The fact that the voltage in the circuit is alternating, we can measure if we connect a voltmeter to the ends of the circuit to measure alternating voltage.
We can proof that an alternating current flows in the circuit if we install a shunt or a current transformer and indirectly measure the current in the circuit.
Rarely anyone had asked the question if the network emits a radiation, and in general we never think about it, so we don't even have specialized devices for this (except for some special measurement devices, which are used in other areas like spectrometry). It is clear, that these photons have a much shorter wavelength than the one calculated by the varying EMF fed to the antenna system since the frequencies of the photons themselves lie in the range of x-rays or even gamma radiation, which can be calculated with the formula ΔE = ν · h, where ν is the frequency, h is Planck's constant.
Let's try to understand these questions again with the help of visual drawings and start from the very beginning - with the action of the variable EMF and the longitudinal waves that arise in this case.
We will assume that alternating voltage is moving at a constant speed (but not electric charges) as longitudinal waves, which are compression and rarefaction of the density of electric charges in a conductor. In this case, outside and inside the conductor, specific physical effects arise, which will be discussed later.
The figure below shows the sections of the conductor, on which negative charges are compressed, coloured in blue, and the sections of rarefaction of negative charges coloured in red, and in the white sections, the density of electric charges is in relative equilibrium.
This is an illustration of a longitudinal charge density wave in a conductor, which is completely analogous to a longitudinal sound wave propagating in a long air channel.
Under normal conditions, negative electric charges in a metal conductor are located near the nodes of the crystal lattice (which have a positive charge), while their density is the same throughout the entire conductor, and the conductor as a whole is electrically neutral.
If a source of constant EMF(DC source) is connected to such a conductor, then the electric charges in the conductor will begin to drift along the nodes of the crystal lattice at a constant speed(electron drift velocity), while the charge density along the entire conductor will also remain constant.
But if a source of variable EMF is connected to a conductor (in a closed circuit), then a longitudinal wave (deformation of the density of electric charges) will appear in the conductor, which is a movement along the conductor, but not of the electric charges themselves, but longitudinal waves of compression and rarefaction of charges. In this case, relative to the crystal lattice of the conductor, the charges themselves will actually remain in place, making only reciprocating movements along its length (within ½ the length of the longitudinal wave). For an external observer, such a movement of charges would appear like the oscillation of a pendulum, its movements are determined by the frequency and amplitude of the variable EMF source. The centre of its oscillations is motionless relative to the conductor!
Let us list again the processes which arise in the conductors:
1) there are moving waves of compression and rarefaction of the density of electric charges in the conductor, alternating along the entire conductor and located at a distance of ¼ wavelengths from each other.
2) these waves continue to move in the conductor "by inertia" at a constant speed in the direction from the generator, due to the exchange of momentum between the adjacent charges;
The term "by inertia" is taken in quotation marks, since in fact the charges continue to move due to the phenomenon of the EMF of self-induction in the conductor, which apparently looks like mechanical motion by inertia. So, these movements turn out to be completely analogous.
3) the resulting imbalance in the density of electric charges inside the conductor leads to a return movement, but not of waves, but charges themselves in the direction towards the source;
4) the return movement of charges restores the pressure in the zone of rarefaction of charges.
5) the movement forward and back of the charges occurs with the same speed and acceleration, the charges in the conductor remain in it in the same section, while making oscillatory movements along the conductor with an amplitude equal to ½ wavelength.
It is important to note that the movement "by inertia" of the compression-rarefaction waves from the EMF source along the conductor at a constant speed is primary, and the reaction in the form of the movement of charges in the opposite direction is secondary. This means that the voltage in the conductor is always primary and the current is secondary.
Let us consider in the illustrations the propagation of longitudinal waves of electric charge density in more detail (by analogy with the propagation of sound waves) in an open and closed long line:
1) the propagation of a longitudinal wave in air at a constant speed of about 340 m / s, which does not depend on the frequency of oscillations of the piston and their amplitude;
The EMF of a generator, like the movement of a piston in a cylinder, deforms the density of electric charges in different ways, we will call the density of charges in the conductor compression, normal and rarefaction of charges. Below is conventionally shown a linear chain of charges standing one after another, the momentum vector P of a moving charge and three states of charge density –
Let's remember that the charges are repelled, therefore, when similar charges approach each other, the Coulomb force acting between them increases, which allows them to exchange momentum without collisions.
Shown below is a section of a two-wire long symmetric line (the dielectric is air) and the deformation of the charge density in it, the oscillation frequency of the generator EMF is 50 Hz, the wavelength λ in such a line is about 6000 km (1/4 λ = 1500 km). Let us remember that in air longitudinal waves in conductors propagate along the entire line at a velocity of about 300,000 km / sec, and this velocity is dependent on the dielectric surrounding the conductor, more precisely on the distributed capacitance of the line. This can be measured, and for example for a PVC insulated wire laying on the ground, the velocity of propagation is 170000km/sec approximately. The most important parameters for the distributed capacitance are the surface and shape exposed to the second conductor or ground plane, the distance between them and the dielectric constant.
The figure shows that in the sections of the circuit where the concentration of charges is indicated, there is a minus sign, where the discharge of charges is indicated, there is a plus sign. Indeed, a negative electric potential can arise only where negative charges are in excess, and a positive potential only where there are fewer negative charges, that is, where only immobile positive metal ions remain at the nodes of the crystal lattice.
Moreover, the direction of longitudinal waves in both conductors of a symmetrical line coincides in direction with the action of the EMF, as shown in the figure below.
This fact is the source of a phenomenon called phase displacement in the line between current and voltage. This will be discussed in more detail. Will be continued.
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Topic: Wave propagation (Read 1138 times)
