Magnetic terms, magnetic material and properties of magnet
- Magnetism and magnets: Magnetism is a force field that acts on some materials and not on other materials. Physical devices which possess this force are called magnets. Magnets attract iron and steel, and when free to rotate, they will move to a fixed position relative to the north pole.
- Classification of magnets Magnets are classified into two groups.
- Natural magnets
- Artificial magnets
- Lodestone (an iron compound) is a natural magnet which was discovered centuries ago.There are two types of artificial magnets. Temporary and permanent magnets.
- Temporary magnets or electromagnets: If a piece of magnetic material, say, soft iron is placed in a strong magnetic field of a solenoid it becomes magnetised by induction. The soft iron itself becomes a temporary magnet as long as the current continues to flow in the solenoid. As soon as the source producing the magnetic field is removed, the soft iron piece will loose its magnetism.
- Permanent magnets: If steel is substituted for soft iron in the same inducing field as in the previous case, due to the residual magnetism, the steel will become a permanent magnet even after the magnetising field is removed. This property of retention is termed retentivenes. Thus, permanent magnets are made from steel, nickel, alnico, tungsten all of which have higher retentiveness.
- Molecular theory of magnetism:
.png)
- In magnetic materials such as iron, steel, nickel, cobalt and their alloys, which are ferromagnetic materials, the molecules themselves are tiny magnets, each of them having a north pole and south pole. This is basically due to their special crystalline structure and to the continuous movements of electrons in their atoms Under ordinary conditions, these molecules arrange themselves in a disorderly manner, the north and south pole of these tiny magnets pointing in all directions and neutralizing one another. Thus a non-magnetized ferromagnetic bar is one in which there is no definite arrangement of the magnetic poles as shown in Fig When iron or steel is magnetized, the molecules are moved into a new arrangement as shown in Fig , which is caused by the force used to magnetize them.
- The earth's magnetic field:
- Since the earth itself is a large spinning mass, it too produces a magnetic field. The earth acts as though it has a bar magnet extending through its centre, with one end near the north geographic pole and the other end near the south geographic pole.
- Classification of magnetic substances Materials can be classified into three groups as follows.
- Ferromagnetic substances: Those substances which are strongly attracted by a magnet are known as ferromagnetic substances. Some examples are iron, nickel, cobalt, steel and their alloys.
- Paramagnetic substances: Those substances which are slightly attracted by a magnet of common strength are called paramagnetic substances. Their attraction can easily be observed with a powerful magnet. In short, paramagnetic substances are similar in behaviour to ferromagnetic materials. Some examples are aluminium, manganese, platinum, copper etc
- Diamagnetic substances: Those substances which are slightly repelled by a magnet of powerful strength only are known as diamagnetic substances. Some examples are bismuth, sulphur, graphite, glass, paper, wood, etc. Bismuth is the strongest of the diamagnetic substances. There is no substance which can be properly called non-magnetic. It may also be noted that water is a diamagnetic material, and air is a paramagnetic substance.
- Magnetic fields: The force of magnetism is referred to as a magnetic field. This field extends out from the magnet in all directions, as illustrated in Fig . In this figure, the lines extending from the magnet represent the magnetic field. The space around a magnet in which the influence of the magnet can be detected is called the magnetic field.
- Magnetic lines: Magnetic lines of force (flux) are assumed to be continuous loops, the flux lines continuing on through the magnet. They do not stop at the poles.
- Magnetic axis: The imaginary line joining the two poles of a magnet are called the magnetic axis. It is also known as the magnetic equator.
- Magnetic neutral axis : The imaginary lines which are perpendicular to the magnetic axis and pass through the centre of the magnet are called the magnetic neutral axis.
- Unit pole: A unit pole may be defined as that pole which, when placed one metre apart from an equal and similar pole, repels it with a force of 10 newtons.
- Attractive property : A magnet has the property of attracting magnetic substances (such as iron, nickel and cobalt) and its power of attraction is greatest at its poles.
- Directive property: If a magnet is freely suspended, its poles will always tend to set themselves in the direction of north and south.
- Induction property: A magnet has the property of producing magnetism in a nearby magnetic substance by induction
- Poles-existing property: A single pole can never exist in a magnet. If it is broken into its molecules, each molecule will have two poles.
- Demagnetising property: If a magnet is handled roughly by heating, hammering, etc. it will lose its magnetism.
- Property of strength: Every magnet has two poles. The two poles of a magnet have equal pole strength. Saturation property: If a magnet of higher strength is further subjected to magnetization, it will never acquire more magnetization due to its being already saturated.
- Property of attraction and repulsion: Unlike poles (i.e. north and south) attract each other, while like poles (north/north and south/south) repel each other..
- Assumed physical properties of magnetic lines of force: The lines of force always travel from the north to the south pole outside the magnet through air and from the south to the north pole inside the magnet. All the magnetic lines of force complete their circuit (form a loop). The magnetic lines do not cross each other. The lines of force travelling in one direction have a repulsive force between them, and, therefore, do not cross. The magnetic lines prefer to pass and complete their circuit through a magnetic material. They behave like a magnetic elastic band.
- Magnetic shielding: Magnetic flux lines can pass through all materials. Magnetic materials have a very low reluctance to flux lines. The lines of flux will be attracted through a magnetic material even if they have to take a longer path. This characteristic allows us to shield things from magnetic lines of force by enclosing them with a magnetic material. This is the way anti-magnetic watches are made. Measuring instruments which are to be shielded are enclosed inside an iron case. Shapes of magnets: Magnets are available in various shapes, with the magnetism concentrated at their ends known as poles.
- The common shapes are listed here.
–
| Type of Magnet | Common Uses |
|---|---|
| Bar Magnet | - Compass needles- Educational demonstrations- Magnetic experiments |
| Horseshoe Magnet | - Magnetic lifting tools- Picking up metal objects- Industrial separators |
| Ring Magnet | - Loudspeakers- Motors and generators- Magnetic levitation |
| Cylindrical Magnet | - MRI machines- Sensors- Electric motors and actuators |
| Specially Shaped Magnet | - Magnetic locks- Medical devices (e.g., dental braces)- Custom machinery |
- Bar magnet: It is in the form of a rectangular block with the magnetism concentrated at the ends, north pole and south pole.
- Horseshoe magnet : A rectangular iron rod bent to the shape of a horseshoe with the magnetism concentrated at their ends forming the north pole and south pole.
- Ring magnet: A ferrous metal formed into a ring as shown in Fig 11c is a ring magnet. Cylindrical type magnet: It is formed by a cylindrical iron rod with concentration of magnetism at the north and south pole ends as shown in above images
Methods of Magnetizing
There are three principal methods used to magnetize a magnetic material:
Touch Method
By Means of Electric Current
Induction Method
1. Touch Method
In this method, magnets are physically touched to the magnetic material. It can be further divided into:
a) Single Touch Method
A single pole of a magnet is stroked along the material (usually a steel bar) from one end to the other repeatedly in one direction.
This aligns the domains inside the material in one direction, inducing magnetism.
b) Double Touch Method
Two magnets are used with like poles (either both North or both South) placed near the center of the material.
They are then stroked outwards simultaneously towards opposite ends.
This creates a stronger and more evenly distributed magnetic field.
c) Divided Touch Method
Two magnets with unlike poles (North and South) are placed near the center of the material.
They are then moved simultaneously outward toward each end.
This method is more effective in producing strong, well-balanced poles at each end of the material.
2. Magnetizing by Electric Current
When a current-carrying wire or solenoid (coil of wire) is wound around a material and a current is passed through it, the magnetic field created by the current magnetizes the material.
The strength of the magnet depends on the amount of current, number of turns, and the material used.
This is the principle behind electromagnets.
3. Induction Method
A magnetic material can become magnetized without direct contact with a magnet.
When placed in the magnetic field of a strong magnet, the domains within the material align, magnetizing it.
This induced magnetism is temporary unless the material is hardened or treated to retain it.
-
General Care for Permanent Magnets
Avoid High Temperatures
-
Exposing magnets to temperatures above their maximum operating limit can cause them to lose magnetism.
-
For example, neodymium magnets can start to demagnetize above 80–200°C, depending on grade.
-
-
Keep Away from Strong External Magnetic Fields
-
Magnetic fields from other magnets or electrical devices (like transformers) can partially or fully demagnetize them.
-
-
Prevent Physical Damage
- Permanent magnets are often brittle (especially ceramic and neodymium types). Avoid impacts, dropping, or allowing them to slam together.
-
Protect from Corrosion
- Especially for neodymium magnets, which are prone to rust. Use coatings (nickel, epoxy) and store in dry environments
Principles and Laws of Electromagnetism
Oersted’s Experiment
Discovery by Hans Christian Oersted (1820): Demonstrated that an electric current produces a magnetic field.
A compass needle deflects when brought near a current-carrying wire, indicating the presence of a magnetic field around the wire.
The deflection direction changes when the direction of the current is reversed.
Magnetic Field Around a Straight Conductor
The direction of magnetic field lines depends on the current’s direction.
Right-hand grip rule:
Thumb points in the direction of the current.
Fingers curl in the direction of the magnetic field around the conductor.
Electromagnetism in Conductors and Coils
Electromagnet Basics
When an electric current passes through a conductor, it generates a magnetic field around it.
Wrapping the conductor into a coil amplifies the magnetic field.
Introducing a magnetic core (e.g., iron) inside the coil further strengthens the magnetic field, forming an electromagnet.
Polarity and Direction
Polarity of an electromagnet depends on the current direction.
Changing current direction reverses the magnetic poles of the electromagnet.
Right-Hand Rules and Corkscrew Rule
Right-Hand Grip Rule (Fig. 3):.png)
.png)
For a straight conductor or coil:
Thumb → current direction.
Fingers → magnetic field direction.
Right-Hand Palm Rule (Fig. 4):.jpeg)
.jpeg)
Used to determine the force on a conductor in a magnetic field.
Fingers → magnetic field direction.
Thumb → direction of motion (conductor).
Palm faces in the direction of the force on a positive charge.
Corkscrew Rule:.png)
.png)
Imagining a corkscrew turned in the direction of current:
Direction in which the corkscrew moves forward → direction of magnetic field.
- Electromagnetism: On passing a current through a coil
of wire, a magnetic field is set up around the coil. If a soft
iron bar is placed in the coil of wire carrying the current,
the iron bar becomes magnetized. This process is known
as `electromagnetism'.
.jpeg)
The soft iron bar remains as a
magnet as long as the current is flowing in the circuit. It
loses its magnetism when the current is switched off from
the coil.
The polarity of this electromagnet depends upon the
direction of the current flowing through it. If the direction of the current is altered, the polarity of the magnetic field
will also be changed.
The magnetic circuits - self and mutually induced emfs- MagnetoMotive Force (MMF): The amount of flux density
set up in the core is dependent upon five factors - the
current, number of turns, material of the magnetic core,
length of core and the cross-sectional area of the core.
More current and the more turns of wire we use, the
greater will be the magnetising effect. We call this product
of the turns and current the magnetomotive force (mmf),
similar to the electromotive force (emf) . MMF = NI ampere-turns
where mmf - is the magnetomotive force in ampere turns
N - is the number of turns wrapped on the core
I - is the current in the coil, in amperes, A.
If one ampere current is flowing through a coil having 200 turns then the mmf is 200 ampere turns.
Reluctance: In the magnetic circuit there is something
analogous to electrical resistance, and is called reluctance, (symbol S). The total flux is inversely proportional
to the reluctance and so if we denote mmf by ampere
turns. we can write ϕ=NI/S
Where ϕ is flux and reluctances S=μoμral
where S - reluctance
I - length of the magnetic path in metres
μo - permeability of free space
μr - relative permeability
a - cross-sectional area of the magnetic path in sq.mm.
The unit of reluctance is ampere turns/Wb.
Magnetic flux:The magnetic flux in a magnetic circuit is
equal to the total number of lines existing on the
cross-section of the magnetic core at right angle to the
direction of the flux. Its symbol is Ø and the SI unit is
weber. ϕ=NI/s
=NIaμoμr/l
where
ϕ - total flux
N - number of turns
I - current in amperes
S - reluctance
μo - permeability of free space
μr - relative permeability
a - magnetic path cross-sectional area in m$^2$
ℓ - length of magnetic path in metres.
Magnetic field strength: This is also known sometimes
as field intensity, magnetic intensity or magnetic field,
and is represented by the letter H. Its unit is ampere turns
per metre.
- Flux density (B): The total number of lines of force per
square metre of the cross- sectional area of the magnetic
core is called flux density, and is represented by the
symbol B. Its SI unit (in the MKS system) is tesla (weber
per metre square). B -
A /φ Weber/ m2 where φ - total flux in webers A - area of the core in square metres B - flux density in weber/metre square.
- Permeability: The permeability of a magnetic material is
defined as the ratio of flux created in that material to the
flux created in air, provided that mmf and dimensions of
the magnetic circuit remain the same. It's symbol is μ and μ = B/H where , B is the flux density and H is the magnetising force Being a ratio it has no unit and it is expressed as a mere
number. The permeability of air μ air = unity. The relative
permeability μr of iron and steel ranges from 50 to 2000.
The permeability of a given material varies with its flux
density.
- Hysteresis: Consider the graphical relation between B
and H for a magnetic material. Since μ = B/H, the
graphical relationship shows how the permeability of a
material varies with the magnetizing intensity H. Assume that the magnetic core is initially completely
demagnetised. As we increase the current . H=NI /L increases and there will be an increase in the flux density,
B. Since the number of turns and the length of core of a
coil are fixed, H is directly proportional to the current or
ammeter reading. The flux density can be measured by
inserting the probe of a flux meter into a small hole drilled
in the core.
A plot of the values of B and H gives the normal
magnetization curve, as shown in Fig 3.There is evidently
a linear portion where B is relatively proportional to H. But
then a condition of saturation occurs when a very large
increase in H is required to significantly increase B. This
point in the curve is called as saturation point.
If the current is now gradually reduced towards zero, H
returns to zero, but B does not. The core exhibits
retentiveness and retains some residual magnetism. The
retentiveness is represented by the distance OR.. If the connections to the coil are reversed, and the current
is again increased, it is found that a certain amount of H
is required to bring the magnetism in the core down to
zero. This is called the coercivity and is represented by
the distance OC.
Further, any increase in the current in the opposite
direction increases the magnetism in the core as before
in the opposite direction, until once again saturation
occurs. - Hysteresis loop: Reduction of the current and
subsequent reversal of the direction will produce a closed
figure called a B-H curve or hysteresis loop. The name
comes from the Greek word `hysteros' meaning `to lag
behind'. That is, the state of the flux density is always
lagging behind the efforts of the magnetising intensity.
The shape of a B-H loop is an indication of the magnetic
properties of the material. Hysteresis results in the dissipation of energy which
appears in the form of heat. The energy wasted in this
manner is proportional to the area of the loop. Thus, the
energy expanded, in joules per cubic metre of material in
one cycle, is equal to the area of the loop in M.K.S. units.
Energy expended/cycle/m2
in joules= Area of hysteresis
loop in m2
.
The shape of the hysteresis loop depends on the nature
of the iron or steel. Iron is subject to rapid reversal of
magnetism and in this case the area of loop is very small. The image provides the formula for hysteresis loss in magnetic materials, which is the energy dissipated per second per unit volume due to magnetic hysteresis. The formula is:
Where:
-
is a constant known as the hysteresis coefficient,
-
is the frequency (Hz),
-
is the maximum flux density (T, teslas).
This equation is used in designing magnetic cores (e.g., in transformers) to estimate energy losses due to hysteresis during magnetic cycling.
- Pulling power of solenoid: When the coil is energised,
it produces a magnetic field which also magnetises the
iron core. The iron core is attracted to the coil and they snap together. Once the core is in the centre of the coil,
the magnetic field is concentrated with that core and
there is no room for further movement The pulling power of a solenoid depends on the number
of turns of the coil, the current, material, flux density of the magnetic core, length and cross-sectional area of the
core. The strength of an electromagnet depends upon its
ability to conduct magnetism. The ability of conduction
depends on mmf, reluctance and permeability of the
magnetic path.
Magnetic Current and Electrical Current:
Magnetic Current
Electrical Current
Flux = mmf / reluctance
Current = emf / resistance
M.M.F. (Magnetomotive Force): Ampere-turns
E.M.F. (Electromotive Force): Volts
Flux (φ): Webers
Current (I): Amperes
Flux Density (B): Wb/m²
Current Density: A/m²
Reluctance (S) = ℓ / (μₐA) or ℓ / (μ₀μᵣA)
Resistance (R) = ρℓ / A
Permeance = 1 / reluctance
Conductance = 1 / resistance
Reluctivity = μ₀μᵣ / A
Resistivity (ρ)
Permeability (μ) = 1 / reluctivity
Conductivity = 1 / resistivity
.jpeg)
The soft iron bar remains as a magnet as long as the current is flowing in the circuit. It loses its magnetism when the current is switched off from the coil. The polarity of this electromagnet depends upon the direction of the current flowing through it. If the direction of the current is altered, the polarity of the magnetic field will also be changed.
- MagnetoMotive Force (MMF): The amount of flux density
set up in the core is dependent upon five factors - the
current, number of turns, material of the magnetic core,
length of core and the cross-sectional area of the core.
More current and the more turns of wire we use, the
greater will be the magnetising effect. We call this product
of the turns and current the magnetomotive force (mmf),
similar to the electromotive force (emf) . MMF = NI ampere-turns
where mmf - is the magnetomotive force in ampere turns
N - is the number of turns wrapped on the core
I - is the current in the coil, in amperes, A.
If one ampere current is flowing through a coil having 200 turns then the mmf is 200 ampere turns.
Reluctance: In the magnetic circuit there is something analogous to electrical resistance, and is called reluctance, (symbol S). The total flux is inversely proportional to the reluctance and so if we denote mmf by ampere turns. we can write ϕ=NI/S
Where ϕ is flux and reluctances S=μoμral
where S - reluctance
I - length of the magnetic path in metres
μo - permeability of free space
μr - relative permeability
a - cross-sectional area of the magnetic path in sq.mm.
The unit of reluctance is ampere turns/Wb.
Magnetic flux:The magnetic flux in a magnetic circuit is equal to the total number of lines existing on the cross-section of the magnetic core at right angle to the direction of the flux. Its symbol is Ø and the SI unit is weber. ϕ=NI/s
=NIaμoμr/l
where
ϕ - total flux
N - number of turns
I - current in amperes
S - reluctance
μo - permeability of free space
μr - relative permeability
a - magnetic path cross-sectional area in m$^2$
ℓ - length of magnetic path in metres.
Magnetic field strength: This is also known sometimes as field intensity, magnetic intensity or magnetic field, and is represented by the letter H. Its unit is ampere turns per metre.
- Flux density (B): The total number of lines of force per square metre of the cross- sectional area of the magnetic core is called flux density, and is represented by the symbol B. Its SI unit (in the MKS system) is tesla (weber per metre square). B - A /φ Weber/ m2 where φ - total flux in webers A - area of the core in square metres B - flux density in weber/metre square.
- Permeability: The permeability of a magnetic material is defined as the ratio of flux created in that material to the flux created in air, provided that mmf and dimensions of the magnetic circuit remain the same. It's symbol is μ and μ = B/H where , B is the flux density and H is the magnetising force Being a ratio it has no unit and it is expressed as a mere number. The permeability of air μ air = unity. The relative permeability μr of iron and steel ranges from 50 to 2000. The permeability of a given material varies with its flux density.
- Hysteresis: Consider the graphical relation between B
and H for a magnetic material. Since μ = B/H, the
graphical relationship shows how the permeability of a
material varies with the magnetizing intensity H. Assume that the magnetic core is initially completely
demagnetised. As we increase the current . H=NI /L increases and there will be an increase in the flux density,
B. Since the number of turns and the length of core of a
coil are fixed, H is directly proportional to the current or
ammeter reading. The flux density can be measured by
inserting the probe of a flux meter into a small hole drilled
in the core.
A plot of the values of B and H gives the normal
magnetization curve, as shown in Fig 3.There is evidently
a linear portion where B is relatively proportional to H. But
then a condition of saturation occurs when a very large
increase in H is required to significantly increase B. This
point in the curve is called as saturation point.If the current is now gradually reduced towards zero, H returns to zero, but B does not. The core exhibits retentiveness and retains some residual magnetism. The retentiveness is represented by the distance OR.. If the connections to the coil are reversed, and the current is again increased, it is found that a certain amount of H is required to bring the magnetism in the core down to zero. This is called the coercivity and is represented by the distance OC. Further, any increase in the current in the opposite direction increases the magnetism in the core as before in the opposite direction, until once again saturation occurs.
- Hysteresis loop: Reduction of the current and
subsequent reversal of the direction will produce a closed
figure called a B-H curve or hysteresis loop. The name
comes from the Greek word `hysteros' meaning `to lag
behind'. That is, the state of the flux density is always
lagging behind the efforts of the magnetising intensity.
The shape of a B-H loop is an indication of the magnetic
properties of the material. Hysteresis results in the dissipation of energy which
appears in the form of heat. The energy wasted in this
manner is proportional to the area of the loop. Thus, the
energy expanded, in joules per cubic metre of material in
one cycle, is equal to the area of the loop in M.K.S. units.
Energy expended/cycle/m2
in joules= Area of hysteresis
loop in m2
.
The shape of the hysteresis loop depends on the nature
of the iron or steel. Iron is subject to rapid reversal of
magnetism and in this case the area of loop is very small. The image provides the formula for hysteresis loss in magnetic materials, which is the energy dissipated per second per unit volume due to magnetic hysteresis. The formula is:
Where:
-
is a constant known as the hysteresis coefficient,
-
is the frequency (Hz),
-
is the maximum flux density (T, teslas).
-
- Pulling power of solenoid: When the coil is energised, it produces a magnetic field which also magnetises the iron core. The iron core is attracted to the coil and they snap together. Once the core is in the centre of the coil, the magnetic field is concentrated with that core and there is no room for further movement The pulling power of a solenoid depends on the number of turns of the coil, the current, material, flux density of the magnetic core, length and cross-sectional area of the core. The strength of an electromagnet depends upon its ability to conduct magnetism. The ability of conduction depends on mmf, reluctance and permeability of the magnetic path.
Magnetic Current and Electrical Current:
Magnetic Current Electrical Current Flux = mmf / reluctance Current = emf / resistance M.M.F. (Magnetomotive Force): Ampere-turns E.M.F. (Electromotive Force): Volts Flux (φ): Webers Current (I): Amperes Flux Density (B): Wb/m² Current Density: A/m² Reluctance (S) = ℓ / (μₐA) or ℓ / (μ₀μᵣA) Resistance (R) = ρℓ / A Permeance = 1 / reluctance Conductance = 1 / resistance Reluctivity = μ₀μᵣ / A Resistivity (ρ) Permeability (μ) = 1 / reluctivity Conductivity = 1 / resistivity