0
2024.09.26
2024.09.29
2024.10.01
article
M. Ljubičić (Amenoum)108. brigade ZNG 43, 35252 Sibinj, Croatia (amenoum.org)mljubicic99{EAT}gmail.com
On electricity and em waves.
physics
electricity, em waves
https://doi.org/10.5281/zenodo.13854394
/authors/Amenoum.html#credits
1
Understanding electricity and electro-magnetic waves
Abstract
A simple and proper explanation of electricity and electro-magnetic waves.
Introduction
Much confusion and false belief exists in public about electricity and electro-magnetic waves (and basically most things in science), even among educated people. It doesn't help that the educating
system teaching basics of such phenomena is basically lying in order to keep things simple. This is an attempt to clarify some things.
Transfer of electro-magnetic energy (electricity)
Electric current is usually associated with movement of electric charges, typically electrons. However, carriers of electro-magnetic force are photons and they are the ones transmitting
energy. Consider an electric circuit consisting of a voltage source and a lightbulb connected to it by extremely long wires. Individual electrons move much slower than light, so if the electro-magnetic
energy would be carried by electrons it would take much longer to power up the lightbulb than it does. The energy is transmitted at the speed of light so the lightbulb appears to be powered
instantaneously.
Electrons do carry kinetic energy, but it is not uniform at room temperatures and a significant part of it then gets transformed to heat. Thus, not only do they not directly carry electricity, they
impede its transfer by transforming it to heat. This is, however, usable in electric heaters. Any individual electron in a wire will not carry energy long distances (eg. from the voltage source to
the lightbulb), instead, they will transfer the energy to other nearby electrons. Thus, transfer of energy/momentum through the wire is similar to the transfer of energy/momentum in a falling stack
of dominoes. For an alternating electric current, however, more accurate analogy is the Newton's pendulum.
Why are the wires needed then? They're not, but without them the energy would simply be dissipated in all directions and hardly anything would be
absorbed by the lightbulb, especially if it's not electrically charged. Wires are used to establish an energy channel, a waveguide, that will guide electro-magnetic energy toward the energy sink - lightbulb, in this case.
With sufficient voltage, a waveguide can be created even without wires. This is evident in lightning, where charge is usually transferred from a cloud to the ground (usually negative). A lightning
starting in the cloud will usually branch sideways at first, but once the main branch reaches the ground, the ionized air channel effectively becomes an electric conductor, through which all the
energy is then transferred to the ground. Branches occur due to differences in potential (voltage between the cloud and molecular clumps in the air), but this difference is greatest between the
cloud and the ground. Once the waveguide between the cloud and the ground is established this becomes the path of least resistance, so no more visible branching occurs. An equivalent phenomenon can
be observed on small scale as well. If the lightbulb is brought close to the voltage source so the contacts are almost touching, sparking will occur and lightbulb will power up. This lightbulb will
stay powered as long as the waveguide remains undisturbed. Even though energy is carried by electro-magnetic waves, it is obvious that the waveguide is formed by unbound electric
charges (ions). Thus, good charge conductors are usually used to transfer electricity due to higher efficiency and lower voltage requirement to create the waveguide.
This seems to suggest that the motion of charges in the wire is unnecessary, all that is needed is acceleration of charges at the source and a waveguide which will guide generated photons toward
the lightbulb. However, the waveguide is made out of ions so it is impossible to avoid ion motion (as they absorb the generated radiation). One could construct a waveguide for light out of an
insulating material (like an fibre-optical cable) but this would be terribly inefficient for energy transfer (it can be, however, an efficient was to transfer information). The problem is that the
photons generated at the voltage source are low frequency photons, so their energy spreads out fast with distance, while the lightbulb is typically too far away and has very limited dimensions and
won't be able to absorb any energy - either as an antenna, or an general absorber of electro-magnetic radiation. Consider the 50 Hz voltage source. It generates photons with a wavelength
of λ = c/f ≈ 6000 km. Thus, you'd need a lightbulb the size of the Moon to absorb energy correlated with photon frequency, that is, assuming voltage source is of that size, but it is
not, which means that the waveform of this photon is crippled at the source (the amplitude is much smaller than the wavelength). Thus, even if the lightbulb would be designed to receive energy
in this form, it wouldn't really work. Electrical appliances don't usually capture energy in this way, they care more about the intensity of radiation, not frequency. However, even if you
generate a lot of photons at the source (increasing intensity), energy density of that radiation is falling of with the square of distance. So, again, the energy captured by the lightbulb would be
very small. And that is the reason why moving charges in the wire are necessary in practical applications of electricity. Photons cross infinitesimal distances when transferred between
one electron and the other in the wire. Therefore, there is no spreading of energy and intensity of radiation is the same at the electrical appliance as it is at the voltage source. Can one still
transfer significant amount of energy using photons alone? Of course, but not with conventional voltage sources, rather with devices that can produce coherent high-frequency photons (something
that lasers do).
Transfer of electro-magnetic information
Electro-magnetic waves always carry energy but they also carry information. If the aim is to convey information, rather than energy, it is much more efficient to use waveguides, or media, which
are good electrical insulators, rather than conductors, to transfer this information. This is because conductors absorb/attenuate electro-magnetic energy and may reflect incoming waves.
As they travel, electro-magnetic waves induce differences in potential in electro-magnetic fields. This difference can be interpreted as pressure that can be felt by electric charges. It is highest
at the peak of an electro-magnetic wave. In air, where charges are tightly bound in atoms and molecules, this pressure, especially in case of lower frequencies, is usually too low to significantly
affect them and thus transfer energy to them. This is not the case in good conductors of electricity, where charges can flow more freely. For tightly bound electrons in atoms pressure itself is
pretty much irrelevant, what matters is frequency. They are insensitive to low frequencies because they are confined to atomic orbitals which have a diameter on the order of picometres and
nanometres (depending on the element, and how excited the atom is). Since wavelengths of low-frequency waves are much bigger than the space electrons are confined to (orbitals), most energy of the
wave simply passes through.
Conductors are used only at the receiver and transmitter, as antennae. At the transmitter, a conductor is used to create radiation by mobilizing charges (accelerating charges create electro-magnetic
radiation), while at the receiver end a conductor is used to absorb electro-magnetic radiation and mobilize the charges. Because electro-magnetic energy travels at the speed of light, the amplitude
of a wave emitted from an electric dipole (with its length being 1/2 of the photon wavelength) in the direction perpendicular to the dipole, is equal to 1/4 of the wavelength.
This is true in case the speed of light is equal in both the dipole and the medium. In reality, this is not the case, however, it is usually a good approximation.
Optimal dipole antenna length is then half the wavelength of radiation, but will work with smaller sizes as well, albeit with amplitude cut-off and lower efficiency. Transmitter dipole larger than
the radiation wavelength will introduce problems (wave distortion, noise) unless the length is the integer multiple of half the wavelength (this will work but will produce additional lobes in
the signal, cancel it in some directions, and lower efficiency).
In practical applications, it is also important that the impedance of the antenna matches the impedance of the voltage source. Impedance is the measure of resistance to propagation of electric
current which also carries information on phase difference between voltage and current (this phase difference arises due to presence of inductivity and capacitance in electric circuits, introduced
mainly by coils and capacitors, respectively). One terminal of the voltage source is connected to one part of the dipole, the other terminal to the other part, so the charges (and associated
electro-magnetic waves) are oscillating between dipole ends and the voltage source. Matching impedance enables the electro-magnetic waves to travel freely from one end of the dipole to the voltage
source. In case the impedance is mismatched, waves will encounter discontinuity between the dipole and the source, resulting in reflection, and thus reduced power transfer to the antenna.
Fig. \fig1: Electric fields produced by a dipole antenna
Intensity of radiation is proportional to to the number of charges mobilized (which is proportional to voltage applied to the dipole), but it is most concentrated in a plane that's passing through
dipole midpoint and that is perpendicular to it.
Obviously, an alternating voltage source will usually be used to generate electro-magnetic waves. A DC source will only generate a single electro-magnetic pulse once connected to
the dipole.
Fig. \fig2: Reflection of an em wave
High-frequency waves carry more concentrated energy and with increasing frequency increasingly behave more like particles than waves (a particle may be interpreted as a localized wave, or
localized wave energy). Thus, they have a lower range and are more likely to be absorbed and reflected. A reflection of a wave encountering a conducting surface is illustrated
in Fig. \fig2. Once the incoming wave encounters the surface it will accelerate a charge within the conductor. This will create a new wave and radiate it in the direction of the original wave and
in the opposite direction. The part emitted in the opposite direction of the original wave can now be interpreted as a reflection of the original wave. Note that the created wave is shifted in
phase by 180° relative to the original wave. Therefore, the part emitted in the same direction as the original wave (to the right, in the figure) will cancel the original wave. This is the case of
a perfect conductor. In reality, some energy will tunnel through, and a reflection will not be perfect (intensity of waves will be lower). Good conductors are good reflectors and are thus used
in Faraday cages - metal enclosures designed to insulate certain space from energetic (high frequency) electro-magnetic radiation. Obviously, if the length of a dipole (surface, or size of
a Faraday cage) is much lower than the wavelength of the electro-magnetic wave, only partial reflection is possible - as the energy of the wave is spread out over larger distances than the charge
current in the conductor is free to move over. This is why low frequency waves will usually pass through standard Faraday cages without much loss.