BS Physics (Foundation) Series

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Module # 26
Electricity and Magnetism
Electricity
The branch of physics which deals with the charges in motion is
called Electricity.
First Law of Magnetism
Like poles repel, unlike poles attract.
Electromagnetism
This branch of Physics deals with observations and laws relating
to electricity and magnetism. In it, we study the laws governing
the charges at rest and in motion and the properties associated
with the charges at rest and in motion.
Magnet
A body that attracts small pieces of iron and points towards north-
south direction when suspended freely is called a magnet.
Properties of a Magnet
(1) When a magnet is suspended, so that, it can move freely, it
aligns itself along the north-south direction. The end of the

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magnet which points towards the north is called the north-pole
while the other is named a south-pole.
(2) An iron piece can be magnetized by rubbing it against a
magnet.
(3) If two magnets are brought near each other; the north pole of
one magnet attracts the south pole of the other. The two north
poles and two south poles repel each other. Thus like poles repel
and unlike poles attract each other.
(4) The magnetism of the magnet is concentrated in the poles of
the magnet.
(5) The two poles of a magnet cannot be separated from each
other. If a magnet is broken into two pieces, two new magnets are
obtained. Each new magnet has both the poles; a north-pole and
a south-pole. This process may be repeated as many times as
desired but each time a magnet with both the poles is obtained.
(6) The magnetic properties are destroyed by heating a magnet.
Methods of Making Magnets
(1) Single Touch Method

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Fig: Making a Magnet by Single Touch Method
We take a hard steel bar AB and rub it with one end of a magnet
in the direction from A to B, keeping the magnet in an inclined
position as shown in figure. On reaching the end B, the magnet is
lifted and the same end is brought back to the end A of the bar in
semicircular path. The process is repeated several times.
The bar AB will then be magnetized. The end A of the bar will
have the same polarity as that of the rubbing pole of the magnet
and the end B of the bar will have polarity opposite to that of the
rubbing pole.
(2) Electrical Method
Fig: Electrical Method of Magnetization
The best and quickest method of making magnets is the electrical
method. In this case, we make use of the magnetic effect of an

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electric current. Take a steel bar of U-shape and wind on it two
segments of a coil of insulated copper wire as shown in figure.
Make sure that the two segments of the coil are wound in
opposite directions. The coil is connected to a battery and strong
current is passed through the coil for a few seconds.
In a similar way, a steel bar can be magnetized by putting it inside
a solenoid and passing a current through the coil. The specimen
will be magnetized. The direction of current in the coil determines
the polarity of the magnet.
Temporary and Permanent Magnets
In non-magnetic substances, such as glass, the molecules do not
align as they align in magnetic substances. Hence, non-magnetic
substances are not magnetized when they are brought near a
magnet. In some substances, such as soft iron the molecules
easily move and line up under the influence of the magnetic field
of a magnet. However, when the external field is removed, the
molecules of the soft iron go back to their original position.
Therefore, the soft iron forms a temporary magnet.
On the other hand, the molecules of some substances, such as
steel require a much greater magnetic force to line up their
molecules. However, when the external magnetic field is

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removed, the molecules of steel will retain their positions.
Therefore, these substances form permanent magnets.
Permanent Magnet
A permanent magnet retains its magnetic properties under all
conditions.
Electromagnet
When a current passes through a solenoid, it produces a
magnetic field inside the solenoid. If a soft iron bar is placed
inside the core of solenoid and current is passed through it, then,
the iron bar becomes a magnet. Such a magnet is called an
electromagnet.
Besides electricity, it is the electromagnet which has played an
important role in the life of modern man. One form of the
electromagnet is a U-shaped soft iron bar which has coils of
insulated wire wound in opposite directions on each of its arms.
Fig: Electromagnet

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Factors for Electromagnet
The following are the factors on which the strength of an
electromagnet depends:
(1) Number of turns per unit length in the coil.
(2) Strength of the current flowing through the coil.
Thus, the more the number of turns in the coil, the greater is the
strength of magnetic field. Similarly, the stronger the current
flowing through the coil, the more will be the strength of the
magnetic field.
Uses of Electromagnet
(1) Electromagnets are employed in electrical appliances, e.g.,
electric bell, electric motor, induction coil, electric fan, loud
speaker, telephone receiver, and television, etc.
(2) Heavy pieces of iron can be lifted and shifted from one place
to another with the help of an electromagnet. They are used in
separating iron form mixtures containing magnetic and non-
magnetic substances. Electromagnets are used to produce strong
magnetic fields for high power motors and generators and in
research laboratories.
(3) Medical practitioners use electromagnet to remove iron
piece from the eyes or wounds of a patient.

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MAGNETISM AND ELECTROMAGNETISM
Magnetism derives its name from Magnesia, a region in Asia
Minor (modern Turkey) where it was found in the form of lumps of
certain iron ore. These lumps of iron ores were found to have the
property of attracting small pieces of iron. It was also found that if
such a lump was suspended so that it could rotate feely, then, it
always pointed towards north and south direction when at rest.
So, it was also called lode stone. Later it was found that these
properties are exhibited not only by iron, but also by cobalt, nickel
and many compounds of these metals. These are called magnetic
materials.
Summarized below are (some of) the characteristics of magnetic
materials. A body that attracts small pieces of iron and points
towards north-south direction when suspended freely is called a
magnet. The end of the magnet pointing towards north is called
the North Pole while the other is called the South Pole. Like poles
of two magnets repel, while, unlike poles attract one another. The
magnetism of the magnet is concentrated in the poles of the
magnet. The two poles of a magnet cannot be separated from
each other. If a magnet is broken into two pieces, two new
magnets are obtained. Each new magnet has both the poles; an
N-Pole & an S-Pole (a North-Pole & a South-Pole).

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Just as electrostatic interactions are represented by means of
fields (electric fields) so also are magnetic interactions
(represented by magnetic fields). The space around a magnet
where the influence of the magnet is felt by another magnet or a
magnetic substance is called magnetic field. The magnetic field is
represented by magnetic field lines (or magnetic lines of force).
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. A magnetic
substance can be magnetized by rubbing it with a magnet or
placing it in a coil through which current flows.
A magnet can be demagnetized either by heating it or by dropping
(striking) it several times.
We know that two point charges located at some distance apart
interact with each other with a force called as electrostatic force.
The nature of this force remains the same even if one of the two
charges moves in space or in any medium. But, in case, both the
charges are in motion, then, the nature of the force of interaction
does not remain the same. Instead, a new force appears between
them. This force that appears as a result of the interaction
between two moving charges is known as magnetic force.

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Magnets were used in order to find direction in navigation. They
were also called lodestone or leading stone. There are magnets
which retain their magnetic properties under all conditions. They
are called permanent magnets. Magnets can be formed artificially
by the passage of electric current. They are called
electromagnets. Electromagnets are widely used in electrical
machinery, but permanent magnets have also many useful
applications. They are particularly convenient for studying the
elementary magnetic phenomena experimentally.
There are some materials having clear magnetic properties,
known as ferromagnetic materials. These are iron, cobalt, nickel
and some of their alloys. The materials having less magnetic
properties are called paramagnetic. These are aluminum,
platinum, manganese and chromium. The materials having
magnetic properties lesser than vacuum are called diamagnetic.
These are bismuth, antimony, copper, zinc, mercury, gold and
silver.
Similarity between Magnetism & Static Electricity
There is a remarkable similarity between magnetism and static
electricity. These are given below.
(a) There are two types of charges, positive and negative. There
are two types of magnetic poles, north and south.

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(b) Like charges and like poles repel each other, opposite
charges and opposite poles attract each other.
(c) Charged objects set up electric fields of force; the magnetic
objects set up magnetic fields of force.
(d) We can charge certain substances by rubbing them
together; certain substances can be magnetized by rubbing with a
magnet.
Molecular Theory of Magnetism
Why some materials are magnetized and others not? To answer
this question, let us discuss molecular theory of magnetism in
brief. This theory was proposed by Wilhelm Weber in nineteenth
century. According to this theory, all matter is made up of small
molecules which are minute magnets. In iron and steel, these little
magnets are strong, while in other materials they are weak. When
a piece of material is not magnetized, these molecules lie in
irregular positions. When the material is magnetized, the
molecules lie with their N ends pointing the same way.
Magnetic Materials & Magnetic Substances
The substances like iron, nickel, cobalt and steel which are
attracted by magnets are called magnetic materials. A magnetic

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substance is made up of small magnets. A magnetic substance,
such as iron, can be magnetized.
Magnetization
A magnetic substance can be magnetized by rubbing it with a
magnet or placing it in a coil through which current flows.
Magnetic Force
The force with which a magnet attracts other magnetic
substances or repels other magnet is known as magnetic force.
OR
The force between two magnetic poles is magnetic force. If a
magnetic needle is placed in a magnetic field, then, a magnetic
force acts on it.
Simple Experiment
Take two bar magnets and identify their north and south poles.
Bring the north pole of one magnet closer to the south pole of the
other magnet. We will find that the two magnets attract each other
even before touching each other. This force of attraction
increases as the distance decreases between the poles. It we
bring north-pole of one magnet closer to the north-pole of the
other, the two poles will repel each other. Same result will be
obtained in case of south-poles. Thus, like two types of electric

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charges, the two magnetic poles repel each other with a magnetic
force if the poles are like and attract each other if the poles are
unlike.
Like gravitational force between two masses and an electric force
between two charges, the magnetic force depends upon distance
between the two poles. If we increase the distance between two
poles, the force decreases. It will be observed that the force is
inversely proportional to the square of distance between the
poles. The force between two poles also depends upon the
strength of the poles. The force is stronger when a powerful
magnet acts on another magnet.
Magnetic Lines of Force or Lines of Magnetic Force
The path of an isolated north magnetic pole in a magnetic field
represents the line of magnetic force or magnetic line of force.
The magnetic lines of force form the magnetic flux.
Properties of Magnetic Lines of Force
Following are the properties of lines of magnetic force.
(1) Outside a magnet, the magnetic lines of force start from the
north-pole and end at its south-pole, while, inside the magnet,
they continue from the south-pole to the north-pole and thus form
a closed curve.

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Magnetic lines of force are continuous and will always form closed
loops.
(2) The intensity of the magnetic field near its poles is
maximum, so the lines of magnetic force are crowded there,
while, its intensity is low at points distant from poles. If the
magnetic field is uniform, then, the lines of magnetic force are
parallel to each other. The density of lines, therefore, indicates the
intensity of the magnetic effect and the total number of lines is a
measure of the total strength of the magnet. The magnetic lines of
force become thicker where the field is strong and become thinner
where the field is weak.

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(3) The magnetic lines of force contract longitudinally, so the
force of attraction between two opposite poles can be explained
with the help of this property.
(4) The lines of magnetic force tend to expand (repel each
other) laterally. The repulsion between similar poles can be
explained with the help of this property.
(5) Two magnetic lines of force do not intersect each other.
(6) The magnetic lines of force pass through iron easily as
compared to air. Magnetic lines of force pass through all materials
both magnetic and non-magnetic.
Electromagnetic Force
When a moving charge passes through a magnetic field, then, a
force acts on it and this force is called electromagnetic force.
Also, a moving charge produces a magnetic field. The
electromagnetic force is the combination of electric force and

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magnetic force and, therefore, it is called electromagnetic force.
Electromagnetic Spectrum
A Scottish physicist, James Clark Maxwell (1831-79), working on
electromagnetism predicted that electrical oscillations would
generate electromagnetic waves. He also derived a formula for
the speed of electromagnetic waves. When electric and magnetic
quantities were measured and the speed of electromagnetic
waves was calculated, it was found to be equal to that of light in
vacuum. This suggested that light might be electromagnetic in
nature. This idea was later confirmed to be correct. The spectrum
of electromagnetic radiation consists of radio wave, microwaves,
infrared waves, visible waves, ultraviolet waves, x-rays and
gamma rays, etc.
Production of Electromagnetic Waves
A changing electric flux creates magnetic field which in turn
creates electric field and electromagnetic waves are generated.
Thus, fundamental requirement for generation of electromagnetic
waves is an electric charge with changing velocity since it will
create changing electric flux. The velocity of an oscillating charge
as it moves to and fro along a wire is always changing and it gives
rise to electromagnetic waves.

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A transmitting antenna is a good example of a system generating
electromagnetic waves through an oscillating charge.
The waves which do not require a medium for their propagation
are called electromagnetic waves, for example, light, heat and
radio-waves.
A changing magnetic flux creates an electric field and a changing
electric flux creates a magnetic field. The electric field gives rise
to a changing electric flux due to which a magnetic field is
produced. This process continues with the motion of electric and
magnetic fields in the surrounding space. Such moving electric
and magnetic fields are known as "electromagnetic waves".
Radar technology is based on the principle of reflection and
detection of electromagnetic waves.
Electromagnetic Wave Spectrum
The total range of frequencies lies in between O hertz to 1022
hertz. This full range is known as the electromagnetic wave
spectrum.
Some of the important electromagnetic waves are sound waves,
radio waves, Light, X-rays, gamma rays and cosmic rays. At
higher frequencies, the electromagnetic wave spectrum includes
nuclear radiation.

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We know that the wavelength of light can be measured with a
diffraction grating having a known number of lines per unit length.
In principle, the wavelength of any electromagnetic wave can be
determined if a grating of proper spacing (of the order of ‘’) is
available.
Classical Electrodynamics
Classical electromagnetism (or classical electrodynamics) is a
branch of theoretical physics that studies consequences of the
electromagnetic forces between electric charges & currents.
Theory of electromagnetism was developed by James Clerk
Maxwell.
A changing magnetic field produces an electric field. Similarly, a
changing electric field generates a magnetic field.
The magnetic field is produced by motion of electric charges, i.e.
electric current.
Magnetic Circuit
The path in which magnetic lines of force are established is called
a magnetic circuit or it is the path of magnetic flux. The opposition
offered to the establishment of magnetic lines of force in a
magnetic circuit is called the reluctance of the circuit.
Magnetic circuit is the path of magnetic lines of force. Both

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resistance and conductance have their counterparts in magnetic
circuits.
Magnetic Compass
The magnetic compass was introduced by Chinese.
Magnetic Declination
The angle between the magnetic and geographic meridians is
called the magnetic declination.
Magnetic Meridian
The magnetic meridian at any place is a vertical plane containing
the magnetic axis of a freely suspended magnet at rest under the
action of the earth's field.
Magnetic Poles
Fig: A magnetized piece of iron points N and S when suspended
freely

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When a magnet is suspended and allowed to rotate freely in a
horizontal plane, it comes to rest with its one end facing north and
the other end facing south. The end facing north is called north
seeking pole or north-pole and the end facing south is called
south seeking pole or south-pole. We mark the end facing north
with the letter N and disturb the orientation of the magnet, then, it
will again come to rest with the end marked N towards north.
When a magnet is dipped in iron filings, the filings are attracted
towards the magnet. There is a greater concentration of the filings
at the two ends than at the rest of the body of the magnet.
It two magnets are brought near each other; the north pole of one
magnet attracts the south pole of the other. The two north poles
and two south poles repel each other. Thus like poles repel and
unlike poles attract each other.
Poles of a Magnet
The poles of a magnet are the regions where the magnetic force
is greatest.
The North Pole of a magnet seeks the north pole of earth and the
South Pole seeks the south pole of the earth.
The poles have equal strengths. Like poles repel and unlike poles
attract each other.

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The places in a magnet where the resultant attractive force
appears to be concentrated are called the poles.
The pole of the magnet that seeks the magnetic north pole of the
earth is called the North Pole and the opposite pole is called the
South Pole.
Opposite magnetic poles have a force of attraction and the similar
poles repel each other.
Magnetic Pole Strength
One unit pole is said to exist when one unit of flux emanates from
(or returns to) a pole surface.
The two regions, usually situated at the ends, where the magnetic
force is greatest are known as the magnetic poles. The earth is
magnetized naturally, with its magnetic poles in roughly the same
regions as the geographical ones. Therefore, a suspended
magnet sets itself approximately north-south. The end that points
north is known as north seeking pole or North Pole. Similarly, the
end that points south is called South Pole. The poles are always
of equal strengths. Unlike poles attract each other while like poles
repel.
Magnetic Space Constant
Magnetic space constant is the permeability of vacuum.

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Magnetizing Force
The magneto motive force per unit length of flux path is called the
magnetizing force or magnetic field strength, denoted by the
symbol H.
The SI unit of magnetizing force is ampere-turns / meter.
Magneto-Hydrodynamics
It is the branch of physics which deals with the behavior of a
conducting fluid under the influence of a magnetic flux.
Magneto Motive Force
Just as the electric current is produced by the electromotive force,
so too the magnetic flux is regarded as being established by a
magnetic motive force mmf, denoted by the symbol F. The
magneto motive force is usually produced by current carrying
turns. If a current of 1 ampere flows through a coil of N turns, the
magneto motive force is the total current linked with magnetic
circuit.
Magneto motive force establishes the magnetic flux. The SI units
of magneto motive force are ampere - turns. Since the number of
turns is dimensionless, magneto motive force is generally
expressed in amperes. The value of the magneto motive force is
proportional to the current and to the number of turns. The

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magneto motive force provided by a permanent magnet is also
measured in ampere-turns that would produce the same effect.
Weber
The SI unit of Magnetic Field B is Tesla which is equal to one
Newton per Ampere meter.
The SI unit of Flux is Nm/A & is called Weber (Wb).
Ferromagnetic Substances
The substances which behave like a magnet in the presence of a
strong field are called ferromagnetic substances. The most
important group of materials for application of electricity and
electronics are Ferromagnetic Materials. Ferromagnetic materials
have a high permeability.
Substances that can be made to form domains are said to be
ferromagnetic, which means "iron magnetic". The only
ferromagnetic materials are iron, nickel and cobalt. However, it is
possible to combine properly two or more non-magnetic elements
and form a ferromagnetic substance.
Experiment
If we bring a nail or pins to the pole of the magnet, then, they will
stick to the magnet. Now, if we bring another nail near this nail,
then, it will stick to it. This nail will attract another nail and so on.

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Nails were not magnets before they were exposed to the bar
magnet and there was no force of attraction or repulsion among
themselves. When the nails are removed from the magnet, they
again do not attract one another and do not retain their
magnetization. Now, we repeat the experiment, but, this time, we
place a small piece of thin paper between the magnet and the first
nail. The chain of nails can be formed without a direct contact with
the pole of the bar magnet.
Hard Ferromagnetic Substances
Iron may be alloyed with certain materials such as aluminum,
silicon and carbon. Once these alloys are magnetized, they retain
their magnetization. These are called hard ferromagnetic
substances.
Soft Ferromagnetic Substances
The ferromagnetic substances which become magnets in the
presence of a magnetic field and lose their magnetism when
removed from the magnetic field are called soft ferromagnetic
substances, i.e., soft iron.
Paramagnetic Materials
Paramagnetic materials, such as aluminum, platinum, etc., have
less magnetic properties. All those materials which do not fall in

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the ferromagnetic or diamagnetic categories are paramagnetic.
The greatest percentages of the substances are paramagnetic.
Non-Magnetic Substances
The substances which are not affected by a magnet are called
non-magnetic substances. Wood, glass, paper etc. are non-
magnetic substances.
Non-Magnetic Material
Paramagnetic and Diamagnetic materials have a very low
permeability. Paramagnetic materials have permeability slightly
greater than one, whereas diamagnetic materials have
permeability slightly less than one. These materials are
considered for all practical purposes as non-magnetic materials.
Air, wood, paper, plastic and glass are non-magnetic materials.
Demagnetization
Magnets can be partially demagnetized by hammering them when
they are pointing in the east-west direction. They can also lose
their magnetism if they are heated strongly. However, these
methods of demagnetization are not recommended as they tend
to damage the steel or iron bars.
The most efficient method of demagnetization of a magnet is to
use an alternating current. The magnet is placed in a solenoid

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through which an alternating current is flowing. A 12volt
transformer may be used to supply the required current from the
main power line. It is recommended that the solenoid be placed in
the east-west direction and the magnet is pulled out while the
current is still flowing.
The alternating current reverses direction at a rate of 100 times
per second and hence causes the magnetism of the material to
reverse the polarity at the same rate. As a result of this rapid
reversal, the magnetization is gradually diminished.
Diamagnetic Materials
Diamagnetic materials, such as bismuth, mercury, etc., have
magnetic properties lesser than vacuum.
Materials made up of non-magnetic atoms, when placed in a
magnetic field, may attempt either to line up in the field or to turn
at right angles to the field. If they try to turn from the direction of
the magnetic field, they are called diamagnetic materials. There
are only a few diamagnetic materials. Some of the more common
are gold, silver, copper, zinc, and mercury.