NCERT Textbook: Electricity Notes | Study Science & Technology for UPSC CSE - UPSC

 Page 1


Electricity
12 CHAPTER
E
lectricity has an important place in modern society. It is a controllable
and convenient form of energy for a variety of uses in homes, schools,
hospitals, industries and so on. What constitutes electricity? How does
it flow in an electric circuit? What are the factors that control or regulate
the current through an electric circuit? In this Chapter, we shall attempt
to answer such questions. We shall also discuss the heating effect of
electric current and its applications.
12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT
We are familiar with air current and water current. We know that flowing
water constitute water current in rivers. Similarly, if the electric charge
flows through a conductor (for example, through a metallic wire), we
say that there is an electric current in the conductor. In a torch, we
know that the cells (or a battery, when placed in proper order) provide
flow of charges or an electric current through the torch bulb to glow. We
have also seen that the torch gives light only when its switch is on. What
does a switch do? A switch makes a conducting link between the cell and
the bulb. A continuous and closed path of an electric current is called an
electric circuit. Now, if the circuit is broken anywhere (or the switch of the
torch is turned off ), the current stops flowing and the bulb does not glow.
How do we express electric current? Electric current is expressed by
the amount of charge flowing through a particular area in unit time. In
other words, it is the rate of flow of electric charges. In circuits using
metallic wires, electrons constitute the flow of charges. However, electrons
were not known at the time when the phenomenon of electricity was first
observed. So, electric current was considered to be the flow of positive
charges and the direction of flow of positive charges was taken to be the
direction of electric current. Conventionally, in an electric circuit the
direction of electric current is taken as opposite to the direction of the
flow of electrons, which are negative charges.
2020-21
Page 2


Electricity
12 CHAPTER
E
lectricity has an important place in modern society. It is a controllable
and convenient form of energy for a variety of uses in homes, schools,
hospitals, industries and so on. What constitutes electricity? How does
it flow in an electric circuit? What are the factors that control or regulate
the current through an electric circuit? In this Chapter, we shall attempt
to answer such questions. We shall also discuss the heating effect of
electric current and its applications.
12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT
We are familiar with air current and water current. We know that flowing
water constitute water current in rivers. Similarly, if the electric charge
flows through a conductor (for example, through a metallic wire), we
say that there is an electric current in the conductor. In a torch, we
know that the cells (or a battery, when placed in proper order) provide
flow of charges or an electric current through the torch bulb to glow. We
have also seen that the torch gives light only when its switch is on. What
does a switch do? A switch makes a conducting link between the cell and
the bulb. A continuous and closed path of an electric current is called an
electric circuit. Now, if the circuit is broken anywhere (or the switch of the
torch is turned off ), the current stops flowing and the bulb does not glow.
How do we express electric current? Electric current is expressed by
the amount of charge flowing through a particular area in unit time. In
other words, it is the rate of flow of electric charges. In circuits using
metallic wires, electrons constitute the flow of charges. However, electrons
were not known at the time when the phenomenon of electricity was first
observed. So, electric current was considered to be the flow of positive
charges and the direction of flow of positive charges was taken to be the
direction of electric current. Conventionally, in an electric circuit the
direction of electric current is taken as opposite to the direction of the
flow of electrons, which are negative charges.
2020-21
Science
200
If a net charge Q, flows across any cross-section of a conductor in
time t, then the current I, through the cross-section is
I
Q
t
=
(12.1)
The SI unit of electric charge is coulomb (C), which is equivalent to
the charge contained in nearly 6 × 10
18
 electrons. (We know that an
electron possesses a negative charge of 1.6 × 10
–19 
C.) The electric
current is expressed by a unit called ampere (A), named after the
French scientist, Andre-Marie Ampere (1775–1836). One ampere is
constituted by the flow of one coulomb of charge per second, that is,
1 A = 1 C/1 s. Small quantities of current are expressed in milliampere
(1 mA = 10
–3
 A) or in microampere (1 µA = 10
–6
 A).
An instrument called ammeter measures electric
current in a circuit. It is always connected in series
in a circuit through which the current is to be
measured. Figure 12.1 shows the schematic
diagram of a typical electric circuit comprising a
cell, an electric bulb, an ammeter and a plug key.
Note that the electric current flows in the circuit
from the positive terminal of the cell to the negative
terminal of the cell through the bulb and ammeter.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
A schematic diagram of an electric circuit
comprising – cell, electric bulb, ammeter and
plug key
QUESTIONS
?
Example 12.1
A current of 0.5 A is drawn by a filament of an electric bulb for 10
minutes. Find the amount of electric charge that flows through the
circuit.
Solution
We are given, I = 0.5 A; t = 10 min = 600 s.
From Eq. (12.1), we have
Q = It
= 0.5 A × 600 s
= 300 C
1. What does an electric circuit mean?
2. Define the unit of current.
3. Calculate the number of electrons constituting one coulomb of charge.
2020-21
Page 3


Electricity
12 CHAPTER
E
lectricity has an important place in modern society. It is a controllable
and convenient form of energy for a variety of uses in homes, schools,
hospitals, industries and so on. What constitutes electricity? How does
it flow in an electric circuit? What are the factors that control or regulate
the current through an electric circuit? In this Chapter, we shall attempt
to answer such questions. We shall also discuss the heating effect of
electric current and its applications.
12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT
We are familiar with air current and water current. We know that flowing
water constitute water current in rivers. Similarly, if the electric charge
flows through a conductor (for example, through a metallic wire), we
say that there is an electric current in the conductor. In a torch, we
know that the cells (or a battery, when placed in proper order) provide
flow of charges or an electric current through the torch bulb to glow. We
have also seen that the torch gives light only when its switch is on. What
does a switch do? A switch makes a conducting link between the cell and
the bulb. A continuous and closed path of an electric current is called an
electric circuit. Now, if the circuit is broken anywhere (or the switch of the
torch is turned off ), the current stops flowing and the bulb does not glow.
How do we express electric current? Electric current is expressed by
the amount of charge flowing through a particular area in unit time. In
other words, it is the rate of flow of electric charges. In circuits using
metallic wires, electrons constitute the flow of charges. However, electrons
were not known at the time when the phenomenon of electricity was first
observed. So, electric current was considered to be the flow of positive
charges and the direction of flow of positive charges was taken to be the
direction of electric current. Conventionally, in an electric circuit the
direction of electric current is taken as opposite to the direction of the
flow of electrons, which are negative charges.
2020-21
Science
200
If a net charge Q, flows across any cross-section of a conductor in
time t, then the current I, through the cross-section is
I
Q
t
=
(12.1)
The SI unit of electric charge is coulomb (C), which is equivalent to
the charge contained in nearly 6 × 10
18
 electrons. (We know that an
electron possesses a negative charge of 1.6 × 10
–19 
C.) The electric
current is expressed by a unit called ampere (A), named after the
French scientist, Andre-Marie Ampere (1775–1836). One ampere is
constituted by the flow of one coulomb of charge per second, that is,
1 A = 1 C/1 s. Small quantities of current are expressed in milliampere
(1 mA = 10
–3
 A) or in microampere (1 µA = 10
–6
 A).
An instrument called ammeter measures electric
current in a circuit. It is always connected in series
in a circuit through which the current is to be
measured. Figure 12.1 shows the schematic
diagram of a typical electric circuit comprising a
cell, an electric bulb, an ammeter and a plug key.
Note that the electric current flows in the circuit
from the positive terminal of the cell to the negative
terminal of the cell through the bulb and ammeter.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
A schematic diagram of an electric circuit
comprising – cell, electric bulb, ammeter and
plug key
QUESTIONS
?
Example 12.1
A current of 0.5 A is drawn by a filament of an electric bulb for 10
minutes. Find the amount of electric charge that flows through the
circuit.
Solution
We are given, I = 0.5 A; t = 10 min = 600 s.
From Eq. (12.1), we have
Q = It
= 0.5 A × 600 s
= 300 C
1. What does an electric circuit mean?
2. Define the unit of current.
3. Calculate the number of electrons constituting one coulomb of charge.
2020-21
Electricity 201
12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
What makes the electric charge to flow? Let us consider the analogy of
flow of water. Charges do not flow in a copper wire by themselves, just as
water in a perfectly horizontal tube does not flow. If one end of the tube
is connected to a tank of water kept at a higher level, such that there is a
pressure difference between the two ends of the tube, water flows out of
the other end of the tube. For flow of charges in a conducting metallic
wire, the gravity, of course, has no role to play; the electrons move only
if there is a difference of electric pressure – called the potential difference –
along the conductor. This difference of potential may be produced by a
battery, consisting of one or more electric cells. The chemical action within
a cell generates the potential difference across the terminals of the cell,
even when no current is drawn from it. When the cell is connected to a
conducting circuit element, the potential difference sets the charges in
motion in the conductor and produces an electric current. In order to
maintain the current in a given electric circuit, the cell has to expend its
chemical energy stored in it.
We define the electric potential difference between two points in an
electric circuit carrying some current as the work done to move a unit
charge from one point to the other –
Potential difference (V) between two points = Work done (W )/Charge (Q)
V = W/Q (12.2)
The SI unit of electric potential difference is volt (V), named after
Alessandro Volta (1745–1827), an Italian physicist. One volt is the
‘Flow’ of charges inside a wire
How does a metal conduct electricity? You would think that a low-energy electron
would have great difficulty passing through a solid conductor. Inside the solid, the
atoms are packed together with very little spacing between them. But it turns out
that the electrons are able to ‘travel’ through a perfect solid crystal smoothly and
easily, almost as if they were in a vacuum. The ‘motion’ of electrons in a conductor,
however, is very different from that of charges in empty space. When a steady current
flows through a conductor, the electrons in it move with a certain average ‘drift speed’.
One can calculate this drift speed of electrons for a typical copper wire carrying a
small current, and it is found to be actually very small, of the order of 1 mm s
-1
. How
is it then that an electric bulb lights up as soon as we turn the switch on? It cannot be
that a current starts only when an electron from one terminal of the electric supply
physically reaches the other terminal through the bulb, because the physical drift of
electrons in the conducting wires is a very slow process. The exact mechanism of the
current flow, which takes place with a speed close to the speed of light, is fascinating,
but it is beyond the scope of this book. Do you feel like probing this question at an
advanced level?
More to Know!
2020-21
Page 4


Electricity
12 CHAPTER
E
lectricity has an important place in modern society. It is a controllable
and convenient form of energy for a variety of uses in homes, schools,
hospitals, industries and so on. What constitutes electricity? How does
it flow in an electric circuit? What are the factors that control or regulate
the current through an electric circuit? In this Chapter, we shall attempt
to answer such questions. We shall also discuss the heating effect of
electric current and its applications.
12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT
We are familiar with air current and water current. We know that flowing
water constitute water current in rivers. Similarly, if the electric charge
flows through a conductor (for example, through a metallic wire), we
say that there is an electric current in the conductor. In a torch, we
know that the cells (or a battery, when placed in proper order) provide
flow of charges or an electric current through the torch bulb to glow. We
have also seen that the torch gives light only when its switch is on. What
does a switch do? A switch makes a conducting link between the cell and
the bulb. A continuous and closed path of an electric current is called an
electric circuit. Now, if the circuit is broken anywhere (or the switch of the
torch is turned off ), the current stops flowing and the bulb does not glow.
How do we express electric current? Electric current is expressed by
the amount of charge flowing through a particular area in unit time. In
other words, it is the rate of flow of electric charges. In circuits using
metallic wires, electrons constitute the flow of charges. However, electrons
were not known at the time when the phenomenon of electricity was first
observed. So, electric current was considered to be the flow of positive
charges and the direction of flow of positive charges was taken to be the
direction of electric current. Conventionally, in an electric circuit the
direction of electric current is taken as opposite to the direction of the
flow of electrons, which are negative charges.
2020-21
Science
200
If a net charge Q, flows across any cross-section of a conductor in
time t, then the current I, through the cross-section is
I
Q
t
=
(12.1)
The SI unit of electric charge is coulomb (C), which is equivalent to
the charge contained in nearly 6 × 10
18
 electrons. (We know that an
electron possesses a negative charge of 1.6 × 10
–19 
C.) The electric
current is expressed by a unit called ampere (A), named after the
French scientist, Andre-Marie Ampere (1775–1836). One ampere is
constituted by the flow of one coulomb of charge per second, that is,
1 A = 1 C/1 s. Small quantities of current are expressed in milliampere
(1 mA = 10
–3
 A) or in microampere (1 µA = 10
–6
 A).
An instrument called ammeter measures electric
current in a circuit. It is always connected in series
in a circuit through which the current is to be
measured. Figure 12.1 shows the schematic
diagram of a typical electric circuit comprising a
cell, an electric bulb, an ammeter and a plug key.
Note that the electric current flows in the circuit
from the positive terminal of the cell to the negative
terminal of the cell through the bulb and ammeter.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
A schematic diagram of an electric circuit
comprising – cell, electric bulb, ammeter and
plug key
QUESTIONS
?
Example 12.1
A current of 0.5 A is drawn by a filament of an electric bulb for 10
minutes. Find the amount of electric charge that flows through the
circuit.
Solution
We are given, I = 0.5 A; t = 10 min = 600 s.
From Eq. (12.1), we have
Q = It
= 0.5 A × 600 s
= 300 C
1. What does an electric circuit mean?
2. Define the unit of current.
3. Calculate the number of electrons constituting one coulomb of charge.
2020-21
Electricity 201
12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
What makes the electric charge to flow? Let us consider the analogy of
flow of water. Charges do not flow in a copper wire by themselves, just as
water in a perfectly horizontal tube does not flow. If one end of the tube
is connected to a tank of water kept at a higher level, such that there is a
pressure difference between the two ends of the tube, water flows out of
the other end of the tube. For flow of charges in a conducting metallic
wire, the gravity, of course, has no role to play; the electrons move only
if there is a difference of electric pressure – called the potential difference –
along the conductor. This difference of potential may be produced by a
battery, consisting of one or more electric cells. The chemical action within
a cell generates the potential difference across the terminals of the cell,
even when no current is drawn from it. When the cell is connected to a
conducting circuit element, the potential difference sets the charges in
motion in the conductor and produces an electric current. In order to
maintain the current in a given electric circuit, the cell has to expend its
chemical energy stored in it.
We define the electric potential difference between two points in an
electric circuit carrying some current as the work done to move a unit
charge from one point to the other –
Potential difference (V) between two points = Work done (W )/Charge (Q)
V = W/Q (12.2)
The SI unit of electric potential difference is volt (V), named after
Alessandro Volta (1745–1827), an Italian physicist. One volt is the
‘Flow’ of charges inside a wire
How does a metal conduct electricity? You would think that a low-energy electron
would have great difficulty passing through a solid conductor. Inside the solid, the
atoms are packed together with very little spacing between them. But it turns out
that the electrons are able to ‘travel’ through a perfect solid crystal smoothly and
easily, almost as if they were in a vacuum. The ‘motion’ of electrons in a conductor,
however, is very different from that of charges in empty space. When a steady current
flows through a conductor, the electrons in it move with a certain average ‘drift speed’.
One can calculate this drift speed of electrons for a typical copper wire carrying a
small current, and it is found to be actually very small, of the order of 1 mm s
-1
. How
is it then that an electric bulb lights up as soon as we turn the switch on? It cannot be
that a current starts only when an electron from one terminal of the electric supply
physically reaches the other terminal through the bulb, because the physical drift of
electrons in the conducting wires is a very slow process. The exact mechanism of the
current flow, which takes place with a speed close to the speed of light, is fascinating,
but it is beyond the scope of this book. Do you feel like probing this question at an
advanced level?
More to Know!
2020-21
Science
202
potential difference between two points in a current carrying conductor
when 1 joule of work is done to move a charge of 1 coulomb from one
point to the other.
Therefore, 1 volt = 
1 joule
1 coulomb
(12.3)
1 V  = 1 J C
–1
The potential difference is measured by means of an instrument called
the voltmeter. The voltmeter is always connected in parallel across the
points between which the potential difference is to be measured.
Example 12.2
How much work is done in moving a charge of 2 C across two points
having a potential difference 12 V?
Solution
The amount of charge Q, that flows between two points at potential
difference V (= 12 V) is 2 C. Thus, the amount of work W, done in
moving the charge [from Eq. (12.2)] is
W = VQ
= 12 V × 2 C
= 24 J.
QUESTIONS
?
12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM
We know that an electric circuit, as shown in Fig. 12.1, comprises a cell
(or a battery), a plug key, electrical component(s), and connecting wires.
It is often convenient to draw a schematic diagram, in which different
components of the circuit are represented by the symbols conveniently
used. Conventional symbols  used to represent some of the most
commonly used electrical components are given in Table 12.1.
1. Name a device that helps to maintain a potential difference across a
conductor.
2. What is meant by saying that the potential difference between two points
is 1 V?
3. How much energy is given to each coulomb of charge passing through a
6 V battery?
2020-21
Page 5


Electricity
12 CHAPTER
E
lectricity has an important place in modern society. It is a controllable
and convenient form of energy for a variety of uses in homes, schools,
hospitals, industries and so on. What constitutes electricity? How does
it flow in an electric circuit? What are the factors that control or regulate
the current through an electric circuit? In this Chapter, we shall attempt
to answer such questions. We shall also discuss the heating effect of
electric current and its applications.
12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT 12.1 ELECTRIC CURRENT AND CIRCUIT
We are familiar with air current and water current. We know that flowing
water constitute water current in rivers. Similarly, if the electric charge
flows through a conductor (for example, through a metallic wire), we
say that there is an electric current in the conductor. In a torch, we
know that the cells (or a battery, when placed in proper order) provide
flow of charges or an electric current through the torch bulb to glow. We
have also seen that the torch gives light only when its switch is on. What
does a switch do? A switch makes a conducting link between the cell and
the bulb. A continuous and closed path of an electric current is called an
electric circuit. Now, if the circuit is broken anywhere (or the switch of the
torch is turned off ), the current stops flowing and the bulb does not glow.
How do we express electric current? Electric current is expressed by
the amount of charge flowing through a particular area in unit time. In
other words, it is the rate of flow of electric charges. In circuits using
metallic wires, electrons constitute the flow of charges. However, electrons
were not known at the time when the phenomenon of electricity was first
observed. So, electric current was considered to be the flow of positive
charges and the direction of flow of positive charges was taken to be the
direction of electric current. Conventionally, in an electric circuit the
direction of electric current is taken as opposite to the direction of the
flow of electrons, which are negative charges.
2020-21
Science
200
If a net charge Q, flows across any cross-section of a conductor in
time t, then the current I, through the cross-section is
I
Q
t
=
(12.1)
The SI unit of electric charge is coulomb (C), which is equivalent to
the charge contained in nearly 6 × 10
18
 electrons. (We know that an
electron possesses a negative charge of 1.6 × 10
–19 
C.) The electric
current is expressed by a unit called ampere (A), named after the
French scientist, Andre-Marie Ampere (1775–1836). One ampere is
constituted by the flow of one coulomb of charge per second, that is,
1 A = 1 C/1 s. Small quantities of current are expressed in milliampere
(1 mA = 10
–3
 A) or in microampere (1 µA = 10
–6
 A).
An instrument called ammeter measures electric
current in a circuit. It is always connected in series
in a circuit through which the current is to be
measured. Figure 12.1 shows the schematic
diagram of a typical electric circuit comprising a
cell, an electric bulb, an ammeter and a plug key.
Note that the electric current flows in the circuit
from the positive terminal of the cell to the negative
terminal of the cell through the bulb and ammeter.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
A schematic diagram of an electric circuit
comprising – cell, electric bulb, ammeter and
plug key
QUESTIONS
?
Example 12.1
A current of 0.5 A is drawn by a filament of an electric bulb for 10
minutes. Find the amount of electric charge that flows through the
circuit.
Solution
We are given, I = 0.5 A; t = 10 min = 600 s.
From Eq. (12.1), we have
Q = It
= 0.5 A × 600 s
= 300 C
1. What does an electric circuit mean?
2. Define the unit of current.
3. Calculate the number of electrons constituting one coulomb of charge.
2020-21
Electricity 201
12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE 12.2 ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
What makes the electric charge to flow? Let us consider the analogy of
flow of water. Charges do not flow in a copper wire by themselves, just as
water in a perfectly horizontal tube does not flow. If one end of the tube
is connected to a tank of water kept at a higher level, such that there is a
pressure difference between the two ends of the tube, water flows out of
the other end of the tube. For flow of charges in a conducting metallic
wire, the gravity, of course, has no role to play; the electrons move only
if there is a difference of electric pressure – called the potential difference –
along the conductor. This difference of potential may be produced by a
battery, consisting of one or more electric cells. The chemical action within
a cell generates the potential difference across the terminals of the cell,
even when no current is drawn from it. When the cell is connected to a
conducting circuit element, the potential difference sets the charges in
motion in the conductor and produces an electric current. In order to
maintain the current in a given electric circuit, the cell has to expend its
chemical energy stored in it.
We define the electric potential difference between two points in an
electric circuit carrying some current as the work done to move a unit
charge from one point to the other –
Potential difference (V) between two points = Work done (W )/Charge (Q)
V = W/Q (12.2)
The SI unit of electric potential difference is volt (V), named after
Alessandro Volta (1745–1827), an Italian physicist. One volt is the
‘Flow’ of charges inside a wire
How does a metal conduct electricity? You would think that a low-energy electron
would have great difficulty passing through a solid conductor. Inside the solid, the
atoms are packed together with very little spacing between them. But it turns out
that the electrons are able to ‘travel’ through a perfect solid crystal smoothly and
easily, almost as if they were in a vacuum. The ‘motion’ of electrons in a conductor,
however, is very different from that of charges in empty space. When a steady current
flows through a conductor, the electrons in it move with a certain average ‘drift speed’.
One can calculate this drift speed of electrons for a typical copper wire carrying a
small current, and it is found to be actually very small, of the order of 1 mm s
-1
. How
is it then that an electric bulb lights up as soon as we turn the switch on? It cannot be
that a current starts only when an electron from one terminal of the electric supply
physically reaches the other terminal through the bulb, because the physical drift of
electrons in the conducting wires is a very slow process. The exact mechanism of the
current flow, which takes place with a speed close to the speed of light, is fascinating,
but it is beyond the scope of this book. Do you feel like probing this question at an
advanced level?
More to Know!
2020-21
Science
202
potential difference between two points in a current carrying conductor
when 1 joule of work is done to move a charge of 1 coulomb from one
point to the other.
Therefore, 1 volt = 
1 joule
1 coulomb
(12.3)
1 V  = 1 J C
–1
The potential difference is measured by means of an instrument called
the voltmeter. The voltmeter is always connected in parallel across the
points between which the potential difference is to be measured.
Example 12.2
How much work is done in moving a charge of 2 C across two points
having a potential difference 12 V?
Solution
The amount of charge Q, that flows between two points at potential
difference V (= 12 V) is 2 C. Thus, the amount of work W, done in
moving the charge [from Eq. (12.2)] is
W = VQ
= 12 V × 2 C
= 24 J.
QUESTIONS
?
12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM 12.3 CIRCUIT DIAGRAM
We know that an electric circuit, as shown in Fig. 12.1, comprises a cell
(or a battery), a plug key, electrical component(s), and connecting wires.
It is often convenient to draw a schematic diagram, in which different
components of the circuit are represented by the symbols conveniently
used. Conventional symbols  used to represent some of the most
commonly used electrical components are given in Table 12.1.
1. Name a device that helps to maintain a potential difference across a
conductor.
2. What is meant by saying that the potential difference between two points
is 1 V?
3. How much energy is given to each coulomb of charge passing through a
6 V battery?
2020-21
Electricity 203
Table 12.1 Symbols of some commonly used components in circuit diagrams
Sl. Components Symbols
No.
1 An electric cell
2 A battery or a combination of cells
3 Plug key or switch (open)
4 Plug key or switch (closed)
5 A wire joint
6 Wires crossing without joining
7 Electric bulb or    
8 A resistor of resistance R
9 Variable resistance or rheostat or 
10 Ammeter
11 Voltmeter
12.4 OHM’S LA 12.4 OHM’S LA 12.4 OHM’S LA 12.4 OHM’S LA 12.4 OHM’S LAW W W W W
Is there a relationship between the potential difference across a
conductor and the current through it? Let us explore with an Activity.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Set up a circuit as shown in Fig. 12.2, consisting of a nichrome wire XY of length, say 0.5 m,
an ammeter, a voltmeter and four cells of 1.5 V each. (Nichrome is an alloy of nickel, chromium,
manganese, and iron metals.)
2020-21