Magnetism

Magnetism

Electromagnetism is produced when an electrical current flows through a simple conductor such as a piece of wire or cable. A small magnetic field is created around the conductor with the direction of this magnetic field with regards to its "North" and "South" poles being determined by the direction of the current flowing through the conductor. 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.

Magnetism

Magnets can be found in a natural state in the form of a magnetic ore, with the two main types being Magnetite 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 inline with the earths 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, manmade 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 molecular magnets become "lined-up" in such a way that they produce a series magnet 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 magnetic materials are composed 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.

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 and materials which are required to retain their magnetism will have a high retentivity and are used to make permanent magnets, while those materials required to loose their magnetism quickly such as soft iron cores for relays and solenoids will have a very low retentivity.

Magnetic Flux

The magnetism in and around a magnetic circuit will have a definite chain producing an organised and balanced pattern of invisible lines around it and which are referred to as the "magnetic field" of the magnet. The shape of this magnetic field is more intense in some parts than others with these areas being called "poles" and each end of a magnet is a pole. They can be seen visually (called a vector field) 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 North-pole and there is always a 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 at the poles of the magnet where the lines of flux are more dense and the general direction for the magnetic flux flow is from the North ( N ) to the South ( S ) 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!. Some important facts emerge when plotting lines of force:

• 1.  -  Lines of force NEVER cross.
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• 2.  -  Lines of force are CONTINUOUS.
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• 3.  -  Lines of force always form individual CLOSED LOOPS around the magnet.
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• 4.  -  Lines of force have a definite DIRECTION from North to South.
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• 5.  -  Lines of force that are close together indicate a STRONG magnetic field.
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• 6.  -  Lines of force that are farther apart indicate a WEAK magnetic field.

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 they REPEL each other.
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• 2.  -  When adjacent poles are not the same they ATTRACT each other.

It can be remembered by the famous expression that "opposites attract" and this interaction of magnetic fields is easily demonstrated with iron fillings. 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 Together

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 N-pole and re-enter at the S-pole. Magnetism can be destroyed by heating or hammering the magnetic material, but cannot be destroyed by simply breaking the magnet into two as each half will become a separate magnet with its own north and south poles. In order to make use of magnetism in electrical or electronic calculations, it is necessary to define the various aspects of magnetism.

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.

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.