NCERT Textbook: Magnetic Effects of Current | Science & Technology for UPSC CSE PDF Download

 Page 1


Magnetic Effects of
Electric Current
12 CHAPTER
I
n the previous Chapter on ‘Electricity’ we learnt about the heating
effects of electric current. What could be the other effects of electric
current? We know that an electric current-carrying wire behaves like a
magnet. Let us perform the following Activity to reinforce it.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Take a straight thick copper wire and place it
between the points X and Y in an electric circuit,
as shown in Fig. 12.1. The wire XY is kept
perpendicular to the plane of paper.
n Horizontally place a small compass near to this
copper wire. See the position of its needle.
n Pass the current through the circuit by
inserting the key into the plug.
n Observe the change in the position of the
compass needle.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
Compass needle is deflected on passing an electric
current through a metallic conductor
We see that the needle is deflected. What does it mean?  It means that
the electric current through the copper wire has produced a magnetic
effect. Thus we can say that electricity and magnetism are linked to each
other. Then, what about the reverse possibility of an electric effect of
moving magnets? In this Chapter we will study magnetic fields and such
electromagnetic effects. We shall also study about electromagnets which
involve the magnetic effect of electric current.
Hans Christian Oersted (1777–1851)
Hans Christian Oersted, one of the leading scientists of the 19
th
century, played a crucial role in understanding electromagnetism. In
1820 he accidentally discovered that a compass needle got deflected
when an electric current passed through a metallic wire placed nearby.
Through this observation Oersted showed that electricity and
magnetism were related phenomena. His research later created
technologies such as the radio, television and fiber optics. The unit of
magnetic field strength is named the oersted in his honor.
Resistor
Long straight
conductor
2024-25
Page 2


Magnetic Effects of
Electric Current
12 CHAPTER
I
n the previous Chapter on ‘Electricity’ we learnt about the heating
effects of electric current. What could be the other effects of electric
current? We know that an electric current-carrying wire behaves like a
magnet. Let us perform the following Activity to reinforce it.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Take a straight thick copper wire and place it
between the points X and Y in an electric circuit,
as shown in Fig. 12.1. The wire XY is kept
perpendicular to the plane of paper.
n Horizontally place a small compass near to this
copper wire. See the position of its needle.
n Pass the current through the circuit by
inserting the key into the plug.
n Observe the change in the position of the
compass needle.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
Compass needle is deflected on passing an electric
current through a metallic conductor
We see that the needle is deflected. What does it mean?  It means that
the electric current through the copper wire has produced a magnetic
effect. Thus we can say that electricity and magnetism are linked to each
other. Then, what about the reverse possibility of an electric effect of
moving magnets? In this Chapter we will study magnetic fields and such
electromagnetic effects. We shall also study about electromagnets which
involve the magnetic effect of electric current.
Hans Christian Oersted (1777–1851)
Hans Christian Oersted, one of the leading scientists of the 19
th
century, played a crucial role in understanding electromagnetism. In
1820 he accidentally discovered that a compass needle got deflected
when an electric current passed through a metallic wire placed nearby.
Through this observation Oersted showed that electricity and
magnetism were related phenomena. His research later created
technologies such as the radio, television and fiber optics. The unit of
magnetic field strength is named the oersted in his honor.
Resistor
Long straight
conductor
2024-25
Science
196
12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES
We are familiar with the fact that a compass needle gets deflected when
brought near a bar magnet. A compass needle is, in fact, a small bar
magnet. The ends of the compass needle point approximately towards
north and south directions. The end pointing towards north is called north
seeking or north pole. The other end that points towards south is called
south seeking or south pole. Through various activities we have observed
that like poles repel, while unlike poles of magnets attract each other.
QUESTION
?
1. Why does a compass needle get deflected when brought near
a bar magnet?
Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2
n Fix a sheet of white paper on a drawing
board using some adhesive material.
n Place a bar magnet in the centre of it.
n Sprinkle some iron filings uniformly
around the bar magnet (Fig. 12.2). A
salt-sprinkler may be used for this
purpose.
n Now tap the board gently.
n What do you observe?
Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2
Iron filings near the bar magnet align
themselves along the field lines.
The iron filings arrange themselves in a pattern as shown
Fig. 12.2.  Why do the iron filings arrange in such a pattern? What does
this pattern demonstrate?  The magnet exerts its influence in the region
surrounding it.  Therefore the iron filings experience a force.  The force
thus exerted makes iron filings to arrange in a pattern. The region
surrounding a magnet, in which the force of the magnet can be detected,
is said to have a magnetic field.  The lines along which the iron filings
align themselves represent magnetic field lines.
Are there other ways of obtaining magnetic field lines around a bar
magnet?  Yes, you can yourself draw the field lines of a bar magnet.
Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3
n Take a small compass and a bar magnet.
n Place the magnet on a sheet of white paper fixed on a drawing
board, using some adhesive material.
n Mark the boundary of the magnet.
n Place the compass near the north pole of the magnet. How does
it behave? The south pole of the needle points towards the north
pole of the magnet. The north pole of the compass is directed
away from the north pole of the magnet.
2024-25
Page 3


Magnetic Effects of
Electric Current
12 CHAPTER
I
n the previous Chapter on ‘Electricity’ we learnt about the heating
effects of electric current. What could be the other effects of electric
current? We know that an electric current-carrying wire behaves like a
magnet. Let us perform the following Activity to reinforce it.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Take a straight thick copper wire and place it
between the points X and Y in an electric circuit,
as shown in Fig. 12.1. The wire XY is kept
perpendicular to the plane of paper.
n Horizontally place a small compass near to this
copper wire. See the position of its needle.
n Pass the current through the circuit by
inserting the key into the plug.
n Observe the change in the position of the
compass needle.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
Compass needle is deflected on passing an electric
current through a metallic conductor
We see that the needle is deflected. What does it mean?  It means that
the electric current through the copper wire has produced a magnetic
effect. Thus we can say that electricity and magnetism are linked to each
other. Then, what about the reverse possibility of an electric effect of
moving magnets? In this Chapter we will study magnetic fields and such
electromagnetic effects. We shall also study about electromagnets which
involve the magnetic effect of electric current.
Hans Christian Oersted (1777–1851)
Hans Christian Oersted, one of the leading scientists of the 19
th
century, played a crucial role in understanding electromagnetism. In
1820 he accidentally discovered that a compass needle got deflected
when an electric current passed through a metallic wire placed nearby.
Through this observation Oersted showed that electricity and
magnetism were related phenomena. His research later created
technologies such as the radio, television and fiber optics. The unit of
magnetic field strength is named the oersted in his honor.
Resistor
Long straight
conductor
2024-25
Science
196
12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES
We are familiar with the fact that a compass needle gets deflected when
brought near a bar magnet. A compass needle is, in fact, a small bar
magnet. The ends of the compass needle point approximately towards
north and south directions. The end pointing towards north is called north
seeking or north pole. The other end that points towards south is called
south seeking or south pole. Through various activities we have observed
that like poles repel, while unlike poles of magnets attract each other.
QUESTION
?
1. Why does a compass needle get deflected when brought near
a bar magnet?
Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2
n Fix a sheet of white paper on a drawing
board using some adhesive material.
n Place a bar magnet in the centre of it.
n Sprinkle some iron filings uniformly
around the bar magnet (Fig. 12.2). A
salt-sprinkler may be used for this
purpose.
n Now tap the board gently.
n What do you observe?
Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2
Iron filings near the bar magnet align
themselves along the field lines.
The iron filings arrange themselves in a pattern as shown
Fig. 12.2.  Why do the iron filings arrange in such a pattern? What does
this pattern demonstrate?  The magnet exerts its influence in the region
surrounding it.  Therefore the iron filings experience a force.  The force
thus exerted makes iron filings to arrange in a pattern. The region
surrounding a magnet, in which the force of the magnet can be detected,
is said to have a magnetic field.  The lines along which the iron filings
align themselves represent magnetic field lines.
Are there other ways of obtaining magnetic field lines around a bar
magnet?  Yes, you can yourself draw the field lines of a bar magnet.
Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3
n Take a small compass and a bar magnet.
n Place the magnet on a sheet of white paper fixed on a drawing
board, using some adhesive material.
n Mark the boundary of the magnet.
n Place the compass near the north pole of the magnet. How does
it behave? The south pole of the needle points towards the north
pole of the magnet. The north pole of the compass is directed
away from the north pole of the magnet.
2024-25
Magnetic Effects of Electric Current 197
Magnetic field is a quantity that has both direction and magnitude.
The direction of the magnetic field is taken to be the direction in which a
north pole of the compass needle moves inside it.  Therefore it is taken
by convention that the field lines emerge from north pole and merge at
the south pole (note the arrows marked on the field lines in Fig. 12.4).
Inside the magnet, the direction of field lines is from its south pole to its
north pole. Thus the magnetic field lines are closed curves.
The relative strength of the magnetic field is shown by the degree of
closeness of the field lines. The field is stronger, that is, the force acting
on the pole of another magnet placed is greater where the field lines are
crowded (see Fig. 12.4).
No two field-lines are found to cross each other. If they did, it would
mean that at the point of intersection, the compass needle would point
towards two directions, which is not possible.
12.2 12.2 12.2 12.2 12.2 MA MA MA MA MAGNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT- - - - -C C C C CARRYING ARRYING ARRYING ARRYING ARRYING
CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR
In Activity 12.1, we have seen that an electric current through a
metallic conductor produces a magnetic field around it. In order to
find the direction of the field produced let us repeat the activity in the
following way –
Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3
Drawing a magnetic field line with the help of a
compass needle
n Mark the position of two ends of the needle.
n Now move the needle to a new position
such that its south pole occupies the
position previously occupied by its north
pole.
n In this way, proceed step by step till you
reach the south pole of the magnet as
shown in Fig. 12.3.
n Join the points marked on the paper by a
smooth curve.  This curve represents
a field line.
n Repeat the above procedure and draw as
many lines as you can. You will get a
pattern shown in Fig. 12.4. These lines
represent the magnetic field around the
magnet. These are known as magnetic
field lines.
n Observe the deflection in the compass
needle as you move it along a field line.
The deflection increases as the needle is
moved towards the poles.
Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4
Field lines around a bar magnet
2024-25
Page 4


Magnetic Effects of
Electric Current
12 CHAPTER
I
n the previous Chapter on ‘Electricity’ we learnt about the heating
effects of electric current. What could be the other effects of electric
current? We know that an electric current-carrying wire behaves like a
magnet. Let us perform the following Activity to reinforce it.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Take a straight thick copper wire and place it
between the points X and Y in an electric circuit,
as shown in Fig. 12.1. The wire XY is kept
perpendicular to the plane of paper.
n Horizontally place a small compass near to this
copper wire. See the position of its needle.
n Pass the current through the circuit by
inserting the key into the plug.
n Observe the change in the position of the
compass needle.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
Compass needle is deflected on passing an electric
current through a metallic conductor
We see that the needle is deflected. What does it mean?  It means that
the electric current through the copper wire has produced a magnetic
effect. Thus we can say that electricity and magnetism are linked to each
other. Then, what about the reverse possibility of an electric effect of
moving magnets? In this Chapter we will study magnetic fields and such
electromagnetic effects. We shall also study about electromagnets which
involve the magnetic effect of electric current.
Hans Christian Oersted (1777–1851)
Hans Christian Oersted, one of the leading scientists of the 19
th
century, played a crucial role in understanding electromagnetism. In
1820 he accidentally discovered that a compass needle got deflected
when an electric current passed through a metallic wire placed nearby.
Through this observation Oersted showed that electricity and
magnetism were related phenomena. His research later created
technologies such as the radio, television and fiber optics. The unit of
magnetic field strength is named the oersted in his honor.
Resistor
Long straight
conductor
2024-25
Science
196
12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES
We are familiar with the fact that a compass needle gets deflected when
brought near a bar magnet. A compass needle is, in fact, a small bar
magnet. The ends of the compass needle point approximately towards
north and south directions. The end pointing towards north is called north
seeking or north pole. The other end that points towards south is called
south seeking or south pole. Through various activities we have observed
that like poles repel, while unlike poles of magnets attract each other.
QUESTION
?
1. Why does a compass needle get deflected when brought near
a bar magnet?
Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2
n Fix a sheet of white paper on a drawing
board using some adhesive material.
n Place a bar magnet in the centre of it.
n Sprinkle some iron filings uniformly
around the bar magnet (Fig. 12.2). A
salt-sprinkler may be used for this
purpose.
n Now tap the board gently.
n What do you observe?
Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2
Iron filings near the bar magnet align
themselves along the field lines.
The iron filings arrange themselves in a pattern as shown
Fig. 12.2.  Why do the iron filings arrange in such a pattern? What does
this pattern demonstrate?  The magnet exerts its influence in the region
surrounding it.  Therefore the iron filings experience a force.  The force
thus exerted makes iron filings to arrange in a pattern. The region
surrounding a magnet, in which the force of the magnet can be detected,
is said to have a magnetic field.  The lines along which the iron filings
align themselves represent magnetic field lines.
Are there other ways of obtaining magnetic field lines around a bar
magnet?  Yes, you can yourself draw the field lines of a bar magnet.
Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3
n Take a small compass and a bar magnet.
n Place the magnet on a sheet of white paper fixed on a drawing
board, using some adhesive material.
n Mark the boundary of the magnet.
n Place the compass near the north pole of the magnet. How does
it behave? The south pole of the needle points towards the north
pole of the magnet. The north pole of the compass is directed
away from the north pole of the magnet.
2024-25
Magnetic Effects of Electric Current 197
Magnetic field is a quantity that has both direction and magnitude.
The direction of the magnetic field is taken to be the direction in which a
north pole of the compass needle moves inside it.  Therefore it is taken
by convention that the field lines emerge from north pole and merge at
the south pole (note the arrows marked on the field lines in Fig. 12.4).
Inside the magnet, the direction of field lines is from its south pole to its
north pole. Thus the magnetic field lines are closed curves.
The relative strength of the magnetic field is shown by the degree of
closeness of the field lines. The field is stronger, that is, the force acting
on the pole of another magnet placed is greater where the field lines are
crowded (see Fig. 12.4).
No two field-lines are found to cross each other. If they did, it would
mean that at the point of intersection, the compass needle would point
towards two directions, which is not possible.
12.2 12.2 12.2 12.2 12.2 MA MA MA MA MAGNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT- - - - -C C C C CARRYING ARRYING ARRYING ARRYING ARRYING
CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR
In Activity 12.1, we have seen that an electric current through a
metallic conductor produces a magnetic field around it. In order to
find the direction of the field produced let us repeat the activity in the
following way –
Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3
Drawing a magnetic field line with the help of a
compass needle
n Mark the position of two ends of the needle.
n Now move the needle to a new position
such that its south pole occupies the
position previously occupied by its north
pole.
n In this way, proceed step by step till you
reach the south pole of the magnet as
shown in Fig. 12.3.
n Join the points marked on the paper by a
smooth curve.  This curve represents
a field line.
n Repeat the above procedure and draw as
many lines as you can. You will get a
pattern shown in Fig. 12.4. These lines
represent the magnetic field around the
magnet. These are known as magnetic
field lines.
n Observe the deflection in the compass
needle as you move it along a field line.
The deflection increases as the needle is
moved towards the poles.
Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4
Field lines around a bar magnet
2024-25
Science
198
12.2.1 Magnetic Field due to a Current through a Straight
Conductor
What determines the pattern of the magnetic field generated by a current
through a conductor? Does the pattern depend on the shape of the
conductor? We shall investigate this with an activity.
We shall first consider the pattern of the magnetic field around a
straight conductor carrying current.
Activity 12.4 Activity 12.4 Activity 12.4 Activity 12.4 Activity 12.4
n Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect
all of them in series as shown in Fig. 12.5 (a).
n Place the straight wire parallel to and over a compass needle.
n Plug the key in the circuit.
n Observe the direction of deflection of the north pole of the needle. If the current flows from
north to south, as shown in Fig. 12.5 (a), the north pole of the compass needle would move
towards the east.
n Replace the cell connections in the circuit as shown in Fig. 12.5 (b). This would result in
the change of the direction of current through the copper wire, that is, from south to
north.
n Observe the change in the direction of deflection of the needle. You will see that now the
needle moves in opposite direction, that is, towards the west [Fig. 12.5 (b)]. It means that
the direction of magnetic field produced by the electric current is also reversed.
Figure 12.5 Figure 12.5 Figure 12.5 Figure 12.5 Figure 12.5 A simple electric circuit in which a straight copper wire is placed parallel to and over a compass
needle. The deflection in the needle becomes opposite when the direction of the current is reversed.
(a) (b)
Activity 12.5 Activity 12.5 Activity 12.5 Activity 12.5 Activity 12.5
n Take a battery (12 V), a variable resistance (or a rheostat), an
ammeter (0–5 A), a plug key, connecting wires and a long straight
thick copper wire.
n Insert the thick wire through the centre, normal to the plane of a
rectangular cardboard.  Take care that the cardboard is fixed and
does not slide up or down.
2024-25
Page 5


Magnetic Effects of
Electric Current
12 CHAPTER
I
n the previous Chapter on ‘Electricity’ we learnt about the heating
effects of electric current. What could be the other effects of electric
current? We know that an electric current-carrying wire behaves like a
magnet. Let us perform the following Activity to reinforce it.
Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1 Activity 12.1
n Take a straight thick copper wire and place it
between the points X and Y in an electric circuit,
as shown in Fig. 12.1. The wire XY is kept
perpendicular to the plane of paper.
n Horizontally place a small compass near to this
copper wire. See the position of its needle.
n Pass the current through the circuit by
inserting the key into the plug.
n Observe the change in the position of the
compass needle.
Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1 Figure 12.1
Compass needle is deflected on passing an electric
current through a metallic conductor
We see that the needle is deflected. What does it mean?  It means that
the electric current through the copper wire has produced a magnetic
effect. Thus we can say that electricity and magnetism are linked to each
other. Then, what about the reverse possibility of an electric effect of
moving magnets? In this Chapter we will study magnetic fields and such
electromagnetic effects. We shall also study about electromagnets which
involve the magnetic effect of electric current.
Hans Christian Oersted (1777–1851)
Hans Christian Oersted, one of the leading scientists of the 19
th
century, played a crucial role in understanding electromagnetism. In
1820 he accidentally discovered that a compass needle got deflected
when an electric current passed through a metallic wire placed nearby.
Through this observation Oersted showed that electricity and
magnetism were related phenomena. His research later created
technologies such as the radio, television and fiber optics. The unit of
magnetic field strength is named the oersted in his honor.
Resistor
Long straight
conductor
2024-25
Science
196
12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES 12.1 MAGNETIC FIELD AND FIELD LINES
We are familiar with the fact that a compass needle gets deflected when
brought near a bar magnet. A compass needle is, in fact, a small bar
magnet. The ends of the compass needle point approximately towards
north and south directions. The end pointing towards north is called north
seeking or north pole. The other end that points towards south is called
south seeking or south pole. Through various activities we have observed
that like poles repel, while unlike poles of magnets attract each other.
QUESTION
?
1. Why does a compass needle get deflected when brought near
a bar magnet?
Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2 Activity 12.2
n Fix a sheet of white paper on a drawing
board using some adhesive material.
n Place a bar magnet in the centre of it.
n Sprinkle some iron filings uniformly
around the bar magnet (Fig. 12.2). A
salt-sprinkler may be used for this
purpose.
n Now tap the board gently.
n What do you observe?
Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2 Figure 12.2
Iron filings near the bar magnet align
themselves along the field lines.
The iron filings arrange themselves in a pattern as shown
Fig. 12.2.  Why do the iron filings arrange in such a pattern? What does
this pattern demonstrate?  The magnet exerts its influence in the region
surrounding it.  Therefore the iron filings experience a force.  The force
thus exerted makes iron filings to arrange in a pattern. The region
surrounding a magnet, in which the force of the magnet can be detected,
is said to have a magnetic field.  The lines along which the iron filings
align themselves represent magnetic field lines.
Are there other ways of obtaining magnetic field lines around a bar
magnet?  Yes, you can yourself draw the field lines of a bar magnet.
Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3 Activity 12.3
n Take a small compass and a bar magnet.
n Place the magnet on a sheet of white paper fixed on a drawing
board, using some adhesive material.
n Mark the boundary of the magnet.
n Place the compass near the north pole of the magnet. How does
it behave? The south pole of the needle points towards the north
pole of the magnet. The north pole of the compass is directed
away from the north pole of the magnet.
2024-25
Magnetic Effects of Electric Current 197
Magnetic field is a quantity that has both direction and magnitude.
The direction of the magnetic field is taken to be the direction in which a
north pole of the compass needle moves inside it.  Therefore it is taken
by convention that the field lines emerge from north pole and merge at
the south pole (note the arrows marked on the field lines in Fig. 12.4).
Inside the magnet, the direction of field lines is from its south pole to its
north pole. Thus the magnetic field lines are closed curves.
The relative strength of the magnetic field is shown by the degree of
closeness of the field lines. The field is stronger, that is, the force acting
on the pole of another magnet placed is greater where the field lines are
crowded (see Fig. 12.4).
No two field-lines are found to cross each other. If they did, it would
mean that at the point of intersection, the compass needle would point
towards two directions, which is not possible.
12.2 12.2 12.2 12.2 12.2 MA MA MA MA MAGNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT GNETIC FIELD DUE TO A CURRENT- - - - -C C C C CARRYING ARRYING ARRYING ARRYING ARRYING
CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR CONDUCTOR
In Activity 12.1, we have seen that an electric current through a
metallic conductor produces a magnetic field around it. In order to
find the direction of the field produced let us repeat the activity in the
following way –
Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3 Figure 12.3
Drawing a magnetic field line with the help of a
compass needle
n Mark the position of two ends of the needle.
n Now move the needle to a new position
such that its south pole occupies the
position previously occupied by its north
pole.
n In this way, proceed step by step till you
reach the south pole of the magnet as
shown in Fig. 12.3.
n Join the points marked on the paper by a
smooth curve.  This curve represents
a field line.
n Repeat the above procedure and draw as
many lines as you can. You will get a
pattern shown in Fig. 12.4. These lines
represent the magnetic field around the
magnet. These are known as magnetic
field lines.
n Observe the deflection in the compass
needle as you move it along a field line.
The deflection increases as the needle is
moved towards the poles.
Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4 Figure 12.4
Field lines around a bar magnet
2024-25
Science
198
12.2.1 Magnetic Field due to a Current through a Straight
Conductor
What determines the pattern of the magnetic field generated by a current
through a conductor? Does the pattern depend on the shape of the
conductor? We shall investigate this with an activity.
We shall first consider the pattern of the magnetic field around a
straight conductor carrying current.
Activity 12.4 Activity 12.4 Activity 12.4 Activity 12.4 Activity 12.4
n Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect
all of them in series as shown in Fig. 12.5 (a).
n Place the straight wire parallel to and over a compass needle.
n Plug the key in the circuit.
n Observe the direction of deflection of the north pole of the needle. If the current flows from
north to south, as shown in Fig. 12.5 (a), the north pole of the compass needle would move
towards the east.
n Replace the cell connections in the circuit as shown in Fig. 12.5 (b). This would result in
the change of the direction of current through the copper wire, that is, from south to
north.
n Observe the change in the direction of deflection of the needle. You will see that now the
needle moves in opposite direction, that is, towards the west [Fig. 12.5 (b)]. It means that
the direction of magnetic field produced by the electric current is also reversed.
Figure 12.5 Figure 12.5 Figure 12.5 Figure 12.5 Figure 12.5 A simple electric circuit in which a straight copper wire is placed parallel to and over a compass
needle. The deflection in the needle becomes opposite when the direction of the current is reversed.
(a) (b)
Activity 12.5 Activity 12.5 Activity 12.5 Activity 12.5 Activity 12.5
n Take a battery (12 V), a variable resistance (or a rheostat), an
ammeter (0–5 A), a plug key, connecting wires and a long straight
thick copper wire.
n Insert the thick wire through the centre, normal to the plane of a
rectangular cardboard.  Take care that the cardboard is fixed and
does not slide up or down.
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Magnetic Effects of Electric Current 199
What happens to the deflection of the compass needle placed at a
given point if the current in the copper wire is changed? To see this, vary
the current in the wire. We find that the deflection in the needle also
changes. In fact, if the current is increased, the deflection also increases.
It indicates that the magnitude of the magnetic field produced at a given
point increases as the current through the wire increases.
What happens to the deflection of the needle if the compass is moved
away from the copper wire but the current through the wire remains the
same? To see this, now place the compass at a farther point from the
conducting wire (say at point Q). What change do you observe? We see
that the deflection in the needle decreases. Thus the magnetic field
produced by a given current in the conductor decreases as the distance
from it increases. From Fig. 12.6, it can be noticed that the concentric
circles representing the magnetic field around a current-carrying straight
wire become larger and larger as we move away from it.
12.2.2 Right-Hand Thumb Rule
A convenient way of finding the direction of magnetic field associated
with a current-carrying conductor is given in Fig. 12.7.
n Connect the copper wire vertically between the
points X and Y, as shown in Fig. 12.6 (a), in
series with the battery, a plug and key.
n Sprinkle some iron filings uniformly on the
cardboard. (You may use a salt sprinkler for this
purpose.)
n Keep the variable of the rheostat at a fixed
position and note the current through the
ammeter.
n Close the key so that a current flows through
the wire. Ensure that the copper wire placed
between the points X and Y remains vertically
straight.
n Gently tap the cardboard a few times. Observe
the pattern of the iron filings. You would find
that the iron filings align themselves showing
a pattern of concentric circles around the
copper wire (Fig. 12.6).
n What do these concentric circles represent?
They represent the magnetic field lines.
n How can the direction of the magnetic field be
found? Place a compass at a point (say P) over
a circle. Observe the direction of the needle. The
direction of the north pole of the compass
needle would give the direction of the field lines
produced by the electric current through the
straight wire at point P. Show the direction by
an arrow.
n Does the direction of magnetic field lines get
reversed if the direction of current through the
straight copper wire is reversed? Check it.
Figure 12.6 Figure 12.6 Figure 12.6 Figure 12.6 Figure 12.6
(a) A pattern of concentric circles indicating
the field lines of a magnetic field around a
straight conducting wire. The arrows in the
circles show the direction of the field lines.
(b) A close up of the pattern obtained.
(a)
(b)
Variable
resistance
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