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| Electro-magnetism, Dynamos and Faraday Disc | |||||
| Some simple reminders on electromagnetism. But, we shall see that everything is not so simple... | |||||
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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. There is no
reciprocity. |
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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.
The permeability depends of the material. It is for the :
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:
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). |
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There are numerous simple methods
to find the relations connecting the orientations of magnetic fields and
electric currents. |
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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. |
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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. |
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| 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). 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. |
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| Permanent magnetic field and continuous current. | |||||
Fig. 2![]() |
Faraday observed, with this device a continuous current
of about one milliampere. |
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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. |
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