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Some elementary physics Earth Magnetism A Supraconductive Inner Core Cocorico !
Electro-magnetism, Dynamos and Faraday Disc
Some simple reminders on electromagnetism. But, we shall see that everything is not so simple...

If a continuous electric current circulates in a conductive loop (Solenoïd), a continuous magnetic field, proportional to this current, is induced according to the perpendicular axis of the plan of the solenoid.
If a continuous magnetic field crosses the plan of a solenoid, no electric current is generated.

There is no reciprocity.

In the opposite figure, the magnetic field is almost uniformly concentrated in the central region of the long solenoid. Outside, the field is weak and divergent.
In the axis of the solenoid, the magnetic induction is a function of :

  1. Of the environment permeability µ.

  2. Of the number n of turns.

  3. Of the solenoide length l.

The permeability depends of the material. It is for the :

  • Vacuum = 4..10-7

  • Gas = 1

  • Iron = 10.000 (At 20° C (At 20°C and except saturation)

  • Nickel = 600

For Ferromagnetic materials, the permeability decreases very fast with the temperature. There is a characteristical temperature, said Curie temperature Tc, over which they lose completely their ferromagnetic properties:

  • Tc - Fer = 770°C

  • Tc - Nickel = 358°C

These characteristics are important because they eliminate at once the hypothesis of the permanent magnet in the ground core. (This one would be constituted by a nickel and iron mixture).

There are numerous simple methods to find the relations connecting the orientations of magnetic fields and electric currents.
The one who is given, below, is neither worse nor better than the others.
We notice that magnetic fields are directed. We say that they are polarized, North and South (or + and -)

The interactions of the solenoids are strictly the same that those of the permanent magnets.

The same polarities of magnetic fields push away themselves. Magnets or solenoids are then submited to a force which tends to take them away.

Complementary polarities magnetic fields re-connect themselves. Magnets or solenoids are then submited to a force which tends to move them closer.

The difference between permanent magnets and solenoids is the fact that solenoids lose their magnetic properties when no electric current run through them.

Magnetic induction and alternative current.
Fig. 1

A solenoid submited to a variable or alternate magnetic field is subjected to an induction current, either in a direction or in the other one. (alternative current).
It is on this principle that are conceived the modern electric generators.

Let us note that these generators are reversible. If we feed them with electric current, they become motors. They are Dynamos. The inventor of these machines is Zénobé Gramme.

In that case, there is reciprocity.

Permanent magnetic field and continuous current.
Fig. 2

Thus to induce an electric currents, a variable magnetic field is needed.
But a surprising experience realized by Michaël Faraday shows that it is not still true. It is the Faraday Disc.
It is a rotating conductive disc (copper) in a permanent magnetic field.

Faraday observed, with this device a continuous current of about one milliampere.
In such conditions, there are only local variations of field , compensated with local inverted equal variations of field. Thus the conventional theory of induction does not allow to explain in a satisfactory way the phenomenon.
We can make it only by taking into account of the variations of the orientations of the axis of the electrons spins, in the atoms of the conductive disc, under the influence of the movement of the lines of force of the magnetic field (J.P. Vigier). It appeals to the Quantum theory.

We see that the experiments of Fig. 1 and Fig. 2 have nothing in common. It is two different phenomena, although that some confuse them under the term of "Dynamo Effect". In what follows we shall always make this distinction by speaking whether of the effect dynamo whether of the Faraday effect.

Documents :
  1. Some publications and Works of Jean-Pierre Vigier (Annals of IHP section A)

Creation date: 10/04/07
Last release: 05/05/16

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