the circuit.

4. Two-pin socket and three-pin socket

A two-pin socket has two holes for connection Fig 10.11 (a) 2 pin socket (b) 3 pin socket
with live and neutral wires. Two pins with
electrical loads are fitted in it. Two-pin sockets
are found in a light circuit. Three-pin sockets
have three holes for the connection with three
pins. Three pins are for the connection with
heavy electrical appliances, the single hole is
connected to the earth wires.

5. Electric lamp

Different types of lamps are found connected in an electric circuit. They change electrical
energy into light energy. They are used as a source of light. Filament lamp, fluorescent
lamp and LED lamps are some common types of electric lamps.

Fig 10.12 (a) LED (b) CFL (c) filament lamp (d) floresent lemp

8 Modern Graded Science and Environment Book 8

6. Heater

Heaters are those electrical appliances which change electrical Fig 10.13 radiator (heater)
energy into heat energy. They have some element or coil of
nichrome for the conversion of energy. Nichrome is an alloy of
nickel and chromium and has more resistance and a high melting
point. When the wire is connected in a closed circuit, it is heated
red and provides heat energy.

7. Electric motor

An electric motor is that electrical device which changes electrical
energy into kinetic energy. Electric motors are used in several electrical
appliances like fan, water pump, mixer, grinder etc. to run them.

8. Others Fig 10.14 electric motor

Electric bell, radio, telephone, television, computers, etc. are some other electrical
appliances. They consume electrical energy to run. They are used for different purposes
in our daily life. Switches are also used in domestic wiring by using which we can turn on
and off the electrical appliances.

Fig 10.15 (a) television (b) electric bell (c) radio

THINGS TO KNOW

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

2. Static electricity is possessed by a body due to the change in the number of electrons.
3. Current electricity is possessed by a body due to the flow of electrons.
4. Cells and dynamos are two main sources of current electricity.
5. Cells change chemical energy into electrical energy and a dynamo changes

mechanical energy into electrical energy.
6. A simple cell has defects like local action and polarization.
7. MnO2 is the depolarizer used in a dry cell. It removes polarization.
8. A dynamo is used as a source of current in bicycles, scooters, etc.

Electricity 9

9. The special pattern of connection of electrical appliances in our houses is called
domestic wiring.

10. Meter box, main switch box, fuse, switches, two-pin sockets and three-pin sockets
are some electrical appliances used in a domestic circuit.

THINGS TO DO

Make your own battery. Cu Zn Cu Zn
i. Take two limes (lemons) and insert limes
a zinc piece (which can be obtained
from the case of an old dry cell) and
a copper piece apart in it.

ii. Connect the two plates with the

lamp of a torch light (3V) with the

help of conducting wire as shown

in the diagram. Observe the lamp.

TEST YOURSELF

1. Fill in the blanks.

a. ............. charge is produced due to more concentration of electrons.

b. Emf of a dry cell is ................. volt.

c. Fuse wire is made of an alloy of ................ and ............ .

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. Tick () the correct answer (MCQs):

a. What is the full form of MCB?

i. main switch box ii. main circuit breaker

iii. miniature circuit iv. mini circuit box

b. Dilute H2SO4 is used as a main chemical in a:

i. photo cell ii. dry cell iii. dynamo iv. simple cell

c. Polarization occurs in a:

i. dynamo ii. simple cell iii. dry cell iv. solar cell

d. Which one of following changes chemical energy in electrical energy?

i. dry cell ii. dynamo iii. motor iv. fuse

e. Which one of the following works as safe guard of a circuit?

i. dynamo ii. fuse iii. main switch iv. meter box

10 Modern Graded Science and Environment Book 8

3. Differentiate between: b. light circuit and power circuit
a. polarization and local action d. static electricity and current electricity
c. simple cell and dry cell

4. Give reasons.
a. Fuse wire is made of an alloy of lead and tin.
b. MnO2 is used in a dry cell.
c. A dry cell is better than a simple cell.
d. The zinc plate in a simple cell is coated with mercury.

5. Answer the following questions.
a. Define electricity ? What are the types of electricity ?
b. What are conductors and insulators?
c. How does a simple cell work ?
d. How is a dry cell constructed ?
e. How does the carbon rod of a dry cell become a positive terminal ?
f. What is the role of MnO2 in a dry cell ?
g. Define local action and polarization ? How do they make the cell defective ?
h. How does a dynamo work ?
i. What is domestic wiring ? Write the functions of the meter box and the main
switch box in the circuit.
j. What is a fuse ? How is MCB better than fuse wire?
k. Describe domestic wiring in short.
l. What is local action ? How can we remove it?
m. Define polarization. How can we remove it ?

6. Diagrammatic questions:

a. Sketch the diagrams of:

i. simple cell ii. dry cell iii. dynamo

b. Illustrate household wiring with the help of a neat diagram.

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

i. What is shown in the figure? Cu Zn
ii. Which chemical is used as 'c'? C
iii. Which defect is found at plate 'Zn'? How

can you remove it?
iv. What defect is found at plate 'Cu'? How

can you remove it?
v. Write the chemical reactions that take

place at 'Cu' and 'Zn'.

Electricity 11

d. A dry cell is shown in the figure. Answer the following questions on the D
basis of it. C
B
i. What are A, B, C and D? A
ii. Write the functions of A, B, C and D.
iii. How does the cell differ from a simple cell? A
iv. Write the chemical reaction that takes
B
place at 'A'. C
v. Write the chemical reaction that takes

place at 'D'.
e. Answer the following questions on the basis of the given

figure.

i. What is shown in the figure?
ii. On what principle is the instrument based?
iii. Name 'A', 'B', and 'C'.
iv. If you have to induce more current by using it,

what change will you do in it?

GLOSSARY

Fundamental : serious and very important

Lightening : flash of very bright light in the sky caused by electricity

Galvanometer : an instrument used to detect the flow of current electricity
in a circuit

Aqueous : containing water, like water

Emf : electromotive force; the chemical energy of a cell which
changes into electrical energy

Local current : the current of charges formed inside a simple cell due to
presence of impurities in it

Internal resistance : opposing the flow of charges inside the source of current

Depolarizers : the chemicals used to avoid polarization of cells

Muslin : a type of fine transparent cotton cloth

12 Modern Graded Science and Environment Book 8

REVISION EXERCISE FOR PHYSICS

1. Tick () the correct answer.

2. Write short answers:
a. Define mechanical advantage and velocity ratio ?
b. How does a dynamo work ?
c. What is the molecular theory of magnetism ?
d. What is MCB? How is it better than fuse wire?
e. How does a simple cell work ?
f. Introduce local action and polarization ?
g. Define magnetic induction.

3. Answer the following questions.
a. A person brings his face close to a mirror. He finds that the image of his face
is magnified. Write the name of the mirror used. Why ?
b. Where should an object be placed in front of a concave mirror so that the
image is real, inverted and magnified ? Draw the ray diagram.
c. A concave mirror is called a converging mirror. Why ?
d. Draw a ray diagram only showing that a pencil appears bent when dipped
partially in a beaker containing water.
e. A doctor uses a mirror to observe the interior parts of the human body. Write
the name of the mirror used.
f. A boy is sitting in a newly built room whose dimensions are 5m x 5m x 5m. If
he whistles, does he hear its echo ? Why ?
g. What is the freezing and boiling temperature of mercury ?
h. Which thermometer, alcohol or mercury, do you prefer to measure the
temperature below -40°C ? Why ?

Electricity 13

i. Write normal human body temperature in centigrade and fahrenheit scales.

j. Define one joule of work.

k. Write about the mathematical relation between power, work and time.

l. Write the name of two fuels which produce heat energy on burning.

m. A plant uses energy obtained from the sun to prepare its food. What is the
name of the energy used by the plant ?

n. Write the name of any two devices in which magnetic energy is used.

o. Define pressure and atmospheric pressure.

p. Nose bleeding may occur while trekking in the Himalayan region. Why ?

q. Describe an experiment to show that the atmosphere exerts pressure on a
body.

r. Describe an experiment to show that pressure exerted by a liquid increases
with depth.

s. Write a formula relating pressure exerted by a liquid, its density, depth of liquid
column and acceleration due to gravity.

t. What are the factors on which pressure exerted by a liquid depends ?

u. The bottom of a dam is made wider than the upper part. Why ?

v. Fuse is used in an electrical circuit. Why ?

w. The atmospheric pressure at sea level is about 105 Nm-2. What does it mean?

x. Even though the atmospheric pressure is more, we do not feel the pressure.
Why ?

y. A student answers that speed is a vector. Is he right ? Why ?

z. Define speed and velocity with their units.

4. Give reasons:

a. A first class lever can give all the advantages of a simple Q
machine.

b. MnO2 is used in a dry cell. CF O
c. A simple cell is not suitable for daily use.

d. The handle of metal cutters is made long P
5. Differentiate between:



14 Modern Graded Science and Environment Book 8

6. Diagrammatic questions:

a. Study the following diagram and answer the questions: N S
i. Which mirror is shown in the ray diagram ?

ii. What is the point 'F' called ?

iii. Write the name of the length OF.

iv. Write the use of the mirror shown. water m
A 2m
b. In the given figure, a magnet is attracting small nails towards
it. Write the name of the energy possessed by the magnet. B
Write about two devices in which such energy is used.

c. Study the given diagram and answer the following : E
i. Find the pressures at point A and point B. L
ii. At which point is the pressure more A or B ?

iii. What would be the pressure at A and B fig. A L
if water in the container is replaced by the E
same depth of mercury ?
Ld
d. Study the diagram to answer the questions: fig. B

i. What type of levers are A and B ?

ii. Which lever can be used to lift a load with
less effort?

iii. Which lever has more velocity ratio ?

iv. Which lever cannot do work faster ?

7. Numerical problems:

a. Calculate MA, VR and efficiency of a lever which is used to lift a load of 600N
by applying an effort of 150N. Effort is at 40 cm and load is at 5 cm from the
fulcrum.

b. A force of 100 N displaces a body through 2 m in its direction. Find the amount
of work done.

c. A crane lifts a weight of 5000 N up to a height of 10 m in 50 seconds. Find the
power of the crane in kilowatt and horse-power. (1 H.P. = 750 W)

d. The velocity of a car is 10 ms-1. Find its displacement in one second.

e. The speed of a running girl is 8 ms-1. How far does she travel in 5 seconds ?

f. A body is moving with a uniform velocity of 10 ms-1. Find its acceleration.

Electricity 15

Unit

4 MATTER

11Lesson MATTER

Total Estimated Pds: 10 [Th. 9 + Pr. 1]

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

write briefly about the structure of an atom and its components (electron,
protons and neutrons).

define atomic number and atomic mass.
define molecular weight and calculate it.
write briefly about electronic configuration of different elements.
introduce Mendeleev's periodic table.
determine valency of first 20 elements by observing their atomic structure.
write the molecular formulae of compounds.
write and balance the chemical equations for a chemical reaction.

Element

There are many types of substances found in our surroundings. They have different
physical states and chemical properties. Some of them are pure and some are mixtures.
Those substances which cannot be changed into two or more simpler substances by a
chemical change are called elements. Iron, oxygen, aluminium, mercury and gold are
some examples of elements. An element is the simplest pure substance that can neither
be split up into two or more simpler substances nor can it be made from two or more
substances by any method. Each element is a distinct kind of matter that contains only
one type of atoms. Hydrogen, nitrogen, copper, sodium, chlorine and magnesium are the
examples of some elements.

At present, 118 elements are known. 92 elements among them have been found naturally
on the earth and in the atmosphere. Other elements have been made artificially in a
laboratory. All the substances are made up of elements. Thus, elements are the basic
units of all the substances which exist today.

At room temperature and standard atmospheric pressure (760 mm Hg), elements are
found in three states: solid, liquid and gas. Elements like hydrogen, helium, nitrogen,

Matter 145

oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon are found in the
gaseous state. Similarly, the elements found in the liquid state are mercury and bromine.
The rest of the elements are found in the solid state.

On the basis of physical and chemical properties, elements have been divided into three
classes: metals, non-metals and metalloids.

a. Metals

Metals are shiny, generally heavy and good conductors of heat and electricity. They are
malleable and ductile. Copper, iron, gold, silver and mercury are the examples of metals.
Usually, they are found in a solid state but some of them like mercury are in a liquid state.

b. Non-metals

Non-metals are non-shiny (dull) and they are usually found in solid, liquid or gaseous
forms. They do not conduct electricity but carbon is an exception. Carbon, iodine, oxygen
and nitrogen are some examples of non-metals.

c. Metalloids

Those elements which have the properties of both metals and non-metals are called
metalloids. Silicon, germanium, arsenic, antimony, etc. are some examples of metalloids.
They are also known as semi-metals. They become a good conductor when they are hot.

The following three points will help you to identify metals and non-metals (metalloids are
neglected):

a. Except helium, tellurium and selenium, the elements whose name ends with
an 'm' sound are metals e.g. magnesium, aluminium, ferrum, etc.

b. Except tin, iron, tungsten and antimony, elements whose name ends with an
'n' sound are non-metals e.g. oxygen, nitrogen, argon, etc.

c. The elements except sulphur, arsenic and phosphorus, which do not end
with an 'm' or 'n' sound are metals e.g. zinc, nickel, cobalt, etc.

Compound

A compound is a pure substance formed of the chemical combination of two or more
elements in a fixed proportion. Water, carbon dioxide, copper sulphate and common salt
are examples of some compounds.

There are thousands of compounds Do you know?
which we use in our everyday life. The

properties of compound are always Gold has 79 protons and 79 electrons.

different from the original substances

from which it is made of. For example, water is a compound because it can be

prepared by the direct combination of hydrogen and oxygen in a proportion of 2 : 1.

Hydrogen is combustible and oxygen is a supporter of combustion. Water, however,

has different properties from hydrogen and oxygen.

146 Modern Graded Science and Environment Book 8