Demystifying electricity and electro-magnetic waves

Abstract A simple and proper explanation of electricity and electro-magnetic waves. Keywords: electricity, em waves 1 Demystifying electricity and electro-magnetic waves M. Ljubičić (Amenoum) 108. brigade ZNG 43, 35252 Sibinj, Croatia (completerelativity.org) [email protected] October 8, 2025 1 Introduction Much confusion and false understanding exists in public about electricity and electro-magnetic waves (and basically about most things in science), even among educated people. It doesn’t help that the educating system teaching basics of such phenomena is often actually lying in order to keep things simple. This is an attempt to clarify some things. 2 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 (e.g., from the voltage source to the lightbulb), instead, they will transfer the energy to other nearby electrons. Thus, transfer of energy/momentum 2 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 way to transfer information). The problem is that the photons generated at the voltage source are low frequency photons, so their energy spreads out, not only fast, but over large distance, while the lightbulb is typically too far away and has very limited dimensions so it won’t be able to absorb any significant energy - either as an antenna, or an general absorber of electro-magnetic radiation. Consider the 50 Hz voltage source. It generates em waves with a wavelength of λ= c/f ≈6000 km. Thus, one would need a lightbulb the size of the Moon to absorb energy correlated with photon frequency. However, the emitter (voltage source) is also, due to small size compared to wavelength, terribly inefficient here - converting most energy to heat/capacitance. Thus, even if the lightbulb would be designed to receive energy in this form, it wouldn’t really work. Electrical appliances 3 don’t usually capture energy in this way, they care more about the intensity of radiation (proportional to current), not frequency. However, even if you generate a lot of photons at the source (increasing intensity), energy density of that radiation is falling off 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) where energy is less spread with distance. It still drops off with the square of distance, but with a different constant of proportionality - smaller than π, as the energy distribution is not isotropic. All photons actually spread isotropically (in all directions) as spherical waves but, depending on frequency and coherence (in case of multiple photons), the energy is more or less localized towards a specific point on the sphere. In extremely high frequencies the localization is so high that photons effectively travel as particles and the point of localization can be interpreted as direction of propagation (from the reference point of the emitter). Regardless of frequency (amount of localization during motion), however, photon absorption implies complete localization ([spherical] waveform collapse) to the point of absorption. The probability of photon absorption at any point on the propagating sphere is proportional to photon energy density at that point. Thus, photon is most likely to be absorbed in the direction of propagation of the point of localization. 3 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 from radiation intensity is pretty much irrelevant, what mat4 ters 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 is, both, in the conductor and the external medium, travelling (spreading) at the speed of light, the optimal length of an dipole antenna is 1/2 of the wavelength. In reality, a difference actually exists between the speed of propagation between different media, but this is usually negligible. 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 a discontinuity between the dipole and the source, resulting in reflection, and thus reduced power transfer to the 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 5 Figure 1: Electric fields produced by a dipole antenna1 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 electromagnetic pulse once connected to the dipole. Figure 2: Reflection of an em wave1 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. 2. 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 6 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. Information is encoded in em waves through variation (modulation) of frequency, polarization or amplitude (intensity) of radiation, but it can also be encoded in modulation of the frequency of emission - implying intermittent (pulsed) emission of radiation. An em wave is polarized when it is not oscillating along a random direction or when that direction is not changing randomly. A wave polarized in a radially-symmetric fashion (e.g., vertical, horizontal) is an example of E-mode polarization as the motion of [radiation producing] charges is usually induced with linear electric field oscillation (as in a dipole antenna). A wave polarized in either a clockwise or counterclockwise fashion is an example of B-mode (rotating) polarization, usually induced by magnetic fields on smaller scales, and gravitational waves/lensing on larger scales. References [1] Introduction to Electricity and Magnetism: MIT 8.02 Course Notes Revised (2013), P. Dourmashkin et al., Pearson Learning Solutions: Boston, Chapter 13 7