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PHYSICS “3rd PART”
The simplest means and most generally employed to produce hertzian radiation, which is that consists in passing via the high frequency currents. It is the method which was used by Hertz in its famous experiments and it is still the method employed today in all the transmitting stations.
In the spirit of many technicians, radiation and current high frequency became synonymous. However, this manner of seeing is hardly acceptable and the thing appears obvious for little that one wants to think of it well.
The high frequency current is not telephone radiation not more than one current is not a sound vibration. To pass from one state to the other, it is necessary to use a transformer of energy which is, in the first case, a radiator of waves or sending antenna, and in the second case, a loudspeaker.
An electric circuit can be the seat of a very intense high frequency current without there being trace of radiation. It is thus interesting to seek to include/understand how one can pass from a high frequency current to a radiated energy.
It is necessary to take care not to want to build a real model of radiation. That could not have any direction. It is however in this error that one falls while tracing, as in figure 10, the “components” of the radiation like two orthogonal fields electric and magnetic (perpendicular) of the same frequency and by comparing this image to that of the radiation.
It is easy to emphasize the nonsense of this design. First of all, according to the laws of Coulomb, the intensity of each field must decrease like the square of the distance whereas the intensity of the component radiated in the absolute vacuum decrease as the distance.
In addition, by superimposing in experiments a magnetic field and an electric field, one does not obtain a radiation. The magnetic force, as the electric force are two absolutely independent demonstrations and which not having any mutual reaction.
It is thus necessary to regard these fields as two particular aspects of the radiation, as different from the radiation itself as a built drawing with two dimensions is different from a model with three dimensions.
We repeat it: one should not try to build a model of radiation. Nevertheless, it is not interdict to seek to include/understand how energy can pass from the electric form, with the radiated form. The radiation is detached energy of its matter support. It is a question of explaining how such a detachment can occur.
Let us imagine that an electron, originally at rest, to put itself to be driven. In other words, the driver which it guide is the seat of an electrical current. The result, we let us know it, it is that a magnetic field will develop all around the driver.
It was believed a long time that this magnetic field invaded all space abruptly. However, one knows, since EINSTEIN, whom there cannot be instantaneous action. All the phenomena being able to be used as signals, whatever they are, are propagated with a determined speed, of which the possible maximum is the speed of the light. It is indeed at the speed of the latter (300 000 km/S) that the magnetic fields are propagated and electric. Consequently, the magnetic field will invade all space gradually. It will appear at the point P before appearing at the P' point (figure 11).
When we reach a stationary regime, the field at the point P, as at the P' point will be constant and will depend only on the speed of the electron and the distance “d” which separates the point considered from the driver.
At the time when the current ceases circulating, the magnetic field disappears but, in normal time, total energy that it represents appears in the form of a extracurrent or of a tension known as of self induction. Thus, when certain electric circuits are cut, one sees appearing a spark of rupture.
One can rather easily imagine the phenomenon of self induction at the time of the cut of the current. The tension fields (imaginary lines) gradually fold up around the electron “e” and, by sweeping the driver, feed the tension of self induction. The demonstrations will all the more be striking that the energy stored in the magnetic field will appear in a more reduced time.
But let us suppose that the movement of the electron can abruptly be stopped. What would become the energy stored in the magnetic field? It would be then impossible for him to appear in the circuit since we suppose that the electron is immobilized. This artifice enables us, to some extent, to completely detach the energy of the magnetic field of its material support. It appears then in the form of radiation.
When we cut an electric circuit abruptly, we reveal a spark of rupture but, at the same time, we create an electromagnetic wave. The experiment teaches us that the simple rupture of a circuit of lighting in the vicinity of a radio operator receiver produces a disturbing noise. The more abrupt the rupture is and the relatively more important the radiated component is.
That is explained. If the rupture is relatively slow, the period of suspension of the current lasts long enough, thanks to the spark of rupture, so that most of the tension fields can return to actuate the electron before it is not constrained with the absolute stop. Thus, the magnetic field corresponding to the point P will have time to return in “e”, whereas that of P' can remain in space (figure 11). The advantage of a brutal stop appears to us thus much better because the intensity of the magnetic field is inversely proportional to the square of the distance.
Let us take again our preceding assumption : an electron is at rest in a driver. To produce a radiated component of maximum intensity, it is necessary:
1) To communicate to the electron with the greatest speed, a speed as high as possible.
Since the electron is supposed party a null speed, one will translate what precedes in another form by saying that it is necessary to communicate to the electron an acceleration as large as possible.
2) After which, it should be stopped in the minimum of time, i.e. to communicate a negative acceleration to him as high as possible in absolute value.
As soon as the electron is stopped, the radiated component will be launched in space. The conditions will be the same ones as at the beginning and all will be able to start again.
To obtain radiation in a continual way, we will thus be brought to launch our electron, to stop it, then to start again. The result will be exactly the same one if, instead of making always progress the electron in the same direction, we alternatively launch it in a direction, then in the other.
But when the electrons of a driver leave alternatively in a direction, then in the other by always preserving the same average position, one agrees to say that the driver is the seat of a AC current. To obtain radiation, AC current should be created.
The intensity of current in a driver represents the quantity of electricity, i.e. the number of electrons which crosses a section in one second. It is conceived, according to that that the driver of figure 10, this intensity will be proportional to the amplitude of the displacement of our presumedly single electron. As it is about a AC current, we will be brought to consider the maximum amplitude of the oscillation (corresponding to the maximum intensity).
It is clear that for the same amplitude, the accelerations transmitted to the electron will be all the more important as the frequency will be larger. Acceleration is indeed the increase speed in the unit of time.
We specified higher than the radiated component became more important when one increased the acceleration communicated to the electron. It is thus certain that the radiation will be easier to highlight if high frequency currents are used.
With a current of frequency the relatively weak, radiation will be unperceivable with the p '', because the energy of the magnetic field will be able, to some extent, to reinstate the circuit. If the frequency is rather large, energy at the P' point will remain in space, i.e. will appear in radiated form.
It rises from the preceding reasoning that the frequency of the radiation is necessarily equal to that of the current which gave him birth.
As the radiation is propagated in space, one can make him correspond the concept wavelength.
We saw in Physics what it was necessary to understand per period, frequency, wavelength (We will see these concepts in the “Electronic” heading).
We learned there that the frequency is the number of period a second and that these two sizes whose symbols are respectively T and F are connected by the relations :
T = 1 / F
F = 1 / T
We defined the frequency as being the number of periods a second. It should now be specified that the word period, if it is didactic, is not the legal unit of the frequency. We will thus employ now this unit which is the hertz (symbol H), name of the German physicist HERTZ (1857-1894). The multiples are the kilocycle (kHz), the megahertz (MHz) and the gigahertz (GHz) which are worth respectively 103 Hz, 106 Hz and 109 Hz.
Lastly, we saw that the wavelength is a distance. More precisely, it is the distance which the wave traverses for one period (or a hertz). Its symbol is (letter “I” of the greek alphabet which is read lambda).
The wavelength , the frequency F and the speed (v) of the electromagnetic waves are bound by the relations :
Since the propagation velocity considered is of 300 000 000 m / s, we can write by respecting the correspondences of the units :
If, always wishing to obtain the wavelength in meter, we have the frequency expressed in kilocycle, i.e. by a number 1 000 times smaller, it are necessary for us also to express speed by a number 1 000 times smaller, i.e., in this case, in kilometer, that is to say :
Lastly, if always wanting to express the wavelength in meter, we have like unit of frequency the megahertz, therefore a number 1 000 times smaller than the precedent, it is necessary for us still to divide the number expressing speed by 1 000 and we obtain then :
Figure 12 shows the spectrum of the electromagnetic waves whose frequencies vary few tens of hertz to more than 5 105 MHz.
We can notice there that the waves corresponding to the visible light and who are between the ultraviolet ray and the infra-red one occupy a relatively small space.
We draw your attention to the fact that in this figure the graduations are not proportional. Thus the telephone frequencies which spread out of 300 Hz with 3 000 Hz occupy the same obstruction as the frequencies allotted to the radars and satellites which they, are between 10 MHz and 100 MHz. If in both cases the relationship between the extreme values varies from 1 to 10, the numbers expressing the differences in frequency between these two extreme values are very distant one from the other.
Thus, for the telephone frequencies and the frequencies radar, we have well :
But if the band of the telephone frequencies is 3 000 - 300 = 2 700 Hz, it is 100 - 10 = 90 MHz for the band radar, although these two bands occupy about the same arc of circle.
This type of representation is necessary because it would be practically impossible to employ a linear scale on a traditional paper sheet of format. Indeed, if in our example we had agreed to account for 1 kHz by 1 cm, we would have had to represent the telephone band by 2,7 cm, which is very feasible, and radar by 90 000 cm bandages it is 900 meters, which is much less realizable !
We thus finish our concepts of physics which, we hope for, will help you with better seizing the other parts of our work (or theoretical lessons of the other programs) and, at the very least, to increase your knowledge.
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