science 10

Let an object be placed beyond 2F of a convex lens perpendicular to
the axis. A ray AM parallel to the principal axis passes through ‘F’
after refraction through the lens. Another ray AO passes through
the optical centre without deviation and meets the another ray at
A'. Thus, A'B' is the real and inverted image of the object AB. Thus,
AB and A'B' are the height of object (O) and height of image (I)
respectively. Similarly BO and OB' are the object distance (u) and
image distance (v) respectively.

In ∆ABO and ∆A'B'O, we have,

1. ∠ABO = ∠A'B'O 1. Both are 90°.

2. ∠AOB = ∠A'OB’ 2. Vertically opposite angles.

3. ∠BAO = ∠B'A'O 3. Remaining angles of the triangles.

∴ ∆ABO is similar to ∆A'B'O

Then, A'B' = B'O
AB OB

∴ Height of image = Image distance
Height of object Object distance

Thus,

Magnification = Height of image = Image distance
Height of object Object distance

i.e. m = I = v
O u

Power of a lens

Power of a lens is defined as its capacity to converge or diverge light
rays. It can also be defined as the reciprocal of focal length of a lens
expressed in metre.

Mathematically,

Power = focal 1 (in metre)
length

∴ P = f 1
(m)

When focal length of a lens is expressed in metre, the power of the
lens is in dioptre (D).

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Some Facts

a. Focal length of a convex lens is positive. So, its power is also positive.
b. Focal length of a concave lens is negative. So, its power is also negative.
c. A lens having more focal length has less power and that having less

focal length has more power.

Relation between object distance (u), image distance (v) and focal

length (f)

The relation of object distance (u), image distance (v) and focal length

(f) can be represented by an equation,

1 = 1 + 1
f u v

It is called lens equation.

Sign convention

Real distance is taken as positive and virtual distance is taken as
negative. Focal length of a convex lens is positive. Therefore, its power
is also positive. Similarly, focal length of a concave lens is negative.
Therefore, its power is also negative.
Solved Numerical Problem 5. 1

An object is kept at a distance of 30cm in front of a convex lens of focal
length 20cm. Where is image formed? Find (i) magnification and (ii)
power of the lens
Solution:

Given, Object distance (u) = 30 cm.
Focal length (f) = 20cm
Image distance (v) =?

Magnification (m) =?

Power (P) =?

We have,

1 = 1 + 1 Or, 1 = 1 + 1 Or, 1 = 1 – 1
f u v 20 30 v v 20 30

Or, 1 = 3-2 Or, 1 = 1
v 60 v 60 ∴ v = 60cm

Again, m= v = 60 =2
u 30

We have, ∴ P = 1 = 1 = 5 Diopter
f = 20cm = 0.2m f 0.2

Hence, the image is formed at 60 cm away from the lens. The

magnification of the image is 2 and power of the lens is 5 Diopter

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Solved Numerical Problem 5. 2

The power of spectacles of a man is -1.5 D. What is the focal length of
the lens? What type of lens is it?

Solution:

Given, Power of the lens (P) = –1.5 D

Focal length (f) = ?

We have, 1 1 1
f f –1.5
P = Or, –1.5 = Or, f = Or, f = – 0.67m

Focal length of the lens (f) = 0.67m = 0.67 × 100cm = 67cm

The lens is concave lens because it has negative focal length and
negative power.

Uses of lens

1. They are used in spectacles.
2. They are used in camera, microscope, telescope, etc.
3. Our eye also uses convex lens.
4. A concave lens is used in peeping hole of door to see the things

outside the door

Differences between convex lens and concave lens

Convex lens Concave lens

1. The lens which is thick in 1. The lens which is thin in the

the middle and thin at the middle and thick at the edge

edge is convex lens. is concave lens.

2. It converges parallel rays of 2. It diverges the parallel rays of

light at a point. light from a point.

3. Its focal length and power 3. Its focal length and power are

are positive. negative.

4. It forms images of different 4. It always forms virtual and

sizes and natures. diminished image of an object.

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5. It is used in camera, 5. It is used in peep hole of door,
microscope, telescope, for the correction of short
for the correction of far sightedness, etc.
sightedness, etc.

Optical instruments

The instruments which produce image of an object are optical instruments.

Camera, eye, projector, microscope, telescope, etc are optical instruments.
Optical instruments use lenses, mirrors or prisms. Camera, projector, eye,
etc are the optical instruments which produce real image. Microscope,
telescope binoculars, etc are the optical instruments which produce
virtual image.

Eye

Eye is also a type of optical instrument. The human eye consists of
different parts. The parts of human eye are shown in following figure:

Lens Sclera

Conjunctiva Vitreous humour
Iris
Fovea
Aqueous humour Blind spot
Pupil Optic nerve

Suspensory ligament
Cornea

Ciliary body Choroid

Retina

In human body, there are a pair of eyes which are located in orbital

cavity. Human eyes are protected by eyebrows and eyelids. Eyebrows

are small hairs which protect eyes from dust, sweat, etc. Eyelids are

the cover of skin which contain eyelashes and protect the eyes from

dust, insects, injury and other foreign bodies. Tear gland produces

tears which keeps the eyes moist and washes away the dirts. Each

eye contains a ball called eyeball. Each eyeball contains three layers.

They are:

i. Sclerotic layer ii. Choroid iii. Retina

i. Sclerotic layer
It is the outermost layer and is made up of tough connective tissue.

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It contains two parts: posterior sclera and anterior cornea. Sclera is
opaque and white part which gives shape to the eyeball and protects
the inner parts.

Cornea is a transparent and thin layer which is bulging outside. It is
not supplied with blood vessels. It is covered externally by a very thin
transparent membrane called conjunctiva. Cornea is also called the
window of the eye because light enters the eyes through cornea.

ii. Choroid

It is the middle layer of eyeball which is made up of thin layer of
connective tissue. It is dark pigmented and supplied with blood
vessels. The dark pigment absorbs light and prevents the reflection
of light within the eye. At the anterior part, this layer gets separated
from cornea forming iris. Iris is a dark coloured disc shaped structure
which lies infront of the lens. It is the pigmented part of eye seen
from outside. In the middle of the iris, there is a small round hole. It
is pupil. Light enters the eyes through pupil . The amount of entering
light is regulated by changing the size of pupil by iris. In bright light
and near vision, size of pupil decreases and its size increases in dim
light and far vision.
Just behind the iris, there is a transparent, crystalline and biconvex
lens made up of proteins. It is suspended by suspensory ligaments and
ciliary muscles produced from choroid layer. Ciliary muscles change
the shape and focal length of eye lens by contraction and relaxation.
This process is called accommodation.
The lens divides the cavity of the eye ball in to two chambers: anterior
and posterior. The anterior cavity is filled by clear watery fluid called
aqueous humour and the posterior cavity is filled by thick, gelatinous
substance called vitreous humour. The aqueous humour inflates the
frontal part of eye ball and keeps the lens moist whereas the vitreous
humour maintains the shape of eyeball, supports retina and maintains
liquid pressure within the eyeball.

iii. Retina

It is the innermost layer of eyeball. It is provided with many layers
of nerve fibres called photoreceptor cells. There are two types of
photoreceptor cells. They are rods and cones. Rods are the rod shaped
photoreceptor cells which are more sensitive to dim light. Similarly,
cones are the cone shaped photoreceptor cells which are sensitive to
bright light and are responsible to distinguish colours. Rods contain

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purple pigment known as rhodopsin which is formed from vitamin
A. Similarly, cones contain violet pigments called iodopsin which is
also formed from vitamin A. Deficiency of vitamin A in rods cause
night blindness and deficiency of vitamin A in cones causes colour
blindness.

Retina has two spots. They are yellow spot and blind spot. Blind spot
does not contain any photoreceptor cells. So, image is not formed
when light falls on blind spot. But yellow spot contains only cones.
The sharpest image is formed when light falls on it.

Optic nerves originate from the posterior part of the retina. These
nerves carry the impulses from the retina to the brain.

Working of eye

When light enters the eyes from an object, it is refracted by cornea,
aqueous humour, lens and vitreous humor respectively. But the
main role for refraction is played by the lens. Then, an inverted, real
and diminished image is formed on the retina. The impulses of the
formation of image are carried by the optic nerves to the brain. The
brain reads the image as erect and of accurate size.

The size of the pupil can be changed with the help of iris. When we
have to see in dimlight, the size of the pupil is made larger and it is
made smaller when we have to see in bright light. The shape and size
of the eye lens can be changed by the ciliary muscles. When we have
to see near objects, the lens is pressed on both sides by the ciliary
muscle and is made thick. Similarly, the lens is made thin when we
have to see the far objects.

Accommodation

A normal eye can see objects keeping at various distances clearly.
The farthest point upto which a normal eye can see clearly is called
far point. For a normal eye, the far point is infinity. When an eye
has to see the objects of the farthest distance, the lens becomes thin
to increase its focal length. In this case,
parallel rays coming from the far objects
are focused on the retina as shown in the
figure.

The nearest point upto which a normal
eye can see clearly is called near point.
The near point for a normal eye is 25cm. Near

point

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The distance between the eye and near point is called least distance of
distinct vision. When an eye has to see the objects of nearer distance,
the lens becomes thick to decrease its focal length. In this case, rays
coming from the nearer objects are focused on the retina as shown in
the figure.

The ability of an eye to focus the image of an object on the retina by
changing the focal length of its lens is called accommodation.

Defects of vision

A normal eye can see any object kept between near point (25cm) and
far point (infinity). The distance between near point and far point is
called range of vision. A normal eye can see any object kept at the
range of vision.

When an eye cannot see the objects lying at the range of vision, it is
called defect of vision. Defect of vision is caused due to two reasons.
They may be (i) inability of ciliary muscle to change the focal length of
eye lens and (ii) elongation or shortening of the size of eyeball. There
are two defects of vision. They are:

(i) Short sightedness (myopia)

(ii) Far sightedness (hypermetropia)

Short sightedness (myopia)

It is a defect of vision in which a person cannot see distant objects
clearly but can see the objects near to the eye. It is also called myopia.
A person suffering from this defect has far point nearer than infinity.

In such eyes, the parallel rays coming from the distant objects are
focused in front of the retina as shown in the diagram (a).

(a) Defect (b) Correction

Causes

This defect may be due to:

1. the thickening of eye lens. In this case, ciliary muscles fail to
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stretch the lens to the required extent.
2. elongation of eye ball. In this case, distance between the eye

lens and eyeball gets increased due to the elongation of eye
ball.

Remedy

To correct this defect, a concave lens of suitable focal length is used as
spectacles. The concave lens diverges the rays coming from the objects.
Due to the combined action of both the lenses (lens in spectacle and
eye lens), the image is formed on the retina. Then, the distant object
can be seen clearly.

Far sightedness (Hypermetropia)

It is a defect of vision in which a person cannot see near objects clearly
but can see distant objects. It is also called hypermetropia. For such
eye, near point is more than 25cm.
In such eyes, the rays coming from the nearby objects are focused
behind the retina as shown in the figure (a).

25cm (b) Correction of far sightedness

(a) Far sightedness
Causes

This defect is due to:

1. becoming the lens thinner. In this case, the ciliary muscles
fail to press the eye lens upto the required thickness. So, the
focal length of eye lens increases.

2. decreasing the size of the eye ball. In this case, the distance
between the eye lens and the retina decreases.

Remedy

To correct this defect, a convex lens of suitable focal length is used as
spectacles. The convex lens converges the rays. Due to the combined
action of both lenses (lens in the spectacles and eyelens), the image
is formed on the retina. Then, the nearby objects can be seen clearly.

Differences between myopia and hypermetropia

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Myopia Hypermetropia

1. It is a defect of vision in which 1. It is a defect of vision in which

a person cannot see distant a person cannot see near

objects clearly but can see objects clearly but can see

nearby objects. distant objects.

2. The focal length of the eye 2. The focal length of the eyelens

lens is less. is more.

3. Theimageofthedistantobjectis 3. The image of nearby object is

formed in front of the retina. formed behind the retina.

4. It is corrected by using a 4. It is corrected by using a

concave lens. convex lens.

Learn and Write

1. A convex lens is called a converging lens. Why?

A convex lens converges the parallel rays to a point after
refraction. Therefore, it is called a converging lens.

2. A concave lens is called a diverging lens. Why?

A concave lens scatters or diverges the parallel beam of light
from a point. So, it is called a diverging lens.

3. The power of a convex lens is positive and that of concave lens is
negative.

It is because the real distance (the distance between the optical
centre and the real image) is taken as positive and the virtual
distance (the distance between the optical centre and virtual
image) is taken as negative.

4. A concave lens as spectacles is used for the correction of myopia.

In myopia, the rays of light refracted from eye lens meet in front
of retina and the person cannot see distant objects. When a
concave lens of a proper focal length is used, it slightly diverges
the parallel rays. The diverged rays after being refracted by the
eye lens meet exactly at retina and the vision becomes clear.

5. A concave lens is used to make peeping hole of doors or windows.

A concave lens always forms an erect and diminished image of
an object. If it is used as a peeping hole of doors or windows, it
gives a wider view of outside.

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Main points to remember

1. A lens is defined as an optical device which is made up of a
transparent medium bounded by spherical surfaces.

2. Convex lens is a lens which is thick in the middle and thin at the edge.
3. Concave lens is a lens which is thin in the middle and thick at the edge.
4. Distance between the optical centre and principal focus is called

focal length.
5. Magnification is defined as the ratio of height of image to the

height of object.
6. Capacity of a lens to converge or diverge light rays is called power of lens.
7. Ability of an eye to focus the image of an object on the retina by

changing the focal length of its lens is called accommodation.
8. Short sightedness is a defect of vision in which a person cannot

see the distant objects clearly but can see the near objects.
9. Long sightedness is a defect of vision in which a person cannot

see the nearby objects clearly but can see distant objects.

Exercise

A. Choose the best alternative.

1. An imaginary line which joins two centres of curvatures of a lens is
called

a. Principal focus b. Optical centre c. Principal axis d. Pole

2. Which of the following is incorrect about real image?

a. It can be obtained on a screen.

b. It is always inverted.

c. It is formed by a convex lens, camera, projector, etc.

d. It is formed by the apparent meeting of refracted rays.

3. The reciprocal of focal length of a lens expressed in metre

a. Magnification b. Power c. Focus d. Principal axis

4. Which of the following is an optical instrument?

a. Camera b. Microscope c. Telescope d. All of these

5. The sharpest image is formed when light falls on yellow spot
because it contains only

a. Cones b. Rods c. Retina d. Choroid

B. Answer theses questions.

1. What is a lens? Mention its types with diagrams.

2. Define: (i) Centre of curvature (ii) Principal axis

(iii) Optical centre (iv) Principal focus (v) Focal length.
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3. Draw ray diagrams of a convex lens when an object is at a point

(i) Beyond 2F (ii) Between F and 2F

(iii) At 2F (iv) Between F and O.

Mention the nature of the images too.

4. What is magnification? Mention the formula for finding the
magnification of an object.

5. A convex lens forms a real and magnified image. Where is the
object placed? Show with diagram.

6. What is power of a lens? Mention its formula.

7. What is lens formula? Mention it.

8. Mention the uses of lenses.

9. What are optical instruments? Mention its types with examples.

10. What is a simple camera? What is the function of shutter in a camera?

11. How does pupil control the amount of light entering the eyes?

12. What is accommodation? How is it possible?

13. Define. (i) Near point (ii) Far point

(iii) Range of vision (iv) Defect of vision.

14. What is myopia? Mention its causes and remedy with diagrams.

16. Ram cannot see the letters written on the blackboard. What is
his defect of vision? How can it be corrected?

C. Differentiate:

1. Convex lens and Concave lens

2. Far sightedness and Short sightedness

D. Give reasons

1. Short sightedness can be removed by using a concave lens.

2. A concave lens is called a diverging lens.

3. A convex lens is called a converging lens.

4. Power of a convex lens is positive.
E. Diagrammatic questions.

1. Complete the following ray diagrams:

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107

2. What defect of vision is shown in the diagram? What are the
causes for it? Mention its remedy with diagram.

F. Numerical Problems

1. Find the power of a lens of focal length 10cm.

2. What is the focal length of a lens having the power 5 D?

3. An object is placed at a distance of 60 cm in front of a convex lens
of focal length 20 cm. Where is the image formed? Mention (i)
magnification of the image (ii) the power of the lens.

4. An object is placed at a distance of 40 cm from a concave lens
of focal length 20cm. Find the image distance.

5. A student wears spectacles of power –2D. What type of defect of the
eye does the student have? Find the focal length of the lens used?

Answers F. 1. 10 diopter 2. 20 cm 3. 30 cm 4. -13.3cm 5.-50cm

Project Work

Take a convex lens and a sheet of plain paper. The plain sheet
of paper acts as a screen. Use a pencil as an object. Place the
pencil at different distances from the lens and observe the
position, size and nature of the image. Write your findings
with necessary diagrams.

Glossary : eye glasses

• Spectacles : thick at the middle and thin at the
• Convex sides

• Concave : thin at the middle and thick at the
sides
• Converge
• Diverge : to collect at a piont
• Aperture
• Principal : to scatter from a point
• Remedy
• Magnification : a small narrow opening

• Accommodation : main

: treatment

: process of making something
appear large

: adjustment

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Chapter

6 Current Electricity
and Magnetism
Alessandro Volta
He is known for the discovery of Methane and
invention of electric cell.
Estimated Periods: 11 (8T+3P)

Objectives

At the end of the lesson, students will be able to:

• explain the effects of current.
• explain working principles of generator, motor and transformer.
• explain uses of electromagnet and electrical appliances in daily life.
• explain safety measures while using electricity
• solve simple numerical problems related with the determination of cost of

electricity.

Electricity is one of the important sources of energy in the modern
age. Various appliances like heater, radio, fan, refrigerator, iron,
etc run with electricity. Similarly, modern means of communication
like telephone, fax, television, computer, radio, etc are also operated
by electricity. The means of entertainment like cinema, television,
video, etc run with electricity. Modern lighting devices like tubelight,
fluorescent light, CFL, etc need electricity. We are making our
life easier and comfortable by using electricity. Thus, electricity is
inseparable part of human life.

DC and AC

The current whose polarity does not change with time is called direct
current. (DC). It is produced by cells, DC generator, etc.

R

+– current →
DC circuit
time →

The current whose polarity as well as magnitude changes with time is

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called alternating current (AC). current →

R

+ ++ +

AC source –– –
AC circuit time →

It flows in one direction during the first half and in the opposite
direction in the next half. Frequency of the current supplied to our
country has frequency 50 Hz. It means the polarity of this current
changes 50 times in a second. It is produced by a generator or dynamo.

Differences between DC and AC

DC AC

1. The current whose polarity 1. The current whose polarity
does not change with time is called changes with time is called
direct current. alternating current.

2. DC voltage cannot be changed 2. AC voltage can be changed
by a transformer. by a transformer.

3. It is generated by a cell or DC 3. It is generated by a
generator. generator or dynamo.

Effects of current

Electrical energy can be transformed into different forms of energy. It
is called effect of electricity. Following effects of current are discussed
here:

(i) heating effect (ii) lighting effect
(iii) chemical effect (iv) magnetic effect

Heating effect

When current passes through some

electrical devices like heater, rice

cooker, immersion rod, iron, etc; Coil
electrical energy is converted into

heat energy. This is called heating Electric iron
effect of current.

All heating devices contain a coil of thin wire called heating element.

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Heating element has high resistance and high melting point. Heating
element is usually made up of nichrome. It is an alloy of nickel
(60%) and chromium (40%). It has high resistance and high melting
point i.e 900°C. Nichrome does not react with oxygen even at a high
temperature of 900°C. But, tungsten reacts with oxygen when it
gets heated. Therefore, tungsten is not used in heating devices even
though it has high resistance and high melting point. The nichrome
as a heating element lasts longer.

Lighting effect

When current passes through some electrical devices like electrical
bulb, fluorescent lamp, etc, the electrical energy is converted into
light energy along with heat. This is called lighting effect of current.
Lighting effect of current plays important role in practical life. It is
used in filament lamp, fluorescent lamp, CFL, etc. They provide light
by converting electrical energy into light energy.

Filament lamp

A coil made up of thin wire is used Nitrogen gas
inside the glass bulb. The thin Filament
wire is called filament. It is made Glass cover
Supporting wire

up of tungsten. Tungsten has high

resistance and high melting point Aluminium support

(3400°C). Due to its high resistance, Terminal pin

it readily gets heated upto 2000°C Filament bulb

when current passes through it and radiates light. The filament

is coiled and is thinner to increase resistance. The filament gets

destroyed due to oxidation when it comes in contact with oxygen at

high temperature. So, inert gases (argon or nitrogen) is filled inside

the bulb. Two pins fix the bulb in the holder.

When current passes through the filament, it gets heated to high

temperature of 2900°C and becomes white. Then, it emits light along

with heat. It is inefficient because more heat is produced than light.

Only 10% of the electricity is converted into light energy whereas

90% is converted into heat energy.

Fluorescent lamp

A fluorescent lamp consists of a long cylindrical glass tube with two
electrodes at its ends. The inner surface of the tube is coated with
fluorescent powder. It is filled with mercury vapour. Therefore, it is

111 Times' Crucial Science Book - 10

also called mercury lamp. It also contains choke coil and starter.

AC source

When high voltage Choke

is supplied to the Glass tube filled with mercury vapour electrode
lamp through

electrodes, the

mercury ionizes fluorescent powder Starter
and produces

ultraviolet rays. Fluorescent lamp
The fluorescent

powder absorbs UV rays and emits visible light.

Fluorescent lamp is more efficient than filament lamp. It emits both

heat and light. About 70% of the total electrical energy is converted

into heat and 30% into light. Thus, fluorescent lamp is three times

efficient than filament lamp. Its life span is longer than that of

filament lamp. It lasts for about 3000 hours whereas a filament lamp

lasts for about 1000 hours.

Differences between filament and fluorescent lamp

Filament lamp Fluorescent lamp

1. It consists of a round or oval 1. It consists of a long
glass bulb. cylindrical glass tube.
2. Tungsten filament emits light. 2. Fluorescent powder emits light.

3. It is filled with nitrogen or inert gases. 3. It is filled with mercury vapour.

4. Its life span is about 1000 hrs. 4. Its life span is about 3000 hrs.

5. It converts 10% of the electrical 5. It converts 30% of the
energy into light and 90% into heat. electrical energy into light and

70% into heat.

Compact Fluorescent Lamp (CFL) is being popular over the fluorescent
lamp in the present days. It is because CFL is much more efficient in
comparison to fluorescent lamp. It converts 90% of electrical energy
into light.

LED lamp

LED light bulb or LED lamp is an electric light that is popularly used
in light fixtures. It emits light using one or more light emitting
diodes (LEDs) assembled into a lamp. It converts 90% of electrical

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energy into light and 10% into heat. The life span and electrical
efficiency of LED lamp is several times greater than the ordinary
lamp. They are remarkably energy-saving lamps as compared to the
fluorescent lamps.
The main advantages of LED lamp over the other electricity-based
light sources are: longer life, smaller size and faster switching, lower
energy consumption, improved physical strength, etc. They are used
popularly in automotive headlamps, general lighting, advertising,
traffic signals, camera flashes, etc.

Magnetic effect of current

When current passes through a conductor, magnetic field is created
around it. It is called magnetic effect of current.
Magnetic effect of current was discovered by Hans Christian
Oersted in 1819. He found that magnetic needle gets deflected when
a current carrying conductor is brought near to it.

Activity 6 .1

Materials required:

Compass needle, copper
wire and a battery.

Procedure

i. Hold a conducting
wire near and above
a compass so that
they are parallel to each other.

ii. Pass electric current through the conducting wire. Observe
what happens?

iii. Disconnect the wire from the battery. What happens?
iv. Again pass the current through the wire reversing the

terminals of the battery. Observe what happens?

Observations:

The magnetic needle gets deflected when current passes through
the conductor. The needle returns to the original position when
there is disconnection of wire from the battery. The deflection of
the needle occurs in opposite direction when the terminals of the
battery are reversed.

Conclusion:

The defection of the magnetic needle takes place due to magnetic
field produced around the current carrying conductor. The
direction of the magnetic field changes when the terminals of a
battery are reversed.

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Magnetic field around a wire

The space around a magnet up to which its influence can be experienced

is called magnetic field. Current flowing through a conductor has also

a magnetic field around it.

Activity 6 .2 To demonstrate magnetic field around a conductor

Materials required: Current carrying wire

Thick cardboard, iron fillings,

battery and copper wire. Thick
cardboard
Procedure

a. Make a small hole in the middle Magnetic field

of a thick card board.

b. Hold a wire vertically by passing Magnetic field around a wire

through the hole of the card board.

c. Sprinkle iron filings all around the wire on the cardboard.

d. Connect the ends of the wire to a battery of 3V.

e. Then tap the cardboard gently and observe.

Observation:

The iron filings arrange forming circles around the wire as

shown in the figure.

Conclusion:

The arrangement of iron filings around the wire in circles is due

to magnetic field produced around the current carrying wire

Magnetic field around a solenoid

A coil of insulated wire is called solenoid. When current passes through
a solenoid, two poles of magnetic field is created as in a bar magnet.
The end of the solenoid in which the direction of magnetic lines of
force is clockwise behaves like South Pole and the other end in which
direction of current is anti-clockwise behaves like North Pole.

Electromagnet

Electromagnet is a temporary magnet which is made by passing current

through a solenoid. It is a temporary magnet because it behaves as a

magnet as long as the current passes through it.

Activity 6 .3 Making an electromagnet. N S Soft iron nail
Materials required: (electromagnet)

Insulated copper wire of 1 meter, a Insulated 6V Iron nails attracted
by electro magnet
soft iron nail, iron pins, etc. wire

Procedure Battery

i. Take a soft iron nail and wind an insulated wire around it for 50

turns.

Times' Crucial Science Book - 10 114

ii. Connect two ends of the wire to a battery of 6V and bring
the nail near small iron pins. What do you observe?

iii. Disconnect the wire from the battery. What happens?
Observation:
The soft iron nail attracts small iron pins when current passes
through the solenoid. It stops attracting when the wire is
disconnected from the battery.

Strength of an electromagnet

Strength of an electromagnet depends upon the following factors:

1. Number of turns of the wire

If the number of turns of insulated wire in a solenoid increases, the
strength of electromagnet also increases. By decreasing the number
of turns, the strength of the electromagnet can be decreased.

2. Magnitude of current

When the magnitude of the current supplied to the solenoid increases,
the strength of the electromagnet also increases. When the magnitude
of the current decreases, the strength of the electromagnet also
decreases.

3. Shape of the electromagnet

When the electromagnet is made U-shaped decreasing the distance
between two poles, the strength of the electromagnet increases.

4. Use of soft iron

When a soft iron core is kept inside the solenoid, the strength of the
electromagnet increases.

Characteristics of electromagnet
1. It is a temporary magnet.
2. Its magnetic strength can be changed as required.
3. Its poles can be reversed.
4. It can be made in required shape and size.

Due to above characteristics, electromagnet is more useful than a
permanent magnet.

Uses of electromagnet
1. It is widely used in various electrical appliances such as loud
speaker, radio, television, electric bells, etc.

115 Times' Crucial Science Book - 10

2. They are used in industries for lifting steel plates, girdlers,
scrap iron, etc.

3. They are used in separation of magnetic bodies from garbage.

Electric bell

An electric bell is a device which produces sound with the help of
electromagnet.

Structure Source Switch

It consists of U-shaped soft iron core around

which an insulated wire is wound to make

it electromagnet. Armature made up of

soft iron is fitted in front of the U - shaped

iron core. Its one end is fixed while other is

connected with a hammer which can move Screw

freely. A gong is fixed near the hammer in Electromagnet Hammer
such a way that the hammer can hit the Gong

gong. The armature is connected with a Electric bell
screw at point ‘A’. One terminal of a battery

is connected to the contact screw and another terminal is connected to

wire of electromagnet. All these parts are fixed to a plastic or wooden

board.

Working

When the switch is turned on, the electric circuit is closed. Then the

soft iron core changes to the electromagnet and attracts armature. Due

to this, the hammer attached with it hits the gong and the bell rings.

As soon as the armature is pulled by the electromagnet, the contact of

armature with screw disconnects at point ‘A’. Then, the soft iron does

not remain as magnet and can attract armature no longer. Thus, the
armature returns to the original position and comes in contact with
screw. Then, the circuit completes and the armature is attracted. Thus,
the hammer hits the gong. The process gets repeated again and again.
Thus, the bell keeps on ringing until switch is turned off.

Interrelation between magnet and electricity

Hans Oersted in 1819 AD discovered that magnetism can be developed

from electricity. After this discovery, many scientists tried to bring

reverse effect, i.e. to produce electricity from magnet. In 1831, Michael

Faraday, an English scientist was able to discover the production

of current electricity from magnets. The process of production of

current electricity in an electric circuit from a magnetic field is called

electromagnetic induction. 116

Times' Crucial Science Book - 10

Dynamo or generator

Dynamo or generator is a device which generates electricity on the
basis of electromagnetic induction. Generally, the device which
produces current in a small scale is called dynamo and that produces
current in a large scale is called generator.

Bicycle dynamo

A bicycle dynamo is a device which produces current in a small scale
and is used to glow bulb fitted to the bicycle.

Construction

It consists of a permanent magnet and a coil of insulated wire wound
around a soft iron core. The coil is kept in the magnetic field of the
magnet. The magnet is connected with the cap of dynamo with an
axle.

Connection Dynamo head
to bicycle frame
Dynamo
Light

Bicycle tyre

NS

Connection Magnet
to bicycle frame
Coil
Wire

Bicycle dynamo

Working

When the wheel of a bicycle rotates, the cap of the dynamo is made to
rotate. This rotates the axle of the magnet which in turn rotates the
permanent magnet. Thus, the magnetic flux passing through the coil
changes and an emf is induced in the coil. The emf produces current
in the coil. This current glows the bulb connected in the bicycle.

Generator

A generator is a device which produces electricity in a large scale.
Electromagnet is used in generator instead of using a permanent
magnet.

Construction

A generator consists of an electromagnet, armature, rings and carbon
brushes. The armature consists of a rectangular coil of insulated wire

117 Times' Crucial Science Book - 10

wound on laminated soft iron core. It can be rotated about its axis
between the poles of the magnet. The ends of the coil are connected
to the copper rings called slip rings. When the armature rotates, the
slip rings also rotate. There are carbon brushes which press lightly
against each ring. These brushes are connected to the external load.

motion

Magnet

Slip rings

Armature

Brushes

Generator

Working

When the armature is rotated in the magnetic field, the magnetic flux
passing through the coil changes and an emf is induced in the coil.
The induced emf causes the production of induced current which is ac
in nature.

In hydropower stations, the rotation of the the armature is done by
turbine which in turn rotates due to flow of water.

In generator or dynamo, the strength of induced current can be
increased by the following ways:

a. by increasing the number of turns in the coil.
b. by increasing the strength of magnetic field.
c. by increasing the speed of the coil in the magnetic field or the

speed of rotation of magnet near the coil.
d. by decreasing the distance between the coil and the magnet.

Motor effect Electric current

When current passes through a S

conductor, magnetic field is created

around it. When a current carrying

conductor is placed in a permanent

magnetic field, there is mutual N Movement
attraction or repulsion between Conductor

the magnetic field due to current

carrying conductor and the permanent magnetic field. As a result,

Times' Crucial Science Book - 10 118

current carrying conductor moves if it is allowed to do so. It is called
motor effect.

Let’s study the given figure carefully. The magnetic lines of force are
passing from the North Pole to the South Pole. A wire is placed straight
down through the magnetic field. When current is passed through
the wire, a magnetic field is developed as shown in the figure. Here
attraction and repulsion between the magnetic field due to current
carrying conductor and the permanent magnetic field occurs. As a
result the current carrying conductor moves.

The motion produced in a current carrying conductor when it is kept
in a permanent magnetic field is called motor effect.

Electric motor, electric fan, vacuum pump, washing machine, etc
work on the principle of motor effect. Electric motor is used in electric
fan, water pump, etc.

It converts electrical energy into kinetic energy.

Electric motor

The flow of electric current through a Coil rotates
Magnetic clockwise

conductor produces a magnetic field field Commutator
reverses
around the conductor. If current is passed current

through a freely movable conducting wire

kept within the magnetic field, the wire Brushes
comes in motion due to the attraction carry
and repulsion of the magnetic fields current to
commutator

around the wire. When a conducting

wire placed in a magnetic field is rotated Electric
current
continuously, electricity is produced in

it. The electric motor is based on this Electric motor

principle. It converts electrical energy into kinetic energy.

Electrical appliances and their uses

Inverter AC Appliances

An inverter is an electrical -+

appliance which converts Battery

alternating current (AC)

into direct current (DC) -+ Inverter Socket
and the direct current Switch
into alternating current.
If there is electric line in

the circuit, the inverter converts the alternating current into direct

119 Times' Crucial Science Book - 10

current and charges the battery. The battery stores electricity in the
form of direct current. In case the electric line is cut off (due to load
shedding or other reason), the inverter converts the direct current of
battery in to alternating current. Then the electrical appliances work
by using alternating current.

Charger

The electrical instrument which is used to
charge a battery is called charger. It is also
used to charge the battery of an electrical
appliance such as mobile phone, radio,
remote control, emergency light, etc. The
charger converts the alternating current of
the electrical line into direct current and
charges the battery. When the battery is fully charged, the cut off
system of the electrical appliance stops the current flow. The charged
battery is used to operate mobile phone, small radio, emergency light,
etc.

Adapter

An adapter is a small device that transforms the household voltage of
alternating current (AC) into low voltage direct current (DC). Since it
changes AC into DC, it is also called an AC adapter. The suitable low
voltage DC provided by adapter is used in some electronic devices such
as radio and other electronic appliances which use low DC voltage. An
adapter allows one device or electronic interface.

Solar cell

A solar cell is a photovoltaic device that changes light energy directly
into electrical energy. A photovoltaic device is an instrument that
produces voltage when exposed to light. The generation of voltage
and electric current in a material while being exposed to light is
called photovoltaic effect. A solar cell is an example of photovoltaic
device. The photovoltaic device was invented by Alexander Edmond
Becquerel in 1839 AD. We can use solar module to solar array to
convert sunlight into electricity.

A solar module is a common device that is used to transform sun light
directly into electricity. A framework in which there are a number of
solar cells arranged in correct order is called solar module. A group of
solar modules forms a solar panel and the group of solar panels is
called a solar array.

Times' Crucial Science Book - 10 120

Conductors and semiconductors are used in a solar module. The most
popular semiconductor used in a module is silicon crystal, which is
laminated into p-type and n-type layers. These layers are placed one
after other. When light irradiates the silicon crystals, electricity is
produced due to photovoltaic effect. Such form of electricity can be
used directly or it can be stored in a battery.

Working of a solar cell

When sunlight falls up on a solar panel, module, etc, the semiconductor
(silicon) absorbs the photons (light energy). The absorbed photons
excite the electrons present in the atomic orbitals of the semiconductor
surface. The excited electrons are unstable in the higher energy
orbitals. Hence, they return to their previous orbitals releasing the
absorbed energy. This process produces electric current which is
captured by electrode and is conducted to the place where it is used.

A solar cell converts solar energy into direct current (DC). If there is
necessity of alternating current (AC), we can convert DC into AC by
an inverter.

Electric power

Electric motor, fans, trolley bus, electric heater, electric bulb,
television, computer, etc use electricity to operate. These devices
use electrical energy and convert it into other forms. For example,
electric heater converts electrical energy into heat energy. Electric fan
converts electrical energy into kinetic energy; electric bulb converts
electrical energy into heat and light energy and so on. Thus, every
electrical device has certain capacity of converting electrical energy
into other forms per unit time. It is called power of the device. Electric
power of a device is defined as the rate at which it converts electrical
energy into other forms. In other words, it can be defined as the rate
at which electrical energy is being supplied to an electrical device, i.e.

Electrical power (P) = Electrical energy supplied (E)
Time (t)

The unit of power is J/s. It is also called Watt.

If E = 1J and t= 1s then P = 1W, i.e. 1W = 1J .
1s

Thus 1 Watt power is the power of an electrical device which converts

1 Joule electrical energy into other forms in 1second.

Kilowatt (KW), Megawatt (MW), Horse power (HP), etc are bigger

121 Times' Crucial Science Book - 10

units of electric power.

1000 Watt = 1 Kilowatt

1000000 Watt = 1 Megawatt

746 Watt = 1 Horse power.

Electric power of a device is also calculated by multiplying potential
difference (V) across its ends and current (I) flowing through it, i.e. P
= IV.

Derivation of P = IV

Voltage (V) = work done (w) or energy supplied (E)
charge (Q)

or E = V×Q …….. (i)
Again,
Power (P) = Energy supplied (E)
Time (t)

P = E ..........(ii)
t

From equation (i) and (ii), we get

P = V ×Q P = V×I (∴I =Q )
t t

What do you mean by the statement that power of an electric
bulb is 60 Watt?

It means the bulb consumes 60 Joule electrical energy in 1 second. In
other words, it converts 60 Joule electrical energy into heat and light
in 1 second.

Electric energy consumption

We have to pay the bill of electricity to the electricity authority regularly
in time on the basis of electric energy consumed. Even though, SI unit
of energy is Joule, we use bigger unit of energy called kilowatt hour
for the measurement of amount electric energy consumed. Kilowatt
hour energy is called only unit for commercial use. One kilowatt hour
is the electrical energy consumed by an electrical device of power 1
kilowatt in 1 hour time.

Energy consumed = Electric power × time
1 KWh = 1 KW × h

Times' Crucial Science Book - 10 122

= 1000W ×3600s.
= 3.6×106J
∴ 1 KWh = 3.6×106 Joule
The amount of electric energy consumed in our house is measured
by the meter box installed in our houses. It can also be calculated
if number of appliances with their power and the duration of their
operation are known. Generally, the different appliances are operated
in different periods of time. Therefore, the electric energy consumed
by each appliance is calculated and total electric energy consumed is
found by adding energy consumed by all appliances.

Electric energy consumed = Electric power × Time
= Power of appliance×No of appliances× Time = P×N×T.

Solved Numerical Problem 6. 1

In a house 12 tube lights of 40 Watt each are lit up for 4 hours a day,
4 television of 40 Watt each are turned on 4 hours a day, 3 irons of 750
Watt each operate 1 hour a day and 2 bulbs of 100 Watt each are lit
up 3 hours a day. Calculate the electricity consumed in a month. How
much money should be paid if the cost of electricity is Rs 7 per unit?

Solution

i. Calculation of electric energy consumed by 12 tube lights

each of 40 Watt per day.

Given,No. of tubelights (N) = 12

Power (P) = 40Watt = 40 KW
1000

Time (T) = 4 hr,

Electric energy consumed (E) = P × N × T = 40 × 12 × 4
1000

= 1.92 KWh

ii. Calculation of electric energy consumed by 4 televisions of 40
Watt each per day.

Given,N = 4

P = 40W = 40 KW
1000

T = 4 hr 40
1000
Electric energy consumed = P ×N × T = 4 × × 4

= 0.64 KWh.

123 Times' Crucial Science Book - 10

iii. Calculation of electric energy consumed by 3 irons of 750

Watt each per day.

Given,N = 3

P = 750 Watt = 750 KW.
1000

T = 1 hr.

Electric energy consumed = P×N×T = 3 × 750 ×1
1000

= 2.25 KWh

iv. Calculation of electric energy consumed by 2 bulbs of 100

Watt each per day.

Given,N = 2

P = 100 KW
1000

T = 3hr = 2 × 100 × 3
Electric energy consumed = P×N ×T 1000

= 0.6 KWh.
Thus, the total electric energy consumed per day = (1.92 + 0.64 + 2.25 +
0.6) KWh

= 5.41 unit
∴ The total electric energy consumed per month = 5.41×30 = 1623
unit.

Then, the cost of electricity = Rs 1623 × 7 = Rs 11361.

Safety measures while using electricity

Colour code of the wires

Domestic electric wires are externally insulated with plastic cover.
Plastic cover prevents from electric shock even if the wire is touched
externally. These insulating covers are of different colours such as
red, yellow, green, blue, white, etc. Due to the following reasons, the
colour codes are used in the cover of electric wire.

a. It is easy to identify the phase, neutral and earthing lines.
b. It is easy to identify the wires in underground wiring.

Generally following colour codes are used for different wires.

Phase or live wire: This wire supplies electricity to the electric load.
We feel electric shock when we touch this wire. The live wire is red or

Times' Crucial Science Book - 10 124

brown in colour.

Neutral wire: This wire carries the electricity back to the supply. It
does not give electric shock on touching. The neutral wire is blue or
black in colour.

Earthing wire: This wire is connected to the body of electrical
devices and carries the leaked current to the earth from the devices.
The earthing wire is green or yellow in colour.

Domestic circuit connection

During wiring in a house, all electrical appliances are connected
in parallel combination and separate switch is connected for each
appliance.

A heavy cable is connected from the transmission line to the meter
box. It contains well insulated two wires; live wire and neutral wire.
The live wire is connected with a fuse. All electricity consumed in a
house flows through this fuse. Generally, the fuse of 15 A is used for
the normal supply of electric current for household purposes.

Now, the live and neutral wires emerging from the meter are sent
to the main switch. A separate fuse is provided for the live wire. The
main switch is connected to the earth through earthing wire. The
wires from the main switch box are connected to the separate circuits
called light circuit and power circuit. Appliances like electric bulb,
television, two pin sockets, etc are connected to light circuit whereas
the high power electrical devices with three pin sockets are connected
to the power circuit. The live terminal of each three pin-socket is
connected to the live wire, the neutral terminal to the neutral wire
and the earthing terminal to the earthing wire.

Things to remember during wiring

Following points should be remembered in household wiring.

a. Switches and fuses should be connected in the live wire.

Current flows to the devices through live wire. If switch and fuse
are connected to the neutral wire, the current flows to the devices
even the switch or fuse disconnects the circuits. Therefore, fuse
and switch should be connected in the live wire to disconnect the
circuit when current is not needed in the device.

125 Times' Crucial Science Book - 10

b. Fuse should be of appropriate capacity.

If the fuses are of lower capacity, the sufficient current cannot
reach to the devices and they cannot work properly. If the fuses
are of higher capacity, the excessive current cannot be controlled
and the devices get damaged.

c. Separate fuse should be used for each room.

It is done so that the appliances of one room still work even though
the fuse of another room goes off.

d. There should be separate light and power circuits.
Power circuits need more current and are wired with thick cables.
But, light circuits need less current and are wired with thin cables.
When separate fuse is used for light and power circuits, the light
circuits are not disturbed. Even if the fuse of power circuit goes off.

e. Electric wiring should not be done in damp places.
Water conducts electricity. Therefore, we may feel electric shock
if wiring is done in damp or wet places.

f. The live wire, neutral wire and earthing wire should be of
different colours.
It is because the live wire causes electric shock while touching it
in naked form. But, other wires are not very dangerous. Therefore
live wire, neutral wire and earthing wire should be used of
different colours.

Fuse

A fuse is a thin wire having low melting point and high resistance.
When the excessive current flows through the circuit, it breaks the
circuit and protects the devices connected in it from being damaged.
Therefore, it is also called safety device. Generally, fuse is made up of
alloy of tin (63%) and lead (37%).
The maximum current that a fuse allows to pass through it is called
rating capacity of the fuse. For example, a fuse has rating capacity of
5A. It means maximum 5A current can flow through it without being
melted. Generally, fuses of capacity 3A, 5A, 10A, 13A, 15A, 30A, etc
are available in the market.

Times' Crucial Science Book - 10 126

Fuse wire
Procelain case

Fuse case with fuse wire Cartridge fuse Symbol of fuse MCB

Now-a-days, cartridge and MCB (miniature circuit breaker) type of
fuse are also in use. The cartridge type of fuse is used in volt guards
and other electrical appliances. MCB is used in the household wiring.
MCB does not get damaged when excessive current flows through it.
However, its switch goes down cutting off the electric supply. When
the switch is pushed up, it starts working again. Thus, MCB is easier
and safer to use in comparison to the conventional fuse.

Transformer

A transformer is an electrical device which is used for increasing or
decreasing A.C voltage. It is of two types. They are:

(i) Step down transformer (ii) Step up transformer

The transformer is based on two laws:
a. The input energy is always equal to the output energy in a

transformer, i.e.

I1V1 = I2V2 where I1 = Input current, I2 = Output current,

V1 = Input voltage and V2 = output voltage.
b. Voltage is directly proportional to the number of turns of

wire.

Structure

Ip IS
AC Vp Vs

Primary Laminated
iron core
coil (Np) Secondary
coil (NS) Times' Crucial Science Book - 10

Structure of transformer

127

It consists of rectangular soft iron core made up of laminated sheets
kept one above another. The lamination of the iron sheets is done
through shellac or varnish. It is done to reduce eddy currents. The loop
of electric current induced within a conductor by a changing magnetic
field in the conductor is called eddy current. Insulated wire is wound
many turns around one arm of the core and its ends are connected to
an AC source. The coil to which AC input is given is called primary
coil. The voltage supplied to the primary coil is called primary voltage.

In another arm of the core, a coil of insulated wire is formed by winding
the wire around the arm of the core. Output current is passed out
from this coil. This coil is called secondary coil. The voltage across the
secondary coil is called secondary voltage.

The number of turns in the secondary coil differs according to the
required secondary voltage.

Principle

When AC voltage is supplied to the primary coil, an alternating
magnetic field is created around the coil. Due to this, the secondary
coil experiences the changing magnetic flux. The changing magnetic
flux in the secondary coil produces AC in this coil. It is called principle
of mutual induction.

The relation between the number of turns in the primary coil, that
in the secondary coil, primary voltage and secondary voltage can be
shown by following equation.

Secondary voltage = No. of turns in secondary coil
Primary voltage No of turns in primary coil

V2 = n2
V1 n1

Types of transformer

There are two types of transformer:

1. Step- up transformer

The transformer which changes low AC voltage to high AC voltage

is called step up transformer. In this transformer, number of turns

in the secondary coil is more than that present in the primary coil.

It is generally used to transmit current from power station to the

long distant residential area.

2. Step - down transformer

The transformer which changes high AC voltage to low AC voltage
Times' Crucial Science Book - 10
128

is called step down transformer. In this transformer, primary
voltage is more than secondary voltage. In such transformer,
the number of turns in the secondary coil is less than that in
the primary coil. It is generally used to decrease the voltage of
alternating current supply in radio, television computer etc.

Uses

Step down transformers are used in voltage regulators for computers,
television, air conditioners, record players, trolley buses, etc. Step up
transformers are used to increase the voltage of AC produced in power
station. At the power station, electricity is generated at 19,500V and
is transformed to 1,32,000V by step up transformer. It is transmitted
to far way places. The voltage is stepped down to 220V in the sub-
station and supplied to the houses.

Learn and Write

1. Filament lamp is filled with inert gases. Why?
Very thin tungsten wire is used inside the filament lamp. It
gets oxidized when it comes in contact with oxygen at higher
temperature. To prevent from this, the filament lamp is filled
with inert gases.

2. Loads are connected in parallel combination in domestic circuit. Why?
When loads are connected in parallel combination, the amount
of voltage received by a load is not unaffected even the number
of the loads is increased. Due to this, brightness of the bulbs
remains unaffected on increasing the number of bulbs. Similarly,
separate switch can be used for individual load.

3. Electromagnet has more advantages over permanent magnet. Why?
Strength of an electromagnet can be altered as our requirements.
Its shape can be changed according to our desire. Due to this,
electromagnet has more advantages over permanent magnet.

4. Number of turns in primary coil and that in secondary coil of a
transformer never remains same. Why?
Transformer changes voltage of alternating current supply and
works on the principle of mutual induction. The magnitude of
output voltage will be different from that of input voltage if only
the number of turns in the primary coil and the secondary coil
is different. If the number of turns remains equal in both coils,
the output voltage will be equal to the input voltage. Therefore,
number of turns in the primary coil and the secondary coil
remain different.

129 Times' Crucial Science Book - 10

5. Nichrome wire is used as a heating element.
Nichrome wire is an alloy of 60 per cent nickel and 40 per cent
chromium metal. It has high resistance and high melting point
so it produces enough heat without melting. It does not react
with oxygen even at high temperature and does not get damaged.

Hence, it is used as a heating element.

6. Separate lines should be provided for lamps and power sockets.
The power sockets provide lines to the high power appliances.
There is more probability of fuse of power line to get damaged. If
separate lines are provided, the lamps remain glowing even after
the fuse goes off in the power line.

7. Switch should always be connected with a live wire.
The switch blocks the supply of electric current to the electric
appliances when it is off. If the switch is connected to the neutral
wire, the electrical appliance still remains live even after the
switch is off. Thus, the worker may get electric shock by touching

appliance although it is switched off.

8. Fuse should always be connected to a live wire.
When the fuse goes off, current is cut off and there is no chance

of electric shock or other harms caused by high electric power.

9. The iron core of a transformer is laminated.
The iron core of the transformer should be laminated (insulated).
If the core is not insulated, a kind of electric current is produced
in the core, it is known as eddy current. This current reduces
the efficiency of the transformer by producing heat. If the core is
a single block of iron instead of laminated sheets, there will be

heating effect on the core and less output will be obtained.

10. A transformer cannot change a DC voltage.
The transformer works on the principle of mutual induction. For
this, the magnetic flux must change continuously in primary coil
to produce current in the secondary coil. As the magnitude and
polarity of direct current remains constant, it does not change
magnetic flux in the primary coil. Hence, it cannot produce
current in secondary coil. Thus, a transformer does not alter the
voltage of direct current.

Main points to remember

1. The path made by connecting load, source and good conducting

Times' Crucial Science Book - 10 130

wire that offers flow of charge is called electric circuit.
2. The current which has fixed magnitude and polarity is called

direct current.
3. The current which has changing magnitude and polarity is

called alternating current.
4. A fuse is a thin wire having low melting point and high resistance which

is used to stop the excessive flow of current in the electric circuit.
5. The process of passing electric current to the ground from the

heavy electric appliance with the help of wire is called earthing.
6. The power consumed per hour by an electric appliance of power

1 kilowatt is called one kilowatt hour or 1 unit electricity.
7. An electric current has the following effects:

a. Heating effect: electric heater, electric iron, etc.
b. Lighting effect: filament lamp, fluorescent lamp, etc.
c. Chemical effect: electrolysis, electroplating, etc.
d. Magnetic effect: electromagnet, electric bell, etc.
8. An electromagnet is a temporary magnet which is made by
passing current through a solenoid.
9. The motion produced in a current carrying conductor when it is
placed in a permanent magnetic field is called motor effect.
10. A transformer is a device which is used for increasing or decreasing the
magnitude of alternating current through mutual induction.

Exercise

A. Choose the best alternative.
1. What is the full form of CFL?
a. Compact filament lamp b. Controllable filament lamp
c. Compact Fluorescent Lamp d. Complete Fluorescent lamp
2. The total voltage is equal to the sum of voltage of individual cells in a
a. Series combination of cells b. Parallel combination of cells
c. Mixed combination of cells d. All of these
3. In commercial use, one Kilowatt Hour of electrical energy is known as
a. 1Joule b. 1 Watt c. 1 Unit d. 1 HP
4. The easier and safer fuse
a. Tin-lead alloy b. MCB c. Cartridge fuse d. Nichrome wire

B. Answer these questions.
1. What is fuse? It is always connected to live wire, why?
2. How is appropriate fuse selected?
3. Tungsten wire is not used in a heater although it has high resistance.
4. Defineheatingelement.Whichmetalisusedtomakeheating element?

131 Times' Crucial Science Book - 10

5. How does earthing help to protect from electric shock?
6. What is meant by short-circuit?
7. Write down any four points that should be considered during

house hold wiring.
8. Write down any two ways to increase the strength of an

electromagnet.
9. What type of connection of loads is made in the domestic

circuit? Why?
10. Lamp is lighted in the running bicycle with the help of dynamo.

The light goes off when the bicycle is stopped. Why?
11. Write any two ways by which the output of a dynamo can be

increased.
12. What is electric power? Write down the unit of electric power

consumptions.
13. State any four methods to increase the magnitude of current

produced by a generator.
14. What is transformer? Which coil, primary or secondary, has

lesser number of turns in a step down transformer?
15. “The use of alternating current would be limited, if transformer

was not invented”. Explain the statement with two clues.
16. What is transformer? How is electric current induced in the

secondary coil of a transformer?
17. What is meant by electrolysis? Mention uses of electrolysis
18. What is motor effect? Where is it used?

C. Differentiate between:
1. AC and DC
2. Filament lamp and Fluorescent lamp
3. Generator and Dynamo
4. Generator and Motor
5. Step-up and Step-down transformer
6. Phase wire and Neutral wire

D. Give reasons:
1. 20W fluorescent lamp gives more light than 50 W filament lamp.
2. The switch is connected to the live wire.
3. Coloured wires should be used in household wiring.
4. The fuse is an important device of domestic circuit.
5. Nichrome wire is used as a heating element.
6. Inert gas is filled in a filament lamp.
7. MCB is used instend of fuse.
8. Tungsten filament is used in filament lamp.

Times' Crucial Science Book - 10 132

9. No electric shock is experienced while touching neutral wire.
10. Separate lines should be used for light and power sockets.
11. The core of the transformer is laminated.

E. Diagrammatic questions.
1. Draw a circuit diagram showing parallel combination of loads
with source and switch.
2. Draw a labelled diagram of:
(i) Step-up transformer
(ii) Step-down transformer
3. Draw a labelled diagram showing the working of electric bell.
4. Which type of transformer is given in the diagram? What is
used to laminate its core? Write its one use.

F. Solve the following numerical problems.
1. In a transformer, the number of primary turns is 880. If
the primary and secondary voltages are 220V and 12V
respectively, calculate the number of turns in the secondary
coil.
2. If the input voltage is 220 V and the ratio of the number of
turns in secondary to primary coil is 3:2, what will be the
output voltage of the transformer?
3. If two irons of 750W each are used 8 hours a month, how much
tariff should be paid? Cost of one unit of electricity is Rs.7.
4. If the input voltage is 11000 V and the ratio of the number of
turns in secondary to primary coil is 3:100, what will be the
output voltage of the transformer?

5. In a hotel, 20 bulbs each of 60W are lighted for 6 hours a day
and 5 fans each of 50W are run for 10 hours a day. Calculate
the total electricity consumed in a month.

6. In a transformer, the number of turns in primary coil is 1000.
If the primary and secondary voltages are 220V and 110V
respectively, calculate the number of turns in the secondary coil.

133 Times' Crucial Science Book - 10

7. In a transformer, the primary winding is 3 times of the
secondary winding. lf the primary voltage is 220V, find the
secondary voltage. Which type of transformer is this? Write
one application of this transformer.

8. A transformer is used to light the bulb of 6V. If the transformer is
connected to 220V mains and the number of turns of primary coil
is 880, what will be the number of turns in the secondary coil?

Answers F. 1. 48 turns 2. 300 V 3. Rs 84. 4. 330 V

5. 291 unit 6. 500 7. 73.33V 8. 24 turns

Project Work

Take a bicycle dynamo if possible. Open it to observe its internal
structure. Are the parts present in the dynamo similar to those given
in the diagram of your text? Draw the diagram of your observation and
explain your findings.

Glossary : voltage
: to run a machine
• Potential : insulated, smeared uniformly with an
• Operate
• Laminated insulating layer
: a smaller-than-usual version of something
• Miniature : household
• Domestic : tired, worn out, used up
• Exhausted : too much
• Excessive : a bar of soft iron or steel placed across the
• Armature
poles of a magnet to maintain its strength

Times' Crucial Science Book - 10 134

Chapter Classification

7 of
Elements

Dmitri Mendeleev

Dmitri Ivanovich Mendeleev was a Russian
chemist and inventor. He formulated the
Periodic Law, created a farsighted version of
the periodic table of elements.

Estimated Periods: 4 (4T+0P)

Objectives

At the end of the lesson, students will be able to:
• explain periodic law,
• explain modern periodic table and the positions of elements in the modern
periodic table.

The systematic classification of elements in definite rows and columns
according to a certain law is known as periodic table. The vertical
columns of the periodic table are called groups whereas the horizontal
rows are known as periods. The periodic table is based on a periodic
law. In periodic table, elements are arranged systematically in
which similar elements are grouped together and there is a gradual
variation in the properties of elements from one member to another.

The present form of periodic table is the outcome of the contribution
of a large number of scientists. Hence, no single scientist can be
given the full credit for the development of the modern periodic table.
In 1802, Dalton postulated atomic theory and calculated atomic
weights of different elements. In 1815 AD, Prout discovered that
atomic weight of an element is simple multiple of atomic weight of
a hydrogen atom. Newland arranged elements on the basis of their
increasing atomic masses. He found that the properties of every eight
element are similar. His law is called law of octaves. In 1869 AD,
Lother Meyer studied the relationship of atomic mass and atomic
volume of elements. But the significant contributors are the Russian
scientist Dmitri Mendeleev and English scientist Henry Moseley.

Mendeleev's Periodic Table

Dmitri Mendeleev, a Russian chemist, studied the physical and
chemical properties of elements and postulated the periodic law in
1869, which is commonly known as Mendeleev's periodic law.

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Mendeleev's periodic law states that the physical and chemical
properties of elements are periodic function of their atomic weights.
The term 'periodic function' means that the properties are repeated
after a regular interval if the elements are arranged in the order
of their increasing atomic weights. The elements having similar
properties lie one below the other in the same column.

Mendeleev classified 63 known elements on the basis of their
increasing atomic weights. Hence, the elements with similar
properties could be placed together in the form of table. This table
is known as Mendeleev's periodic table. Mendeleev's periodic table
may be defined as the systematic classification of all the elements
on the basis of their increasing atomic weights in the form of table.
Mendeleev used the term 'periods' for the horizontal rows and 'groups'
for the vertical columns of the elements. The elements having similar
properties are placed in the same group while the elements with
gradual change in their properties are placed in the periods.

There were eight groups and 6 periods in Mendeleev's periodic table.
There was no place for the inert (noble) gases as those gases were not
discovered. The periods 4, 5, 6 and 7 were divided into two halves to
place more elements.

Mendeleev left some gaps in his periodic table for some elements which
he thought would be discovered later on. He predicted the names
Eka-aluminium, Eka-boron, Eka-silicon, etc for the undiscovered
elements.

Mendeleev’s original form of Periodic Table

Group Group Group Group Group Group Group Group

I II III IV V VI VII VIII

Period 1 H

Period 2 Li Be B C N O F

Period 3 Na Mg Al Si P S Cl

Period 4 K Ca 1* Ti V Cr Mn Fe Co
3* As Se Br Ni
Cu Zn 2*

Period 5 Rb Sr Y Zr Nb Mo 4* Ru Rh
Ag Cd In Sn Sb Te I Pd

Period 6 Cs Ba La Hf Ta W Re Os Ir
Au Hg Th Pb Bi Po At Pt

*Names given by Mendeleev for the undiscovered elements:

1. Eka - Aluminium 2. Eka - Boron

3. Eka - Silicon 4. Eka - Manganese

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Merits of Mendeleev's periodic table

1. Systematic study of the elements

In the periodic table, similar elements are kept together in a group.
Hence, we can have an idea about the properties of an element
and its compounds by knowing its group. In a given period, there
is a gradual variation of properties from one element to another.

2. Prediction of undiscovered elements

Mendeleev left some gaps for the undiscovered elements and
predicted their properties based on his periodic law. Later on,
all those elements were discovered during his lifetime. Those
elements were as follows:

Eka - Boron : Scandium (21)

Eka - Aluminium : Gallium (31)

3. Correction of certain faulty atomic weights

Mendeleev found atomic weights of some elements to be doubtful.
Based on his periodic law, he corrected the doubtful atomic
weights of those elements. The atomic weights of the elements
such as beryllium, indium, platinum, etc were corrected.

Demerits of Mendeleev's periodic table

1. Position of hydrogen: Mendeleev placed hydrogen in group 1
with alkali metals without any justification. But hydrogen could
be placed in group 17 as well. So, the position of hydrogen has not
been properly defined.

2. Similar elements were separated and dissimilar elements
were placed together: Alkali metals and other metals such as
copper, silver and gold were placed in the same group. But the
properties of these metals are quite different from those of alkali
metals.

3. Position of isotopes: If elements are classified on the basis of
atomic weight, isotopes of the same element should have separate
positions in the periodic table. But there is no idea about the
position of isotopes in the Mendeleev's periodic table.

4. Some elements having higher atomic weights have been
placed before those with lower atomic weights. For

137 Times' Crucial Science Book - 10

Times' Crucial Science Book - 10 Modern Periodic Table: Long Form Periodic Table

IA IIA IIIB IVB VB VIB VIIB VIII IB IIB IIIA IVA VA VIA VIIA 'O'

138

example, Co (58.9) is placed before Ni (58.7), and Te (127.6) is
placed before I (127.0).

5. There was no proper place for lanthanides and actinides
in Mendeleev's original periodic table. If these elements
were arranged according to their atomic weight, they would
violate the Mendeleev's periodic law.

Modern Periodic law

An English scientist, Henery Moseley, in 1913 proposed that atomic
number is more fundamental property of elements than the atomic
weight. This led to the modification of Mendeleev's periodic table and
gave rise to a new law known as modern periodic law.

The modern periodic law states that the physical and chemical
properties of elements are periodic function of their atomic number.
It means that when the elements are arranged in the order of their
increasing atomic number, the properties of elements are repeated.
The classification of elements based on the modern periodic law is
called modern periodic table. Modern periodic table is also known as
long form of periodic table.

At present, IUPAC (International Union of Pure and Applied
Chemistry) has simplified the modern periodic table and made it
more logical. IUPAC is an international scientific organization which
determines global standards for names, symbols and units.

Characteristics of modern periodic table

The modern periodic table consists of 7 horizontal rows and 18
vertical columns.

1. Periods

The horizontal rows of elements in the periodic table are known as
periods. There are seven periods in the modern periodic table. They
are numbered as 1, 2, 3, 4, 5, 6 and 7. The period (number) of an
element is equal to the total number of shells present in an atom
The elements of the same period have the same number of shells but
different chemical properties.

A period may be either short or long depending upon the number of
elements. The number of elements present in different periods and
the type of the periods is given below:

Period No. of elements Types of period
1 2 Very short period
2 8 Short period

139 Times' Crucial Science Book - 10

38 Short period
4 18 Long period
5 18 Long period
6 32 Very long period
7 32 Very long period

The above table shows that the periods can be categorized into three
types short periods, long periods and incomplete long period. Each of
these periods contains fixed number of elements.

The long form of periodic table is based on the electronic configuration
of atoms. The number of elements in each period is equal to the
number of electrons required to fill the various orbitals of the shell
starting with a new energy level. For example, the first shell contains
only two electrons at maximum and hence there are two elements in
the first period. Similarly, the second shell contains 8 electrons and
there are 8 elements in the second period and so on.

2. Group

The vertical columns in the periodic table are known as groups. There
are altogether 18 groups in the modern periodic table. The groups
are also named in numerals as 1, 2, 3, 4, ..................., 16, 17, and 18.

At present, most people use IUPAC classification of elements. In
this system, the groups are numbered as 1, 2, 3, ……, 18 as stated
above. The group 18 is also named as group zero (0). There is no
subgrouping in this system. However, in the long form periodic table,
groups of elements are named in roman numerals as I, II, III, etc.
The groups I-VII have subgroups A and B. Group VIII does not have
any subgroup but contains three vertical columns. In this unit, we
shall use both systems of classification.

a. Group 1 and 2: The elements of group 1 and 2 are highly
reactive metals. They are named as alkali metals and alkaline
earth metals respectively. Each group occupies a single vertical
column in the periodic table.

Group 1 elements: The elements of group 1 are known as alkali
metals because these metals react with water to form alkali.
They are highly reactive electro-positive metals. It is because
these elements contain single electron in valence shell which can
easily be lost to become cation.

Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), Cesium
(Cs) and Francium (Fr) are the elements of group 1.

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Group 2 elements: The elements of group 2 are called alkaline
earth metals because they form basic hydroxides which are less
soluble in water. The elements of this group are beryllium (Be),
magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba)
and radium (Ra). All these elements have two electrons in their
valence shells.

Group 17 elements: The elements of group 17 are known
as halogens because they form salts by reacting with group 1
elements. They are highly reactive electronegative non-metals.
It is because these elements contain seven electrons in their
valence shell which can easily attract one electron from metals
to gain octet. Then they become electronegative.

Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine
(At) are the elements of group 17.

b. Group 'O' (Group 18): The 'O' group constitutes single column
of non-reactive or inert gases. These elements are called noble or
inert gases because they do not take part in chemical reactions
due to their completely filled valence shell. The elements of this
group are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon
(Xe) and radon (Rn). Except helium, all these elements have 8
electrons in their valence shell.

3. Transition elements

The elements of group 3 to 12 in the modern periodic table are called
transition elements. All these elements are the metals. The transition
elements are also known as d-block elements because the outermost
electron of these elements enters in d-sub shell.

4. Lanthanides and actinides

A series of fifteen elements including lanthanum (atomic number 57
to71) in the periodic table is known as lanthanides. All these elements
have similar chemical properties. These elements are also known
as rare earth elements. Similarly, a series of 15 elements including
actinium (atomic number 89 to 103) in the periodic table is known as
actinides. These elements are also known as transuranium elements.
The lanthanides and actinides are together known as inner-transition
elements.

The lanthanides and actinides are placed at the bottom of the
periodic table in two separate rows. Each row consists of a series of
14 elements.

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All the elements of sub-group A in the periodic table are called
representative elements. They have only one incomplete shell,
i.e. outermost shell. These elements have their outermost orbit
incompletely filled while the inner orbits are completely filled.

Difference between Modern and Mendeleev's periodic table:

Modern periodic table Mendeleev's periodic table

1. It is based on the increasing 1. It is based on the increasing

atomic number of elements. atomic mass.

2. It consists of seven periods 2. It consists of seven periods
and nine groups. and eight groups in the original
forms.

3. The position of hydrogen, 3. There is no justifiable position

lanthanides and actinides is of hydrogen, lanthanides and

properly justified. actinides.

4. There is proper position for 4. There is no idea about the

the isotopes of an element. position of isotopes of an element.

Differences between groups and periods:

Groups Periods

1. The vertical columns of 1. The horizontal rows of elements

elements in the periodic table in the periodic table are called

are called groups. periods.

2. All the elements of a group 2.The number of valence electrons

have same number of valence of elements in a period increases

electrons. from left to right.

3. All the elements of a group 3. The elements of a period have

have same valency. different valency.

4. The elements of a group have 4.The elements of the same
similar properties. period have different contrasting
properties.

5. The size of atoms increases 5. The size of atoms decreases

from top to bottom. from left to right.

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Differences between representative and transition elements:

Representative elements Transition elements

1. The elements of sub-group A 1. The elements of sub-group B
in the periodic table are called and group VIII in the periodic
representative elements. table are called transition
elements.

2. They include both metals 2. They all are metals.
and non metals.

3. Only valence shell is 3. They have two shells
incompletely filled in these incompletely filled.
elements.

4. They have a single definite 4. They have variable valency.
valency.

5. Their salts are mostly 5. Their salts are colourful.
colourless.

Sub shell and electronic configuration

A shell is a well defined fine circular path in which the electrons
move around the nucleus. lt is also known as orbit. The shells can be
represented as K, L, M, N ....... etc. They can also be represented in
numbers such as 1, 2, 3, 4, ……. etc.

The maximum number of electrons that can be accommodated in a
shell of an atom is given by 2n2 rule. Since the 2n2 rule of electronic
configuration could not include the electronic configuration of all the
elements, the concept of s, p, d and f sub-shells was introduced.

The sub-shells are composed of orbitals. The region around the
nucleus where the probability of finding electrons is maximum
is called orbital. The maximum number of electrons that can be
accommodated in s, p, d and f sub-shells is 2, 6, 10 and 14 respectively.
The sub-shells present in each shell is given below:

Shell Number of Sub-shells present
sub-shell
K s sub shell
L 1 s and p sub shells
M 2 s, p and d sub shells
N 3 s, p and f sub-shells
4

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Maximum number of electrons in sub-shell is as follows:

Sub-shell Maximum number of electrons
s2
p6
d 10
f 14

The stable state of having two electrons in the first orbit is duplet
state. For example, helium has duplet state. Similarly, a stable set
of eight electrons in the outermost shell of an atom is called octet.
For example, neon, argon, etc have octet. The tendency of an atom
to acquire 8 electrons in the outermost shell is called octet rule.
The atoms can acquire octet by gaining, losing or mutual sharing of
electrons and it is the cause of chemical combinations.

Electronic configuration

The distribution of electrons in various shells and sub shells of an
atom is called electronic configuration Aufbau principle tells us the
sequence of filling up of atomic orbitals.

According to this principle "the atomic orbitals are filled up in the
ground state in the order of increasing energy levels".

The sequence of filling the atomic sub shells or orbitals follows a
specific order which can be obtained from the following diagram.

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d 6f

Sequence of filling of atomic orbitals

Thus, the correct order is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s,
4f, ....... etc.

The electronic configuration of some elements (atomic no. 1 to 30) in
terms of sub shells is below:

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1. Hydrogen (H)→ 1s1 2. Helium (He) →1s2

3. Lithium (Li)→ 1s2, 2s1 4. Beryllium (Be) →1s2, 2s2

5. Boron (B) →1s2, 2s2, 2pl 6. Carbon(C) →1s2, 2s2, 2p2

7. Nitrogen (N) →1s2, 2s2, 2p3 8. Oxygen (O) →1s2, 2s2, 2p4

9. Fluorine (F) →1s2, 2s2, 2p5 10. Neon (Ne) →1s2, 2s2, 2p6

11. Sodium (Na) → 1s2, 2s2, 2p6, 3s1

12. Magnesium (Mg) → 1s2, 2s2, 2p6, 3s2

13. Aluminum (Al) →1s2, 2s2, 2p6, 3s2, 3p1

14. Silicon (Si) →1s2, 2s2, 2p6, 3s2, 3p2

15. Phosphorus (P) → 1s2, 2s2, 2p6, 3s2, 3p3

16. Sulphur (S) → 1s2, 2s2, 2p6, 3s2, 3p4

17. Chlorine (Cl) →1s2, 2s2, 2p6, 3s2, 3p5

18. Argon (Ar) →1s2, 2s2, 2p6, 3s2, 3p6

19. Potassium (K) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1

20. Calcium (Ca) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2

21. Scandium (Sc)→ 1s2, 2s2, 2p6, 3s2,3p6, 4s2, 3d1

22. Titanium (Ti) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d2

23. Vanadium (V) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d3

24. Chromium (Cr) → ls2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5

25. Manganese (Mn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d5

26. Iron (Fe) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6

27. Cobalt (Co) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d7

28. Nickel (Ni) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8

29. Copper (Cu) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10

30. Zinc (Zn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10

145 Times' Crucial Science Book - 10

In chromium and copper, the actual electronic configuration differs
slightly from the expected ones, because half filled or completely
filled 'd' orbital is more stable than d4 or d9 state.

Classificationofelementsonthebasisofelectronicconfigurations

The elements in the modern periodic table are divided into four
blocks. They are:

1. s–block element 2. p–block element

3. d–block element 4. f–block element

The last electron present in the sub-shell determines the block of an
element.

1. s -block elements

The elements in which the last electron enters in s-sub-shell of
outermost shell are called s-block elements. These elements contain
1 or 2 valence electrons. The elements of group 1 and 2 are s-block
elements. Thus, the alkali metals and alkaline earth metals are
s-block elements. These are very reactive metals.

Example:

H→1s1 Be → 1s2, 2s2

Li→1s2, 2s1 Mg → 1s2, 2s2, 2p6, 3s2

Na → 1s2, 2s2, 2p6, 3s1 Ca → 1s2, 2s2, 2p6, 3s2, 3p6,
4s2 and so on.

2. p -block elements

The elements in which the last electron enters into p-sub-shell of
their outermost shell are called p-block elements. The elements of
group 13 to 18 (group O) belong to p-block elements. The p-sub-shell
of these elements is gradually filled up. Since the p-sub shell can
hold maximum of 6 electrons, the p-block elements constitute six
columns in the periodic table.

Examples:

B →1s2, 2s2, 2p1 C → 1s2, 2s2, 2p2

N→ ls2, 2s2, 2p3 O → 1s2, 2s2, 2p4

F→ ls2, 2s2, 2p5 Ne →1s2, 2s2, 2p6

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