The Nature of Magnetism
Magnetism plays an important role in Electrical and
Electronic Engineering because without it components such as relays, solenoids,
inductors, chokes, coils, loudspeakers, motors, generators, transformers, and
electricity meters etc, would not work if magnetism did not exist.
Then every coil of wire uses the effect of
electromagnetism when an electrical current flows through it. But before we can
look at Magnetism and especially Electromagnetism in more detail we need to remember back to our
physics classes of how magnets and magnetism works.
The Nature of Magnetism
Magnets can be found in a natural state in the
form of a magnetic ore, with the two main types beingMagnetite also called “iron oxide”, ( FE3O4 ) and Lodestone, also called “leading stone”. If these two
natural magnets are suspended from a piece of string, they will take up a
position in-line with the Earth’s magnetic field always pointing north.
A good example of this effect is the needle of
a compass. For most practical applications these natural occurring magnets can
be disregarded as their magnetism is very low and because nowadays, man-made
artificial magnets can be produced in many different shapes, sizes and magnetic
strengths.
There are basically two forms of magnetism,
“Permanent Magnets” and “Temporary Magnets”, with the type being used dependant
upon its application. There are many different types of materials available to
make magnets such as iron, nickel, nickel alloys, chromium and cobalt and in
their natural state some of these elements such as nickel and cobalt show very
poor magnetic quantities on their own.
However, when mixed or “alloyed” together with
other materials such as iron or aluminium peroxide they become very strong
magnets producing unusual names such as “alcomax”, “hycomax”, “alni” and
“alnico”.
Magnetic material in the non-magnetic state
has its molecular structure in the form of loose magnetic chains or individual
tiny magnets loosely arranged in a random pattern. The overall effect of this
type of arrangement results in zero or very weak magnetism as this haphazard
arrangement of each molecular magnet tends to neutralise its neighbour.
When the material is Magnetised this random arrangement of the molecules
changes and the tiny unaligned and random molecular magnets become “lined-up”
in such a way that they produce a series magnetic arrangement. This idea of the
molecular alignment of ferromagnetic materials is known as Weber’s Theory and is illustrated below.
Magnetic Molecule Alignment of a Piece of Iron
and a Magnet
Weber’s theory is based on the fact that all
atoms have magnetic properties due to the spinning action of the atoms
electrons. Groups of atoms join together so that their magnetic fields are all
rotating in the same direction. Magnetic materials are composed of groups of
tiny magnets at a molecular level around the atoms, and a magnetised material
will have most of its tiny magnets lined up in one direction only to produce a
north pole in one direction and a south pole in the other direction.
Likewise, a material that has its tiny
molecular magnets pointing in all directions will have its molecular magnets
neutralised by its neighbouring magnet, thereby neutralising any magnetic
effect. These areas of molecular magnets are called “domains”.
Any magnetic material will produce a magnetic
field itself which depends on the degree of alignment of magnetic domains in
the material set up by orbital and spinning electrons. This degree of alignment
can be specified by a quantity known as magnetisation, M.
In an unmagnetised material, M = 0, but some of the domains remain aligned over
small regions in the material once the magnetic field is removed. The effect of
applying a magnetising force to the material is to align some of the domains to
produce a non-zero magnetisation value.
Once the magnetising force has been removed,
the magnetism within the material will either remain or decay away quiet
quickly depending on the magnetic material being used. This ability of a
material to retain its magnetism is called Retentivity.
Magnetic Flux
All magnets, no matter what their shape, have
two regions called magnetic poles with the magnetism both in and around a
magnetic circuit producing a definite chain of organised and balanced pattern
of invisible lines of flux around it. These lines of flux are collectively
referred to as the “magnetic field” of the magnet. The shape of this magnetic
field is more intense in some parts than others with the area of the magnet
that has the greatest magnetism being called “poles”. At each end of a magnet
is a pole.
These lines of flux (called a vector field)
can not be seen by the naked eye, but they can be seen visually by using iron
fillings sprinkled onto a sheet of paper or by using a small compass to trace
them out. Magnetic poles are always present in pairs, there is always a region
of the magnet called the North-pole and there is always an
opposite region called the South-pole.
Magnetic fields are always shown visually as
lines of force that give a definite pole at each end of the material where the
flux lines are more dense and concentrated. The lines which go to make up a
magnetic field showing the direction and intensity are called Lines of
Force or more commonly
“Magnetic Flux” and are given the Greek symbol, Phi ( Φ ) as shown below.
Lines of Force from a Bar Magnets Magnetic
Field
As shown above, the magnetic field is
strongest near to the poles of the magnet were the lines of flux are more
closely spaced. The general direction for the magnetic flux flow is from the North ( N ) to theSouth ( S ) pole. In addition, these magnetic
lines form closed loops that leave at the north pole of the magnet and enter at
the south pole. Magnetic poles are always in pairs.
However, magnetic flux does not actually flow
from the north to the south pole or flow anywhere for that matter as magnetic
flux is a static region around a magnet in which the magnetic force exists. In
other words magnetic flux does not flow or move it is just there and is not
influenced by gravity. Some important facts emerge when plotting lines of
force:
·
Lines of force NEVER cross.
·
Lines of force are CONTINUOUS.
·
Lines of force always
form individual CLOSED LOOPS around the magnet.
·
Lines of force have a
definite DIRECTION from North to South.
·
Lines of force that
are close together indicate a STRONG magnetic field.
·
Lines of force that
are farther apart indicate a WEAK magnetic field.
Magnetic forces attract and repel like
electric forces and when two lines of force are brought close together the
interaction between the two magnetic fields causes one of two things to occur:
·
1. – When
adjacent poles are the same, (north-north or south-south) they REPEL each other.
·
2. – When
adjacent poles are not the same, (north-south or south-north) they ATTRACT each other.
This effect is easily remembered by the famous
expression that “opposites attract” and this interaction of magnetic fields can
be easily demonstrated using iron fillings to show the lines of force around a
magnet. The effect upon the magnetic fields of the various combinations of
poles as like poles repel and unlike poles attract can be seen below.
Magnetic Field of Like and Unlike Poles
When plotting magnetic field lines with a
compass it will be seen that the lines of force are produced in such a way as
to give a definite pole at each end of the magnet where the lines of force
leave the North pole and re-enter at the South pole. Magnetism can be destroyed
by heating or hammering the magnetic material, but cannot be destroyed or
isolated by simply breaking the magnet into two pieces.
So if you take a normal bar magnet and break
it into two pieces, you do not have two halves of a magnet but instead each
broken piece will somehow have its own North pole and a South pole. If you take
one of those pieces and break it into two again, each of the smaller pieces
will have a North pole and a South pole and so on. No matter how small the
pieces of the magnet become, each piece will still have a North pole and a
South pole, crazy!.
Then in order for us to make use of magnetism
in electrical or electronic calculations, it is necessary to define what are
the various aspects of magnetism.
The Magnitude of Magnetism
We now know that the lines of force or more
commonly the magnetic flux around a magnetic material is given the Greek
symbol, Phi, ( Φ ) with the unit of flux being the Weber, ( Wb ) after Wilhelm Eduard Weber. But the number of lines of
force within a given unit area is called the “Flux Density” and since flux
( Φ ) is measured in
( Wb ) and area
( A ) in metres
squared, ( m2 ), flux density
is therefore measured in Webers/Metre2 or ( Wb/m2 ) and is given the symbol B.
However, when referring to flux density in
magnetism, flux density is given the unit of the Tesla after Nikola Tesla so therefore one Wb/m2 is equal to one Tesla, 1Wb/m2 = 1T. Flux density is proportional to the lines of
force and inversely proportional to area so we can define Flux Density as:
Magnetic Flux Density
The symbol for magnetic flux density is B and the unit of magnetic flux density is the
Tesla, T.
It is important to remember that all
calculations for flux density are done in the same units, e.g., flux in webers,
area in m2 and flux density in Teslas.
Magnetism Example No1
The amount of flux present in a round magnetic
bar was measured at 0.013 webers. If the material has a diameter of 12cm,
calculate the flux density.
The cross sectional area of the magnetic
material in m2 is given as:
The magnetic flux is given as 0.013 webers,
therefore the flux density can be calculated as:
So the flux density is calculated as 1.15
Teslas.
When dealing with magnetism in electrical
circuits it must be remembered that one Tesla is the density of a magnetic
field such that a conductor carrying 1 ampere at right angles to the magnetic
field experiences a force of one newton-metre length on it and this will be
demonstrated in the next tutorial about Electromagnetism.
What is magnetic flux?
Magnetic flux is a measurement
of the total magnetic field which passes through a given area. It is a useful
tool for helping describe the effects of the magnetic force on something
occupying a given area. The measurement of magnetic flux is tied to the
particular area chosen. We can choose to make the area any size we want and
orient it in any way relative to the magnetic field.
If we use the field-line picture of a magnetic field then every field line passing through
the given area contributes some magnetic flux. The angle at which the field
line intersects the area is also important. A field line passing through at a
glancing angle will only contribute a small component of the field to the
magnetic flux. When calculating the magnetic flux we include only thecomponent of the magnetic field vector which is normal to our test area.
If we choose a simple flat
surface with area AAA as our test area and there is an angle \thetaθtheta between the normal to the surface and a magnetic field vector
(magnitude BBB) then the magnetic flux is,
\vec{B}B, with, vector, on top\vec{A}A, with, vector, on top\Phi = \vec{B}\cdot\vec{A}
\Phi = B A \cos{\theta}Φ=BAcosθ
In the case that the surface is perpendicular to the field then
the angle is zero and the magnetic flux is simply B ABAB, A. Figure 1 shows an example of a flat test
area at two different angles to a magnetic field and the resulting magnetic
flux.
Figure 1: Magnetic flux through
given areas (blue) oriented at an angle (left) and normal to (right) the
magnetic field.
Figure 1: Magnetic flux through given
areas (blue) orientwed at an angle (left) and normal to (right) the magnetic
field.
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