Magnetism

Characteristics of a Magnetic Field Produced By Current Carrying Conductors

When a magnetic compass is brought near a current carrying wire, the compass needle
gets deflected. Why does this happen?

The compass needle suffers deflection because a current carrying wire produces
magnetic field lines around it.

Construct a simple open circuit with two open ends M and N respectively. Take a piece
of aluminium wire and connect it with the two open ends. Now, bring a magnetic
compass close to the aluminium wire and wait till the compass needle comes to rest.
Now, allow the current to flow through the wire and notice the direction of deflection of
the North pole of the compass needle with respect to the direction of the current flowing
through the wire.

Now, switch off the current and reverse the direction of the current by reversing the
polarities of the battery. Now, switch on the current and observe the deflection of the
North pole of the compass needle. You will find that the compass needle in this case
gets deflected in the opposite direction. Why does this happen?

This happens because the direction of the magnetic field lines produced around a
conductor depends on the direction of the current flowing through it. Hence, the
direction of the magnetic field lines gets reversed when the direction of the current
flowing through the conductor is reversed.

Pattern of magnetic field lines around a current carrying a straight conductor

Similar to bar magnets, a current carrying a straight conductor also has magnetic field
lines around it. What factors are responsible for the pattern of magnetic field lines?
To understand the pattern of magnetic field lines around a current carrying a straight
conductor, we will perform the following activity:

Using a rheostat box, a 12 V battery, and a key, construct a simple open electric circuit
with open ends M and N respectively (you can obtain a rheostat box from your science
laboratory). Now, take a cardboard sheet and insert an aluminium wire through the
centre of the sheet.

Connect the open ends of the wire with the open ends M and N of the circuit. Now,
spread a few but equal iron filings over the sheet in such a way that the sheet stays
horizontal. Now, turn the knob of the rheostat box to maximum to allow the current to
pass through the circuit and the aluminium wire.

Now, turn the knob of the rheostat box slowly to lower the resistance and observe the
movement of the iron filings. You will see that the iron filings arrange themselves in a
regular pattern. Can you identify that regular pattern?

You will find that the iron filings align themselves in concentric circles around the
conductor. What does this represent?

This represents that concentric magnetic fields are produced in the form of concentric
circles around a current carrying the conductor.

How is the direction of concentric magnetic fields around the conductor related
with the direction of current?

To understand this, slowly bring a magnetic compass near the aluminium wire and
observe its deflection. Now, turn the knob of the rheostat slowly from its maximum value
to its minimum value and simultaneously observe the direction of deflection of the North
Pole of the compass needle. The compass needle deflects clockwise with an increased
magnitude.

Now, switch off the current and reverse the polarities of the battery so that the direction
of the current gets reversed. Keep the rheostat at its maximum value. Turn the key on
and increase the current by turning the knob of the rheostat box towards its minimum
value.

You will find that as current increases, the deflection of the needle in an anti-clockwise
direction also increases. Now, reduce the current slowly. What change in deflection of
the needle do you observe?

You can see that the deflection in the needle also decreases. Therefore, we can say
that the strength of a magnetic field produced around a straight current carrying
conductor is directly proportional to the magnitude of the current.

Similarly, if you move the compass away from the current carrying a wire, then you will
notice that the deflection in the compass needle decreases and vice-versa.

Hence, from the above discussion, we find that the magnitude of the magnetic field
produced by a straight current carrying a wire at a given point is

(i) directly proportional to the strength of the current passing through the wire

(ii) inversely proportional to the distance of this point from the wire

The direction of magnetic fields around a conductor is given by the right hand
thumb rule.

Right hand thumb rule

Magnetic field due to a current carrying a circular loop

When a straight wire is bent, it becomes a circular loop. What happens when a
current is allowed to pass through a circular loop? Will the circular magnetic
fields produced around the current carrying a straight conductor also bend when
the conductor is bent?

To understand the pattern of magnetic field lines around a current carrying a straight
conductor, we will perform the following activity:
Take a rectangular cardboard sheet with two holes M and N respectively. Insert the two
ends of a circular loop of the wire into the holes respectively. Now, connect both ends
with a circuit (as shown in the following figure). Before closing the key, spread some
iron filings over the sheet.

Hold the board gently and observe the pattern formed by the iron filings on it. You will
find a regular pattern. Can you explain the origin of this regular pattern?

When a current is passed through a circular loop of wire, magnetic field lines are
produced that pass through the loop in the direction normal to the loop area (given by
Maxwell’s right hand rule).

Every point on the current carrying wire produces its own concentric field lines with its
centre at the point. The produced magnetic field lines are circular near the current
carrying the loop. As we move away from the wire, concentric circles representing
magnetic field lines become bigger, indicating that the strength of the produced
magnetic force decreases gradually.

The magnitude of the field lines produced by a circular loop at its

centre is

1. directly proportional to the amount of current

2. inversely proportional to the radius of the loop

Magnetic field due to a current carrying a solenoid

A solenoid is a long straight insulated wire, often wrapped around a cylinder-shaped
body. A copper coil is wound around a cylindrical core, such that the core’s diameter is
lesser than its length. It produces a magnetic field when electric current is passed
through it. A strong magnetic field is produced inside a solenoid when a current is
allowed to pass through it.

The given diagram shows the produced magnetic field lines when a current flows
through a solenoid. The field lines are parallel inside (indicating that the magnetic field is
uniform inside the solenoid) and curved outside the solenoid. This is similar to the
magnetic field lines produced by a bar magnet.

The direction of the magnetic field produced by the solenoid depends on the direction of
the current flowing through it. Depending on the direction of the flow of current, one end
of the solenoid acts like the North Pole of a magnet while the other end acts like the
South Pole. In the figure given above, magnetic lines of force get emanated from the left
end and terminate at the right end of the current carrying the solenoid. Hence, the
right end of the solenoid acts as the North Pole while the left end acts as the South
Pole.

Each turn of the coil behaves like a magnet when electric current is made to flow
through it. In the above figure, the current is flowing in a clockwise direction; thus, the
left hand side of each coil becomes the south pole, and conversely, the right hand side
of each pole becomes the north pole. If we consider small coils as small bar magnets,
with their opposite poles kept one after the other, then the entire coil behaves like a bar
magnet.
If we take this analogy further (of taking one coil as one small bar magnet), then the
more the number of small bar magnets combining to form a large bar magnet, the
stronger will be the magnetic field of the complete bar. Hence, we can conclude that the
more the number of coils in a solenoid, the stronger is the magnetic field produced by it.

When a solenoid is wound on soft iron, the magnetic field is very strong; but we cannot
retain the magnetic property after the current is stopped. In such a situation, a solenoid
is called an electromagnet. However, if we use a bar of steel instead soft iron, we can
retain its magnetic properties even after the current is stopped. In other words, the bar
of steel becomes a permanent magnet, but its magnetic strength is lesser than that of
soft iron.

The strength of the magnetic field produced by a current carrying a solenoid

(i) is directly proportional to the number of turns in the solenoid

(ii) is directly proportional to the strength of the current in the solenoid

(iii) depends upon the nature of the core material

For example,the use of core materials such as soft iron increases the magnetic field.

Soft iron core is placed inside the transformer core to increase the magnetic field.

Construct a simple circuit using a solenoid. Place a magnetic compass near the
solenoid and close the switch. The compass needle will deflect in a direction. Now,
move the compass slowly near one end of the solenoid and observe the position of the
needle. You will observe that the needle gets aligned horizontally along the length of the
solenoid.

Try to explain this with the help of Maxwell’s right hand rule. Now, bring the compass
near the other end. You will find that the compass needle gets aligned horizontally in the
opposite direction. Can you explain why the needle reverses its direction when
moved from one end to the other of the current carrying the solenoid?
Games C. Maxwell (1831 – 1879) was one of the great physicists
and mathematicians of the 19th century. He gave a unified model for
electricity and magnetism called electromagnetism with his four
sets of electromagnetic equations. He predicted the direction of the
magnetic field produced around a current carrying a wire.

Permanent Magnets and Electromagnets

Amar watches his father fix an electric bell in their new house. He wonders how a bell
produces such a loud sound when its switch is pressed on.

In this section, we will learn about the construction and working of an electric bell.

An electric bell works on the principle of electromagnetism.

Let us show you the working of an electric bell.

Hans Christian Oersted (1777-1851) was one of the greatest

physicists of his time. Until 1820, it was believed that only natural
magnets were able to possess magnetism. In 1820, Oersted
suggested that a wire behaves similar to a magnet when an electric
current passes through it. He was the first scientist to demonstrate
the magnetic effect of current.

Now, if you switch off the current in the previous experiment, then what would you
expect to observe? Would the needle return to its previous position? If yes, then
why?
You will observe that the needle returns to its original position. This happens because
the wire carries no current when the switch is off. Hence, the phenomenon of the
magnetic effect of current does not apply here.

Reverse the terminals of the cell by reversing the cell

and bring the compass near the circuit again, as shown
in the given figure.

Switch on the current in the circuit and observe the

deflection in the compass needle. Does the needle
deflect in the same direction as in the previous
case?

You will observe that the needle is deflected in a direction opposite to that in the earlier
case. This happens because the direction of current in the wire is opposite to that in the
earlier case.

The direction of deflection of a compass needle depends on the direction of the

current flowing in the wire.

The phenomenon of the magnetic effect of current is used in various fields in our day-to-
day life. One such use of the magnetic effect of current is illustrated below.

Puneet visited a junkyard on a school education trip. In the junkyard, he saw the arm of
a crane, with a large magnet at its bottom, move over a heap of junk and collect objects
made of iron. The magnet used in a junkyard crane is not a natural or a permanent
magnet. It is a temporary magnet, which is called an electromagnet. As its name
suggests, its magnetic nature depends on the presence of an electric current.

Construction of an electromagnet

Take a long piece of insulated copper wire and an iron nail. The wire must be insulated
i.e., it must be covered by plastic in order to prevent short-circuiting, which is caused by
the contact of wires. Make a coil from this wire by winding it around the iron nail. Now,
 construct an electric circuit that consists of a cell, a switch, and the two ends of the coil,
as shown in the given circuit diagram.

After constructing the electric circuit, switch on the current in the coil by closing the
switch and bring a paper clip near one end of the nail. What do you observe? Does
the paper clip get attracted towards the end of the nail and get attached to it?

When the switch is ON, the nail in the circuit behaves like a magnet. When a magnetic
material such as an iron nail is placed within a current-carrying coil, it behaves like a
magnet called electromagnet.

Types of an electromagnet

• Bar-shaped or I-shaped electromagnet: To create such an electromagnet, a thin

insulated copper wire is wound in a form of a solenoid on a soft iron bar. When current
passes through this wire, the bar starts behaving like an electromagnet.

• Horse-shoe or U-shaped electromagnet: This electromagnet is constructed when a

thin insulated copper wire is spirally wound on the arms of the horse-shoe shaped soft
iron core. The wire is wound in such a way that when the winding is seen from both the
ends, they appear to be in opposite sense on the two arms.
 What will happen to the paper clip when you open the switch? Will it remain
attached to the nail?

• A magnetic material will act as an electromagnet till the time current continues to
pass through it.
• When the current stops flowing, the material loses all its magnetic properties and
behaves like a normal material.

The principle of electromagnetism is used in various devices such as electric

bell, electric fan, etc.

The strength of an electromagnet depends on the

• total number of turns in the coil of wire

• strength of the current in the coil
• characteristics of the magnetic material over which the wire is wound

Fleming's Left Hand Rule

It is clear from the above activity that a current carrying the rod experiences a force
when placed between two poles of strong magnets. The direction of displacement of the
rod reverses when the direction of current is reversed. This suggests that the direction
of force exerted on the rod is related with the direction of current.

Repeat the same process by reversing the direction of the magnetic field. You can
easily observe that the rod will displace towards the right when the current flows from Q
to P, and displaces towards the left when the current flows from P to Q. What does this
suggest?
This suggests that the direction of force exerted on the rod is related with the
direction of the magnetic field. Thus, we can say that the directions of current,
magnetic field, and magnetic force are perpendicular to each other.

Experimentally, it is found that the magnitude of this force depends upon three factors:

Combining equations (1), (2) and (3) we get F ∝ IBl = KIBl

where K is a constant and its value in SI unit is 1.

So, F = IBl

Apply Fleming’s left hand rule to find the direction of this magnetic force.

Fleming’s left hand rule

This rule states that if you stretch the thumb, index finger, and middle finger of your left
hand such that they are mutually perpendicular to each other, then your index finger
represents the direction of the field, the middle finger represents the direction of the
current, and the thumb represents the direction of the force experienced by the
conductor.

A charged particle is moving between two poles of magnets (as shown). Using
Fleming’s left hand rule, find out the direction of the magnetic force exerted on
the particle?
Do You Know:

Magnetic field is always produced by an electric current. The ions

dissolved in blood move along the nerve cells. Thus, they
produce a magnetic field, although a weak one. They produce
sensations that we sense through the skin. This is because when
we touch, the nerve cells transport an electric impulse to the
specific part of our body that we have to use. A short lived
magnetic field is produced there, which allows us to sense.
Magnetic resonance imaging (MRI) is primarily a technique that
uses magnetism, which is produced in our nerve cells to obtain
computerised images of internal parts of our body such as the
heart, brain etc.

Electric Motor

An electric motor is a rotating device that converts electrical energy into mechanical
energy. A number of household devices such as fans, mixers, grinders, washing
machines etc. consist of a motor.
Do you know how an electric motor works?

Motor principle

The basic principle on which the electric motor works is the magnetic effect of current. A
current carrying a rectangular coil starts rotating when placed in a magnetic field. This
happens because a continuous magnetic force acts upon it. Rotation of the coil also
results in the rotation of the shaft attached to it. Thus, through this process, a motor
converts electrical energy into mechanical energy.

Construction of an electric motor

The given figure illustrates the internal parts of a simple electric motor. A motor consists
of a rectangular coil MNST of insulated copper wire. The coil is placed between two
magnetic poles such that the magnetic field acts normal on lengths MN and ST.

The coil is connected with two carbon brushes at points A and B respectively. The inner
sides of these carbon brushes are in contact with half rings C and D, which are
insulated and in contact with an axle (not shown in the figure).

Functioning of an electric motor

When a current is allowed to flow through the coil MNST by closing the switch, the coil
starts rotating anti-clockwise. Why does this happen? This happens because a
downward force acts on length MN and at the same time, an upward force acts on
length ST. As a result, the coil rotates anti-clockwise.

The current in length MN flows from M to N, and magnetic field acts from left to right
normal to length MN. Hence, according to Fleming’s left hand rule, a downward force
acts on length MN. Similarly, the current in length ST flows from S to T, and magnetic
field acts from left to right normal to its length. Hence, an upward force acts on length
ST. These two forces cause the coil MNST and the axle to rotate anti-clockwise.

After half-rotation, the position of length MN and ST get interchanged. Simultaneously,

half ring D comes in contact with brush A and half ring C comes in contact with brush B
respectively. Hence, the direction of current in coil MNST gets reversed and flows
through TSNM.

An electric device that reverses the direction of current in a circuit is called a

commutator.

Thus, the split ring acts as a commutator of the electric motor. Now, due to the reverse
direction of current in lengths MN and ST, an upward force acts on length MN, which
pushes it up and a downward force acts on length ST, which pushes it down. As a
result, the coil MNST further rotates anti-clockwise. The reversal of the current through
the coil MNST repeats at each half-rotation, while its anti-clockwise rotation continues.

A commercial motor uses

1. an electromagnet and permanent magnetic poles
2. large number of turns in the rectangular coil
3. A core of soft material such as iron on which the coil is wounded. The core becomes an
electromagnet when a current flows through the coil. The coil–core system is known as
the armature of motor.

Electromagnetic Induction

Let us perform an activity first.

What would you expect if the same experiment is repeated with the South Pole of
the magnet? Will the direction of deflection of the pointer be reversed?

From the above activity, we can conclude that the motion of the magnet sets up a
current in the coil, whose direction depends on the nature of the incoming pole as well
as on the direction of the motion of the magnet. What happens when the magnet is
kept stationary and the solenoid is moved across it?

It is observed that a current is induced in the solenoid when it is moved across a

stationary magnet. What does this indicate?

This indicates that a potential difference and current are induced in the solenoid, when
either the solenoid or the magnet or both are moving.

The phenomenon of the generation of induced current in a conductor by

changing the magnetic field or by moving a conductor in the magnetic field is
known as electromagnetic induction.

The voltage generated in the circuit during electromagnetic induction is

called electromotive force (e.m.f). Factors influencing induced e.m.f. are:

i) Number of turns of the coil,

ii) Rate of change of magnetic field linked with the coil.

This phenomena was first observed by Faraday. From his observations he enunciated
two laws of electromagnetic induction.

First Law: A changing magnetic flux linking with a coil induces an electromotive force in
the coil.

Second Law: The induced electromotive force is proportional to the rate of change of
magnetic flux linked with the coil.
The direction of induced e.m.f is given by Lenz's law according to which the direction of
induced e.m.f. (or induced current) is such that it opposes the cause which produces it.

Michael Faraday (1791 – 1867) was one of the greatest physicists

of the 19th century. Initially, he worked as a book binder in a book
shop. He was so interested in experiments of science that he used
to read the books that came for binding. He was the first to show
that current is induced in a coil when a solenoid, magnet, or both
are moved relative to each other. He also discovered the law of
electrolysis.

Take a long non-conducting cylindrical rod. Insert two coils of wires M and N on this rod.
Ensure that the number of turns in coil M is greater than the turns in coil N. Now,
connect a battery and a key to the ends of coil M and connect a galvanometer to the
ends of coil N (as shown in the given figure). Allow the current to pass through coil M by
closing the switch and observe the pointer of the galvanometer. You will see that the
pointer comes back to zero after deflecting by a small amount. What does this
indicate?

Deflection of the pointer of the galvanometer indicates that a temporary current is set up
in coil N. Now, open the switch to stop the current in coil M. You will see that the pointer
again deflects, but in the opposite direction and comes back to zero. This indicates that
a current has set up in coil N in the opposite direction.

From the above activity, we observed that as soon as the current in coil M reaches from
zero to maximum or from maximum to zero, a current gets induced in coil N. Here, coil
M is called the primary coil, while coil N is called the secondary coil.
As a current is always associated with a magnetic field, the change in current in coil M
causes a change in the associated magnetic field. This in turn changes the magnetic
field around coil N. The change in the magnetic field around coil N generates an
induced electric current in the coil. This phenomenon is known as mutual induction.

The working of a transformer is based on the phenomenon of mutual induction.

The direction of the current induced with respect to the directions of the magnetic field
and motion of the coil is given by Fleming’s right hand rule.

Fleming’s right hand rule

The direction of current induced in a conductor can be obtained by holding the thumb,
the index finger, and the middle finger of your right hand mutually perpendicular to each
other. In this situation, the thumb indicates the direction of the motion of the conductor,
the index finger points along the magnetic field, and the middle finger points along the
current induced in the conductor.

A solenoid is moved from left to right in a magnetic field.

What will be the direction of current induced in the
solenoid?

The phenomenon of electromagnetic induction is used in electric generators for

the generation of electricity.
When current in a circuit flows only in one particular direction and does not change its
magnitude, it is called Direct current or DC.

When current periodically changes its direction and magnitude about a mean value
(called r.m.s. value), it is called Alternating current or AC.

Difference between A.C. and D.C.

Direct Current (D.C.) Alternating Current (A.C.)

Current of constant magnitude Magnitude of current varies periodically with time

Always flow in one direction Direction of current reverses periodically

Can be obtained from cell or battery Can be obtained from A.C. generator

Advantage of alternating current over direct current

If a direct current is generated at the power station, its voltage cannot be increased for
transmission, and so due to passage of high current in the transmission lines, there will
be a huge loss of electrical energy in the form of heat in transmission lines. However,
we can increase or decrease the voltage of alternating current for transmission and thus
it reduces heat loss in the transmission line.

Do you know that the current we receive in our houses is AC? Even the current
produced in emergency generators and in hydroelectric
power houses is alternating current. The alternating current that we use in our homes
has a frequency of 50 Hz. It means that the current changes its direction 50 times in a
second. The supply of AC voltage is controlled by a transformer.

Electric Generator

An electric generator is a machine that generates electricity by rotating its rotor in a

magnetic field. Thus, it converts mechanical energy into electrical energy.

Principle

The basic principle on which electric generator works is electromagnetic induction.

When rotor rotates in a magnetic field, current is induced in the rotor.

Construction
A generator consists of a rectangular coil MNST of insulated copper wire placed
between two strong magnetic poles. The two ends of the coil MNST are connected with
brushes A and B of rings C and D respectively. The inner sides of the rings are
insulated. They are attached with an axle X, which can be rotated mechanically.
Brushes A and B are connected with a galvanometer that can measure the flow of
current in coil MNST.

Working

When the axle is rotated, lengths MN and ST move up and down respectively. Since
lengths MN and ST are moving in a magnetic field, a current gets induced in these
lengths caused by an electromagnetic induction. The direction of the induced current in
both the lengths is given by Fleming’s right hand rule.
Since length MN is moving upwards in the magnetic field that acts from left to right, the
direction of the induced current will be from M to N. Similarly, the direction of the
induced current in length ST will be from S to T. Hence, an induced current will set up in
the coil in the direction MNST, which produces deflection in the galvanometer.

After half-rotation, length MN starts moving down, whereas length ST starts moving up.
The direction of the induced current in the coil gets reversed i.e., the induced current will
now flow from T to M via S and N i.e., TSNM. Therefore, we can conclude that after
each half-rotation, the direction of the induced current is reversed. This current is
called an alternating current (AC). An AC reverses its direction after equal time
intervals. A machine with this arrangement is called an AC generator.
To get a current that flows in one direction only, a split ring is used.

In this arrangement, brush A always remains in contact with the length moving up,
whereas brush B always remains in contact with the length moving down. Here, split
rings C and D act as a commutator.

In this case, the direction of the current induced in the coil will be from M to T via N and
S for the first half-rotation, and from T to M via S and N for the second half-rotation of
coil MNST. Hence, we get a unidirectional current called direct current (DC). A
machine with such an arrangement is called a DC generator.

Do You Know:

Electricity supplied to our homes, school buildings, company buildings etc. is AC, which
reverses its direction after every second with a frequency of 50 Hz. Most power stations
in the world generate AC than DC. This is because AC can be transmitted over very
long distances without much loss of energy.

Transformers

Principle − It works on the principle of electromagnetic induction. When current in one

circuit changes, an induced current is set up in the neighbouring circuit.

There are two types of transformers.

Step-up transformer:

Step-up transformers are used to increase the ac voltage. In this, the number of turns in
the secondary coil is greater than that in the primary coil as shown in the figure.
Step-down transformer:

Step-down transformers are used to decrease the ac voltage. In this, the number of
turns in secondary coils is less than that in the primary coil as shown in the figure.

Working

An alternating emf is supplied to the primary coil PP’. The resulting voltage is produced
as an induced voltage in the secondary coil SS’. Voltage at the primary and the
secondary coils depend on the number of turns. Magnetic flux that is linked with the
primary coil is also linked with the secondary coil. The induced emf in each turn of the
secondary coil is equal to that of the primary coil.

Let,

VP = Alternating emf applied to primary coil

VS = Alternating emf induced to secondary coil

NP − Number of turns in the primary coil

NS − Number of turns in the secondary coil

From the working of a transformer, we observe that the voltage across the coil is directly
proportional to the number of turns in the coil.

It means that VP α NP.

So, VP = KP NP (i)

Here, KP is the constant of proportionality.

VS α NS

So, VS = KS NS (ii)

Here, KS is the constant of proportionality.

Dividing equation (ii) by (i)

• For step-up transformer K > 1.

∴ Vs > Vp

• For step-down transformer K < 1.

∴ Vs < Vp

Also, power transfer through the transformer is suppose to be constant.

Input electrical power = Output electrical power

VpIp = VsIs

Transformers are used in telegraph, telephone, power stations, etc.

Losses in a transformer:

Copper loss − Heat in copper wire is generated by working of a transformer. It can be

diminished using thick copper wires.
Iron loss − Loss is in the bulk of iron core due to the induced eddy currents. It is
minimised by using a thin laminated core.

Hysteresis loss – Alternately, magnetising and demagnetising the iron core cause loss
of energy. It is minimised using a special alloy of iron core with silicon.

Magnetic loss − It is due to the leakage of magnetic flux.

Induction Coil:

Induction coil is a device used to convert low DC Voltage to high AC Voltage. It is

similar to a transformer and works on the same principle. It usually consists of two coils,
the primary coil and the secondary coil. Primary coil consists of few turns of a thick
insulated copper wire wound over a soft iron core which is connected to the battery.

The secondary coil has large number of turns which are wound over the primary coil.
An interrupter is used for making and breaking the current in the primary coil
automatically.

This current magnetizes the iron core and produces a large magnetic field throughout
the induction coil.