3. When an object is at 2F F O F A'
2F
When an object is placed at 2F of a B B'
convex lens, the image is formed at A
2F at the another side. The image 2F A'
is real, inverted and of same size as 2F
that of object.
B'
4. When an object is at any point between F and 2F
When an object is at any point
between F and 2F of a convex B
lens, the image is formed A F
beyond 2F at another side. The 2F F O
image is real, inverted and
bigger in size, i.e. magnified.
5. When an object is at ‘F’
When an object is placed at ‘F’, the B
rays coming after refraction through A F
the lens become parallel to each 2F F O 2F
other. These rays meet at infinity. To image at
Thus, the image is formed at infinity infinity
at the next side of the lens. The image is real, inverted and highly
magnified.
6. When an object is kept between ‘F’ and ‘O’
When an object is placed B'
between ‘F’ and ‘O’ in front
of a convex lens, the image is
formed on the same side of the A' B O F O
object. The image is virtual, 2F FA 2F
erect and magnified.
In this position, convex lens
can be used as a magnifying
glass.
From the above ray diagrams, it can be said that when the object
approaches the lens, the images formed are gradually larger and
farther away from the lens.
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Fact &Reason
Where should object be placed in convex lens so that virtual, erect and magnified
image can be obtained?
The object should be placed between O and F so that virtual, erect and magnified
image can be obtained.
Rules to draw ray diagram in a concave lens
a. A ray parallel to the principal axis from the tip of the object is
drawn which gets diverged after refraction through the lens.
The diverged ray meets the principal axis after producing at
point ‘F’.
b. A ray from the tip of the object should be drawn passing
through the optical centre. It passes without deviation.
A O O F
BF F
(a) (b)
c. An image is formed at a point where the ray passing through
the optical centre and the produced ray meet.
Images formed by a concave lens
A concave lens always forms virtual, erect and diminished image. The
image is always formed on the same side.
1. When the object is at infinity
When an object is at infinity, the image is formed at F O
the principal focus (F) on the same side of the lens.
The image is virtual, erect and highly diminished.
2. When an object is at a point between infinity and principal focus (F)
When an object is placed at a point between F O
infinity and principal focus (F), the image is A
formed between the lens and the principal
focus (F) on the same side of the lens. The
B
image is virtual, erect and diminished.
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Differences between real image and virtual image
Real image Virtual image
1. The image which can be 1. The image which cannot
obtained on a screen is be obtained on a screen
called real image. is called virtual image.
2. It is always inverted. 2. It is always erect.
3. It is formed by a convex lens, 3. It is formed by a concave
camera, projector, etc. lens, microscope,
telescope, etc.
4. It is formed by meeting the 4. It is formed by the
actual refracted rays. apparent meeting of
refracted rays.
5. It is usually formed at the 5. It is generally formed
another side or behind the on the same side of the
lens. object in the lens.
Magnification
Images formed by a lens will be larger or smaller than the size of the
object depending upon the positions of the objects. The variation in
the size of the image with respect to the size of object is expressed by
the term magnification.
Magnification is defined as the ratio of height of image to the height of object.
Mathematically,
Height of image ⇒ m = I
Magnification = Height of object O
Relation of magnification with the size of image
1. When the magnification is less than 1, the size of image is
smaller than the size of object, i.e. I < O when m < 1.
2. When magnification is more than 1, the size of image is larger
than the size of object, i.e I> 0 when m > 1.
3. When magnification is equal to 1, the size of image is equal to
the size of the object, i.e, I = 0 when m = 1
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Method of measuring magnification
M
A
B F B'
2F 2F
F O
A'
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 v
O u
i.e. m = =
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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 or P =f 100
(m) (cm)
When focal length of a lens is expressed in metre, the power of the
lens is in dioptre (D).
One diopter Power: The power of lens is 1 diopter if its focal length
is 1m.
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.
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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,
1f = 1 + 1 O r, 2 10 = 310 + 1 Or, 1 = 1 – 1
u v 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 0.2
f = 20cm = 0.2m
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
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, P= 1 Or, –1.5 = 1 Or, f = 1 Or, f = – 0.67m
f f –1.5
Focal length of the lens (f) = 0.67m = 0.67 × 100cm = 67cm
Here, the lens is concave lens because it has negative focal length and
negative power.
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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.
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
An 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:
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Ciliary muscles
Iris Suspensory Retina
Cornea ligaments
Eye Blind Spot
Pupil lens Optic nerve
Aqueous
humour Vitreous
humour
Eye
Each eye contains a ball called eyeball. Each eyeball contains three
layers. They are: i. Sclerotic layer ii. Choroid iii. Retina
Sclera is the outermost part of eye ball. It is an opaque and white
part which gives shape to the eyeball and protects the inner parts.
Cornea is a transparent front part of sclera through which light
enters the eye. Choroid is the middle part of the eye ball. It contains
ciliary muscles which holds eye lens. The lens divides the cavity of
the eye ball in two chambers: anterior and posterior. The anterior
cavity is filled with clear watery liquid called aqueous humour and
the posterior part is filled with thick, gelatinous substance called
vitreous humour. Retina is the innermost layer of eyeball. Image
of an object forms on it. It is provided with many nerve cells called
photoreceptor cells. Rods and cones are such photoreceptor cells.
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
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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.
Fact &Reason
What happens when ciliary muscle gets damaged?
Ciliary muscle is responsible for changing the focal length of eye lens. If ciliary
muscle gets damaged, a person suffers from both myopia and hypermetropia.
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
The distance between the eye and near point
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.
Range of vision
A normal eye can see an object kept anywhere between near point
and far point. The near point for a normal eye is 25 cm and far point
is infinity. The distance between near point and far point which can
be focused by our eyes is called range of vision. The normal eyes can
focus the objects lying inside the range of vision.
Defects of vision
A normal eye can see any object kept between near point (25cm) and
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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. Thickening of eye lens. In this case, ciliary muscles fail to
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 the spectacles. The concave lens diverges the rays coming from
the objects. Due to the combined action of both the lenses (lens in
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spectacle and eye lens), the image is formed on the retina. Then, the
distant object can be seen clearly.
Fact &Reason
A person can clearly read a book but has difficulties in seeing the object of far
distance. What type of defect of vision is it? How can it be corrected?
He has myopia. It can be corrected by using concave lens (diverging lens) of
appropriate power.
Fact &Reason
Myopia cannot be corrected by using convex lens, why?
When suction is caused at the end of the soda straw, partial vacuum is created
inside the straw. The lemonade goes up inside the straw to fill the vacuum due
to the pressure exerted by the atmosphere on the earth surface. But there is
no atmospheric pressure on the surface of the moon, so it is impossible to suck
lemonade with soda straw.
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. Shortening of eye lens. 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
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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.
Fact &Reason
Why does a person suffering from hypermetropia prefer to use glasses while
reading book?
A person suffering from hypermetropia has difficulty in seeing the object of near
distance due to the decrease in converging power of the eye lens. So, the person
uses convex lens (converging lens) while reading the book.
Differences between myopia and hypermetropia
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. The image of the distant 3. The image of nearby object is
object is formed in front of the formed behind the retina.
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 focal length of convex lens is positive and focal
length of concave lens is 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
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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.
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. Very short answer questions (1 mark)
1. What is lens?
2. Define power of lens. Write its formula and SI unit.
3. The magnification of the lens is less than one. What does it
mean?
4. Define optical center.
5. What is focal length of a lens?
6. In which condition is the image formed by a convex lens
erected ?
7. What is focusing?
8. Write the relationship between power of lens and its focal
length.
9. Write one function of Ciliary muscles and Iris each.
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10. What is magnification of lense? Mention its formula.
11. In which point should the object be kept in front of the convex
lens to have magnification one?
12. What is myopia or short-sightedness?
13. Where is the image formed in myopia?
14. Where should an object be placed to get its inverted, real and
magnified image in front of a convex lens?
15. Why is focusing done?
16. Define one dioptre power of a lens.
17. Name the factor on which the power of a lens depends.
18. What is meant by defects of vision?
19. Write one function of aqueous humour and vitreous humour.
20. Define far point of an eye.
21. What is near point of an eye?
22. What do you mean by accommodation?
B. Short answer questions (2 marks)
1. Write two differences between converging lens and diverging
lens.
2. How do ciliary muscles focus image of an object?
3. Write any two differences between principal focus and focal
length.
4. A person can read the number of a distant bus clearly but he
finds difficulty in reading a book. Name the type of the defect
of vision. How can this defect be corrected?
5. Differentiate between myopia and hypermetropia.
C. Long answer questions (3 marks)
1. What type of defect of vision is long-sightedness? How is
long-sightedness corrected? Explain with ray diagram.
2. What type of defect is myopia? How is myopia corrected ?
Explain with ray diagram.
3. Write any three uses of a convex lens.
4. Complete the given ray diagram and write the characteristics
of the image formed.
(a) (b) (c)
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D. Very long answer questions (4 marks)
1. Draw a ray diagram of an image formed when an object is
placed at a distance of 10 cm from a convex lens having focal
length 20 cm. Write any two nature of image. Calculate the
power of the lens.
2. Draw ray diagrams of a convex lens when an object is at a
point
(i) Beyond 2F (ii) At infinity
(iii) At 2F (iv) Between F and O.
3. Draw a neat and labelled ray diagram showing the image
formed by a convex lens when the object is placed between F
and 2F. Also, write down the characteristics and utility of the
image formed.
4. Study the given diagram and answer the following questions.
i ) What type of defect of vision is in
the diagram of eye?
ii) What is the cause of this defect of
vision?
iii) What is to be done to remedy such
defect?
iv) Draw a diagram to show the
remedy of this defect.
5. Draw a ray diagram of an image formed when an object is
kept at a distance of 50 cm from convex lens having focal
length 20 cm. Write any two nature of image. Calculate the
power of lens.
6. Draw a ray diagram of an image formed when an object is
kept at a distance of 40 cm from a convex lens having focal
length 20 cm. Write any two nature of image. Calculate the
magnification of image.
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.
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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
D. 1. 5 Diopter 5. 5 Diopter 6. 1
Numerical problems
1. 10 diopter 2. 20 cm 3. 30 cm (i) 0.5 (ii) 5 Diopter
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
• Spectacles : eye glasses
• Convex
• Concave : thick at the middle and thin at the sides
• Converge
• Diverge : thin at the middle and thick at the sides
• Aperture
• Principal : to collect at a piont
• Remedy
• Magnification : to scatter from a point
• Accommodation : a small narrow opening
: main
: treatment
: process of making something
appear large
: adjustment
112Times' Crucial Science Book - 10
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 lamp, CFL(Compact fluorescent lamp) 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
called alternating current (AC).
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R current →
+ ++ +
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.
Fact &Reason
The frequency of AC current in Kathmandu is 50 Hz. What does this statement
mean?
This statement means that the polarity and magnitude of AC current in
Kathmandu changes 50 times per second.
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
Electric iron
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electrical energy is converted into heat energy. This is called heating
effect of current.
All heating devices contain a coil of thin wire called heating element.
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 Nitrogen gas
is used inside the glass Filament Glass cover
bulb. The thin wire is called Supporting wire
filament. It is made up of
tungsten. Tungsten has high
resistance and high melting Aluminium support
point (3400°C). Due to its Terminal pin
high resistance, it readily gets
Filament bulb
heated upto 2000°C 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.
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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
also called mercury lamp. It also contains choke coil and starter.
When high voltage AC source
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.
The fluorescent Fluorescent lamp
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.
Fact &Reason
Inner wall of fluorescent lamp is coated with fluorescent powder, why?
Inner wall of fluorescent lamp is coated with fluorescent powder so that it converts
uv rays into white light.
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 electrical energy into light and
heat. 70% into heat.
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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.
Fact &Reason
Why is Compact fluorescent lamp more efficient than simple fluorescent lamp?
Compact fluorescent lamp is more efficient than simple fluorescent lamp because
it can convert 90% of electrical energy into light energy.
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
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.
Fact &Reason
Name the devices based on magnetic effect of current.
The devices based on magnetic effect of current are microphone, electric bell,
loudspeakers, television etc.
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Activity 6 .1
To show that deflection
of the magnetic needle
takes place due to
magnetic field.
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 deflection 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.
Magnetic field around a solenoid
A spiral & coiled adjustment 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.
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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.
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.
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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.
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 Armature
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 A
such a way that the hammer can hit the Gong
Hammer
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.
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Interrelation between magnet and electricity
Hans Christian 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.
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
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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 alternating current 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
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
Working Generator
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.
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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 Motor effect
carrying conductor and the permanent magnetic field. As a result,
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 Magnetic Coil rotates
current through a field clockwise
conductor produces a
magnetic field around
the conductor. If current Brushes Commutator
is passed through a freely carry reverses
movable conducting wire current to current
commutator
kept within the magnetic field, the wire
comes in motion due to the attraction and Electric Electric motor
repulsion of the magnetic fields around current
the wire. The electric motor is based on
this principle. It converts electrical energy into kinetic energy.
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Electrical appliances and their uses
Inverter AC Appliances
An inverter is an electrical -+
appliance which converts Battery
alternating current (AC)
into direct current (DC) Socket
Switch
and the direct current -+ Inverter
into alternating current.
If there is electric line in Electrical inverter
the circuit, the inverter converts the alternating current into direct
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 Charger
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
124Times' Crucial Science Book - 10
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.
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 upon 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.
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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
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.
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 of 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
= 1000W ×3600s.
= 3.6×106J
∴ 1 KWh = 3.6×106 Joule
The amount of electric energy consumed in our house is measured
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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 hrs
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 hrs 40
1000
Electric energy consumed = P ×N × T = 4 × × 4
= 0.64 KWh.
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iii. Calculation of electric energy consumed by 3 irons of 750
Watt each per day.
Given,
N=3 750
1000
P = 750 Watt = KW.
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 = 3hrs 100
1000
Electric energy consumed = P×N ×T = 2 × × 3
= 0.6 KWh.
Thus, the total electric energy consumed per day = (1.92 + 0.64 + 2.25 + 0.6)
KWh
= 5.41 unit
Therefore, the total electric energy consumed per month = 5.41×30
= 162.3 unit.
Then, the cost of electricity = Rs 162.3 × 7 = Rs 1136.1.
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.
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.
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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.
g. Connections at plugs, switches, sockets must be made
tight.
h. Naked wires and joints should be covered with insulating
tape.
i. The main switch should be switched off in case of short
circuiting and fire.
j. Water should not be poured in case of fire due to
electricity.
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.
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Structure IS
Vs
Ip
AC Vp
Primary Laminated
iron core
coil (Np) Secondary
coil (NS)
Structure of transformer
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.
Fact &Reason
What material is used to laminate the core of transformer?
The core of the transformer is laminated with varnish or shellac.
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
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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
Vs = ns
Vp np
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
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. Electromagnet has more advantages over permanent magnet. Why?
Strength of an electromagnet can be altered as our requirements.
131 Times' Crucial Science Book - 10
Its shape can be changed according to our desire. Due to this,
electromagnet has more advantages over permanent magnet.
3. 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.
4. 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.
5. 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.
6. 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
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.
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4. The power consumed per hour by an electric appliance of power
1 kilowatt is called one kilowatt hour or 1 unit electricity.
5. 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.
6. An electromagnet is a temporary magnet which is made by
passing current through a solenoid.
7. The motion produced in a current carrying conductor when it is
placed in a permanent magnetic field is called motor effect.
8. A transformer is a device which is used for increasing or decreasing the
voltage of alternating current through mutual induction.
Exercise
A. Very short answer questions (1 mark )
1. What do you mean by short circuiting?
2. Why is 220V written in most of the electrical appliances?
3. How does earthing help to protect from electric shock?
4. What is electric motor ?
5. What is a solenoid ?
6. What is a fuse?
7. Which metal is used as a filament?
8. What is electromagnetic induction?
9. What is magnetic effect of current electricity?
10. What is motor effect?
11. Write the name of any two devices which are based on motor
effect.
12. What is the full form of MCB ? Why is it used?
13. What is overloading?
14. What is heating element? What is it made of ?
15. What is filament?
16. In what effect of electricity is an electric bell based?
17. What is a charger? Write its use.
18. What is a transformer? Write its types.
19. Which coil, primary or secondary, has lesser number of turns
in a step down transformer?
20. What is step up transformer?
21. Name the materials which are used to laminate the core of a
transformer.
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22. What is lighting effect of current ?
23. What is dynamo ? On which principle does it work?
B. Short answer questions ( 2 marks)
1. Why is tungsten wire used in filament bulb?
2. Mention any two reasons of using nichrome wire in electric
heater.
3. Why is the inner wall of a fluorescent lamp coated with
fluorescent powder?
4. Why is it not advised to operate electric switches with wet
hands?
5. Write two differences between primary coil and secondary coil
in the step down transformer on the basis of their structure.
6. What type of a wire is used in an electric heater? Why?
7. The number of turns in the primary coil and secondary coil of
a transformer are never made equal, why?
8. Why does the bulb of a bicycle connected to a dynamo glow
brightly when speed of the bicycle increases and becomes dim
when the speed is decreased?
9. Write two reasons for not using copper wire as a fuse wire.
10. Why is tungsten metal not used in heater?
11. Mercury vapour is filled in a fluorescent lamp, why?
12. A fluorescent lamp with power 60 watt gives more brightness
than a filament lamp with same power, why?
13. Write any two differences between phase wire and fuse wire.
14. How does earthing help to protect from electric shock?
15. Inert gases are filled in a filament lamp, why?
16. Fluorescent lamp is also called mercury lamp, why?
17. A soft iron core is used in a transformer, why?
18. "The use of alternating current would be limited, if
transformer was not invented”. Justify the statement.
19. Write two differences between the electricity generated by
the generator and cell.
20. Write two differences between nichrome wire and fuse wire.
21. On what two principles the transformer is based?
22. The core of transformer is laminated, why?
23. Write two uses of transformer.
24. What is meant by the frequency of alternating current
distributed in Kathmandu Valley is 50 Hertz (Hz) ?
134Times' Crucial Science Book - 10
C. Long answer questions (3 marks)
1. Draw a neat and labelled figure showing step-up and step-
down transformer.
2. Mention any three methods for increasing the amount of
current produced by a bicycle dynamo or generator.
3. Draw a diagram of a step up transformer and mention its any
two parts.
4. Write down three uses of electromagnets.
5. State any three methods to increase the magnitude of
current produced by a generator.
6. Write any three points that we should remember during
household wiring
7. Draw a neat and labelled diagram of an electric bell.
8. Differentiate between:
i) AC and DC
ii) Filament lamp and Fluorescent lamp
iii) Generator and Dynamo
iv) Generator and Motor
v) Step-up and Step-down transformer
vi) Phase wire and Neutral wire
D. Very long answer questions (4 marks)
1. Draw a labelled diagram of:
(i) Step-up transformer
(ii) Step-down transformer
2. Draw a labelled diagram showing the working of electric bell.
3. Which type of transformer is given in the diagram? What is
used to laminate its core? Write its one use.
135 Times' Crucial Science Book - 10
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.
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?
9. The number of turns in the primary coil of a certain
transformer is 150 times more than that in the secondary
coil. Calculate the input emf in the primary coil if the emf
generated in the secondary coil is 220V AC. Which type of
transformer is it? Why?
10. In a transformer, the number of turns in primary coil is three
times more than the number of turns in secondary coil. If the
primary voltage of transformer is 240 volt, find the secondary
voltage.
11. In a house, 6 bulbs each of 100 watt run for 8 hours daily and
3 heaters of 1500 watt run for 5 hours daily, how much unit
of electricity is consumed in thirty days?
12. The number of turns in the secondary coil is 100 times more
than that of primary coil. Calculate the output voltage, if 220
136Times' Crucial Science Book - 10
volt input voltage is passed through this transformer. Where
is this transformer used?
Answers Numerical problems
1. 48 turns 2. 330 V 3. Rs 84. 4. 330 V
7. 73.33V 8. 24 turns
5. 291 unit 6. 500 11. 819 unit 12. 22000V
9. 33000V 10. 80V
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
• Potential difference : voltage
• Operate : to run a machine
• Laminated : insulated, smeared uniformly with an
insulating layer
• Miniature : a smaller-than-usual version of something
• Domestic : household
• Exhausted : tired, worn out, used up
• Excessive : too much
• Armature : a bar of soft iron or steel placed across the
poles of a magnet to maintain its strength
137 Times' Crucial Science Book - 10
Chapter Classification
of
7
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.
138Times' Crucial Science Book - 10
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
Cu Zn 2* 3* As Se Br Ni
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
139 Times' Crucial Science Book - 10
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(IA) with alkali metals without any justification. But hydrogen
could be placed in group 17(VIIA) 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
140Times' Crucial Science Book - 10
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:
141 Times' Crucial Science Book - 10
Times' Crucial Science Book - 14210 Modern Periodic Table: Long Form Periodic Table
s block elements
IA IIA IIIB IVB VB VIB VIIB VIII IB IIB IIIA IVA VA VIA VIIA 'O'
p block elements
d block elements
Lanthanides
f-block elements
Actinides
Period No. of elements Types of period
1 2 Very short period
2 8 Short period
3 8 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 very 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.
Characteristics of periods
1. Valence electrons: There occurs gradual change in electronic
configuration of elements in a period even though the number of
shells remains same. The number of valence electrons increases
when we move from left to right in a period. The number of
valence electrons changes as 1, 2, 3, 4, 5, 6, 7, 8, as we move from
left to right.
2. Valency: Valency of elements changes as 1, 2, 3, 4, 3, 2, 1, 0 as
we move from left to right. In the beginning, the valency increases
from 1 to 4 then decreases to 0.
3. Atomic size: The atomic size or radii of the elements decreases
when we move from left to right in a period. When we move
from left to right in a period, the number of shells remain same.
But, there is increase in the number of electrons and protons in
elements. This causes increase in electrostatic attraction force
between electrons and the nucleus. Thus, nuclear attraction
power increases from left to right in a period. As a result, shells
are held tightly towards nucleus and size of the nucleus decreases.
143 Times' Crucial Science Book - 10
4. Electronegativity: Tendency of elements to take electrons to
make their outermost orbit complete is called electronegativity.
Electronegativity increases when we move from left to right in a
period.
5. Reactivity: Reactivity decreases on moving from left to right in a
period from group I (1) A to IVA (14) then it increases from group
IV (A) to group VIIA (17). At last, elements of group 0 (18) are
inert gases.
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(IA) and 2(IIA): The elements of group 1(IA) and 2(IIA)
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(IA) 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),
Caesium (Cs) and Francium (Fr) are the elements of group 1(IA).
Fact &Reason
Why is hydrogen kept in group IA in periodic table?
Hydrogen has one valence electron and can lose one electron. So, hydrogen is
placed in group IA.
144Times' Crucial Science Book - 10
Group 2(IIA) 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.
Fact &Reason
Why is calcium placed in group IIA in modern periodic table?
The outermost shell of calcium consists of two electrons. So, calcium is placed in
group IIA in modern periodic table.
Group 17(VIIA) elements: The elements of group 17(VIIA)
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(VIIA).
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.
Fact &Reason
The element of group 0 (18) is not affected by their atomic size, why?
The outermost orbit of the elements of group 0 is completely filled with electrons
due to which it neither loses electrons nor gains electrons. So, the element of
group 0 (18 ) is not affected by their atomic size.
3. Characteristics of groups
1) Valence electrons: The electrons present in the outermost
orbit an element are called valence electrons. When we move
from top to bottom, the number of valence electrons remains
same in all elements.
145 Times' Crucial Science Book - 10
2) Valency: Valency of elements of a same group remains same
because they have equal number of valence electrons.
3) Atomic size: When we move from top to bottom in a group,
the number of shells of elements increases. Since number of
shells increases, the atomic size also increases.
4) Metallic character: The tendency of elements to lose
electrons is called metallic character. When we move from
top to bottom in a group, tendency of losing electrons
increases. Thus, metallic character increases when we move
from top to bottom in a group.
5) Non- metallic character: The tendency of elements to take
electrons in the valence shell is called non-metallic character.
When we move from top to bottom in a group, the tendency of
attracting electron decreases. Thus, non- metallic character
decreases.
4. Transition elements
The elements of group 3(IIIB) to 12(IIB) in the modern periodic table
are called transition elements. All these elements are metals. The
transition elements are also known as d-block elements because the
outermost electron of these elements enters in d-sub shell.
5. 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 transitional 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.
146Times' Crucial Science Book - 10
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