Characteristics
of a magnetic field

They go through the
path of least resistance between opposite magnetic poles. They attempt to form
closed loops from pole to pole.

They don’t cross one another.

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They have the same strength.

Their density decreases when they go from an area of higher permeability
and then to one of a lower permeability.

With increasing distance from the poles their density increases.

They flow from the south pole to the north pole within a material and north
pole to south pole in air like the diagram shows – https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagneticFieldChar.htm

Magneto motive force (mmf)

Magneto motive force (mmf) is what allows their to be a
magnetic flux in a magnetic circuit.  The
mmf  is solved as F = NI

N is the amount of turns of wire in the coil, I is the current in the coil. The
unit for mmf is ampere-turns (At).

The mmf is related to the
electromotive force and could be called the thing that sets up the flux.

When the current or number of turns is increased
the mmf is larger and if the rest of the magnetic circuit does not change, the
magnetic flux increases with it.

Permeability

‘Permeability is the ability for a material to keep the
formation of a magnetic field within itself.

It is the degree of magnetization that a material
gathers in response to a magnetic field.’ – https://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29

Soft iron has a much higher permeability than air. If soft iron is
placed in a magnetic field, it will concentrate the flux lines.

Henries per meter (H/m) is what permeability is measured in.

B/H curves and loops

The BH Curve is a graph of the Magnetic Flux
Density (B) and Magnetic Field Strength (H).

This graph plays a big role in selecting
materials for electric machines.  The graph shows that as the field
strength is increased it shows the change in flux density.

The current in the coil is proportional to the msf.

If the centre is starts off not magnetised and the current is slowly
increased, the flux density will go
up.

This continues until the core material becomes full.

Ferromagnetic materials

A strong form of
magnetisation is known as ferromagnetism, the magnetic field of a magnet can bring out magnetism in some materials,
the magnetism causes the material to become attracted to the magnet, these materials
are known as ferromagnetic materials.

Ferromagnetism is the only magnet type that
creates forces strong enough to be felt.

Reluctance

Magnetic reluctance is the measure of how difficult it is for
magnetic flux to go through material.

Magnetic flux will try to follow the path of least reluctance, just
like the way that current goes through the path of least resistance.  The magnetic field causes the flux to go down the path of least magnetic
reluctance.

The greater the permeability the lower its reluctance.

The unit for magnetic reluctance is inverse henry, H?1

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