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Conventionally, to obtain a current in a conductor one creates an electric field along the conductor, for example by connecting it to a battery. Here the aim is to drive the electrons along a conductor, thus obtaining this current, by attracting or repelling them by an electric field external to the conductor. By separating the control circuit from the driven circuit, this would be equivalent to the principle of induction between coils, but with the advantage of being able to pass DC current.
The current impossibility of DC current induction seems to me to be the result of a missing link, not in the theory of electromagnetism, but in the practice we have of it. The spatial gradient of the potential vector, considered in another thread, also seeks this same result.
Electrons move very slowly in a conductor, in proportion to the intensity of the current. We should therefore drive them with the same slowness. Let's quantify it.
For the current/speed correspondence, we have to go through the current density J = I/S where I is the current and S the cross-section of the conductor, and the drift velocity of electrons is given by v = J/ρ where ρ is the charge density in the conductors, which depends on the nature of the conductors. For copper, ρ = 1.4 * 1010 C/m³.
Example: 20A in 2.5 mm² wire gives us a velocity of 0.6 mm/s. As an indication, I produce here the distance travelled by an electron during one period of an AC signal. At 50 Hz, this corresponds to an oscillation of 2µm amplitude. In TV at 500 MHz, we get to the nanometre level.
Note that the drift velocity is very low compared to the thermal agitation speeds which are of the order of 105 m/s.
If, to begin modestly, we are looking to obtain a current of only 1 mA, it is in our interest to use the thinnest possible conductor, so as to increase the current density and the speed. There is magnet wire with a diameter of 0.05 mm, which would give us a current density J=10-3 / (pi*(0.025*10-3 )²)=509296 A/m². The drift velocity of the electrons would be v = J/ρ = 0.036 mm/s.
But even so, if we use a coil with a diameter of 5 cm, we will need a field rotating around it at only 0.18 Hz or about 1/6th of a revolution/s. For a mechanical realisation, such a slow rotation will have to use a stepper motor, or gears, which is not easy. But for a field rotating from quadrature signals, it is perfectly within the reach of a signal generator.
If the field rotates too fast, the electrons will not follow because of the resistance of the conductor. The question now is to optimise the coupling of the external field to the free electrons in the conductor.
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Topic: Archimedes' screw applied to electricity (Read 4431 times)
