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Quantifying
Magnetic Properties
(Magnetic
Field Strength, Flux
Density, Total Flux and Magnetization)
Until now, only the qualitative features of the
magnetic field have been discussed. However, it is necessary to
be able to measure and express quantitatively the various characteristics
of magnetism. Unfortunately, a number of unit conventions are used (as shown in the table below). SI units will be used in this material.
The advantage of using SI units is that they are traceable back
to an agreed set of four base units - meter, kilogram, second,
and Ampere.
| Quantity |
|
SI Units
(Sommerfeld)
|
SI Units
(Kennelly)
|
CGS Units
(Gaussian)
|
| Field |
H |
A/m
|
A/m
|
oersteds
|
Flux Density
(Magnetic Induction) |
B |
tesla
|
tesla
|
gauss
|
| Flux |
f |
weber
|
weber
|
maxwell
|
| Magnetization |
M |
A/m
|
-
|
erg/Oe-cm3
|
The
units for magnetic field strength H are ampere/meter. A
magnetic field strength of 1 ampere/meter is produced at the center
of a single circular conductor with a one meter diameter carrying a
steady current of 1 ampere.
The number of magnetic lines
of force cutting through a plane of a given area at a right angle
is known as the magnetic flux density, B.
The flux density or magnetic induction has the tesla as its unit.
One tesla is equal to 1 Newton/(A/m). From these units, it can
be seen that the flux density is a measure of the force applied
to a particle by the magnetic field. The Gauss is the CGS unit for
flux density and is commonly used by US industry. One gauss represents
one line of flux passing through one square centimeter of air
oriented 90 degrees to the flux flow.
The total number of lines
of magnetic force in a material is called magnetic flux,
f. The strength of the flux is determined by the number
of magnetic domains that are aligned within a material. The
total flux is simply the flux density applied over an area. Flux
carries the unit of a weber, which is simply a tesla- meter2.
The magnetization is a measure of the extent to
which an object is magnetized. It is a measure of the magnetic
dipole moment per unit volume of the object. Magnetization carries
the same units as a magnetic field: amperes/meter.
Conversion between CGS
and SI magnetic units.
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