BASIC ELECTRICAL THEORY 5

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Magnetism

 

Certain metals and metallic oxides have the ability to attract other metals. This
property is called magnetism, and the materials which have this property are
called magnets. Some magnets are found naturally while others must be
manufactured.

Magnetism is a result of electrons spinning on their own axis around the nucleus (Figure 18).

 

electrons spinning on their own axis around the nucleus

 

In magnetic materials, the atoms have certain areas called domains. These domains are aligned
such that their electrons tend to spin in the same direction (Figure 19).

magnetic domains

 

The alignment of these domains results in the formation of magnetic poles at each end of the
magnet. These poles are called the north pole and the south pole. The law of magnetism states
that like magnetic poles repel and unlike magnetic poles attract one another (Figure 20).

 

magnetic attraction and repultion

 

magnetic flux

image

 

Magnetic materials are those materials that can be either attracted or repelled by a magnet and
can be magnetized themselves. The most commonly used magnetic materials are iron and steel.
A permanent magnet is made of a very hard magnetic material, such as cobalt steel, that retains
its magnetism for long periods of time when the magnetizing field is removed. A temporary
magnet is a material that will not retain its magnetism when the field is removed.

 

permeability

 

 

 

 

 

 

Magnetic materials are classified as either magnetic or nonmagnetic based on the highly magnetic
properties of iron. Because even weak magnetic materials may serve a useful purpose in some
applications, classification includes the three groups described below.

Ferromagnetic Materials: Some of the ferromagnetic materials used are iron, steel, nickel,
cobalt, and the commercial alloys, alnico and peralloy. Ferrites are nonmagnetic, but have the
ferromagnetic properties of iron. Ferrites are made of ceramic material and have relative
permeabilities that range from 50 to 200. They are commonly used in the coils for RF (radio
frequency) transformers.

Paramagnetic Materials: These are materials such as aluminum, platinum, manganese, and
chromium. These materials have a relative permeability of slightly more than one.

Diamagnetic Materials: These are materials such as bismuth, antimony, copper, zinc, mercury,
gold, and silver. These materials have a relative permeability of less than one.

 

Electromagnetism

The relationship between magnetism and electrical current was discovered by a Danish scientist
named Oersted in 1819. He found that if an electric current was caused to flow through a
conductor, the conductor produced a magnetic field around that conductor (Figure 21).

magnetic field around that conductor

 

Polarity of a Single Conductor

A convenient way to determine the relationship between the current flow through a conductor
and the direction of the magnetic lines of force around the conductor is the left-hand rule for
current carrying conductors, as illustrated in Figure 22. The student should verify that the left hand
rule holds true for the examples shown in Figure 21.

 

left-hand rule for current carrying conductors

 

Magnetic Field and Polarity of a Coil

Bending a straight conductor into a loop has two results: (1) magnetic field lines become more
dense inside the loop, and (2) all lines inside the loop are aiding in the same direction.

When a conductor is shaped into several loops, it is considered to be a coil. To determine the
polarity of a coil, use the left-hand rule for coils (Figure 23).
Figure 23

 

left-hand rule for coils

 

Adding an iron core inside of a coil will increase the flux density. The polarity of the iron core
will be the same as that of the coil. Current flow is from the negative side of the voltage source,
through the coil, and back to the positive side of the source (Figure 24).

 

left-hand rule to find north pole of an electromagnet

 

Magnetomotive Force

Magnetomotive force (mmf) is the strength of a magnetic field in a coil of wire. This is
dependent on how much current flows in the turns of coil: the more current, the stronger the
magnetic field; the more turns of wire, the more concentrated the lines of force. The current
times the number of turns of the coil is expressed in units called “ampere-turns” (At), also known
as mmf. Equation (1-13) is the mathematical representation for ampere-turns (At).

 

magnetomotive force

field intensity

 

field intensity

Generally, different types of materials have different values of reluctance (Figure 25). Air gap
is the air space between two poles of a magnet. Since air has a very high reluctance, the size
of the air gap affects the value of reluctance: the shorter the air gap, the stronger the field in the
gap. Air is nonmagnetic and will not concentrate magnetic lines. The larger air gap only
provides space for the magnetic lines to spread out.

 

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