Electric field
Magnetic field
The magnetic field, the electric field and the field of gravitation
can be considered as a zone in which a magnet, an electrified
body or an object with mass is subjected to forces (Petit Larousse
dictionary).
What is a field?
In order to understand the concept of a field, let us take the
example of a campfire. Near the fire, one can feel the radiated
heat but does not see it. While moving away gradually from the fire,
one perceives less and less heat. In this case, it is a thermal
field. It is exactly the same for electric
field and magnetic induction: the intensity of the field is
significant near its source, and decreases rapidly as one moves
away from it.
How do we define electric field and magnetic
induction?
When a bedside lamp is connected to an electrical supply network
through a socket-outlet, there is only an electric field. It
is possible to compare an electric field with the pressure present
in a sprinkler pipe when it is connected to a water distribution system
and the tap is turned off. The electric field is related to the
potential difference, whose unit of measure is the volt.
It is generated in the presence of electric charges, and is measured
in volts per meter (V/m). The higher the supply voltage
of an apparatus is, the more intense the electric field that results
from it becomes.
When a lamp is switched on (i.e. when a current passes through
the electric wire), there occurs at the same time an electric and
magnetic field. The magnetic induction is related to the passage
of the current (i.e. the motion of the electrons) through the
electric wire. In the example of the sprinkler pipe, the magnetic
induction would correspond to the movement of the water through
the pipe. The unit of measurement for the magnetic induction is the tesla
(T). However the magnetic induction that we usually
measure are in the range of the microtesla (µT), that is to
say, one millionth of a tesla. Another unit sometimes used is the gauss
(G). One gauss is equivalent to 100 microteslas.
The term "Magnetic field" is often used instead of "Magnetic induction" (or "Magnetic flux density"). That is why you can find magnetic field (H) expressed in tesla (or Gauss), which is the unit of magnetic induction (B).
The rapport between magnetic induction (B in tesla) and magnetic field (H in ampere per metre) is the permeability (µ in henry per metre - a universal constant with a value of 1.25666 10-6 H/m in the majority of
materials, e.g. air, vacuum, gas, copper, ground).
From Ministère
de l'industrie, des Postes et des Télécommunications
et du Commerce Extérieur et du Ministère des Affaires
Sociales, de la Santé et de la Ville
The term "Magnetic field" is often used instead of "Magnetic induction" (or "Magnetic flux density"). That is why you can find magnetic field (H in Ampere/meter) expressed in Tesla (or Gauss with 10 -4 T = 1 G), which is the unit of magnetic induction (B).
Magnetic field H and magnetic induction field B are linked, in a given material, by the equation:
B = µ * H
where µ is the magnetic permeability of the material (in Henry/meter).
The magnetic permeability of a material is the capability of this material to channel magnetic induction, in other words, to concentrate magnetic flux lines and thus to increase the value of magnetic induction. It means that this value depends on the material in which it is produced.
The chanelling of the magnetic field in a material which is also a conductor is especially reduced, because of contact current (lien...) when frequency of field variation, permeability and conductivity are high.
(Further information in the dictionary of the site)
|
|
