Relation between magnification (m) and the size of the image
a. If M = 1 ⇒ I = O, i.e., No change in the size of the image
b. If M > 1 ⇒ I > O, i.e., Image is bigger than the object
c. If M < 1 ⇒ I < O, i.e., Image is smaller than the object
The magnification (M) produced by a lens is equal to the ratio of the image distance (v) to the
object distance (u). So, the magnification of a lens is also calculated by the following formula:
Magnification (M) = Image distance(v) or M= v
Object distance(u) u
5.13 Method of Measuring Magnification
Magnification produced by a lens is A F Convex lens F I
the ratio of height of the image to the B v 2F
height of the object. It is also the ratio 2F O
of the image distance to the object Object C D
distance. It can be seen in the following
mathematical process. Fig. 5.19
In the given diagram, OA is an object, u
ID is the image of OA and C is the
optical center of the convex lens. Let 'u'
and 'v' be the object distance and image
distance respectively. Then from the
given diagram, we get
S.N. Statement Reason
i. ∠ACO = < DCI i. Vertically opposite angles
ii ∠AOC = < DIC ii. Both are right angles
iii. ∠OAC = < IDC iii. Remaining angles of two triangles
∴ ∆AOC is similar to ∆ IDC
∴ ID = IC (∵ Corresponding sides of similar triangles)
OA OC
iimmaaggeeddisistatnacnece
∴ Magnification = =OIDA O=ICC v = oobbjjeeccttddisistatnacnece
u
Hence, magnification is calculated by measuring the image distance and the object distance.
5.14 Lens Formula
The formula that gives the relation between the object distance (u), image distance (v) and
focal length (f) of a lens is called the lens formula. The lens formula is written as:
PHYSICS Oasis School Science - 10 93
1 = 1 + 1
f v u
This lens formula applies to both convex and concave lenses.
5.15 Sign Convention for Lenses
Now-a-days, the New Cartesian Sign Convention is used for measuring distances in the ray
diagrams of convex and concave lenses. According to the New Cartesian Sign Convention:
i) All the distances are measured from the optical center of the lens.
ii) The distances measured in the same direction as that of the incident light are taken
as positive.
iii) The distances measured against the direction of the incident light are taken as
negative.
iv) The distances measured upward and perpendicular to the principal axis are taken as
positive.
v) The distances measured downward and perpendicular to the principal axis are taken
as negative.
Worked out Numerical 3
A convex lens of focal length 10cm is placed at a distance of 12cm from a wall. How far from
the lens should an object be placed so as to form its real image on the wall?
Solution:
Given,
Image distance (v) = +12 cm [ ∵ The image is real.)
[∵ It is a convex lens.)
Focal length (f) = +10 cm
Object distance (u) = ?
According to the lens formula,
1 = 1 + 1
f v u
or 1 = 1 + 1
10 12 u
or 1 = 1 − 1
u 10 12
or 1 = 6−5
u 60
or, u = 60 cm.
Thus, the object should be placed at a distance of 60cm from the lens.
94 Oasis School Science - 10 PHYSICS
Differences between Convex lens and Concave lens
S.N. Convex lens S.N. Concave lens
1. A lens which is thick in the middle 1. A lens which is thin in the middle but
but thin at the edges is called a thick at the edges is a called a concave
convex lens. lens.
2. It converges the parallel rays of 2. It diverges the parallel rays of light
light falling on it. falling on it.
3. It has a real focus. 3. It has a virtual focus.
4. The power of this lens is positive. 4. The power of this lens is negative.
5. It forms a real or virtual image 5. It always forms a virtual image.
depending on the position of the
object.
Differences between Real image and Virtual image
S.N. Real image S.N. Virtual image
1. The image which can be formed 1. The image which can't be formed on
on the screen is called a real the screen is called a virtual image.
image.
2. It is always inverted. 2. It is always erect.
3. It is formed by the actual intersection 3. It is formed when the rays of light
of rays of light at a point. appear to meet at a point.
5.16 Optical Instruments
Instruments that are used to produce an image of an object with the help of a lens or lenses are
called optical instruments, e.g., a camera, telescope, microscope, binoculars, etc. Most of the
optical instruments work on the basis of refraction and reflection of light. In this unit, we will
discuss the structure and working mechanism of the human eye and defects of vision with ray
diagrams.
Human eye
A human eye is also an optical instrument. The eye ball is more or less spherical in shape
having a diameter of about 2.2 cm. The front part of the eye is called the cornea. It is made of a
transparent substance and is bulging outwards. The light coming from objects passes through
the cornea. There is a colored diaphragm called the iris which controls the size of the pupil. It
appears black since no light is reflected from it.
The eye-lens is a convex lens which is made of proteins. The lens is supported and adjusted
by ciliary muscles. The focusing is done by the ciliary muscles. In this respect, it is different
PHYSICS Oasis School Science - 10 95
from a photographic camera. The image is formed on the retina of the eye. The retina is made
up of two types of photosensitive cells called rods and cones. There are two types of liquid
in the human eye; one is present in between the cornea and lens called aqueous humour and
another in between the lens and retina called vitreous humour. These liquids help the eye ball
to retain its shape.
Ciliary muscles
Fig. 5.20 Structure of a human eye
The image formed on the retina activates the light sensitive cells, which generate electrical
impulses. The retina sends those electrical impulses to the brain through the optic nerve.
Although the image formed on the retina is inverted, the brain intercepts the erect and correct
image of the object.
When we enter a dark room from a bright light, at first we cannot see things clearly. After
some time, our vision improves. This is due to the fact that in a bright light the pupil of our
eye becomes small. So, when we enter a dark room, very little light enters our eyes, and we
cannot see things clearly. After sometime, the pupil of our eye expands and becomes larger.
More light enters our eye, and we can see things clearly. When we go into a bright light from
darkness the case will be just the opposite.
Lens
Pupil
Image
Object
Fig. 5.21 Working mechanism of a human eye
Accommodation
A normal eye is one which does not have any defect of vision. Thus, a normal eye can see
distant objects as well as nearby objects clearly. An eye can form the image on the retina for
96 Oasis School Science - 10 PHYSICS
objects at various distances by changing the focal length of the eye lens. The changing of the
focal length of the eye lens is done by the ciliary muscles.
When we see distant objects, the ciliary muscles are relaxed and the eye lens becomes very
thin. It helps to increase the focal length. As a result, the converging power of the eye lens
decreases.
Ciliary muscles are relaxed
Parallel rays from a Image
distant object
Object at infinity
Here eye lens has large
focal length but small
converging power
Fig. 5.22 In this position eye lens has large focal length
but small converging power
When we see nearby objects, ciliary muscles become tense due to which the eye lens becomes
thick and focal length decreases. It increases the converging power of the eye lens.
Ciliary muscles
are tense
Diverging rays from Image
nearby object
O
25 cm Here eye lens has short
focal length but large
converging power
Fig. 5.23 In this position eye lens has short focal
length but large converging power
The ability of an eye to focus the image of objects at various distances on the retina by changing
the focal length of the eye lens is called accommodation.
Far point: The distant point from the eye which can be seen clearly is called the far point of the
eye. For a normal eye, the far point is at infinity.
Near point: The nearest point upto which an eye can see the object clearly is called the near
point of the eye. For a normal eye, the near point is at a distance of 25 cm from the eye.
Fact File - 6 Fact File - 7
Least distance of distinct vision: The Range of vision: The distance between
minimum distance at which an object is the far point and the near point is called
clearly visible for a normal eye is called the range of vision. For a normal human
the least distance of distinct vision. For a eye, the range of vision is about 25 cm to
normal eye, it is about 25 cm. infinity.
PHYSICS Oasis School Science - 10 97
Reasonable Fact-1
Explain why we cannot see our surroundings clearly when we enter a dark cinema hall
from a bright sunshine, but our vision improves after some time.
Ans: The size of the pupil of our eye is small in the bright sunshine. So, when we enter the
dark cinema hall, very little light enters our eyes through the small pupil, and we cannot see
properly. After some time, the pupil of our eye expands and becomes larger to allow more
light to pass through it, and we can see clearly.
Reasonable Fact-2
An object placed at a distance of less than 25cm cannot be seen by the normal human eye.
What is the reason behind it?
Ans: The maximum accommodation of a normal human eye is reached when an object
is located about 25cm from the eye. After this, the ciliary muscles cannot make the eye
lens bulge more. Therefore, an object placed at a distance of less than 25cm cannot be
seen clearly by the normal eye because all the power accommodation of the eye has already
been used.
5.17 Defects of Vision
The disorder in which a person cannot see nearby or distant objects clearly is called defect of
vision. There are two most common defects of vision which are:
1. Myopia or short sightedness
2. Hypermetropia or long sightedness
1. Myopia
Myopia is the defect of vision in which a person can see nearby objects clearly but cannot
see distant objects. It is also called short sightedness or near sightedness. For such an eye,
the far point is less than infinity.
Causes of myopia
i. Due to the shortening of the focal length of the eye lens or high converging power of the
eye lens
ii. Due to the elongation of the eye balls
In this case, the ciliary muscles attached to the eye lens do not relax sufficiently to make
the eye lens thinner to reduce the converging power. So, the image of a distant object
is formed in front of the retina due to the great converging power of the eye lens. As a
result, the person cannot see distant objects clearly.
myopia /maɪˈəʊpiə/ - the inability to see things clearly when they are far away
98 Oasis School Science - 10 PHYSICS
Eye lens
Image Image
Fig 5.24 (a) An eye affected by myopia Concave lens
Fig 5.24 (b) Correction of myopia
Remedy: Myopia can be corrected by using a concave lens of suitable focal length. When
a concave lens of suitable power is placed in front of the eye having myopia, the parallel
rays of light coming from the distant object are first diverged by the lens. Due to the
combined action of both lenses, the image of the distant object is formed at the retina. As
a result, the person can see the distant objects clearly.
2. Hypermetropia
Hypermetropia is the defect of vision in which a person can see distant objects clearly
but cannot see nearby objects. It is also called long-sightedness or far-sightedness. For
such an eye, the near point is more than 25 cm.
Causes of hypermetropia
i. Due to the increase in the focal length of the eye lens or low converging power of
the eye lens
ii. Due to the contraction of the eye ball
Eye lens Eye lens
Image
Image
Convex lens
(a) An eye affected by hypermetropia (b) Correction of hypermetropia
Fig. 5.25
Remedy: Hypermetropia can be corrected by using a convex lens of a suitable focal
length. When a convex lens of suitable focal length is placed in front of the eye, the rays
of the light coming from a nearby object are first converged by the convex lens and then
by the eye lens. Due to the combined converging action of both the lenses, the image is
formed at the retina. As a result, the person can see nearby objects clearly.
permanent /ˈpɜːmənənt/ - for all time in the future or lasting for a long time
PHYSICS Oasis School Science - 10 99
SUMMARY
• Light is a form of energy which produces the sensation of vision.
• The process of bending of rays of light when they travel from one medium
to another is called refraction of light.
• A lens is a piece of transparent glass bounded by two refracting surfaces
which are usually spherical.
• A lens which is thick in the middle and thin at the edges is called a convex
lens.
• A lens which is thin in the middle and thick at the edges is called a concave
lens.
• Since the rays of light do not actually pass through the focus of a concave
lens, it has a virtual focus.
• An image is a physical likeness or representation of an object produced by
a lens after refraction.
• An image which can be obtained on the screen is called a real image.
• An image which cannot be obtained on the screen is called a virtual image.
• The power of a lens is defined as the reciprocal of its focal length in meters.
• The magnification produced by a lens is defined as the ratio of the size of
the image to the size of the object.
• Instruments that are used to produce the image of an object with the help
of a lens or lenses are called optical instruments.
• In the human eye, the image is formed on the retina. The retina is made up
of two types of photosensitive cells called rods and cones.
• The ability of an eye to focus the image of objects at various distances on the
retina by changing the focal length of the eye lens is called accommodation.
• Myopia is the defect of vision in which a person can see nearby objects
clearly but cannot see the distant objects.
• Hypermetropia is the defect of vision in which a person can see distant
objects clearly but cannot see nearby objects.
100 Oasis School Science - 10 PHYSICS
Exercise
Group-A
1. What is a lens? How many types of lenses are there?
2. Define convex lens with a diagram.
3. What is a concave lens?
4. What is centre of curvature?
5. What is principal axis?
6. Define principal focus.
7. What is focal length?
8. Define optical centre.
9. What is focusing?
10. What is an image? Name the two types of images formed by lenses.
11. What is a real image?
12. Name the type of lens which forms a real image.
13. What is virtual image?
14. Name the type of lens which forms a real image.
15. What is an optical instrument?
16. What is meant by the defect of vision? Name its types.
17. What is myopia or short-sightedness?
18. What is hypermetropia or far-sightedness?
19. Write two main causes of hypermetropia.
20. Write down the main causes of myopia.
21. What formula is used to find the fluctuation of the shape of a body?
22. What is magnification of a lens?
23. Write down the formula of power and magnification of lens.
Group-B
1. What is meant by a virtual image?
2. What is meant by the power of a lens? Write down its formula and SI unit.
3. Write two differences between convex lens and concave lens.
4. Write two differences between real image and virtual image.
5. Write two differences between image formed by a convex lens and image formed by a
concave lens.
PHYSICS Oasis School Science - 10 101
6. Write two differences between myopia and hypermetropia.
7. What is meant by short-sightedness? Draw a neat diagram showing this type of defect.
8. Which lens is used to remove myopia? Why?
9. Why is convex lens called a converging lens? Write with figure.
10. Why is concave lens called a diverging lens? Write with figure.
11. A concave lens is used to remove myopia. Why?
12. Far-sightedness can be removed by using a convex lens. Why?
13. Write two differences between principal focus and focal length.
14. What type of defect is found in the given diagram of the eye? Write with reason.
A
15. In which defect of vision is such lens used? Why?
O
16. If a man with long-sighted eye wants to read a textbook, what should be the distance
between book and the lens? Give reason.
17. Short-sightedness cannot be corrected by using a convex lens, why?
18. Explain with reason the paper burns when sunlight is focused on a piece of paper with
the help of a convex lens.
Group-C
1. What type of defect of vision is shown in the given diagram? Draw a labeled diagram for
the correction of such defect.
2. Focal length of a convex lens is 2 cm. An object of height 1 cm is placed at a distance of
1 cm from the lens. Draw a ray diagram and write the nature of the image formed.
3. What type of defect is found in the given diagram of the eye? State the cause of the defect
and draw a diagram to show how this defect can be corrected.
Retina
Image
4. Write any two applications of a convex lens. A man uses a lens of power 1.5D. Find the
focal length of that lens. (Ans: 0.66)
102 Oasis School Science - 10 PHYSICS
5. Write any two utilities of concave lens. Which type of lens is shown in diagram? Calculate
the power of the lens. (Ans: –50D)
A
FF
2cm
Group-D
1. Draw a clear ray diagram of the image formed when an object is placed 40 cm away from
a lens having the focal length 20 cm. Calculate the magnification of the image. (Ans: 1)
2. Study the given defect of vision in the given figure and answer the following questions.
Retina
Image
i) Name the defect of vision shown.
ii) Write two causes of this defect.
iii) Redraw the diagram showing its correction.
3. A student of last bench of a class cannot read the letters written on the board. Which type
of defect of eye does s/he have? Draw a figure showing such type of defect of vision.
Complete the given ray diagram.
Obejct F O
4. If a man with long-sighted eye wants to read a textbook, what should be the distance
between book and the lens? Give reason. Draw a diagram of a short-sighted eye and with
the defect corrected.
5. Explain with reason when sunlight is focused on a piece of paper with the help of a con-
vex lens, the paper will burn. After examine the eye of a student, a doctor suggested him
to use spectacles of power 1.5D and answer the following questions.
i) What types of defect is there in his eye?
ii) Draw a diagram to show the correction of this defect after using the spectacles.
PHYSICS Oasis School Science - 10 103
6UNIT Estimated teaching periods
Theory 10
Practical 3
CURRENT ELECTRICITY
AND MAGNETISM Michael Faraday
(1791-1867 AD)
Objectives
After completing the study of this unit, students will be able to:
• demonstrate and describe the effects of current electricity.
• introduce electromagnetic and electric appliances of daily use.
• state the safety measures of using electric appliances.
• solve the problems related to electricity tariff and transformer.
• describe the functions of a dynamo, motor and transformer with their diagrams.
• electromagnetic induction and explain its uses.
6.1 Introduction
In today's world, electricity is a vital source of Fact File - 1
energy. Electricity is used in our homes, offices,
industries, laboratories and many other places. One of the important discoveries related
It is used to produce heat and light, to operate to current was made accidentally by Hans
various types of machines, electronic goods, etc. Christian Oersted in 1820 AD. While
When two opposite poles of an electric source, preparing a lecture, he witnessed the
current in a wire disturbing the needle
such as a battery, are connected by a conductor, of a magnetic compass. He discovered
charges flow continuously through the conductor. electromagnetism, a fundamental
Such continuous flow of charges produces a interaction between electricity and
current, which is called electric current. The flow magnetism.
of electrons in a definite direction is called electric
current. When current is passed through a resistance, it causes a heating effect. This fact was
studied by James Prescott Joule in 1840 AD.
6.2 Electric Circuit
An electric circuit is usually a combination of electric components, such as a cell or battery,
switch, bulb, etc. These components are connected by a good conducting wire to allow the
flow of current.
104 Oasis School Science - 10 PHYSICS
Let us observe the following figure:
Switch Cell
Copper wire
Bulb
Fig. 6.1 Electric circuit
The above figure shows an electric circuit having a dry cell (source), a bulb (load), a switch and
connecting wire, i. e., copper wire. A path made by connecting the electric source, load and
switch with a good conducting wire, which can offer a continuous flow of electric current, is
called an electric circuit. It is of two types:
1. Open electric circuit
2. Closed electric circuit
1. Open electric circuit
An electric circuit in which no current is flowing
is called an open electric circuit. In such a circuit, Source
the switch may be turned 'OFF' or there may be
breakage in the wire. Thus, the electric load does Switch
not work in such a circuit. Birds are found sitting off Bulb
on high transmission lines. Because of the open Fig. 6.2 Open circuit
electric circuit, they do not get an electric shock.
2. Closed electric circuit Source Switch (on)
An electric circuit in which electric current is Bulb
flowing continuously is called a closed electric
circuit. In such a circuit, the electric load is in Fig. 6.3 Closed circuit
working condition due to the continuous flow of
electric current.
6.3 AC and DC Fact File - 2
If the current changes direction after equal intervals of The frequency of AC is 50 Hz
time, it is called alternating current (AC). A generator means that 50 cycles are made by
or dynamo produces alternating current. The polarity the current in one second.
of an alternating current is not fixed. Thus, due to the
change of polarity in AC, it has non-zero frequency.
resistor /rɪˈzɪstər/ - a device that has resistance to an electric current
PHYSICS Oasis School Science - 10 105
If the current always flows in the same direction, it is called a direct current (DC). A cell or a
battery produces direct current because the current always flows in the same direction. The
polarity of a direct current is fixed.
Differences between AC and DC
S.N AC S.N. DC
1. The current which changes 1. The current which does not change
magnitude and polarity is called magnitude and polarity is called direct
alternating current (AC). current (DC).
2. It is produced by AC generator, 2. It is produced by cells, battery, DC
dynamo, etc. generator, etc.
3. It can be used in a transformer to 3. It cannot be used in a transformer.
increase or decrease the voltage.
4. It has non-zero frequency. 4. It has zero frequency.
6.4 Domestic Electric Circuit
The AC circuit made in the industries, factories, houses, etc. is called domestic electric circuit.
Electric power generated by a turbine is connected by cables through high transmission lines
at very high voltage. The voltage is decreased to 220 V by using step down transformers
and supplied for household purposes. To operate any electrical appliances, we need two
connecting wires. They are live wire (L) and neutral wire (N). A third wire is also connected in
the power circuit, which is called earthing wire (E). Earthing wire saves us from electric shock
and prevents the flow of excessive current, i.e., overloading through the appliances.
A fuse which is connected before the kilowatt hour (KWh) meter is known as corporation fuse.
This fuse protects the meter from getting damaged. Other fuses which are connected after the
kWh meter are called consumer's fuses. These fuses are generally connected after the main
switch box. The main switch helps to cut off the current throughout the household circuit. The
main switch box is made up of iron, and a connection is made with the iron box (main switch
box) to the earth by a conductor, which is known as earthing. Finally, the wires which emerge
out of the main fuse box are connected to the distribution board as required.
Corporation Consumers' fuse Socket
Fuse L
L N
E
N
Bulb
E Fan
Meter Main Main
box switch box fuse
(kWh meter)
Distribution board
Fig. 6.4 Domestic electric wiring
106 Oasis School Science - 10 PHYSICS
All the electric appliances are connected to a separate circuit
in a parallel manner. So, separate switches are used to control
the flow of current. All the current flows through the fuse of
the meter box. So, it should be of the value 15 A. It is very
important to note here that usually there are separate circuits Symbol of cell
in a house, the lighting circuit with a 6 A fuse and the power
circuit with a 16 A fuse. The lighting circuit is one which Fig. 6.5 (a) Fig. 6.5 (b)
allows less amount of current to flow, but the power circuit
draws more current through it. The lighting circuit is used for
running electric bulbs, tube lights, fans, radio, etc. On the other hand, the power circuit is
used for running, the electric iron, room heater, refrigerator, etc. Each distributive circuit is
provided with a separate fuse so that if a fault like short circuiting occurs in one circuit, its
corresponding fuse blows off, but the remaining circuits remain unaffected.
6.5 Short Circuit
The direct connection of a live and neutral wire in the AC, or positive terminal, and negative
terminal in the DC is called a short circuit. In this condition, the resistance of the so-formed
circuit is very low and hence allows a large amount of current to pass through. It causes
heating in the circuit which may start a fire.
Functions of three different wires
Live (or phase) wire (L) : It carries current to the device from the source.
Neutral wire (N) : It carries current from the device to the source.
Earthing wire (E) : It carries leaked current to the earth.
6.6 Things to be Remembered During Household Wiring
1. Switch should be connected to a live (or phase) wire so that it cuts off the current in the
device when the switch is turned off.
2. Fuse should be connected to a live wire so that it cuts off the overflow of current to the
device since current always flows from the live wire to the device.
3. Fuse should be of proper rating so that the devices are protected fully.
4. Separate fuses should be used for separate flats so that the light bulb and other devices
remain working in other rooms even if a fuse goes off in one flat or room.
5. The lines for light bulbs and power sockets should be separated so that the light bulbs
remain glowing even after a fuse in the power line goes off and vice-versa.
6. Wiring should be done in a dry and safe place so that a short circuit and leakage of
current do not occur.
7. The naked wires or broken wires should be insulated properly with tape.
8. The connection of plugs, switches and sockets should be strong.
9. We should wear rubber gloves while working with electricity or electric appliances to
avoid electric shock.
10. We should use electric appliances and wires of high quality.
PHYSICS Oasis School Science - 10 107
6.7 Colour Code of Wires for Household Wiring
Metallic wires are covered by plastic to prevent the risk of short circuit and electric shock.
Generally, wires of certain colors are used for phase line, neutral line and line for earthing. In
household wiring, red or brown wires are used for phase (live) line, blue or black wires for
neutral line and green or green with yellow stripes are used for earthing. It is done for the
following reasons:
Earthing wire
Neutral wire Live wire
Fig 6.6 Electric wires and color code
(i) It becomes easy to identify the specific wires of underground wiring.
(ii) It helps to know the types of wire (phase, neutral or earthing).
(iii) It makes wiring and maintenance easy and reliable.
Reasonable Fact-1
A fuse is always connected to a live line, why?
Ans: A fuse is always connected to a live line to cut the electric supply in the electric
appliances when the fuse melts.
Reasonable Fact-2
A switch is always connected to a live line, why?
Ans: The switch is always connected to a live line to disconnect the electric supply to
electric appliances when the switch is turned off.
Reasonable Fact-3
An electric switch should not be touched with wet hands, why?
Ans: An electric switch should not be touched with wet hands because water acts as a
conducting layer between the hand and the live wire of the switch through which the
current passes through the body, and the person may get a fatal electric shock.
6.8 Electric Power Consumption
The rate at which electrical work is done or the rate at which electrical energy is consumed, is
called electric power consumption. It is measured in kilowatt hour (kWh). It is calculated by
108 Oasis School Science - 10 PHYSICS
the given formula: Electric power = Work done by electric current
Time taken
or, Work done by electric current = Electric power × Time taken
∴ Electrical energy = Work done by electric current
= Electric power × Time taken
So, the electrical energy consumed by an electrical appliance depends on the following factors:
i. Power rating of the appliance (P)
ii. Number of devices used (N)
iii. Time for which the device is used (t)
∴ The electrical energy consumed = P × N × t
The commercial unit of electrical energy is kilowatt hour (kWh) or unit.
Fact File - 3
One kilowatt hour is the amount of electrical energy consumed by an electrical appliance
having a power rating of 1 kW in 1 hour, [1 kW = 1000 watt]. It is also called one unit power
consumption. For simplicity, kWh is commonly called a ‘unit’.
∴ 1kWh = 1 unit
Worked out Numerical 1
Calculate the electrical energy consumed by 5 tube lights of 40 W each when they are used
for 50 hours.
Solution:
Here, Power (P) = 40 W
= 40 kW
1000
Number of tube lights (N) = 5
Time (t) = 50 h
We have,
Electrical energy consumed = P × N × t
= 40 × 5 × 50
1000
= 10 kWh
∴ Electrical energy consumed = 10 kWh = 10 unit.
PHYSICS Oasis School Science - 10 109
Worked out Numerical 2
In a building, 12 tube lights of 40 W each are used for 4 hours daily, 3 TVs of 40 W each are
used for 4 hours daily, 2 irons of 750 W each are used for 2 hours a week and 2 electric bulbs
of 100 W each are used for 2 hours daily. Calculate the total cost of electricity in one month.
The cost of 1 unit is Rs. 7.
Solution:
i. For tube lights
Electrical energy consumed in 1 day = P × N × t
= 40 × 12 × 4 [∵ 1000 W = 1 kW]
1000
= 1.92 kWh
ii. For TV
Electrical energy consumed in 1 day = P × N × t
= 40 × 3 × 4
1000
= 0.48 kWh
iii. For iron
Electrical energy consumed in 1 week = P × N × t
= 750 × 2 × 2
1000
= 3 kWh
iv. For electric bulbs
Electrical energy consumed in 1 day = P × N × t
= 100 × 2 × 2
1000
= 0.4 kWh
Total electrical energy consumed in 1 month,
= 1.92 × 30 + 0.48 × 30 + 3 × 4 + 0.4 × 30
= 96 kWh
= 96 unit
∴ Total cost in 1 month = Total unit × Rate
= 96 × Rs. 7
= Rs 672.
6.9 Effects of Electric Current
Electricity can be used to produce light, heat and to run various electric appliances. The electric
current can give different effects. Some of them are:
a. Heating effect b. Lighting effect c. Magnetic effect
110 Oasis School Science - 10 PHYSICS
a. Heating effect
When electric current is passed through a high resistance wire, like nichrome wire, the
resistance wire becomes very hot and produces heat. This is called heating effect of
electric current. The wire which is used to produce heat by passing current through it
is called heating element. Most of the heating devices have nichrome wire as a heating
element. Nichrome is an alloy of nickel and chromium metals. It has high resistance and
doesn't oxidize even at a very high temperature of about 900°C. But a tungsten wire gets
oxidized at high temperature and burns out. Similarly, copper wire has low resistance
so it cannot convert electrical energy into heat energy. Owing to these reasons, nichrome
wire is a suitable wire as a heating element.
(a) (b) (c) (d)
Fig. 6.7 Devices having heating element
b. Lighting effect
When current is passed through a wire of high resistance (as in the filament of electric
bulb), it becomes extremely hot and produces light. This effect is called lighting effect of
electric current. Such effect of current is used for lighting electric lamps. Electric lamps
are of two types. They are filament lamp and fluorescent lamp.
i. Filament lamp : When current is passed through the thin glass
wire of an electric bulb, it emits light. The wire is called a
filament. It is made up of tungsten metal. The temperature Filament wire
of the filament becomes 2900°C. Light is produced due to
its very high temperature. The melting point of tungsten Nitrogen or
is about 3400°C. A filament lamp is filled with inert gas, Argon
such as nitrogen, argon or neon, so that no oxidation takes Wire
place. If it is filled with air, the filament wire gets oxidized
due to the reaction in oxygen in the air. A filament lamp Fig. 6.8 Filament lamp
converts 90 % of electrical energy into heat energy and
10 % into light energy. Its average life span is about 1000
hours.
Reasonable Fact-4
Inert gases are filled in a filament lamp, why?
Ans: Inert gases like argon, neon, nitrogen, etc. are filled in an electric bulb to prevent the
filament (made up of tungsten) from vaporizing when heated to a high temperature (about
29000C).
PHYSICS Oasis School Science - 10 111
Reasonable Fact-5
Why is tungsten wire used in a filament bulb? Give any two reasons.
Ans: Tungsten wire is used in a filament bulb because:
(i) It has very high resistivity.
(ii) It has a very high melting point. So, it can be heated upto 34000C to get light.
Reasonable Fact-6
The tungsten filament becomes white hot while passing current through it, but the copper wire
connecting the filament does not. Why?
Ans: The tungsten filament becomes white hot because it has very high resistance, but the
copper wire has very low resistance. So, the copper wire does not become very hot.
ii. Fluorescent lamp : The structure of a fluorescent lamp or tube light is shown in the
figure below. This lamp contains a glass tube filled with mercury vapour. Fluorescent
lamp is also called a mercury lamp because of the mercury vapour in it. When electrons
flow through the vapour, they collide and produce ultra-violet (UV)rays. When the ultra-
violet rays fall on the inner wall of the glass surface (which is coated with fluorescent
powder like zinc silicate or cadmium silicate), visible light is produced. A fluorescent
lamp converts 70% of electrical energy into heat energy and the remaining 30% into
light energy. Its average life span is about 3000 hours. Now-a-days, Compact Fluorescent
Lamps (CFL) are used in place of fluorescent lamps.
Starter
Electrode
Tube light
Fluorescent power A C 220 V
Choke coil
Fig. 6.9 Fluorescent lamp
Fig. 6.10 CFL
Differences between Filament lamp and Fluorescent lamp
S.N. Filament lamp S.N. Fluorescent lamp
1. It is filled with inert gases like 1. It is filled with mercury vapour at low
argon, nitrogen inside the bulb. pressure.
2. It converts 10% of electrical 2. It converts 30% of electrical energy
energy into light energy and 90% into light energy and 70% into heat
into heat energy. energy.
3. Its average life span is about 1000 3. Its average life span is about 3000
hours. hours.
4. It is cheap. 4. It is expensive.
fluorescent /flʊəˈresnt/ - producing bright light by using some forms of radiation
112 Oasis School Science - 10 PHYSICS
Reasonable Fact-7
A fluorescent lamp of power 60 watts gives more brightness than a filament lamp
with the same power, why?
Ans: A filament lamp converts only 10% of the electrical energy into light energy, but
a fluorescent lamp converts 30% of the electrical energy into light energy. Therefore,
a fluorescent lamp of 60W gives more light than a filament bulb of 60W.
Reasonable Fact-8
Mercury vapour is filled in a fluorescent lamp, why?
Ans: Mercury vapour is filled in a fluorescent lamp in order to produce UV-rays. In
this process, when electrons from the cathode strike the mercury atoms, UV-rays are
produced.
Reasonable Fact-9
Why is the inner wall of a fluorescent lamp coated with fluorescent powder?
Ans: The inner wall of a fluorescent lamp is coated with fluorescent powder in order
to produce visible light when UV-rays strike the mercury vapour.
c. Magnetic effect
When current is passed through a wire, it produces a magnetic field around it. This
effect of an electric current is called magnetic effect. It was discovered by Hans Christian
Oersted in 1820 AD. He found that a current carrying conductor was able to deflect a
magnetic needle. Please note that a magnetic needle can be deflected only by a magnetic
field. Therefore, a current flowing through a wire always gives rise to a magnetic field
around it.
Current carrying conductor
N
S
Fig. 6.11 Deflection of magnetic needle
due to current carrying conductor
Magnetic field around a wire
Let us take a straight vertical wire AB which passes through a horizontal cardboard sheet
C. The ends of the wire AB are connected to a battery. When the current is passed through
the wire AB, it produces a magnetic field around it. The magnetic lines of force can be
shown by sprinkling iron filings on the cardboard C. The iron filings arrange themselves
in circles around the wire showing that the magnetic lines of force are circular in nature.
The tangent that is drawn on the circle gives the direction of the magnetic field.
PHYSICS Oasis School Science - 10 113
6.10 Electromagnet
A temporary magnet made by passing an electric Iron nail
current through a solenoid with a core of soft iron is
called an electromagnet. An electromagnet is made by Battery
using a cylindrical form of insulated copper coil having
a large number of close turns called solenoid. When Coil of insulated
current is passed through the coil, a magnetic field is copper wire
produced. If a soft iron rod, called core, is placed inside
a solenoid, the strength of the magnetic field becomes Switch
very large because the iron core gets magnetized by
induction. The combined form of the solenoid and a soft Fig. 6.12 Electromagnet
iron core is called an electromagnet.
To make an electromagnet, an iron rod and insulated Fact File - 4
copper wire are required. The insulated copper wire
should be wound around the iron. When the two The core of an electromagnet must
ends of the copper coil are connected to a battery, an be of soft iron since soft iron loses
electromagnet is formed. all of its magnetism when the
current in the coil is switched off.
An electromagnet is temporary because the solenoid
containing soft iron core in it acts as a magnet as long as the current is flowing in the solenoid.
If the switch is turned off the solenoid no more behaves as a magnet. All the magnetism of the
soft iron core disappears as soon as the current in the coil is switched off.
Activity 1
• Take an iron rod. Make 250 windings of insulated copper wire around it. The coil
should have free ends. Connect the terminals of the coil to the two dry cells connected
in a series.
• Now, bring a small iron nail near the iron rod. Does the iron rod behave as a magnet?
What can you conclude from this activity?
Factors affecting the strength of an electromagnet
i. Total number of turns in the coil: By increasing the number of turns in the coil, the
strength of an electromagnet is increased.
ii. Current in the coil: If the current in the coil is increased, the strength of the electromagnet
increases.
iii. Nature of core material: By using soft iron in the core of the solenoid, the strength of the
electromagnet is increased.
Why is electromagnet highly useful?
Electromagnets are widely used in electric appliances, such as electric bells, loudspeakers,
electric motors, electric fans, etc. because of the following reasons:
a. An electromagnet is temporary in nature. So, the magnetic property can be developed
when required.
114 Oasis School Science - 10 PHYSICS
b. Shape and size of the electromagnet can be altered as required.
c. The strength of an electromagnet can be increased. So, an electromagnet of the desired
power can be made.
Uses of electromagnets
i. Electromagnets are used for constructing electric bells, loudspeakers, electric motors,
electric fans, etc.
ii. They are used to lift and transport heavy loads like big machines, steel girders, etc.
iii. Electromagnets are used for magnetizing steel bars and removing steel splinters from the
eye.
iv. They are used to separate magnetic substances from non-magnetic ones.
6.11 Electric Bell
It is one of the applications of an electromagnet. It converts electrical energy to sound energy.
Electric bells are used in a telephone, office, home, school, etc.
Switch –+ Spring
Battery
Electromagnet Soft iron
armature
Hammer
Fig. 6.13 Electric bell
Working
When the switch of an electric bell is pressed, the circuit is completed, and current flows
through the coil of the electromagnet. It gets magnetized and attracts the iron strip towards
itself. This brings the hammer in contact with the gong, and the sound of the bell is heard.
Owing to the displacement of the iron strip from its original position, it loses contact with the
screw, and the circuit of the electromagnet breaks. At that time, current stops flowing, and the
electromagnet loses its magnetism.
Hence, the iron strip comes back to its original position, which once again completes the
circuit. The entire process is repeated as long as the switch is pressed. The hammer strikes the
gong again and again. In this way, the ringing of the bell is heard.
6.12 Electromagnetic Induction
You have seen that an electric current can produce a magnetic effect as in an electromagnet. Is
the reverse situation possible?
PHYSICS Oasis School Science - 10 115
Coil Conductor
Magnet
B
SN
A
Galvanometer Galvanometer
Fig. 6.14 (a) Fig. 6.14 (b) Electromagnetic induction
Figure 6.14 (a) shows an experiment conducted by Michael Faraday. In this experiment, a coil
of conducting wire is connected to the galvanometer. A galvanometer is a device which is used
to detect current. In this instrument, the zero is at the center of the scale, so the current can be
measured in either directions. When a strong bar magnet is pulled out and pushed into the
coil alternately, the galvanometer needle shows deflection due to induction of current in the
coil. This experiment helped Faraday to put forward his theory of electromagnetic induction,
which finds innumerable applications today. The process of inducing current in a closed coil
due to the relative motion between the magnet and the coil is called electromagnetic induction.
Activity 2 Magnet
S
• Take a match box. Make many turns of an
insulated copper wire around it as shown Match box
in the figure. Connect the galvanometer Turns of wire
with the two ends of the wire. Move a bar
magnet in and out of the box. Is there any Galvanometer
deflection in the galvanometer? Yes, there Fig. 6.15 Electromagnetic induction
is deflection in the galvanometer due to the
electromagnetic induction. The needle of
the galvanometer deflects in the opposite
direction as before and after the magnetic
motion in and out of the box.
6.13 Generator or Dynamo
A generator, or dynamo, is a device which converts mechanical energy (or kinetic energy) into
electrical energy. It works on the principle of electromagnetic induction, i.e., when a straight
conductor is moved perpendicular to the magnetic field, current is induced in the conductor.
A generator produces current on a large scale whereas a dynamo does it on a small scale.
Bicycle dynamo
A dynamo is used in a bicycle to produce current to light a bulb. It works on the principle of
electromagnetic induction, i.e., when a conductor is moved perpendicular to the magnetic
116 Oasis School Science - 10 PHYSICS
field, current is induced in the coil. A dynamo has a Wheel rubs against the tyre
permanent magnet which moves near to a closed coil. to make the magnet turn
Due to the motion, the magnetic flux linked to the coil
changes. As a result, current is induced in the coil. An Dynamo
armature is connected to a tyre of a bicycle to rotate the Magnet
magnet. The current is induced in the bicycle dynamo so Coils of wire
long as there is motion in the tyre. Wire going to lamp
Fig. 6.16 Bicycle dynamo
Activity 3
• Connect the rotating end of the dynamo to the tyre of a bicycle and rotate the tyre at
variable speed. Observe the brightness of the bulb by increasing and decreasing the
speed. What do you observe?
Generator
In a generator, a strong permanent magnet is used. When the
turbine is rotated by external force, the rotor fixed to the magnet
will also rotate. As a result, current is induced in the coil due to
the change in the magnetic flux linked to the coil.
The magnitude of current induced by a generator, or dynamo, Fig. 6.17 Generator
can be increased by the following methods:
i. By increasing the number of turns in the coil
ii. By increasing the speed of the rotation
iii. By decreasing the distance between the magnet and the coil
iv. By increasing the strength of the magnetic field by using a powerful magnet.
6.14 Motor Effect Current S Magnetic lines
of force
Acurrent carrying wire exerts a force on a
magnetic compass and deflects it from its
usual north-south position. So, a current
carrying conductor exerts mechanical
force on a magnet if the magnet is free to N Motion
move. The force can produce a motion Conducting wire
in the magnet. The reverse of this is also
true, i.e., a magnet exerts a mechanical Fig. 6.18 Motor effect
force on a current carrying wire if the
wire is free to move. This force can produce a motion in the wire. This result can be obtained
by applying Newton's third law of motion. If a current carrying conductor exerts a force on a
PHYSICS Oasis School Science - 10 117
magnet, the magnet will exert an equal and opposite force on the current carrying conductor.
In 1821 AD, Michael Faraday discovered that when a current carrying conductor is placed
perpendicular to the magnetic field, the conductor moves. This effect is called motor effect.
When current is passed through a conductor, a magnetic field is developed around it.
Attraction and repulsion occur between the magnetic field of a magnet and that of the current
carrying conductor. As a result, the wire moves. The direction of motion of the conductor can
be found by Fleming's left hand rule.
6.15 Electric Motor
An electric motor is a device which converts electrical
energy into mechanical energy. Every motor has a spindle
which rotates continuously when the current is passed
through it. An electric motor works on the principle of
motor effect.
Fig. 6.19 Electric motor
Fig. 6.20 Internal structure of an electric motor
An electric motor is used in an electric fan, washing machine, refrigerator, mixer grinder, etc.
Fact File - 5
It is to be noted that when current is passed through a generator, or dynamo, it becomes
an electric motor. So, an electrical generator can be run "backwards" to form a motor.
Fact File - 6
An electric motor works on the principle that, when a rectangular coil is placed in a
magnetic field and current is passed through it, a torque (i.e., a twisting force that causes
machinery to rotate) acts on the coil, which rotates it continuously.
118 Oasis School Science - 10 PHYSICS
Differences between a Dynamo (generator) and an Electric motor
S.N. Dynamo (generator) S.N. Electric motor
1. It is a device which converts electrical
It is a device which converts 1. energy into mechanical energy.
2.
3. mechanical energy into electrical It works on the principle of motor effect.
energy. Direction of motion of conductor can be
found by using Fleming’s left hand rule.
It works on the principle of Faraday’s 2.
laws of electromagnetic induction.
Direction of current can be found by 3.
using Fleming’s right hand rule.
6.16 Transformer
A transformer is a device that converts high voltage AC to low
voltage AC of the same frequency and vice versa. It works on the
principle of mutual induction, i.e., whenever the current (magnetic
flux) through a coil changes, an e.m.f. is induced in the neighboring
coil. According to this principle, when the electric current in a closed
circuit (primary coil) changes, the changing current (AC) creates a
changing magnetic field. A second circuit (secondary coil) in reach of
this magnetic field will experience this change in the magnetic field Fig. 6.21 Transformer
as a change in its related magnetic flux. Therefore, an electromotive
force is set up in the second circuit (secondary coil) called induced e.m.f. (or transformer emf).
If the two ends of the secondary coil are connected through an electrical load, current will
flow. A transformer is widely used in electric and electronic devices, such as the radio, TV,
trolley bus, factories, computers, etc.
Structure of a transformer
A transformer consists of two separate coils winding on a A.C. input A.C. output
laminated iron core. The two coils are called primary coil Iron core
and secondary coil. The coil of the transformer in which AC
voltage is given is called primary coil and the coil from which Secondary
AC voltage is taken is called secondary coil. The iron core, Coil
when used in a single block, develops eddy current, and the
Primary Coil
core is heated up. The heat so produced damages the internal Magnetic field in core
insulation besides causing loss of energy. In order to minimize Fig. 6.22 Structure of a transformer
the heating effect, thin insulated metal sheets are adjusted to
make the core of a transformer. These iron sheets are coated with insulating materials like
varnish or shellac. This process is called lamination. The core of a transformer is laminated to
minimize energy loss due to the production of eddy current.
Mutual induction: A changing current (AC) in the first circuit (the primary) creates a changing
magnetic field. This changing magnetic field induces a voltage change in the second circuit
(the secondary). This effect is called mutual induction.
Primary voltage (V1) and Secondary voltage (V2)
The input voltage of a transformer is called primary voltage (V1) and the output voltage is
called secondary voltage (V2). The number of turns in the secondary coil (n2) can be calculated
PHYSICS Oasis School Science - 10 119
by the following relation:
Secondary Voltage(V2 ) = Number of turns in sec ondary coil (n2 )
Pr imary Voltage (V1 ) Number of turns in primary coil (n1 )
∴ V2 = n2
V1 n1
By the appropriate selection of the number of turns, a transformer thus allows an alternating
voltage to be stepped up or stepped down.
A transformer is based on the following two laws:
1. In a transformer, the input power (I1 V1) is equal to the output power (I2V2), where
I1 = input current, V1 = input voltage, I2 = output current and V2 = output voltage.
2. The magnitude of the induced e.m.f. is directly proportional to the ratio of the number
of turns in the secondary coil to the number of turns in the primary coil.
Types of transformer
There are two types of transformers: step up transformer and step down transformer.
a. Step up transformer: The transformer which converts low voltage AC to high voltage
AC is called a step up transformer. Such a transformer has more number of turns in the
secondary coil than in the primary coil, i.e., n2 > n1. It is used in electric grids to save
electrical energy during power transmission through overhead wires. It is also used in
television sets, refrigerators, etc. and for producing X-rays in X-ray tubes.
AC source 220V 11000V
Input Output
Fig.6.23 Step up transformer
b. Step down transformer: The transformer which converts high voltage AC to low voltage
AC is called a step down transformer. Such transformers have less number of turns in the
secondary coil than in the primary coil, i.e., n2 < n1. It is used in radios, cassette players,
TV sets, etc. It is also used at power sub-stations to step down the voltage before its
distribution to consumers.
AC 11000V Output
source 220V
Input
Fig. 6.24 Step down transformer
120 Oasis School Science - 10 PHYSICS
Fact File - 7
In a transformer, the input energy and output energy are equal. The voltage is directly
proportional to the ratio of number of turns.
Fact File - 8
A transformer cannot change the e.m.f. of a dry cell because the magnitude of the DC does
not change.
Activity 4
• Open the outer cover of a step down transformer which is not in use and find out its
working principle and structure.
Differences between Step up transformer and Step down transformer
S.N. Step up transformer S.N. Step down transformer
1. It converts low voltage AC into 1. It converts high voltage AC into low
high voltage AC. voltage AC.
2. The number of turns in the 2. The number of turns in the secondary coil
secondary coil is more than that in is less than that in the primary coil.
the primary coil.
3. It is used in power transmission 3. It is mostly used in household devices
lines to increase voltage. which run at less than 220V.
Worked out Numerical 3
Calculate the number of turns in the secondary coil if a transformer is connected to the AC
main of 220V to produce 12V with the primary coil of 1000 turns. Also, write down the type
of the transformer.
Solution:
Given, Primary voltage (V1) = 220 V
= 1000
No. of turns in the primary coil (n1) = 12 V
Secondary voltage (V2) =?
No. of turns in the secondary coil (n2)
We know, V2 n2
V1 n1
=
or, n2 = V2 × n1
V1
= 12 ×1000
220
= 54.5
≈ 55 turns
solenoid /ˈsəʊlənɔɪd/ - a piece of wire, wound into circles, which acts as a magnet when carrying an electric current
PHYSICS Oasis School Science - 10 121
Hence, the number of turns in the secondary coil is 55 .
The type of transformer is step down because secondary voltage is less than the primary
voltage.
Worked out Numerical 4
The number of turns in the secondary coil is 150 times less than that in the primary coil. If
the secondary voltage is 220V, calculate the primary voltage.
Solution:
Given,
Secondary voltage (V2) = 220V
Primary voltage (V1) = ?
Let the number of turns in the secondary coil (n2) be x
Then number of turns in the primary coil (n1) is 150x
We know,
∴ Primary voltage = 33000 V.
V2 = n2
V1 n1
or V1 = V2 n1 = 220 ×150x
n2 x
= 33000V
Worked out Numerical 5
The number of turns in the primary coil of a transformer is double that of the secondary
coil. Calculate the inpur voltage if 220V is generated by the secondary coil.
Solution:
Given,
Let, number of turn in secondary coil (n2) be x
Then number of turns in primary coil (n1) is 2x
Input voltage (V1) = ?
Output voltage (V2) = 220V
V1n2 = V2n1
or, V1 = V2n1 = 220×2x = 440
n2 x
∴ The input voltage (V1) = 440V.
splinter /ˈsplɪntə/ - a small thin sharp piece of metal, wood, etc. that is broken off a larger piece
122 Oasis School Science - 10 PHYSICS
6.17 Battery Charger
A battery charger is a device used to put energy into a secondary
cell or rechargeable cell by forcing an electric current through it.
It is commonly used to recharge the battery of a mobile phone,
auto mobile and other electronic equipment.
The charging protocol (how much voltage or current for how
long, and what to do when charging is complete) depends on the
size and type of battery being charged. Fig. 6.25 Battery charger
A simple charger works by supplying a constant DC or pulsed
DC power source to a battery being charged. A simple charger typically does not alter its
output based on charging time or the charge on the battery. Solar chargers convert light energy
into DC current.
There is a cut-off system inside the battery charger, which cuts the supply of electricity when
the battery is fully charged.
6.18 Inverter
A power inverter, or inverter, is an electronic device that changes alternating current (AC) into
direct current (DC) and direct current (DC) to alternating current (AC).
Fig. 6.26 Inverter
The input voltage, output voltage and frequency, and overall power handling depend on
the design of the specific device. The inverter does not produce any power, the power is
provided by the source. A power inverter can be entirely electronic or may be a combination
of mechanical effects and electronic circuitry.
A typical power inverter device requires a relatively stable DC power source capable of
supplying enough current for the intended power demands of the system. The input voltage
depends on the design and purpose of inverters. For example;
– + AC appliance
Battery
+ Socket
Inverter
–
Switch
Fig. 6. 27
PHYSICS Oasis School Science - 10 123
12 VDC (Volt direct current) is for smaller consumer and commercial inverters that typically
run from a rechargeable 12V lead acid battery. Similarly, 24, 36 and 48 VDC are common
standards for home energy systems. An inverter is used during load shedding or when the
main supply of electricity is cut off.
SUMMARY
• A path made by connecting the electric source, load and switch with a good
conducting wire, which can offer a continuous flow of electric current, is called an
electric circuit.
• An instrument that works with the help of an electric current is called an electric load.
• The current which has fixed magnitude and polarity is called direct current (DC).
• The current which changes its magnitude and polarity is called alternating current (AC).
• The direct connection of the live wire and neutral wire in AC, or positive terminal,
and negative terminal in DC is called short circuit.
• The amount of electrical energy consumed when an electrical appliance rating of
1 kW is used for 1 hour is called 1kWh, or 1 unit.
• The wire which is used to produce heat by passing current through it is called
heating element. It is made of nichrome wire.
• A temporary magnet which is made by using an electric current is called an
electromagnet.
• The process of producing current in a closed coil due to the relative motion between
the magnet and coil is called electromagnetic induction.
• A generator, or dynamo, is a device which converts mechanical energy into
electrical energy.
• When a current carrying conductor is placed perpendicular to a magnetic field, the
conductor moves. This effect is called motor effect.
• An electric motor is a device which converts electrical energy into mechanical energy.
• A transformer is a device that converts high voltage AC into low voltage AC or vice
versa.
• There are two types of transformers. They are step up transformer and step down
transformer.
• A battery charger is a device used to put energy into a secondary cell or rechargeable
cell by forcing an electric current through it.
• A power inverter, or inverter, is an electronic device that changes direct current (DC)
to alternating current (AC).
• An inverter is used during load shedding or when the main supply of electricity is
cut off.
124 Oasis School Science - 10 PHYSICS
Exercise
Group-A
1. What is current electricity?
2. Write any two effects of current electricity.
3. What is heating effect of current electricity?
4. What is generator?
5. Define transformer.
6. What is the magnetic effect of current electricity?
7. What is filament? Which material is used as filament?
8. Which principle is dynamo based on?
9. Define step-up transformer.
Group-B
1. Write two differences between tungsten and nichrome.
2. State any two differences between step up and step down transformer.
3. The filament lamp of 40 W gives more light than the fluorescent lamp of 40 W, why?
4. What is the cause of filling the tube of fluorescent lamp with mercury vapour and
filament bulb with nitrogen gas?
5. The coil of the transformer is coated by enamel, why?
6. Write any two differences between dynamo and generator.
7. The number of turns in primary and secondary coil of a transformer is never made equal,
why?
8. Nichrome wire is used as a heating element by not tungsten, why?
9. Write any two differences between inverter and charger.
Group-C
1. Write any three advantages of heating effect of current electricity.
2. Write any three methods of increasing the electricity produced by generator.
3. An electric bulb is rated 60W. When such 7 bulbs are used 2 hours a day, calculate the
monthly bill for 30 days. The cost of 1 unit is Rs. 10. (Ans: Rs. 252)
4. Write any three safety measures that should be adopted while using electricity.
5. How does a bicycle dynamo work? Describe in brief.
PHYSICS Oasis School Science - 10 125
Group-D
1. Describe the working mechanism of an electric bell with a figure.
2. Describe the structure of a step up transformer with a labeled figure.
3. What happens when oxygen gas is used into the filament bulb? 10 tube lights of 20 W
each are used in a house for 5 hours daily, calculate the bill of one month for the electricity
with the rate of Rs. 7.30 per unit. (Ans: Rs. 4158)
4. Name the type of transformer which is used to play a radio with 12 volts in the electric
line of 240 volts. An electric bulb draws a current of 0.8A from 220 V main line. The bulb
is used 8 hours daily, calculate the total bill for 30 days, if per unit costs Rs. 7.0.
(Ans: Rs. 295,68)
5. Describe in brief the working mechanism of an inverter. The number of turns in the
primary winding of a certain transformer is 150 times more than that in the secondary
winding. Calculate the input e.m.f. in the primary winding if the e.m.f. generated in the
secondary winding is 220V AC. (Ans: Rs. 409.09 turns)
6. Draw a neat figure showing the structure of a transformer which is used to play a radio.
Name the type of the transformer shown in the given figure. Calculate the secondary
voltage of this transformer. (Ans: 222.22V)
1000V 11000V Output=?
450 turns 100 turns
126 Oasis School Science - 10 PHYSICS
Part 2 : Chemistry
Scope and sequence of the subject matter
Area Unit Syllabus
Change of Matter
7. Classification • Periodic law
of Elements • Modern Periodic Table
8. Chemical • Types of chemical reaction
Reaction • Rate of chemical reaction, causative factors, catalyst
9. Acid, Base and • Properties and uses of Acid, Base and Salt
Salt • Neutralization reaction and its utility in daily life
Matter Around Us 10. Some Gases • Preparation, properties and uses of Carbon dioxide and
Ammonia gas
11. Metal • Occurrence, properties and uses of main metals (Iron,
Aluminium, Copper, Silver and Gold)
12. Hydrocarbon • Hydrocarbons and compounds formed by hydrocarbon
and its • Methane, alcohol, glycerol, glucose
Compounds
13. Materials Used • Cement, Glass, Fibre, Ceramics, Plastics, Soap, Detergent,
in Daily Life Insecticide
• Types of Nitrogen, phosphorus and potassium fertilizers
and their uses
• Chemical pollution (Synthetic cleanser, plastics, chemical
fertilizers, synthetic colours and feed and insecticides
• Biodegradable and non-biodegradable wastes
SEE Specification Grid-2074
S.N. Unit (K) (U) (A) (HA) Total Remarks
7. Classification of Elements 4×1 4×2 2×3 1×4 22
8. Chemical Reaction
9. Acid, Base and Salt
10. Some Gases
11. Metals
12. Hydrocarbon and its Compounds
13. Materials Used in Daily Life
CHEMISTRY Oasis School Science - 10 127
7UNIT Estimated teaching periods
Theory 4
Practical 0
CLASSIFICATION
OF ELEMENTS Mendeleev
Objectives (1834–1907 AD)
After completing the study of this unit, students will be able to:
• explain Modern and Mendeleev's Periodic Law and their table.
• describe atomic structure and electronic distribution.
• explain position of elements in Modern Periodic Table.
7.1 Introduction
The classification of elements into various blocks, periods and groups according to their
characteristics is considered a significant achievement in the history of chemistry. Previously
very few elements were discovered. So, individual study of these elements was possible. With
the development of science and human knowledge, many new elements were discovered.
Thereafter, the study of an individual element became difficult. To make study easy, fast
and clear, chemists felt the need of grouping these elements. About 118 elements have been
discovered till today and they have similarities as well as dissimilarities among each other.
So, the grouping of elements on the basis of their similarities and dissimilarities is the called
classification of elements.
Many chemists have made attempts to classify the known elements. Among them Antoine
Lavoisier was the first person who classified all the 32 known elements into metals and
non-metals. Since then many systematic classifications have been given by various scientists.
7.2 Contribution of Different Chemists in Classification of Elements
1. John Dalton's atomic theory
In 1802 AD, Dalton propounded the atomic theory and said that the atoms of different
elements have different atomic weight.
2. Prout's hypothesis
In 1815 AD, Prout propounded a hypothesis which stated, "The atomic weight of an
element is the simple multiple of the atomic weight of a hydrogen atom." After some time
it was found that the atomic weight of all the atoms was not in fixed whole numbers. So,
Prout's hypothesis could not hold much importance.
element / ˈ e l ɪ m ə n t / - chemical substance that consists of atoms of only one type
128 Oasis School Science - 10 CHEMISTRY
3. Dobereiner's law of triads
A German chemist, John Dobereiner (1829 AD), classified elements on the basis of
the relationship between atomic weights of certain elements and their properties. He
arranged elements in increasing order of their atomic weights in a well marked group of
three elements called triads. In each triad, the atomic weight of the middle element was
found to be the arithmetic mean of the atomic weights of the other two elements.
Elements Lithium (Li) Sodium (Na) Potassium (K)
Atomic weights 7 23 39
In the triad of Li, Na and K, the atomic weight of 'Na', i.e., 23, is the arithmetic mean of
the atomic weights of Li and K, i.e., 7 + 39 = 23.
2
4. Newland's law of octaves
In 1863 AD, John Newland arranged elements on the basis of their increasing atomic
masses. He found that the properties of every eighth element were similar to the properties
of the first element. This repetition of the properties of elements is similar to the repetition
the eighth note in an octave in music. So his law is called the ‘law of octaves’.
Newland's law of octaves states, "Elements when arranged in increasing order of their
atomic weights show resemblance in physical and chemical properties between the
eighth and the first element similar to the eighth and first notes on a musical scale."
Li Be B C N O F
Sa Re Ga Ma Pa Dha Ni
Na Mg Al Si P S Cl
Sa Re Ga Ma Pa Dha Ni
K Ca
Sa Re
In this classification, if lithium is the first element, then sodium will be eighth element.
Similarly, potassium is the eighth element after sodium. These elements have similar
physical and chemical properties.
5. Lothar Meyer's curve Atomic volume Cs
Rb
In 1869 AD, German chemist Lothar
Meyer plotted a graph of atomic volume K
versus atomic mass of various elements. Na
He found that elements having similar
properties were in a similar position in
the graph.
From the above graph, it becomes clear Li
that alkali metals (Li, Na, K, Rb, and
Cs) having similar properties occur in a Atomic mass
similar positions, i.e., in the peak position (Lothar Meyer's Curve)
of the graph.
octave / ˈ ɒ k t ɪ v / - the difference between the first and the last notes in a series of eight notes on a scale
CHEMISTRY Oasis School Science - 10 129
7.3 Periodic Table
The table, or chart, which is obtained after arranging elements on the basis of similarities
and dissimilarities is called the periodic table. It is called the periodic table because elements
having similar characteristics are repeated after a certain interval in the table. In the periodic
table, there are horizontal rows, i.e., periods and vertical columns, i.e., groups.
7.4 Mendeleev's Periodic Table
Russian chemist Dmitri Ivanovich Mendeleev studied the physical and chemical properties
of 63 known elements and their compounds. After the study, he arranged all the known
elements on the basis of increasing atomic weights. As a result, he found that elements with
similar properties occur at regular intervals. On the basis of the above observation, Mendeleev
formulated the periodic law in 1869 AD. According to Mendeleev's periodic law, "The
physical and chemical properties of elements are a periodic function of their atomic weights."
Periodic function means, if elements are arranged in the order of increasing atomic weights,
the properties of the elements go on changing with their atomic weights. But after a certain
interval, they repeat the properties of the previous elements and fall one below another in the
same vertical column. With the help of their above periodic law, Mendeleev arranged elements
according to increasing atomic weights. Hence, he found a table known as Mendeleev's periodic
table. In Mendeleev's periodic table, elements are classified into horizontal rows called periods
and vertical columns called groups. Elements with similar properties were in the same group
and elements with gradual change in properties were in the same period. In Mendeleev's
periodic table, the periods 4, 5 and 6 were divided into two halves to place more elements. Half
of the elements were placed in the upper left corner and half of the elements were placed in the
lower right corner of each box.
Many elements were not discovered when Mendeleev formed the periodic table. So, he left
some gaps for the undiscovered elements and proposed names for them, e.g., Eka–Boron,
Eka–Aluminium, Eka–Silicon, Eka–Manganese, etc.
A part of Mendeleev's periodic table
Group I Group II Group III Group IV Group V Group VI Group VII Group VIII
H
Period 1 Li Be B C N O F
Period 2 Na
Period 3 K Mg Al Si P S Cl
Period 4
Ca 1* Ti V Cr Mn Fe Co
Period 5 Rb Cu Zn 2* 3* As Se Br Ni Rh
Sr Y Zr Nb Mo 4* Ru
Ag Cd In Sn Sb Te I Pd
Ba La Hf Ta W Re Ir
Period 6 Cs Os
Au Hg Th Pb Bi Po At Pt
Name given by Mendeleev: 1* Eka - Boron,
2* Eka - Aluminium, 3* Eka - Silicon, 4* Eka - Manganese
periodic table /ˌpɪərɪˈɒdɪk ˈteɪbl/ - a list of all the chemical elements, arranged according to their characteristics
130 Oasis School Science - 10 CHEMISTRY
Merits of Mendeleev's periodic table
1. Mendeleev's periodic table was the first scientific and systematic study of all known 63
elements.
2. In Mendeleev's periodic table, there were many gaps for undiscovered elements. After
their discovery, they got their proper place in the table.
3. Mendeleev's periodic table helped to correct the doubtful atomic weights of some
elements on the basis of their position in the periodic table, e.g., gold (Au), platinum
(Pt), uranium (U), etc.
4. With the help of Mendeleev's periodic table, properties of several elements were studied
correctly.
Demerits of Mendeleev's periodic table
1. Mendeleev could not arrange hydrogen properly because it shows the properties of both
alkali metals and halogens.
2. Mendeleev could not arrange isotopes because they have different atomic weights but
the same atomic number, e.g., 12 C, 13 C,164 C .
6 6
3. Mendeleev could not arrange Lanthanides and Actinides.
4. In Mendeleev's periodic table, some elements do not obey the periodic law. In some
places, elements having more atomic weight were placed before those of less atomic
weight. For example, 582.79Co was placed before 582.86Ni .
5. In Mendeleev's periodic table, more reactive alkali metals (Li, Na, K, etc.) and less
reactive coinage metals (Cu, Ag, Au) were placed together in the same group.
6. The eighth group of Mendeleev's periodic table was not divided into sub-groups.
7. Inert gases or noble gases were absent.
7.5 Modern Periodic Table
A group of chemists led by Henry Moseley studied the chemical and physical properties of
all known elements. They found that properties of elements depended more correctly on
the atomic number than on atomic weight. He said that atomic number is the fundamental
property of the atom. Afterward, they proposed a new law called the modern periodic law.
Modern periodic law states that, "Physical and chemical properties of the elements are a
periodic function of their atomic numbers." After the discovery of the modern periodic law,
elements were arranged on the basis of increasing atomic numbers. As a result, they found that
elements having similar properties fall one after another in the same vertical column called
group. Elements having gradual change in the properties were in the horizontal row called
period. Hence, a more correct table than Mendeleev's table was obtained, which is known as
the modern periodic table. So, "The table which is obtained after arranging elements on the
basis of increasing atomic numbers is called the Modern periodic table."
coinage /ˈkɔɪnɪdʒ/ - the coins of a particular type
alkali /ˈalkəlaɪ/ - the base that dissolves in water
CHEMISTRY Oasis School Science - 10 131
132 Oasis School Science - 10 CHEMISTRY s-Block Modern Periodic Table 0/18
IA/1
p-Block
1
IIIA/13 IVA/14 VA/15 VIA/16 VIIA/17
IIA/2
2
d-Block
3
IIIB/3 IVB/4 VB/5 VIB/6 VIIB/7 VIII/8 VIII/9 VIII/10 IB/11 IIB/12
4
5
6
7
Lanthanide series
Actinide series
f-Block
Alkali Alkaline Transition Basic Metalloids Non Halogens Noble Lanthan- Actinides
metals earth metals metals metals gases ides
metals
Correction of defects of Mendeleev's Periodic Table by Modern Periodic Table
In the Modern periodic table, elements have been arranged on the basis of increasing atomic
numbers. As a result, all the defects of Mendeleev's periodic table were removed. The defects
and their reasonable corrections are given below.
1. Position of Hydrogen: Since hydrogen has the least atomic number, i.e., 1, it is kept in
group 1 of the modern periodic table, but its position is still controversial due to its dual
characteristics, i.e., characteristics of group 1 and group 17.
2. Position of Isotopes: Isotopes are elements having the same atomic number but different
atomic weights, e.g., 12 C, 13 C, 14 C . So, without any doubt, all isotopes of one element are
kept in one place. 6 6 6
3. Position of Lanthanides and Actinides: Elements of Lanthanide series and elements of
Actinide series are kept below the main block of the periodic table as they have different
properties from other elements of the periodic table.
4. Correction of Periodic Law: Some elements do not obey Mendeleev's periodic law but
when they are arranged according to atomic number they obey the modern periodic law.
For example, 3198.9Ar is placed before 391.91K, 582.79Co is placed before 58.6 Ni .
18
5. Position of Alkali metals and Coinage metals: In the modern periodic table, alkali
metals (Li, Na, K, etc.) are kept in group IA and coinage metals (Cu, Ag, Au) are in group
IB of the periodic table.
6. In the modern periodic table, the eighth group is divided into three vertical columns.
Differences between Modern Periodic Table and Mendeleev's Periodic Table
S.N. Modern Periodic Table S.N. Mendeleev's Periodic Table
1. It is based on increasing atomic 1. It is based on increasing atomic
number. weight.
2. In this table, there are eighteen 2. In this table, there are eight vertical
vertical columns. columns.
3. In this table, there is fixed 3. In this table, there is no fixed position
position for hydrogen, isotopes, for hydrogen, isotopes, lanthanides
lanthanides and actinides. and actinides.
Position of elements in modern periodic table
1. Position of hydrogen: Hydrogen having the least atomic number, i.e., 1 is placed in
group 1 of the modern periodic table. But it also shows the properties of group 17 of the
periodic table.
Reasonable Fact-1
Write any two reasons why hydrogen is kept in group 1 in the periodic table.
Ans: Two reasons for keeping hydrogen in group 1 in the periodic table are:
(i) Hydrogen has one electron in its shell.
(ii) Hydrogen can lose one electron like other elements of group 1.
CHEMISTRY Oasis School Science - 10 133
Reasonable Fact-2
Alkali metals are more reactive, why?
Ans: The atoms of alkali metals have only one electron in their outermost orbit. Therefore,
alkali metals are more reactive to obtain octet state.
2. Position of metals, non-metals and metalloids: Metals are placed on the left side of
the modern periodic table. For example, Alkali metals (Li, Na, K, Rb, Cs, and Fr) are
kept in group 1. Alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra) are kept in group 2
of the periodic table. The metals of group 1 are called alkali metals because they react
with water and produce alkalis. Similarly, the metals of group 2 are called alkaline earth
metals because they are found on the surface of the earth in the form of oxides and react
with water to produce alkalis.
Non-metals are placed on the left side of the modern periodic table. For example,
halogens (F, Cl, Br, I and At) are placed in group 17. The elements of group 17 are called
halogens because they react with hydrogen to form acids, from which simple salts like
NaCl, KCl, KBr, etc can be produced. Metalloids (like Si, Ge, As, Te, etc.) are placed in
between metals and non-metals.
3. Position of Lanthanides and Actinides: Elements of Lanthanides and Actinides series,
having different properties from other elements of the periodic table, are kept in f-Block
below the main table in a separate box. The 14 elements from cerium (58Ce) to lutetium
(71Lu) after lanthanum (57La) are called Lanthanides and other 14 elements from thorium
(Th90) to lawrencium (103Lr) after actinium (89Ac) are called Actinides.
Lanthanides are rare earth metals whereas Actinides are radioactive metals. The
properties of Lanthanides resemble each other very closely but are different from the rest
of the elements due to the preferential filling of f-orbitals. So, they are placed separately
to avoid unnecessary sidewise expansion of the periodic table.
7.6 Periods and Groups of Modern Periodic Table
Periods
In the modern periodic table, there are seven horizontal rows, which are called periods. In a
period, elements having gradual change in properties are placed one after another. According
to the number of elements, periods of the modern periodic table are divided into four groups.
They are very short, short, long and very long.
S.N. Period Number of elements Types of period
1. First 2 Very short period
2. Second 8 Short period
3. Third 8 Short period
4. Fourth 18 Long period
halogen /ˈhaelədʒən/ - a set of five elements of groupVIIA, viz. F, Cl, Br, I and At
metalloid /ˈmetləɪd/ - a chemical element which has properties both of metals and non-metals
134 Oasis School Science - 10 CHEMISTRY
5. Fifth 18 Long period
6. Sixth 32 Very long period
7. Seventh 26 Very long but
incomplete period
Groups
In the modern periodic table, there are a total of 18 vertical columns (IUPAC system), which
are called groups. In a group, elements having similar properties are placed one below the
other.
In the modern periodic table, representative elements are kept in groups 1, 2, 13, 14, 15, 16
and 17. These elements are called representative elements, or normal elements, as they have
only one incomplete outer shell. Noble gas elements are kept in 0 (zero) group of the Modern
periodic table. Similarly, transitional elements are kept in groups 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12.
Differences between Periods and Groups
S.N. Periods S.N. Groups
1. Horizontal rows of the periodic ta- 1. Vertical columns of the periodic table are
ble are called periods. called groups.
2. In periods, the atomic size of 2. In groups, the atomic size of elements
elements decreases while moving increases while moving from top to
from left to right. bottom.
3. Valency of elements increases first 3. Valency of elements remains the same in
(upto group IV) and then decreases a group.
(upto group 0).
4. The elements of the same period 4. The elements of the same group have
have different properties. similar properties.
7.7 Sub-shell
Each and every main shell contains one or more than one sub-shell, which are denoted by s,
p, d and f.
The main shells along with their sub-shells are listed in the given box.
Main Shells Sub-shells (orbitals)
K (n=1) s
L (n=2) s and p
M (n=3) s, p and d
N (n=4) s, p, d and f
orbital /ˈɔːbɪtl/ - connected with the path of electrons around the nucleus of an atom
CHEMISTRY Oasis School Science - 10 135
The maximum number of electrons that can be accommodated by each sub-shell is given
below:
Sub-shells (orbitals) Maximum number of electrons
s (sharp) 2
p (principal) 6
d (diffuse) 10
f (fundamental) 14
The above data show that K shell (n = 1) contains only one sub-shell (1s) with maximum two
electrons. L shell (n = 2) contains two sub-shells (2s and 2p) with maximum eight electrons. M
shell (n = 3) contains three sub-shells (3s, 3p and 3d) with maximum eighteen electrons. N shell
(n = 4) contains four sub-shells (4s, 4p, 4d and 4f) with maximum thirty two electrons. The last
electron present in a sub-shell determines the block of an element.
Aufbau principle
This principle was given by Wolfgang Pauli and Niels Bohr in the early 1920s. The different
sub-shells of an atom have different energy. Electrons always try to enter the sub-shell which
has less energy.
Aufbau principle states, "The electrons in an atom are so distributed that they occupy shells
in the order of their increasing energy." It means that the shells having low energy are filled
faster than the shells having high energy in the following sequence.
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p.
136 Oasis School Science - 10 CHEMISTRY
Table: Electronic configuration of some elements on the basis of sub-shells (s, p, d, and f) is
given below:
Elements Atomic K L M N O P
No. 1s 2s 2p 3s 3p 3d 4s 4 p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f
H 11
He 2 2
Li 3 2 1
Be 4 2 2
B 5 22 1
C 6 22 2
N 7 22 3
O 8 22 4
F 9 22 5
Ne 10 2 2 6
Na 11 2 2 6 1
Mg 12 2 2 6 2
Al 13 2 2 6 2 1
Si 14 2 2 6 2 2
P 15 2 2 6 2 3
S 16 2 2 6 2 4
Cl 17 2 2 6 2 5
Ar 18 2 2 6 2 6
K 19 2 2 6 2 6 1
Ca 20 2 2 6 2 6 2
Sc 21 2 2 6 2 6 1 2
Cr 24 2 2 6 2 6 5 1
Fe 26 2 2 6 2 6 6 2
Ni 28 2 2 6 2 6 8 2
Cu 29 2 2 6 2 6 10 1
Ag 47 2 2 6 2 6 10 2 6 10 1
7.8 Classification of Elements Based on Electronic Configuration
After the discovery of sub-shells, elements were divided into four groups. They are:
1. s - Block elements
2. p - Block elements
3. d - Block elements
4. f - Block elements
CHEMISTRY Oasis School Science - 10 137
Period s-Block p-Block elements 0
n=1 elements
n=2 IIIA/13 IVA/14 VA/15 VIA/16 VIIA/17 18
n=3 IA/1
IIA/2
n=4
n=5 d-Block elements
n=6
n=7 IIIB/3 IVB/4 VB/5 VIB/6 VIIB/7 VIII IB/11 IIB/12
10
89
Lanthanide series (58 to 71)
Actinide series (90 to 103)
f-Block elements
s-Block elements
The elements whose last electron enters into s-orbital are called s-Block elements. The
elements of groups 1 and 2 are s-Block elements. Either one or two valence electrons are filled
in s-orbital of s-Block elements. s-Block is located on the left side of the Modern periodic
table. The elements of s-Block form electropositive ions by losing one or two electrons. All the
elements of s-Block are metals except H and He. Example: Sodium (Na) is kept in s-Block as
its last electron enters the s-orbital, i.e., 11Na–1s2,2s2 2p6, 3s1.
p-Block elements
The elements whose last electron enters the p-orbital of the outermost shell are called p-Block
elements. The elements of groups 13, 14, 15, 16, 17 and elements of zero (0) group or group 18
except Helium are p-Block elements. p-Block is situated on the right side of Modern periodic
table. This block consists of metals, metalloids and non-metals. Example: Aluminium (Al) is
kept in p-Block as its last electron enters into p-orbital, i.e. 13Al– 1s2, 2s2 2p6, 3s2 3p1.
Reasonable Fact-3
Argon atom can exist freely in nature, why?
Ans: The outermost orbit of Argon atom is filled by having eight electrons. Due to this, it does not
take part in chemical reaction. Therefore, argon atom can exist freely in nature.
d-Block elements
Elements whose last electron enters the d-orbital of the penultimate shell are called d-Block
elements. This block consists of elements of group 3 to 12. d-Block is located between s-Block
and p-Block. The elements of d-Block are called transitional metals as they are present between
s-Block and p-Block. Example: Copper (Cu) is kept in d-Block as its last electron enters the
d-orbital, i.e., 29Cu–1s2 , 2s2, 2p6 , 3s2 , 3p6, 3d10, 4s1.
138 Oasis School Science - 10 CHEMISTRY
f-Block elements
Elements whose last electron enters the f-orbital of the ante-penultimate shell are called f-Block
elements. The elements of lanthanide and actinide series are kept in f-Block. This block is
located at the bottom of Modern periodic table just below the d-Block elements. The properties
of f-Block elements are similar to the transition metals of d-Block. However, their last electron
enters f-orbital. Therefore, f-Block elements are kept separately below the d-Block elements or
transitional elements. So, f-Block elements are also called inner-transition elements.
7.9 Characteristics of Periods and Groups
1. Atomic radius
Atomic radius is the distance between the outermost shell and the center of the nucleus
of an atom. It determines the size of an atom. It is measured in picometer (pm) [1m = 10-12
pm]. The atomic radius (atomic size) decreases on moving from left to right in a period
of the Modern periodic table.
Elements of 3rd Na Mg Al Si P S Cl
period (2, 8, 1) (2, 8, 2) (2, 8, 3) (2, 8, 4) (2, 8, 5) (2, 8, 6) (2, 8, 7)
Atomic radius (pm) 157 136 125 117 110 104 99
Na Mg Al Si P S Cl
Atomic size decreases
The atomic number of elements increases as we move from left to right in a period. It
means that the number of protons and electrons increases, but extra electrons are added
into the same shell. As a result, the electrons are pulled towards the nucleus due to more
positive charge in the nucleus of the atom present towards the right side of the period. Due
to this, the atomic size of elements decreases on moving from left to right in a period.
The atomic radius of elements increases gradually on moving from top to bottom in a
group of the Modern periodic table.
Group 1 Atomic radius (pm) Atomic size increases while
moving down in a group
Li 123 Li The smallest atom
Na 157 Na
K 202 K
Rb 216 Rb
Cs 235 Cs
Fr 256 Fr The biggest atom
CHEMISTRY Oasis School Science - 10 139
As we move down in a group, a new shell is added to the atom at every step. As a
result, the size of the atom increases gradually from top to bottom of the same group.
However, as we move down in a group, the nuclear charge also increases, but the
nuclear attraction becomes less as compared to the increase in atomic size. The smallest
atomic size is found at the top of a group and the biggest atomic size is found at the
bottom of the same group.
2. Ionization energy or nuclear power
The amount of energy required to remove the outermost loosely bound electron of an
isolated gaseous atom is called ionization energy. It increases in a period on moving
from left to right but decreases in a group on moving from top to bottom.
Period Ionization energy increases from left to right in a period
Group
Ionization energy decreases from top to bottom in group
3. Valence electrons
The electrons present in the outermost shell (or valence shell) of an atom are called
valence electrons. Number of valence electrons remains the same in a group, but it
increases continuously in a period.
Period Number of valence electrons increases from left to right in a period
Group
Number of valence electrons remains the same in a group
4. Valency
The total number of electron(s) gained, lost or shared by atoms during a chemical reaction
is called valency. In a period, the valency increases from 1 to 4 and then decreases to zero
(0). It is because the number of valence electrons increases from 1 to 8 on moving from
left to right in a period. But in a group, the valency of all elements remains the same as
they have equal number of valence electrons in the same group.
Period Valency increases from 1 to 4 and then decreases to zero
Group
Valency of elements remains the same in a group
5. Electronegativity
The amount of energy required for an atom to attract the outer electron of another atom
is called electronegativity (EN). It increases on moving from left to right in a period and
decreases from top to bottom in a group.
metallic /məˈtae lɪk/ - connected with metals
140 Oasis School Science - 10 CHEMISTRY
Period Electronegativity increases from left to right in a period
Group
Electronegativity decreases from top to bottom in a group
6. Metallic character
Metallic character is the tendency of an atom to lose one or more electrons. It increases
when we move from top to bottom in a group and decreases from left to right in a period.
Period Metallic character decreases from left to right in a period
Group
Metallic character increases from top to bottom in a group
7. Non-metallic character
Non-metallic character is the tendency of an atom to gain one or more electrons when
supplied with energy. It increases in a period on moving from left to right and decreases
on moving from top to bottom in a group.
Period Non-metallic character increases on moving from left to right in a period
Group
Non-metallic character decreases on moving from top to bottom in a group
7.10 Reactivity of Elements
Metals have a tendency to lose electrons, and non-metals have a tendency to gain or share
electrons. A metal which loses electrons easily is called an active metal and a non-metal which
gains electrons easily is called an active non-metal. There are several factors on which the
activity of elements depend. Among them, some common factors are the atomic size, nuclear
charge and number of electrons present in the valence shell.
Reactivity of metals in a group
The chemical reactivity of metals increases on moving down in a group of modern periodic
table. When we move from top to bottom in the groups of metals, the atomic size increases
gradually. Due to this, nuclear attraction to the valence electron decreases. As a result, the
tendency of metal atoms to lose electron/s increases. Therefore, chemical reactivity of metals
increases on moving from top to bottom in a group.
Example: In group 1, the chemical reactivity of alkali metals increases from lithium (Li) to
francium (Fr).
CHEMISTRY Oasis School Science - 10 141
Group 1 Least reactive metal
Li
Na
K Chemical reactivity of metals
increases on moving down in
a group
Rb
Cs
Fr Most reactive metal
Reasonable Fact-4
Lithium and sodium are kept in group 1, why?
Ans: Lithium and sodium both contain one valence electron, and both of them react with
water to produce corresponding alkali and hydrogen gas. So, Lithium and sodium are
placed in group IA of the modern periodic table.
Reasonable Fact-5
Elements of Group IA, IIA and IIIA (or group 1, 2 and 13) are more reactive as we go down
in the group of the periodic table, why?
Ans: Elements of Group IA, IIA and IIIA (or group 1, 2 and 13) are more reactive as we go
down in the group because of the following reasons:
(i) Atomic radius of these elements increases as we go down in the group.
(ii) Ionization potential decreases as we go down in the group.
142 Oasis School Science - 10 CHEMISTRY
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Science and Environment 10
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