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On a background green, permanent magnets from a DVD electric engine : one can see row of magnetic poles,
i.e 5 north pôlesand 5 south poles. This 5 mm thickness disc spin above a set of 10 flat coils,
matching to the 10 magnetic poles. The engine so made is actually very flat.
On right hand, the magnetic disc normally saw ; if we pass a screwdriver rod on its surface, on can feel the shift from a pole to the following. If we pass a magnet
instead, we can feel separately pole north and south.
Magnetization, flux, circuits, magnetic poles.
The iron filings visualizes the flux of the magnetic field around a magnet.
This flux, which wouldn't circulate, is however oriented for a compass needle position changes depending on its location : the flux "get out" from
one end of "magnet"
with the symbol + that get in the other end with a sign - The magnetic field occupies the space that surrounds the magnet, here in a volume "kiwi" shaped, i.e
close to the ellipse curb;. Inside the bar, one squeeze a bit, and even a lot. The lines are parallel.
A magnetic field is still a closed circuit and there is no magnetism which would have an alone pole.
If one break a magnet in two parts, two magnets are created.
- Our planet is amagnet.
Its magnetic field shields us from the attacks of different radiations, whose magnetic violence of the sun, and life would be impossible without it.
Drawing on the left, and the earth and its magnetosphere (beyond 800/1000 km altitude).
This image is a colorful representation of our Earth - small ball half white, half black, as illuminated by the sun - with its magnetic north pole that enters and exits at the south pole (near the geographic pole),
so there is not quite what one would imagine, but from the satellites and the theory of Mr. Van Allen, is the representation that we are in fact: it is blown like the flame of a
candle by the solar wind, a little even through due to the inclination of the axis of the earth (click
to see: solar wind, magnetosphere, Van Allen two areas; gray, stationary shock wave, neutral zone and plasma zone )
Right on the magnetic pole, if possible, raise the usual question: "what direction indicates a magnetic needle to the North Pole" ?
answer: "the south"! A two-axis compassit would plunge vertically into the ground, or would panic as others relate, both may be true, depending on location.
Average value of horizontal magnetic field at ground level: 50 microtesla (5 divided by 1 followed by 5 zeros = 5x10 -5 = power -5 ) is not much, however, the Tesla
is a very large unit and the strongest magnets Scientists rarely exceed a few tens of tesla; a good regular magnet (they does not say what it is ..),
can only make two.
(*) There is no "normal" area : because our Earth's magnetic field has a lot of anomalies
: it is not at the place the geographic is (the north of the maps) ;
it turns slowly around the geographic pole, has also a varying periodic value.
It is zigzagging in many places for several reasons : the content of the deep soil of our planet, and the variations of the gravity which makes our planet bumpy.
In an area, the magnetic field can be in the process of growth or decline.
During the Earth history, the poles have reversed many times, the latest date back 800,000 years; these upside down are
unpredictable and range from tens of millions years to hundreds of centuries.
They are deduced from rocks crystals magnetic orientation, frozen at a volcanic cooling time.
The method is based on the magnetic resetting that occur when the the material (stone, clay) is heated beyond a certain temperature
(Curie point) that make it loss the magnetism quality.
While cooling, it recover it and naturaly "take" the present field.
If a has been heated by fire it loses its magnetism traces of the original and takes the field this cooling.
At each reversal - over 200 are known - the field weakens, exposing more of our planet to the sun's deadly
effluvia, but any loss of life has been observed (without field magnetic, no life as we know). By cons our electronic
equipment and may be electric and could not stand it.
hibis comments : "we were not there to see any damage, but as the process takes some time, at least tens of thousands of years, one can observe that civilizations have ample time to disappear and reborn
(unless 10,000 years is in prehistoric times)".
A CNRS newspaper, January 2008.
The magnetic variation "V" is the angular difference, at a given location, between the magnetic north aimed by the
needle and the geographic north (true north).Example : "the magnetic variation at New York is 13 degrees West".
You see that the gap is is very important.
The variation West is said positive (+) ; that means if you are going full South on the map in New York (180
degrees) the needle of your compass must shows degrees 193 (180 + 13).
To deduct the magnetic orientation from the geographical orientation measured on a map, you must add the magnetic variation if it is West(and substract it if is East).
- Magnetic mass.
Interaction between poles ; Power of flux.
Two identical poles + or - repulse temselves
We have now to quantify the concept of magnetic mass as well as both distance and forces.
The CGS magnetic mass unit is that which, placed in the vacuum (air here is comparable),
facing an equal mass at one centimetre gap, exerts on a force of 1 dyne (about 1 milligram). For example, the poles of a compass has a mass of a few dynes.
1 Newton is 98.1 grams.
A dyne = 1 Newton divided by 100000 (10 power -5) = 0.981
For the powerful magnets, it is preferable to use the Newton, legal unit, along with the meter.
Two opposite poles attract themselves.
Physicist Coulomb has given the following law : "the force between two interacting magnetic masses m and m' depends
on the square of their distance
The formula is : F = m.m'/ d2.
If the force has the value "f" at 1 cm, it is only f/4 (divided by 4) at 2 cm !
and only /16 at 4 cm.
The force weakens very quickly with the distance
all matters (substances), are permeable to a magnetic field, bur they can more or less alter its value.
This permeability is represented by the Greek letter  "μ". It is a coefficient which, in a ferrous metal or else, represent the intensification of the field.
μ is therefore a figure far greater than in air, in which by convention μ = 1.
The induction B
(Greek letter β "beta"), is given by the formula B = μ.H.
(B = H multiplied by μ).
It is in fact the phenomenon observed inside a magnet over the external magnetic "spectrum",
although less dense and extensive.
The permeability of substances/matters isn't constant. It varies with the intensity of the applied magnetic field: the permeability decreases as the field
There are three kinds of materials, ferromagnetic (strong magnetization in the same direction as the field such as nickel iron cobalt),
paramagnetic (low magnetization in the same direction as the field such as platinum chrome aluminum oxygen)
and diamagnetic (low magnetization in the opposite direction of the field such as copper, silver, lead or water, every metalloids except oxygen, organic
In atoms the rings of atoms orbits (spin) are in all directions (quantum theory).
If the interaction of orbits between them is strong enough, their orbital ring are unchanged, but their magnetic moment increase or weaken.
If the interaction is strong, some of electrons have their the orbital ring turned toward the magnetic field.
The magnetizable metals are very rare, such as iron, would have closer
atoms (2.7 to 3 angströms).
One angstrom = 1 divided by 1 followed by 10 zeros.
- A prestigious brand of wrist watches offers a model for scientists whose casing protects against magnetic induction up to 1000 gauss.
- Inside a PC disk drive, nearly all the metallic pieces are paramagnetic and are not attracted by the ultra powerful magnet.
flux - A magnetic induction B is expressed in B Tesla (T) .
The magnetic induction is tipically represented by the terrestrial magnetic field or by the field powered by the wire of a coil driven by an electric current.
- an induction flux is expressed in B "Weber" (Wb);
this unit precise that one tesla goes through one meter square.
A winding surface S, through which pass a portion of this field, receives a magnetic flux Φ or φequal to (number or fraction of tesla) multiplied by its surface S (number or fraction of m2).
Formula Wb= T x S (its not the usual writing because one can talk about a fields B, but it is more simple to understand.
When the surface S it is not perpendicular with the direction of the field, one multiply the result S.T by the value of cosine α
"alpha" (angle between a perpendicular arrow to the S coil surface, and the field direction.
In our case, α is = zero and the cosine = 1 (no change)
Finally, all initial B, T etc.. are usually topped by an arrow to remember that they represent vectors (arrows).
If you have an accute interest in dismantling the magnets from a disk drive of a PC, you'll have a good idea of the nowdays magnetization.
It's pretty incredible. Have a try before
they disappear ! (simple screws to remove from the casing, and inside, needless to look for fasteners for the magnets, there is none, because the two magnets
hold only with their strength ! must try to wedge leveling with a screwdriver to remove the magnets and their force falls abruptly. Stuck
on a very thin sheet (réfrigérateur..), they are very difficult to remove. To use them, I glued two thicknesses of card on the magnet (red)
and is still strong !
Because of their huge progress in power and manufacturing, along to those of electronic circuits, magnets are used everywhere : possibility hitherto unimaginable, permanent magnets are used to built electric motors fitted out to new
most powerfull electric cars. They are spreading throughout the industry and domestic gears (of which washing machine).
They are cheaper, safer and less noisy.