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.