NCERT Textbook: Electric Current & its Effects Notes | Study Science & Technology for UPSC CSE - UPSC

 Page 1


SCIENCE 160
Electric Current
and its Effects
14
Y
ou might have tried the game
‘How steady is your hand?’
suggested in Chapter 12 of
Class VI. If not, you may try it out now.
Paheli and Boojho had also set up the
game by connecting an electric circuit
as suggested in Class VI. They had lots
of fun trying it out with their families
and friends. They enjoyed it so much
that they decided to suggest it to a
cousin of theirs who stayed in a
different town. So, Paheli made a neat
drawing showing how the various
electric components were to be
connected (Fig.14.1).
Fig. 14.1  Setup to check how steady
your hand is
Can you draw this circuit
conveniently? It made Boojho wonder if
there was an easier way to represent
these electric components.
14.1 SYMBOLS OF ELECTRIC
COMPONENTS
Some common electric components can
be represented by symbols. In Table 14.1,
some electric components and their
symbols are shown. You may come
across different symbols for these
components in different books. However,
in this book, we shall be using the
symbols shown here.
Look at the symbols carefully. In the
symbol for the electric cell, notice that
there is a longer line and a shorter but
thicker parallel line. Do you recall that
an electric cell has a positive terminal
and a negative terminal? In the symbol
of the electric cell, the longer line
represents the positive terminal and the
thicker, shorter line represents the
negative terminal.
For a switch the ‘ON’ position and
the ‘OFF’ position are represented by the
symbols as shown. The wires used to
connect the various components in a
circuit are represented by lines.
In Table 14.1, a battery and its
symbol are also shown. Do you know
what a battery is? Look at the symbol of
a battery. Can you make out what a
battery could be? For some of the
activities we may need more than one
cell. So, we connect two or more cells
together as shown in Fig.14.2. Notice
2020-21
Page 2


SCIENCE 160
Electric Current
and its Effects
14
Y
ou might have tried the game
‘How steady is your hand?’
suggested in Chapter 12 of
Class VI. If not, you may try it out now.
Paheli and Boojho had also set up the
game by connecting an electric circuit
as suggested in Class VI. They had lots
of fun trying it out with their families
and friends. They enjoyed it so much
that they decided to suggest it to a
cousin of theirs who stayed in a
different town. So, Paheli made a neat
drawing showing how the various
electric components were to be
connected (Fig.14.1).
Fig. 14.1  Setup to check how steady
your hand is
Can you draw this circuit
conveniently? It made Boojho wonder if
there was an easier way to represent
these electric components.
14.1 SYMBOLS OF ELECTRIC
COMPONENTS
Some common electric components can
be represented by symbols. In Table 14.1,
some electric components and their
symbols are shown. You may come
across different symbols for these
components in different books. However,
in this book, we shall be using the
symbols shown here.
Look at the symbols carefully. In the
symbol for the electric cell, notice that
there is a longer line and a shorter but
thicker parallel line. Do you recall that
an electric cell has a positive terminal
and a negative terminal? In the symbol
of the electric cell, the longer line
represents the positive terminal and the
thicker, shorter line represents the
negative terminal.
For a switch the ‘ON’ position and
the ‘OFF’ position are represented by the
symbols as shown. The wires used to
connect the various components in a
circuit are represented by lines.
In Table 14.1, a battery and its
symbol are also shown. Do you know
what a battery is? Look at the symbol of
a battery. Can you make out what a
battery could be? For some of the
activities we may need more than one
cell. So, we connect two or more cells
together as shown in Fig.14.2. Notice
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 161
Many devices such as torches,
transistors, toys, TV remote controls, use
batteries. However, in some of these
devices the electric cells are not always
placed one after the other as shown in
Fig. 14.2. Sometimes the cells are placed
side by side. Then how are the terminals
of the cells connected? Look carefully
inside the battery compartment of any
device. There is usually a thick wire or
a metal strip connecting the positive
terminal of one cell to the negative
terminal of the next cell (Fig.14.3). In
order to help you to place the cells
correctly in the battery compartment,
‘+’ and ‘–’ symbols are usually printed
there.
How can we connect the cells to
prepare batteries for our activities? You
may make a cell holder, as shown in
Fig.14.4, using a wooden block, two iron
strips and rubber bands. It is necessary
Fig. 14.3  Connecting two cells together to make
a battery
Fig. 14.2  (a) A battery of two cells
(b) A battery of four cells
(a)
(b)
Table 14.1 Symbols for some
electric circuit components
S.No. Electric component Symbol
1. Electric cell
2. Electric bulb
3. Switch in ‘ON’ position
4. Switch in ‘OFF’ position
5. Battery
6. Wire
that the positive terminal of one cell is
connected to the negative terminal of the
next cell. Such a combination of two or
more cells is called a battery.
2020-21
Page 3


SCIENCE 160
Electric Current
and its Effects
14
Y
ou might have tried the game
‘How steady is your hand?’
suggested in Chapter 12 of
Class VI. If not, you may try it out now.
Paheli and Boojho had also set up the
game by connecting an electric circuit
as suggested in Class VI. They had lots
of fun trying it out with their families
and friends. They enjoyed it so much
that they decided to suggest it to a
cousin of theirs who stayed in a
different town. So, Paheli made a neat
drawing showing how the various
electric components were to be
connected (Fig.14.1).
Fig. 14.1  Setup to check how steady
your hand is
Can you draw this circuit
conveniently? It made Boojho wonder if
there was an easier way to represent
these electric components.
14.1 SYMBOLS OF ELECTRIC
COMPONENTS
Some common electric components can
be represented by symbols. In Table 14.1,
some electric components and their
symbols are shown. You may come
across different symbols for these
components in different books. However,
in this book, we shall be using the
symbols shown here.
Look at the symbols carefully. In the
symbol for the electric cell, notice that
there is a longer line and a shorter but
thicker parallel line. Do you recall that
an electric cell has a positive terminal
and a negative terminal? In the symbol
of the electric cell, the longer line
represents the positive terminal and the
thicker, shorter line represents the
negative terminal.
For a switch the ‘ON’ position and
the ‘OFF’ position are represented by the
symbols as shown. The wires used to
connect the various components in a
circuit are represented by lines.
In Table 14.1, a battery and its
symbol are also shown. Do you know
what a battery is? Look at the symbol of
a battery. Can you make out what a
battery could be? For some of the
activities we may need more than one
cell. So, we connect two or more cells
together as shown in Fig.14.2. Notice
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 161
Many devices such as torches,
transistors, toys, TV remote controls, use
batteries. However, in some of these
devices the electric cells are not always
placed one after the other as shown in
Fig. 14.2. Sometimes the cells are placed
side by side. Then how are the terminals
of the cells connected? Look carefully
inside the battery compartment of any
device. There is usually a thick wire or
a metal strip connecting the positive
terminal of one cell to the negative
terminal of the next cell (Fig.14.3). In
order to help you to place the cells
correctly in the battery compartment,
‘+’ and ‘–’ symbols are usually printed
there.
How can we connect the cells to
prepare batteries for our activities? You
may make a cell holder, as shown in
Fig.14.4, using a wooden block, two iron
strips and rubber bands. It is necessary
Fig. 14.3  Connecting two cells together to make
a battery
Fig. 14.2  (a) A battery of two cells
(b) A battery of four cells
(a)
(b)
Table 14.1 Symbols for some
electric circuit components
S.No. Electric component Symbol
1. Electric cell
2. Electric bulb
3. Switch in ‘ON’ position
4. Switch in ‘OFF’ position
5. Battery
6. Wire
that the positive terminal of one cell is
connected to the negative terminal of the
next cell. Such a combination of two or
more cells is called a battery.
2020-21
SCIENCE 162
that the rubber bands hold the metal
strips tightly.
You could also buy cell holders from
the market for making batteries of two
or more electric cells. Place the cells in
them properly, such that the positive
terminal of one cell is connected to the
negative terminal of the next cell.
Connect a piece of wire each to the two
metal clips on the cell holder as shown
in Fig.14.5.  Your battery is ready for
use.
The symbol used for representing a
battery is shown in Table.14.1.
Let us now draw a circuit diagram of
an electric circuit using symbols shown
in Table 14.1.
Activity 14.1
Make the electric circuit shown in
Fig. 14.7. You used a similar circuit in
Class VI to make an electric bulb glow.
Do you remember that the bulb glows
only when the switch is in the ‘ON’
position? The bulb glows as soon as
the switch is moved to the ‘ON’
position.
 Copy this electric circuit in your
notebook. Make also a circuit diagram
of this circuit using symbols for the
various electric components.
Is your diagram similar to the one
shown in Fig. 14.8?
It is much easier to draw a circuit
diagram using symbols. Therefore, we
generally represent an electric circuit by
its circuit diagram.
Fig. 14.9 shows another circuit
diagram. Is it identical to the circuit
diagram shown in Fig.14.8? In which
way is it different?
Would the bulb glow in this electric
circuit? Recall that the bulb glows only
when the switch is in the ‘ON’ position
and the electric circuit is closed.
Fig. 14.4  A cell holder
Fig. 14.5  Another type of cell holder
Paheli and Boojho wonder whether
the batteries used in tractors, trucks
and inverters are also made from cells.
Then why is it called a battery? Can
you help them to find the answer to
this question? 
Fig. 14.6  Truck battery and its cutout
2020-21
Page 4


SCIENCE 160
Electric Current
and its Effects
14
Y
ou might have tried the game
‘How steady is your hand?’
suggested in Chapter 12 of
Class VI. If not, you may try it out now.
Paheli and Boojho had also set up the
game by connecting an electric circuit
as suggested in Class VI. They had lots
of fun trying it out with their families
and friends. They enjoyed it so much
that they decided to suggest it to a
cousin of theirs who stayed in a
different town. So, Paheli made a neat
drawing showing how the various
electric components were to be
connected (Fig.14.1).
Fig. 14.1  Setup to check how steady
your hand is
Can you draw this circuit
conveniently? It made Boojho wonder if
there was an easier way to represent
these electric components.
14.1 SYMBOLS OF ELECTRIC
COMPONENTS
Some common electric components can
be represented by symbols. In Table 14.1,
some electric components and their
symbols are shown. You may come
across different symbols for these
components in different books. However,
in this book, we shall be using the
symbols shown here.
Look at the symbols carefully. In the
symbol for the electric cell, notice that
there is a longer line and a shorter but
thicker parallel line. Do you recall that
an electric cell has a positive terminal
and a negative terminal? In the symbol
of the electric cell, the longer line
represents the positive terminal and the
thicker, shorter line represents the
negative terminal.
For a switch the ‘ON’ position and
the ‘OFF’ position are represented by the
symbols as shown. The wires used to
connect the various components in a
circuit are represented by lines.
In Table 14.1, a battery and its
symbol are also shown. Do you know
what a battery is? Look at the symbol of
a battery. Can you make out what a
battery could be? For some of the
activities we may need more than one
cell. So, we connect two or more cells
together as shown in Fig.14.2. Notice
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 161
Many devices such as torches,
transistors, toys, TV remote controls, use
batteries. However, in some of these
devices the electric cells are not always
placed one after the other as shown in
Fig. 14.2. Sometimes the cells are placed
side by side. Then how are the terminals
of the cells connected? Look carefully
inside the battery compartment of any
device. There is usually a thick wire or
a metal strip connecting the positive
terminal of one cell to the negative
terminal of the next cell (Fig.14.3). In
order to help you to place the cells
correctly in the battery compartment,
‘+’ and ‘–’ symbols are usually printed
there.
How can we connect the cells to
prepare batteries for our activities? You
may make a cell holder, as shown in
Fig.14.4, using a wooden block, two iron
strips and rubber bands. It is necessary
Fig. 14.3  Connecting two cells together to make
a battery
Fig. 14.2  (a) A battery of two cells
(b) A battery of four cells
(a)
(b)
Table 14.1 Symbols for some
electric circuit components
S.No. Electric component Symbol
1. Electric cell
2. Electric bulb
3. Switch in ‘ON’ position
4. Switch in ‘OFF’ position
5. Battery
6. Wire
that the positive terminal of one cell is
connected to the negative terminal of the
next cell. Such a combination of two or
more cells is called a battery.
2020-21
SCIENCE 162
that the rubber bands hold the metal
strips tightly.
You could also buy cell holders from
the market for making batteries of two
or more electric cells. Place the cells in
them properly, such that the positive
terminal of one cell is connected to the
negative terminal of the next cell.
Connect a piece of wire each to the two
metal clips on the cell holder as shown
in Fig.14.5.  Your battery is ready for
use.
The symbol used for representing a
battery is shown in Table.14.1.
Let us now draw a circuit diagram of
an electric circuit using symbols shown
in Table 14.1.
Activity 14.1
Make the electric circuit shown in
Fig. 14.7. You used a similar circuit in
Class VI to make an electric bulb glow.
Do you remember that the bulb glows
only when the switch is in the ‘ON’
position? The bulb glows as soon as
the switch is moved to the ‘ON’
position.
 Copy this electric circuit in your
notebook. Make also a circuit diagram
of this circuit using symbols for the
various electric components.
Is your diagram similar to the one
shown in Fig. 14.8?
It is much easier to draw a circuit
diagram using symbols. Therefore, we
generally represent an electric circuit by
its circuit diagram.
Fig. 14.9 shows another circuit
diagram. Is it identical to the circuit
diagram shown in Fig.14.8? In which
way is it different?
Would the bulb glow in this electric
circuit? Recall that the bulb glows only
when the switch is in the ‘ON’ position
and the electric circuit is closed.
Fig. 14.4  A cell holder
Fig. 14.5  Another type of cell holder
Paheli and Boojho wonder whether
the batteries used in tractors, trucks
and inverters are also made from cells.
Then why is it called a battery? Can
you help them to find the answer to
this question? 
Fig. 14.6  Truck battery and its cutout
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 163
electric current passes through it. When
the bulb gets fused, its filament is
broken.
Fig. 14.8  Circuit diagram of electric circuit
shown in Fig. 14.7
Fig. 14.9 Another circuit diagram
If the filament of the bulb is broken,
would the circuit be complete?   Would
the bulb still glow?
You might have noticed that a
glowing electric bulb become warm. Do
you know why?
14.2 HEATING EFFECT OF ELECTRIC
CURRENT
Activity 14.2
Take an electric cell, a bulb, a switch
and connecting wires. Make an electric
circuit as shown in Fig.14.9. This
activity has to be done using only one
cell. Keep the switch in the ‘OFF’
Fig. 14.7  An electric circuit
In the bulb there is a thin wire, called
the filament, which glows when an
CAUTION
Never touch a lighted electric bulb
connected to the mains. It may be very
hot and your hand may get burnt
badly. Do not experiment with the
electric supply from the mains or a
generator or an inverter. You may get
an electric shock, which may be
dangerous. Use only electric cells for
all the activities suggested here.
§ Notice that the key or switch can
be placed anywhere in the circuit.
§ When the switch is in the ‘ON’
position, the circuit from the
positive terminal of the battery to
the negative terminal is complete.
The circuit is then said to be closed
and the current flows throughout
the circuit instantly.
§ When the switch is in the ‘OFF’
position, the circuit is incomplete.
It is said to be open. No current
flows through any part of the
circuit.
2020-21
Page 5


SCIENCE 160
Electric Current
and its Effects
14
Y
ou might have tried the game
‘How steady is your hand?’
suggested in Chapter 12 of
Class VI. If not, you may try it out now.
Paheli and Boojho had also set up the
game by connecting an electric circuit
as suggested in Class VI. They had lots
of fun trying it out with their families
and friends. They enjoyed it so much
that they decided to suggest it to a
cousin of theirs who stayed in a
different town. So, Paheli made a neat
drawing showing how the various
electric components were to be
connected (Fig.14.1).
Fig. 14.1  Setup to check how steady
your hand is
Can you draw this circuit
conveniently? It made Boojho wonder if
there was an easier way to represent
these electric components.
14.1 SYMBOLS OF ELECTRIC
COMPONENTS
Some common electric components can
be represented by symbols. In Table 14.1,
some electric components and their
symbols are shown. You may come
across different symbols for these
components in different books. However,
in this book, we shall be using the
symbols shown here.
Look at the symbols carefully. In the
symbol for the electric cell, notice that
there is a longer line and a shorter but
thicker parallel line. Do you recall that
an electric cell has a positive terminal
and a negative terminal? In the symbol
of the electric cell, the longer line
represents the positive terminal and the
thicker, shorter line represents the
negative terminal.
For a switch the ‘ON’ position and
the ‘OFF’ position are represented by the
symbols as shown. The wires used to
connect the various components in a
circuit are represented by lines.
In Table 14.1, a battery and its
symbol are also shown. Do you know
what a battery is? Look at the symbol of
a battery. Can you make out what a
battery could be? For some of the
activities we may need more than one
cell. So, we connect two or more cells
together as shown in Fig.14.2. Notice
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 161
Many devices such as torches,
transistors, toys, TV remote controls, use
batteries. However, in some of these
devices the electric cells are not always
placed one after the other as shown in
Fig. 14.2. Sometimes the cells are placed
side by side. Then how are the terminals
of the cells connected? Look carefully
inside the battery compartment of any
device. There is usually a thick wire or
a metal strip connecting the positive
terminal of one cell to the negative
terminal of the next cell (Fig.14.3). In
order to help you to place the cells
correctly in the battery compartment,
‘+’ and ‘–’ symbols are usually printed
there.
How can we connect the cells to
prepare batteries for our activities? You
may make a cell holder, as shown in
Fig.14.4, using a wooden block, two iron
strips and rubber bands. It is necessary
Fig. 14.3  Connecting two cells together to make
a battery
Fig. 14.2  (a) A battery of two cells
(b) A battery of four cells
(a)
(b)
Table 14.1 Symbols for some
electric circuit components
S.No. Electric component Symbol
1. Electric cell
2. Electric bulb
3. Switch in ‘ON’ position
4. Switch in ‘OFF’ position
5. Battery
6. Wire
that the positive terminal of one cell is
connected to the negative terminal of the
next cell. Such a combination of two or
more cells is called a battery.
2020-21
SCIENCE 162
that the rubber bands hold the metal
strips tightly.
You could also buy cell holders from
the market for making batteries of two
or more electric cells. Place the cells in
them properly, such that the positive
terminal of one cell is connected to the
negative terminal of the next cell.
Connect a piece of wire each to the two
metal clips on the cell holder as shown
in Fig.14.5.  Your battery is ready for
use.
The symbol used for representing a
battery is shown in Table.14.1.
Let us now draw a circuit diagram of
an electric circuit using symbols shown
in Table 14.1.
Activity 14.1
Make the electric circuit shown in
Fig. 14.7. You used a similar circuit in
Class VI to make an electric bulb glow.
Do you remember that the bulb glows
only when the switch is in the ‘ON’
position? The bulb glows as soon as
the switch is moved to the ‘ON’
position.
 Copy this electric circuit in your
notebook. Make also a circuit diagram
of this circuit using symbols for the
various electric components.
Is your diagram similar to the one
shown in Fig. 14.8?
It is much easier to draw a circuit
diagram using symbols. Therefore, we
generally represent an electric circuit by
its circuit diagram.
Fig. 14.9 shows another circuit
diagram. Is it identical to the circuit
diagram shown in Fig.14.8? In which
way is it different?
Would the bulb glow in this electric
circuit? Recall that the bulb glows only
when the switch is in the ‘ON’ position
and the electric circuit is closed.
Fig. 14.4  A cell holder
Fig. 14.5  Another type of cell holder
Paheli and Boojho wonder whether
the batteries used in tractors, trucks
and inverters are also made from cells.
Then why is it called a battery? Can
you help them to find the answer to
this question? 
Fig. 14.6  Truck battery and its cutout
2020-21
ELECTRIC CURRENT AND ITS EFFECTS 163
electric current passes through it. When
the bulb gets fused, its filament is
broken.
Fig. 14.8  Circuit diagram of electric circuit
shown in Fig. 14.7
Fig. 14.9 Another circuit diagram
If the filament of the bulb is broken,
would the circuit be complete?   Would
the bulb still glow?
You might have noticed that a
glowing electric bulb become warm. Do
you know why?
14.2 HEATING EFFECT OF ELECTRIC
CURRENT
Activity 14.2
Take an electric cell, a bulb, a switch
and connecting wires. Make an electric
circuit as shown in Fig.14.9. This
activity has to be done using only one
cell. Keep the switch in the ‘OFF’
Fig. 14.7  An electric circuit
In the bulb there is a thin wire, called
the filament, which glows when an
CAUTION
Never touch a lighted electric bulb
connected to the mains. It may be very
hot and your hand may get burnt
badly. Do not experiment with the
electric supply from the mains or a
generator or an inverter. You may get
an electric shock, which may be
dangerous. Use only electric cells for
all the activities suggested here.
§ Notice that the key or switch can
be placed anywhere in the circuit.
§ When the switch is in the ‘ON’
position, the circuit from the
positive terminal of the battery to
the negative terminal is complete.
The circuit is then said to be closed
and the current flows throughout
the circuit instantly.
§ When the switch is in the ‘OFF’
position, the circuit is incomplete.
It is said to be open. No current
flows through any part of the
circuit.
2020-21
SCIENCE 164
position.  Does the bulb glow? Touch
the bulb. Now move the electric switch
to the ‘ON’ position and let the bulb glow
for a minute or so. Again touch the bulb.
Do you feel any difference? After moving
the switch back to the ‘OFF’ position,
touch the bulb again.
Activity 14.3
Make a circuit as shown in Fig.14.10.
Take about 10 cm long piece of nichrome
wire and tie it between the nails. (You
can get nichrome wire from an electric
repair shop or you can use a piece of
discarded coil of an electric heater.)
Touch the wire. Now switch on the
current in the circuit by moving the
switch to the ‘ON’ position. After a few
The wire gets hot when an electric
current passes through it. This is the
heating effect of the electric current.
Can you think of any electric appliance
where the heating effect of the electric
current is used?  Make a list of such
appliances.
Fig. 14.10
seconds touch the wire. (Do not hold it
for a long time.) Switch off the current.
Touch the wire again after a few
minutes.
You might have seen an electric
room heater or an electric heater used
for cooking. All these contain a coil of
wire. This coil of wire is called an
element. You may have noticed that
when these appliances are switched on
CAUTION
Do not keep the switch in the ‘ON’
position for a long time, otherwise the
cell may become weak very quickly.
Boojho could not see element in an
electric iron.  Paheli told him that
electrical appliances, such as
immersion heaters, hotplates, irons,
geysers, electric kettles, hair dryers,
have elements inside them. Have you
ever seen the element in any
appliance?
Fig. 14.11 Element of electric iron
2020-21