The document discusses electric and magnetic fields. It defines an electric field as the region around a charged object where another charge will experience a force. A magnetic field is defined as the region around a magnet or current-carrying conductor where its magnetic effects can be detected. The document then goes on to describe experiments that demonstrate electric and magnetic fields, such as using iron filings to visualize field lines or observing the deflection of a compass needle near a magnet. It also discusses the concept of electric and magnetic field lines and how fields are produced by charges and currents.

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Module # 27
Electric Field & Magnetic Field
Electric Field
An electric charge sets up an electric field in the space
surrounding it and an electric force is exerted on any charged
body placed in the field. Thus, the space or a region around a
charged body in which it exerts a force on a stationary electric
charge is called electric field.
OR
The space surrounding an electric charge where another charge
experiences a repulsive or attractive force is called the electric
field of that charge.
Explanation
From Newton's law of universal gravitation and Coulomb's law,
we can calculate the magnitude and direction of the gravitational
and electrical forces, respectively. These laws are limited to an
interaction between two point charges or masses. There are
some fundamental and basic questions to be answered about
these laws.
(1) What is the origin of these forces?

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(2) How these forces are transmitted from one body to
another or from one charge to another?
The answer to (1) is unknown; we acknowledge the existence of
force as it is.
To answer (2), two theories are important and are described
briefly as under:
(a) Action at a distance, and
(b) The field effect or field theory.
Action at a distance means that the force between two charged
bodies is conveyed directly and instantaneously (i.e. with no time
delay). But, this is not in accordance with the experiment.
Action at a distance idea has never been a satisfying one for
scientists.
Michael Faraday (1791 -1867) introduced field theory concept.
According to field theory concept, the interaction between two
charges q and q0 separated by some distance is explained in two
steps in the following manner.
Step (1): The charge q produces an electric field in the space
surrounding it, &
Step (2): The field interacts with a charge q0 which is brought in
the field and produces force F on it.

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If the ends of a conducting wire are connected to the terminals of
a battery, an electric field is set up within the wire, which causes a
continuous motion of electrons through the wire.
Alternatively, any region in which electric charges experience
mechanical forces is called an electric field.
An electric field can be represented by lines of force, also known
as electric flux lines. The direction of flux lines is defined as being
from a positive charge to a negative charge. The direction of a
flux line shows the direction of the field. The flux lines leave the
positive charge and terminate on negative charge.
Direction of Electric Field
The direction of an electric field at any point is the direction of the
force on a small positive charge placed at the point.
Electric Line of Force
The electric line of force is the path along which a tiny positive
charge will move in an electric field. The electric lines of force
start from a positive charge and end on a negative charge. The
electric lines of force expand laterally.
Electric Field Intensity
To measure the strength and direction of an electric field at a
point, a unit positive charge is placed at that point. The direction

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in which this unit positive charge moves or tends to move is the
direction of the field. The strength of the field is the magnitude of
the force experienced by the unit positive charge when placed at
that point. A single vector quantity containing information both
about the field strength and its direction at that point is denoted by
E and is known as electric field intensity. Quantitatively, it is
defined as the force experienced by a unit positive charge placed
at that point.
Magnetic Field
The region or space around a magnet where the effects of its
magnetism such as the deflection of a compass needle can be
detected is called a magnetic field. A magnetic field always exists
around a current carrying conductor. The magnetic field is
represented by magnetic field lines or magnetic lines of force.
Thus, a magnetic field is represented by magnetic lines of force.
These lines are imaginary and come out of a North Pole, pass
through the space around the magnet and enter the South Pole
and travel through the magnet to North Pole.
The field around a moving charge is called magnetic field, as
opposed to an electric field around stationary charge. The
magnetic field is denoted by B and is a vector quantity. Its

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experimental demonstration was carried out by Hans Oersted, a
Danish professor of Physics.
When a compass is brought near a current carrying conductor,
the needle sets itself at right angles to the conductor. It shows
that a magnetic field is present around a conductor. If a conductor
is passed through a hole in a sheet of cardboard and a current is
passed through the conductor, the shape and direction of the field
may be determined by setting the compass at various points on
the cardboard and noting its deflection. This shows that the
magnetic field exists in concentric circles around the conductor.
When the current is flowing downward, the field direction is
clockwise. However, if the supply polarity is reversed so that
current flows upward, the field is found to be counter clockwise.
Furthermore, it can be demonstrated that the strength of the
magnetic field is greater near the current-carrying wire and
decreases as the distance from the conducting wire increases.
Field Lines
The path along which an isolated north pole of a magnet moves in
the magnetic field is called the field line. The field lines are
directed from N-pole of the magnet towards the S-pole, the field
lines do not intersect one another.

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Let us perform the following experiments to clarify the concept of
a magnetic field.
Experiment 1
Place a bar magnet on a table and bring a compass needle close
to it. It will be seen that the compass needle changes its direction.
If the compass needle is placed at different points around the
magnet, then, it will point in different directions at different points.
This indicates the effect of magnetic force of the magnet on the
compass needle. It is, therefore, concluded that there is a certain
space around the magnet where the effect of the magnet can be
experienced.
Experiment 2
Place a clean glass plate over a bar magnet and sprinkle iron
filings over it. Now, tap the plate gently. The iron filings tend to
arrange themselves in a definite pattern. The arrangement of the
iron filings around the magnet indicates clearly the existence of
magnetic field. If the arrangement of iron filings is examined
carefully, then, it can be concluded that they arrange themselves
along definite lines. These lines are called magnetic lines of force.
Experiment 3

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Place a deep circular tray of water on a table. Arrange a bar
magnet on its rim as shown in fig. Fill the tray with water so that
its surface remains about two or three centimeters below the
magnet. Pass a magnetized needle through a big cork in such a
way that its north pole comes out through the cork. Make the cork
float on the water so that the north pole of the magnetized needle
remains above the surface of water while its south pole remains
submerged.
Fig: Direction of Magnetic Flux
Bring the cork close to the north-pole of the bar magnet and leave
it. It will be noticed that the cork moves away from the north pole
of the bar magnet along a curved path and approaches its south-
pole. If the cork is brought back to its original position and allowed
to move, then, it will retrace its earlier path and will reach the
same point as it reached at the end of its previous journey. If the
experiment is repeated by placing the cork initially at different

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points near the north-pole, then, it will reach the south-pole by
traversing a different definite path. This is shown by the doted
curve in the fig. The curve (shown by dotted path) along which the
north pole of the magnetized needle moves in the magnetic field
of the bar magnet is called a line of force or magnetic line of force
or line of magnetic force.
In a laboratory, the magnetic field of a bar magnet is represented
by drawing lines of magnetic force with the help of a compass
needle.
Experiment 4
Fig: Magnetic flux pattern by plotting compass method
Fix a large piece of paper on the table. Place a bar magnet in its
middle in such a way that its north pole points towards the
geographic south. Place a magnetic compass near the north pole
of the bar magnet and tap it gently. The south-pole of the
magnetic compass points towards the north-pole of the bar
magnet.

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After it settles down, mark dots with the help of a pencil on the
paper at the ends of the compass needle. Now, move the
compass in such a way that its south pole faces the point marked
against the north pole of the magnetic needle. Repeat this
process till the needle of the magnetic compass reaches the
south pole of the bar magnet. A smooth curve can be obtained by
joining these points. This curve represents a magnetic line of
force. The curve shown in the fig: can be drawn about the bar
magnet. The magnetic lines of force originate from the north pole
of a magnet and end on its south pole. If a magnetic compass is
placed on a magnetic line of force, then, its north-pole always
points along the direction of magnetic line of force. The concept of
the magnetic lines of force is very useful in explaining the action
of magnetic force at a distance.
Force on a Current Carrying Conductor in a Magnetic Field
There are several models used by scientists to understand the
force on a moving electric charge or on a conductor carrying a
current in a magnetic field. One of these models uses the right
hand rule.
The extended thumb is placed in the direction of the motion of a
positive charge or the conventional direction of the electric
current. The fingers are placed in the direction of the magnetic

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field at that position. The direction of the force experienced is then
given by the direction in which the palm of the hand would push.
Fig: Current carrying conductor in a magnetic field
If detailed observations and measurements are carried out, then,
it will be found that the force is stronger when the field strength is
strong and reduces with a decrease in the field strength. We
conclude the following from the observations.
(1) The current carrying conductor situated in a magnetic field
experiences a force whenever it is placed at an angle to the
direction of the field.
(2) The force is always directed perpendicular to the direction of
the current and to the magnetic field.
(3) The magnitude of the force produced is proportional to the
current and the field strength.
(4) The direction of the force exerted on a current carrying wire
in a magnetic field can be determined by using the right hand rule.

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Magnetic Field due to Solenoid
The magnetic field of a straight wire is very weak. But, the
magnetic field due to the same current in a coil of wire is stronger.
A coil of insulated copper wire in the form of a long cylinder is
called a solenoid. When an electric current is passed through a
solenoid, the magnetic field is very similar to that of a bar magnet.
One end of the solenoid acts like a north-pole and the other end
as the south-pole as shown in the figure.
Fig: Magnetic Field of a Solenoid
It means that a solenoid carrying current can be used as a bar
magnet. The magnetic field inside the solenoid is very strong and
uniform because the lines of force are parallel and close to one
another. Outside the solenoid the magnetic field is very weak. The
magnetic field strength of a coil depends upon the electrical
current and the number of turns in the solenoid.

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MAGNETIC FIELD DUE TO CURRENT
The interaction of one charge with some other charge is generally
described by associating fields around the charges. Just as an
electric charge brought in the field around a fixed charge interacts
with the field and experiences an electrostatic force, similarly, a
moving charge interacts with a field around another moving
charge and experiences magnetic force. The field around a
moving charge is called magnetic field, as opposed to an electric
field around stationary charge. The magnetic field is denoted by B
and is a vector quantity. Its experimental demonstration was
carried out by Hans Christian Oersted, a Danish professor of
Physics. He showed that an electric current in a wire deflects a
near-by compass needle. Thus, a current-carrying wire has a
magnetic field around it which is the manifestation of interaction
between moving charges in the conductor. The (magnetic) lines of
force form concentric circles around the conducting wire and the
magnetic field remains effective so long as the current keeps
flowing through the wire.
Furthermore, it can be demonstrated that the strength of the
magnetic field is greater near the current-carrying wire and
decreases as the distance from the conducting wire increases.

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The direction of the magnetic lines of force is given by right hand
rule which is stated as follows: “If the wire is grasped in the right
hand with the thumb pointing in the direction of the current, then,
the fingers of the hand will circle the wire in the direction of the
magnetic field".
An electromagnetic field is a magnetic field caused by a current
flow in a wire. Whenever, electric current flows, a magnetic field
exists around the conductor and the direction of the magnetic field
depends upon the direction of the current flow.
Oersted, working on a simple cell, observed that, if a current
carrying wire is placed parallel to magnetic needle or compass,
then, deflection is produced in the compass. The direction of
deflection depends upon direction of current and also whether the
wire is placed above or below the magnetic needle. The deflection
of the magnetic needle is due to magnetic field around the wire.
This leads us to the conclusion that a magnetic force
accompanies a wire which carries a current. Ampere was the first
to feel presence of magnetic force due to passage of current
through a wire. If two wires, in which currents are flowing in the
same directions, are placed parallel and close to each other, then,
they will attract each other like the opposite poles of magnets. If
the currents are flowing in the opposite directions, then, there will

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be force of repulsion between them. This force between the wires
cannot be electric in nature and the field around the wires is not
electric but magnetic. The force disappears when current in one
of the wires is stopped.
Reasons
We know that current is produced by flow of electric charges. The
number of electrons in a conductor is always equal to the number
of protons irrespective of whether the current is flowing or not
through the conductor. The electric fields due to positive and
negative charges neutralize each other. Moreover, by reversing
direction of current in a wire, the nature of charges cannot be
changed to convert force of attraction into force of repulsion
between the wires. The force disappears when current in one of
the wires is stopped. These observations indicate that electric
current produces a field around a wire which is not an electric field
but a magnetic field. Hence, a magnetic field is produced around
a wire in which current is flowing. Since, an electric current
through a wire produces magnetic effects, so we should expect it
to be surrounded by magnetic lines of force.
The following experiment shows that circular lines of magnetic
force are formed around a current carrying wire.

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Experiment
Pass a straight copper wire through a card board and connect the
ends of the wire with the terminal of a battery. Spread the iron
fillings on the card board and tap it gently. The iron fillings arrange
themselves along the concentric circles having their centre at the
point where the copper wire passes through the card board. This
means that magnetic field exists around the wire through which
current flows and the concentric circles are the lines of force.
These lines can also be drawn with the help of a magnetic
compass.
(a) (b) (c)
Direction of the Magnetic Field
The direction of the magnetic field or magnetic lines of force is
indicated by the direction in which the north-pole of the compass
points when placed near the current carrying wire. The
experiments show that:

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If the current flows from top to the bottom in the vertical wire,
then, the direction of the magnetic lines of force is found to be
clockwise.
If the current flows from bottom to the top in the vertical wire,
then, the direction of the magnetic lines of force is found to be
counter clockwise (or anti-clockwise).
Magnetic Field of the Earth
Fig: Earth Magnetic field
A freely suspended magnet, or a compass, always lines up in
north-south direction. If it is disturbed, it again comes to the north-
south direction after a few oscillations. A small compass in the
presence of a magnetic field will rotate until its N-pole points in
the direction of the field. If there is no magnet present, then, the
needle is influenced only by the earth's magnetic field which
causes the needle to point in the north-south direction. This

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discovery was used by the early navigators to find the direction of
North.
The earth behaves as a large bar magnet along the north south
direction with the north-pole towards the south geographic pole as
shown in the figure. The actual cause for the earth's magnetic
field is not exactly known. All the theories, which are currently
followed, assume that the earth's magnetic field is similar to that
of an imaginary bar magnet situated at its centre. It is quite clear
that it has two polarities like an ordinary bar magnet.
Field Theory
There are two important branches of physics:
(1) Mechanics
(2) Field theory
The field theory explains the origin, nature and properties of fields
such as gravitational field, electromagnetic field and nuclear field.
Electric Polarization
When a dielectric is placed in an electric field in such a way that
the field is directed from up to downward direction, then under the
influence of this external field, negative charges appear on the
upper face (side) and positive charges on the lower side (face) of

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the dielectric. The phenomenon is known as electric polarization
and dielectric is said to be polarized.
Magnetic Field Strength or Magnetic Field Intensity
The amount of force experienced by a unit pole placed in air at
the point of consideration is called magnetic intensity.
The unit for magnetic intensity is ampere - turns per meter.