mod 3 rg 2.10 magnetism

Permanent magnets now have many more applications and are an essential component of numerous electrical devices important to aviation, including

motors, generators, microphones, loudspeakers, hard drives, sensors, and switches.

The discovery of a connection between electricity and magnetism began with Michael Faraday's early work in 1831 and developed into a major discipline within physics called

‘electromagnetism’

An understanding of electromagnetism is now fundamental to the design and maintenance of electrical components and systems used frequently in

aviation

Magnetism is a phenomenon caused by

the motion of electric charges

The region in which magnetic effects are observed is called a

“magnetic field”

A magnetic field is observed near an

electric current or a permanent magnet.

Some elements are described as being magnetic (principally iron, nickel, and cobalt). This means that items containing these elements are attracted to

permanent magnets and can be magnetised (made into permanent magnets).

electrons moving in their orbits, within the atoms, generate a magnetic field, so each atom is effectively a small,

permanent magnet.

In most elements, such magnetic fields from the many electrons present cancel out but in magnetic elements, there is

a small, residual effect due to the particular arrangements of electrons in the atoms of these elements.

A permanent magnet attracts

magnetic materials

The force of attraction of a permanent magnet is strongest at

each end (pole) of the magnet

Experiments show that like poles

repel and unlike poles attract

in addition to being an important navigational aid, a compass can be used to map the direction

of magnetic force around a magnet

a compass used to map the direction of a magnetic force is referred to as a

plotting compass

A plotting compass can be used to plot the magnetic field associated with a magnet. The resulting plot, illustrated below, shows

the direction of the force on a north pole, anywhere around the magnet. Note that the north pole of the plotting compass symbol is the end carrying the arrowhead.

The lines produced by such a plot of a magnetic field are referred to as

“lines of magnetic flux”

It is helpful for this topic to picture a flow of magnetic flux along these lines from

north to south around the magnet—although nothing is physically flowing in this direction, it is an essential model for understanding magnetism.

Permanent magnets are produced in a variety of different shapes, and the shape affects

the magnetic field pattern

Where the lines of magnetic flux are closer together, the magnetic field is

stronger

Also, the flux density is greater where

the lines of magnetic flux are closer together (more lines of magnetic flux flow through a smaller area).

Lines of magnetic flux are closed loops that

continue inside a magnet, leave the magnet at the north pole, and enter the south.

Externally, lines of magnetic flux always point

away from the north pole and towards the south pole of a permanent magnet.

Magnetic lines of flux have the following properties:

• They repel each other sideways

• They try to shorten themselves

• They always take the path of least reluctance

• They appear to originate at the North Pole and terminate at the South Pole

• They never cross each other

• They always form closed loops

• There is no known insulator to them

Earth's magnetic field, also known as the Geo-magnetic Field and extends out into space, creating a region known as the

magnetosphere

The magnetic field is generated in our planet's interior by a

self-exciting (geo)dynamo process. Electrical currents flowing in the slowly moving molten iron generate the magnetic field.

The Earth's outer core is in a state of turbulent convection as the result of radioactive heating and chemical differentiation. This sets up a process that is a bit like

a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy.

The motion of the electrically conducting iron in the presence of the Earth's magnetic field induces electric currents. Those electric currents generate their own magnetic field, and as the result of this internal feedback,

the process is self-sustaining so long as there is an energy source sufficient to maintain convection.

The crust of the Earth has some

permanent magnetisation

the Earth’s core generates its own magnetic field, sustaining the main part of the field we measure at the

surface

However, at most places on the Earth's surface, the compass doesn't point exactly toward geographic north. This is due to

“pockets of magnets (Lodestones) in the crust or rocks

Lodestones are

Lodestones are naturally magnetised pieces of the mineral magnetite. ... They are naturally occurring magnets, which can attract iron.

Lodestones in the crust or rocks. causes compasses not to point directly at the geographical north pole. This is called

deviation

The deviation of the compass from true north is an angle called

'declination' (or "magnetic declination")

'declination' (or "magnetic declination") has been a nuisance to navigators for centuries, especially since it varies with

both geographic location and time. It is a particular problem for aircraft is they were relying on navigation from magnetic compasses.

the earths magnetic lines of force are parallel to the earths surface at the

magnetic equator (zero degrees no dip)

the magnetic lines of force angle of inclination to the earths surfaces increase towards the

poles (90 degrees maximum dip)

If the Earth is pictured as a permanent magnet, then the north pole of the Earth is the south pole of this permanent magnet (since it attracts the north pole of a compass). Therefore, the magnetic field lines of the Earth flow

away from the south pole and towards the north pole. The north magnetic pole is the point at which the Earth's magnetic field lines point vertically downwards.

If a permanent magnet is freely suspended, it eventually comes to rest with one end pointing towards the north pole of the Earth. This end is called

the north-seeking pole, or north pole. The other end is called the south-seeking pole, or the south pole.

a freely suspended magnet rest in a

north-south direction

property of a permanent magnet is utilised in the construction and operation of a magnetic compass, which may be used to

find the direction of magnetic north and therefore all other directions.

With global navigation, aircraft or ships, using magnetic compasses, there are problems with the Earth’s magnetic field

These magnetic fields shift constantly to add to the complication of compass correction.

A magnetic compass contains a very small, light magnet called a

needle which is suspended on a small axle or pin. It is free to turn and points north and south in alignment with the Earth's magnetic field if no stronger magnetic field is present. If a permanent magnet is placed nearby, then the compass needle aligns with the field around the permanent magnet.

Some materials in the Earth’s crust are inherently magnetic but most are not. Iron consists of many very small magnets – known as

Weiss domains – within which all the magnetic fields of the individual atoms are aligned in the same direction (Domain Theory).

On a larger scale, an iron bar has no magnetic field because the magnetic fields from each domain cancel each other out, due to

being aligned in random directions

Subjecting an iron bar to a strong magnetic field causes the magnetic domains in the iron to

align in the same direction and the small magnetic fields all then reinforce one another. As a result, a magnetic field is observed around the iron bar and it is said to have been magnetised. The iron bar is then referred to as a permanent magnet.

the Weiss domains aligned so that their

magnetic fields are all pointing in the same direction.

methods of magnetising a materials are:

• Stroking with another magnetic.

• By hammering a piece of magnetic material held in the same direction as a magnetic field i.e., the Earth’s field.

• By heating a piece of magnetic material, and allowing it to cool in a magnetic field.

• By passing an electric current through a coil wound around the magnetic material (an electro-magnet) and by far, the most common way.

The magnetism cannot be isolated in one area and the whole iron bar is responsible for the magnetic force; therefore,

the flux lines must continue inside the iron (as mentioned previously).

However, if a permanent magnet is broken into many pieces, each piece

acts as a smaller, permanent magnet, with a north and south pole.

If magnetism must be removed from a workpiece or a tool, it has to be demagnetised.

This can be done by the following methods:

• Turning the curent off of an electro-magnet.

• Slowly moving the item away from a strong alternating magnetic field. Large items require a very strong field. This procedure is known as ‘degaussing’ and is often necessary for components after a lightning strike.

• Using mechanical force (striking or dropping) to shift the domains from their ordered direction and thus reducing the net magnetic field. The procedure must be frequently repeated to fully demagnetise the item.

• Heating beyond the Curie temperature; the disadvantage of this is that the magnetism may recur if the temperature falls below the Curie temperature. For iron, this temperature is 769℃. Regular exposure to high temperatures can cause permanent loss of magnetism.

Slowly moving the item away from a strong alternating magnetic field. Large items require a very strong field. This procedure is known as

‘degaussing’ and is often necessary for components after a lightning strike.

Using mechanical force (striking or dropping) to shift the domains from their ordered direction and thus reducing the net magnetic field. The procedure must be frequently repeated to

fully demagnetise the item.

Heating beyond the Curie temperature; the disadvantage of this is that the magnetism may recur

if the temperature falls below the Curie temperature. For iron, this temperature is 769℃.

Regular exposure to high temperatures can cause

permanent loss of magnetism.

For a given ferromagnetic material the alignment of the domains abruptly disappears at a certain temperature which is called the

Curie temperature for the material.

iron curie temprature

769℃

nickel curie temprature

356℃

cobalt cure temperature

1075℃

currie temperature of some ferrites

50-600℃

Magnetic shielding is used to

create a shield for a device or component that requires protection from magnetic interference.

n the example below, the steel ring attracts the lines of magnetic flux to itself because it has high permeability (is a good conductor of magnetic flux); consequently, any component placed inside the ring itself

should not be subjected to a magnetic field applied in the direction shown

Nothing can stop magnetic flux from flowing from north to south, but good conductors of magnetic flux can

redirect the lines of magnetic flux.

Although somewhat superseded by glass cockpit technology in modern aircraft, some legacy aircraft still have electro-mechanical instruments, such as ammeter and voltmeters. These will be affected by stray magnetic fields, such as the

earth’s and fields created in cables when a current is passing through them. By encasing them in a metal case the result described above is achieved. However, for it to be efficient, certain parameters must be met.

The different values of reluctance of air and soft iron are made use of in

magnetic screening.

The different values of reluctance of air and soft iron are made use of in magnetic screening. Air has a high reluctance whilst soft iron has a low reluctance. Therefore, if the equipment to be screened is surrounded by soft iron, most of the flux will pass through

the soft iron, rather than the air inside it, since lines of flux take the path of least reluctance.

Reluctance is analogeous to

resistance

soft iron is a material that readily magnetises when under the influence of a magnetising force, but also demagnetises when

that force is removed.

There are three types of magnetic materials, each with its own characteristics:

• Ferromagnets

• Paramagnets

• Diamagnets

Each type has some special effect on the magnetic lines of force.

Ferromagnetic Materials: are natural magnetic properties of

iron. The term 'Ferro' comes from the Latin for iron, 'ferrum'.

A ferromagnetic material is one that readily

magnetises

(its constituent atoms easily orient their electron spins to conform to an external magnetic field force – hence Easily magnetised).

Soft ferromagnetic materials become demagnetised spontaneously when

removed from a magnetic field.

Hard ferromagnetic materials can retain their magnetism, making them useful in the production of

permanent magnets

There are wide applications of ferromagnetic materials in the industry. They are widely used in devices like an

electric motor, generators, transformers, telephone, loudspeakers, magnetic stripe at the back of credit cards.

Most of the ferromagnetic materials are metals. Common examples of ferromagnetic substances are

iron, cobalt, and nickel. Metallic alloys and rare earth magnets are also classified as ferromagnetic materials.

Magnetite is a ferromagnetic material which is formed by the

oxidation of iron into an oxide

Magnetite has the greatest

magnetism among all the natural minerals on earth.

All ferromagnets have a maximum temperature where the ferromagnetic property disappears as a result of

thermal agitation. This temperature is called the Curie temperature.

The ‘Curie’ temperature of iron is about

1043 K.

The ‘Curie’ temperature provides the amount of energy it takes to

break up the long-range ordering in the material.

Ferromagnets will tend to stay magnetised to some extent after

being subjected to an external magnetic field.

This tendency to remember their magnetic history is called

hysteresis

The fraction of the saturation magnetisation which is retained when the driving field is removed is called the

remanence of the material and is an important factor in permanent magnets.

Ferromagnetic materials respond mechanically to an impressed magnetic field, changing the length slightly in the direction of the applied field. This property, called

magnetostrsiction

Magnetostrsiction is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetisation.

magnetostriction, leads to the familiar

hum of transformers as they respond mechanically to AC voltages, and various frequencies.

Paramagnetic Materials have a small and positive susceptibility to magnetic fields. These are metals that are

weakly attracted to magnets. They include aluminium, titanium, oxygen and iron oxide.

atoms of paramagnetic substances contain electrons most, but not all, that

spin in the same direction. This gives the atoms some polarity

example you couldn't pick up a tiny piece of aluminium with a magnet. Since some of the atoms can be turned to point their poles in the same direction, these metals are very weak magnets. Their attractive force is only measured with

sensitive instruments.

paramagnets materials are slightly attracted by a magnetic field, and the material does not retain the magnetic properties when

the external field is removed.

Paramagnetic properties are due to the presence of some

unpaired electrons, and from the realignment of the electron orbits, caused by the external magnetic field

Diamagnets are repelled by a

magnetic field

If diamagnetic materials are placed in a magnetic field, an induced magnetic field occurs

in the opposite direction, causing a weak repulsing force. These are materials that would normally be considered non-magnetic, such as water, plastics and many metals such as gold, copper and mercury.

All materials have diamagnetic properties. If this is the only response to a magnetic field, they are considered diamagnets. If the material also has ferromagnetic or paramagnetic properties, it will

overcome the weak diamagnetism.

One way to check whether an object is diamagnetic or paramagnetic is to

observe how it aligns itself in a magnetic field. Diamagnetic objects will rotate to avoid magnets.

A diamagnetic material will twist to get the weaker magnetic field, a paramagnetic and ferromagnetic will

will rotate to align itself with the magnetic field.

para and ferro magnetic materials having unpaired

electrons in their outer orbits, whereas with a diamagnetic material, all the electrons are paired. It is these paired electrons that cause the repulsion.

An electromagnet is a type of magnet in which the magnetic field is produced by an

electric current.

Electromagnets usually consist of insulated wire wound into a

coil

A current through the wire creates a magnetic field, which is concentrated

in the hole in the centre of the coil.

Each loop of such a coil is called a

turn

The wire turns are often wound around a magnetic core made from a

ferromagnetic or ferrimagnetic material, such as iron. The magnetic core concentrates the magnetic flux lines and increases the strength of the electromagnet.