Magnetic Properties of Materials
As kids, we loved playing with magnets. It seemed fascinating to
watch the magnets get attracted to each other. But, today we know that
magnets have so much more to them. Not only this, now we, also
learn about the magnetic properties of various materials. Did you
know even the Earth has a magnetic field? Let us learn about all these
interesting concepts in this chapter.
What is a Magnet?
A magnet is a material or object that can produce a magnetic field.
This magnetic field is invisible. However, it is mainly responsible for
the most notable property of a magnet. According to these magnetic
properties, a magnet possesses a force that pulls on other
ferromagnetic materials, such as iron, and attracts or repels other
magnets.
What is a Magnetic Field?
A magnetic field is the magnetic effect of electric currents and
magnetic materials. Hence, the magnetic field at any given point is
specified by both a direction and a magnitude (or strength). Therefore,
it is clear that it is a vector field. You can produce magnetic fields by
moving electric charges and the intrinsic magnetic moments of
elementary particles associated with a fundamental quantum property,
their spin.
What is Magnetic Field Intensity?
The magnetic field intensity at a point is defined as the force
experienced by a unit of the North Pole at that point. The tangent is
drawn on the line of forces usually gives the direction of magnetic
field intensity. It is measured in Telsa (T) or Gauss.
● Magnetic Pole Strength: This (symbol: p) is a physical quantity
that measures the strength of the pole of a bar magnet (or a
hypothetical magnetic monopole).
● Magnetic Moment: The magnetic moment of a magnet is a
quantity that determines the torque that it experiences in an
external magnetic field. Hence, we can find magnetic moments
in a loop of electric current, a bar magnet, an electron
(revolving around, a molecule).
Is the Magnetic Moment Scalar or Vector?
The magnetic moment is a vector quantity, having a magnitude and
direction. Therefore, the direction of the magnetic moment points
from the South to the North Pole of the magnet. Hence, the magnetic
field produced by the magnet is proportional to its magnetic moment.
Therefore, depending on the above magnetic properties, magnets can
be broadly classified:
● Diamagnetic
● Para-magnetic
● Ferro-magnetic
● Ferri-magnetic
● Anti-Ferro Magnetic
Diamagnetic Substance
Diamagnetic substances are those that are repelled by magnets. This is
because they produce negative magnetization. Hence, the net magnetic
moment is zero in diamagnetic substance. Every element in the
periodic table possesses the property of diamagnetism. However,
elements like Cu, Al2O3, Si, Zn have stronger diamagnetic property.
Paramagnetic Substance
Paramagnetic substances have a little magnetic moment. This is
because the magnetic moment does not cancel out completely. The
magnetic moments in the paramagnetic material are randomly aligned.
Example of paramagnetic materials includes Al, Cr, Mo, Ti, Zr.
Ferromagnetic Substance
Unlike diamagnets or paramagnets, you can magnetize ferromagnetic
substances, even when you remove the magnetic field. This
phenomenon is called Hysteresis. However, at one point or
temperature, the ferromagnetic materials lose their magnetic
properties. This temperature is the Curie point or Curie Temperature.
Ferri-Magnetic Substance
The main difference between a ferromagnetic material and
ferri-magnetic material is based on the alignment of the magnetic
domains. While some magnetic domains in ferri-magnetic material
points in the same direction, some point in the opposite direction. In
the case of ferromagnetic material, all the magnetic domains point in
the same direction.
Anti-Ferromagnetic Substance
In Anti-Ferromagnetic substances, the magnetic moments of atoms or
molecules are usually related to the spin of the electrons. Therefore,
they align in a regular pattern with neighboring spins in opposite
directions. MnO or Manganese Oxide is an example of an
anti-ferromagnetic substance.
Solved Questions For You
Q1. What is a magnetic material?
Ans: A magnetic material is a material which can attract or repel other
substances, under the influence of its magnetic field. Therefore, the
actions of attracting or repelling a substance depend on the
arrangement of electrons. Here, this is the Magnetic Moment of the
substance. Therefore, when you bring it under the influence of the
external magnetic field, you produce this moment.
Q2. What are the properties of Magnet?
Ans: The properties of a magnet are:
● The north pole of one magnet attracts the south pole of other
magnet and vice-versa.
● The magnet possess its own magnetic field lines due to various
factors like the flowing of current, orbital spin of an electron,
magnetic moment etc
● The behavior of a magnet is always studied with respect to the
earth’s magnetic field.
Q3. What do you mean by paramagnetic material?
Ans: A paramagnetic material is a substance which has a little
magnetic susceptibility. Therefore the net magnetic moment in the
paramagnetic substance does not cancel out completely. Hence. they
have very little magnetic strength. Example of paramagnetic materials
includes Al, Cr, Mo, Ti, Zr.
Magnet and Its Properties
We all have played with the magnets. But, have you ever wondered
about its origin? The story of the discovery of the magnetic substances
is interesting but more interesting is the scientific study of the cause of
this force. Each magnet produces a force which is unique. Let us study
the magnet and its properties.
The Story of Magnet
There is an island called Magnesia in Greece. Years ago, shepherds
here complained that their wooden shoes with nails stayed stuck to the
grounds. They were unable to walk further. This was the story behind
the discovery of magnetism. So, how did this relate to the presence of
magnetic fields? Actually, this island had lots of magnetic ore
deposits!
Sounds really interesting! Doesn’t it? Let us now look at this topic in
greater detail and discuss magnets and their properties. The property
of an object by virtue of which it can attract a piece of iron or steel is
called magnetism. The object itself is called a magnet. Now, we will
look at the types of magnets.
Natural Magnets
A natural magnet is an ore of iron that attracts small pieces of iron,
cobalt, and nickel towards it. It is usually an oxide of iron named
Fe3O4. Magnetite or lodestone is a natural magnet.
Artificial Magnets
A magnet that is prepared artificially form the artificial magnets.
Examples include an electromagnet, a magnetic needle, horseshoe and
bar magnets etc. According to the molecular theory, every molecule of
a magnetic substance, irrespective of whether or not it is magnetized.
The poles are the two points near but within the ends of the magnetic
materials, at which the entire magnetism can be assumed to be
concentrated. The poles always occur in pairs and they are of equal
strength. Like poles repel each other and unlike poles attract each
other. This is all the basic information about magnets. Now, we will
look deeper into the properties of magnets.
Video on Magnetic Effects of Current and Magnetism
Properties of Magnets
● Magnets attract magnetic substances like steel, cobalt, iron etc.
This is also known as its attractive property.
● When a bar magnet is freely suspended, it points in the
north-south direction. The tip which points to the geographic
north is called the north pole and the tip which points to the
geographic south is called the south pole. This is also known as
its directive property.
● There is a repulsive force when north poles (or south poles) of
two magnets are brought close together. Conversely, there is an
attractive force between the north pole of one and the south
pole of the other. This states that unlike poles attract each other
and like poles repel each other.
● We cannot isolate the north or south pole of the magnets. If
magnets are broken into two halves, we get two similar bar
magnets with somewhat weaker properties. Unlike electric
charges, isolated magnetic north and south poles known as
magnetic monopoles do not exist.
Solved Examples For You
Q: If a bar magnet is cut lengthwise into 3 parts, the total number of
poles will be:
A) 2 B) 3 C) 4 D) 6
Sol: D) Magnetic monopoles can’t exist. Therefore, no matter how
low you go to (atomic or subatomic level), you will always find a
magnetic dipole. Magnets have to have at least one North and one
South Pole. So when the bar magnet is cut into 3 parts, every part will
have its own poles and thus the total number of poles will be 6.
Magnetization and Magnetic Intensity
We have all played with magnets as children. Some of us even play
with them now! But, what makes them ‘magnetic’? Why don’t all the
materials and substances possess a magnetic field? Have you ever
wondered about it? In this chapter, we will cover the topics of
magnetization and magnetic intensity.
Magnetization
As we know, magnetization results from a magnetic moment. The
motion of electrons in the atoms is what induces this. The net
magnetization results from the response of a material to the external
magnetic field. It also takes into consideration any unbalanced
magnetic dipole moment that is inherent in the material due to the
motion of its electrons as mentioned earlier.
The concept of magnetization helps us in classifying the materials on
the basis of their magnetic property. In this section, we will learn more
about magnetization and the concept of magnetic intensity. The
magnetic behavior of a magnet is characterized by the alignment of
the atoms inside a substance. This is what we will look at in this
chapter.
Magnetic Intensity
The magnetic intensity at a point is defined as the force that unit north
– Pole experiences when it is placed in that field. The intensity of the
magnetic field at P due to single pole is given by:
We say that the magnetic field B can be written as:
Intensity of Magnetic Field due to a Magnet at Different Points
In Longitudinal Position
+/-m = Magnitude of the south and north poles
r = Distance of point P from the center of the magnet
l = Length of the bar magnet
∴ Intensity of the magnetic field at point P is given by,
where M is the magnetic moment = 2*m*l
Case: For a small magnet, r2 ≫ l2
In Transverse Position
The intensity of the magnetic field at point P is given by,
When the magnet is of short length,
In Any General Position
The direction of is always parallel to the axis from the north to the
south pole in the magnet.
Definition of Intensity of Magnetisation
The Magnetic moment of a magnet undergoes a change when it is
placed in a magnetic field. This change that is, the magnetic moment
change per unit volume is the Intensity of Magnetisation.
The formula of Intensity of Magnetisation
Where, m – Pole strength and A – Area of the cross-section. The S.I
unit of intensity of magnetization is Ampere/ meter or A/m
Solved Examples for You
Question: What is the difference between Magnetic Intensity and
Intensity of Magnetisation?
Answer: The magnetic intensity defines the forces that the poles of a
magnet experiences in a magnetic field whereas the intensity of
magnetization explains the change in the magnetic moment of a
magnet per unit volume.
Question: What do you mean by induced magnetization?
Answer: Induced magnetization is a process where you can magnetize
a non-magnetic material. You can do so when you bring it under the
influence of an external magnetic field.
Difference Between Permanent Magnet and Electromagnet
Difference between Permanent Magnet and Electromagnet is magnetic
field and strength. In Electromagnet, the magnetic field is created by a
wire-wound coil but the magnetic field of Permanent (Bar) Magnet
cannot be changed. The strength of Permanent Magnet depends on the
material used for its creation, on the other hand, the strength of
Electromagnet varies according to the flow of electric current into it.
Three types of Permanent (Bar) Magnets are Ceramic Magnet,
Flexible magnets, Neodymium iron boron magnet, and Samarium
cobalt magnet. Each has its own applications and types. Let’s see the
more differences between the Permanent Magnet and Electromagnet.
Permanent Magnet
These types of magnets can retain their magnetism and magnetic
properties for a longer time. Strongly magnetized hard materials make
up permanent magnets. A perfect example of a permanent magnet is
the Bar Magnet. This magnet widely explains the behavior of
magnets. Actually, we call permanent magnets as Bar Magnets also.
Electromagnet
When a battery is connected to a solenoid (a coil of wire is wounded
around a nail), the apparatus behaves like a magnet. This is due to the
magnetic field produced by the current flowing through the coil. The
nail retains its magnetism until there is a current flowing through the
coil, but once there is no current, the nail loses its magnetism.
You can produce electromagnets when you wound a coil of wire
across an iron core. Let us now look at the differences between a
permanent magnet and electromagnet.
Difference between Permanent Magnet and Electromagnet
Permanent (Bar) Magnet Electromagnet
They are permanently magnetized. These are temporarily magnetized.
These are usually made of hard materials. They are usually made of soft materials.
The strength of the magnetic field line is
constant i.e. it cannot be varied. The strength of the magnetic field lines can be
varied according to our need.
The poles of a Permanent magnet cannot be
changed. The poles of an electromagnet can be altered.
Example of a permanent magnet is a Bar
Magnet
Example of a temporary magnet is solenoid
wounded across a nail and connected to a
battery.
Similarities between Permanent Magnets and Electromagnets
Both the magnets possess imaginary magnetic field lines. The magnets
have north and south-pole whose behavior depends on the Geographic
north-pole and south-pole of the earth. Both the magnets exhibit the
properties of magnetism.
Advantages of Electromagnets over Permanent Magnets
You can get electromagnets at cheaper rates than the permanent
magnets. This is because the cost of materials used in the
electromagnet is lesser. You can alter the magnetic strength of an
Electromagnet according to your need. This is not possible in case of a
permanent magnet.
Disadvantages of Electromagnets
● Electromagnets require a large number of copper couplings.
This makes them unfit for use in small spaces. They also
require a lot of maintenance. The short-circuit may damage the
electromagnet.
● Electromagnets require a continuous supply of current. This
may, at some point in time, affect the magnets and its field due
to various factors like ohmic heating, Inductive voltage spikes,
core losses, the coupling of coils, etc.
Disadvantages of Permanent Magnets
● You can produce the magnetic field of a permanent magnet
only below a certain temperature. Therefore, you cannot use
this type of magnets for hot-device applications.
● These permanent magnets tend to corrode with time. The
strength of the maximum magnetic field is, thus, reduced. You
cannot vary the poles of the permanent magnet.
Types of Permanent Magnets
In this section, we will discuss the various types of permanent magnet.
They are:
● Ceramic Magnet: These magnets are the most inexpensive
permanent magnets. We use them in food processing industries,
resonance imaging etc.
● Flexible Magnets: The door seals used in the refrigerator are
flexible magnets. You can develop these through a combination
of rubber polymers, plastic, and magnetic powders.
● Neodymium Iron Boron Magnet (NdFeB): It a type of rare
earth magnet. You can oxidize it very easily. It is a very
expensive material. We commonly use it in jewelry making,
bookbinding etc.
● Samarium Cobalt (SmCo) Magnet: This is a type of rare earth
magnet. It is resistant to temperature and oxidation. They have
a higher magnetic strength. You can use them in high-end
motors, turbomachinery etc.
Application of Electromagnets
Transformers use electromagnets most commonly. The coils in the
transformer produce varying magnetic fields when you supply the
current. This induces a voltage. We use transformers primarily used to
regulate the alternate voltages in the electric power system.
You can achieve the desired voltage as and when required. We can do
this by changing the amount of current. You can also use
electromagnets in magnetic locks, relays, magnetic levitation, electric
bells, loudspeakers etc.
Solved Example For You
Q: Which among the following consists of soft iron?
A) Permanent Magnet B) Electromagnet C) Temporary Magnet
D) All
Solution: B) The soft iron inside the coil makes the magnetic field
stronger as it itself becomes a magnet when the current starts to flow.
Soft iron is suitable because it loses its magnetism as soon as the
current stops flowing.
The Bar Magnet
We all have played with magnets in our childhood. All of us had a Bar
Magnet as our favorite toy! It was quite surprising to wonder how a
tiny piece of the magnet could attract so many iron scraps. We basked
in the magic when one magnet repelled another! It was indeed a
glorious time! Well, today it is time to decode the theory of magnets!
Introduction
In this chapter, we will discuss all magnets, particularly bar magnets.
We will look at their properties and other related terms. Let us start
with the types of magnets. Let us look at the types of magnets in brief:
● Natural Magnets: They occur in nature and have a weak
magnetic field. Examples include lodestones.
● Artificial magnets: They are produced by man-made means and
have a stronger magnetic field. You can shape them as
required. When shaped in form of a bar, they are called bar
magnets.
Bar Magnet
A bar magnet is a rectangular piece of the object. It is made up of iron,
steel or any other ferromagnetic substance or ferromagnetic
composite, having permanent magnetic properties. The magnet has
two poles: a north and a south pole. When you suspend it freely, the
magnet aligns itself so that the north pole points towards the magnetic
north pole of the earth.
Properties of Bar Magnet
A bar magnet’s properties are similar to those of permanent magnets.
Let us look at them in the below section. The bar magnet has a north
pole and a south pole at two ends. Even if you break a bar magnet
from the middle, both the pieces will still have a north pole and a
south pole, no matter how many pieces you break it in.
The magnetic force of it is the strongest at the pole. When the magnet
is suspended freely in the air with a thread, it will not come to rest
until the poles are aligned in a north-south position. A Mariner’s
Compass uses this property to determine direction.
When you place two bar magnets close to each other, their unlike
poles will attract and like poles will repel each other. A bar magnet
will attract all ferromagnetic materials such as iron, nickel, and cobalt.
Magnetic Field Lines
Let us understand the concept of magnetic field lines using the activity
described below. Let us sprinkle iron filings on a sheet of paper and a
bar magnet in between. When we tap the paper, we notice that the
fillings get aligned in the form of many lines. The patterns of the
filings show us the magnetic field lines that surround this bar magnet.
The magnetic field lines can be defined as imaginary lines that can be
drawn along the magnetic field that is acting around any magnetic
substance. The magnetic field lines possess certain properties. They
are mentioned below.
● The magnetic field lines of a magnet form continuous closed
loops.
● The tangent to the field line at any point represents the
direction of the net magnetic field B at that point.
● Larger the number of field lines crossing per unit area, the
stronger is the magnitude of the magnetic field B.
● The magnetic field lines do not intersect.
Pole Strength
The Pole strength of a bar magnet can be measured by moving it along
an ‘infinite’ wire and carefully measuring the amount of current that is
created. The formula for calculating the pole strength is:
where p = the Strength of the magnetic pole, W = the work-done while
moving the magnet around the wire, I = the electric current in the
wire.
Alnico and Neodymium Bar Magnet
It will be incomplete to talk about bar magnets and not mention
Alnico bar magnet and neodymium bar magnet. The primary
components of Alnico magnets are aluminum, nickel, cobalt, and iron.
These magnets produce a strong magnetic field and retain their
magnetic property even under extreme heat. Neodymium bar magnets
are made with a mixture of neodymium, boron, and iron. These are
both extremely powerful magnets but very brittle.
Solved Examples For You
Q 1: Assertion: It is not necessary that every magnet has one north
pole and one south pole.
Reason: It is a basic fact that magnetic poles occur in pairs.
A. Both the statements are true and the reason is a correct
explanation of the assertion.
B. The statements are true but the reason is not the correct
explanation of the assertion.
C. Assertion is true but the reason is wrong.
D. The reason is true but the assertion is wrong.
Sol: D) As you move on in Physics, you will be able to show that
magnetic monopoles i.e. the isolated North Pole or the isolated South
Pole of a magnet can’t exist. But a magnet may have more than one
pole when it is called the multipole. Therefore the answer is D.
Q 2: Why are some magnets shaped like a horseshoe?
Sol: Some magnets are shaped like a horseshoe because of the high
magnetic flux at the end. The lifting power of a horseshoe is more
than that of a simple rectangular bar magnet. Thus, having a horseshoe
shape makes them powerful.
The Earth’s Magnetism
Are you aware that the earth is also a magnet? Did that surprise you?
How do you think then that the suspended bar magnet always points in
the north-south direction? This is the concept of the Earth’s
Magnetism that we are going to discuss in this chapter. It is really
interesting to study and analyze this concept of earth’s magnetism.
Earth’s Magnetism
The reason, why a bar magnet points in the north-south direction, is
because of the influence of earth’s gigantic magnetic field. It is
believed that the electric currents circulating from earth’s core to
space give rise to the earth’s magnetic field.
This magnetic field saves the earth from the solar wind that could
cause the ozone layer of the earth to strip away. The SI unit of the
earth’s magnetic field is Tesla.
Theory of Earth’s Magnetism
There is no concrete reason for the cause of the earth’s magnetism.
However, there are a few theories that revolve around it. Some of the
theories are mentioned below:
● The Dynamo Effect: The outer core of the earth has molten
Iron and other heavy elements in liquid form. The inner core
solidifies under the influence of gravity. Therefore, the motion
of metallic fluids in the outer core of the earth causes an
electric current. Thus, the earth gets its own magnetic field
lines.
● Ionization of the Outer Layers: This theory tells us that the
rotation of the earth in its own axis produces strong electric
current due to the ionization of the outer layers of earth. This
produces magnetism due to the movement of the ions.
However, the magnetic field will be very weak. The Dynamo
Effect is the more acceptable theory.
Components of Earth’s Magnetic Field
Near the surface of Earth, the magnetic field of the planet can be
resolved in different directions. These are the components that are
responsible for the magnitude and direction of the magnetic field of
the earth at a given location:
● Magnetic Declination
● Horizontal Component of Earth’s Magnetic Field
● The angle of Dip or Magnetic Inclination
Magnetic Declination
When a magnetic needle is suspended freely in the air, it always points
in the north-south direction free form all other attracting forces. This
is known as Magnetic Meridian.
Magnetic Declination is defined as the angle made by the Magnetic
meridian with the geographic meridian. Here geographic meridian is
defined as the plane passing through the north and south poles of the
earth.
The Angle of Dip or Magnetic Inclination
Take a magnetic needle and suspend it freely so that it can rotate about
a horizontal axis as shown in the diagram below:
The angle of dip or magnetic inclination
The angle that the north pole of the needle makes with the horizontal
axis is known as the Angle of Dip or Magnetic Inclination.
The Formula for Earth’s Magnetic Field
The magnetic intensity of the earth’s magnetic field makes an angle
known as Angle of Dip (δ) with the horizontal axis. We can separate
the intensity of the earth’s magnetic field into two components:
● Horizontal Component(H)
● Vertical Component(v)
There are times when these elements undergo regular or irregular
changes at all places on earth. Why does this happen? Following are
some of the important variations of the elements.
Variations in Earth’s Magnetic Field
● Secular Variation: The magnetic axis undergoes a periodic
change because of its spin around its own axis from east to
west. The time cycle of this variation is 960 years.
● Eleven-year Sunspot Cycle: Once in every eleven years, the
earth faces the sunspot which is a region of the strong magnetic
field. Thus the magnetic activity of the earth is very much
influenced by this variation.
● Daily and Annual Variation: The ultraviolet rays from the sun
ionize the earth’s atmosphere. As a result of which the current
is generated which further produces the magnetic field. This is
the result of daily and annual variations.
● Lunar Variations: Apart from the sun, the moon also influences
the magnetic activity of earth. Due to the tidal motions of the
earth’s ionized layer during a lunar eclipse, there is variation in
earth’s magnetic field. This variation is the Lunar Variation.
● Irregular and Aperiodic Variation: During a particular period of
time when the solar activity of the sun is more active, the
radiations from the sun ionize the atmosphere of the earth. This
causes current when the earth revolves around its own axis
resulting in the magnetic field.
Solved Example For You
Q: Why the earth has a magnetic field?
Answer: There are various theories suggesting the reason behind
earth’s magnetic field. However, the two most considered theories are:
● The current generated due to the motion of liquids inside the
earth’s core.
● The current generated due to the movement of the ionized
particle of earth’s atmosphere when the earth rotates around its
own axis.