Wonders of Electricity (WOE) #1: Fundamentals of electricity generation

Hello readers, I hope you are in a good mood. Because today I am going to start these posts in English that I found very interesting to share and comment. We are going to write about electricity, but not in a very rigid way, it won't be a straight line. I have focused this on creating a wide path of thoughts. I will not write here about technical and conventional details of electricity. Instead, I prefer to engage this as an amazing trip with a lot of conceptions and hypotheses, the WOE saga.

However, my reasoning is also based on scientific knowledge and my engineering background, that have been very useful to comprehend this mysterious phenomenon. Nonetheless, having 8 years of experience and practice wasn't enough to understand deeply what electricity really is. But we are here to solve this, and to discover other astonishing facts and features of these negative charges, also known as electrons.

For starting this travel, I will not fall into the monotonous and boring questions that are usually stated. Then, a more stimulating approach is needed for capturing your attention. But the wonders of electricity will accomplish this by themselves. I do not need to force an argument to be interesting and suggestive. Let's start with something that you probably shouldn't know.

How do we produce electricity?

This question is very affordable to me, as I have been studying things such as generators, inductors, power systems, breakers, and electricity consumption. But maybe you have not confronted these questions, and also you are wondering how we can generate electricity. Cheap and green electricity. Also, our current energy and climate crisis are good motives to incentive your worries about your personal economic problems and the nature that surrounds you. I really understand your points. But remember that these are not guidelines that you can follow. Just imagine, relax and enjoy.

The ask for this question is far from simple. Nowadays in our industry, we can produce electricity with two principles: By transmitting rotation to the shaft of an electric generator (the most common one) and by inducing a photoelectric effect with a structure of multiple semiconducting doped layers. This last mechanism is the foundation of solar photovoltaic power plants.

Electricity generation with inertial mass

Electric generators are based in two types of electrical machines, that are usually used: Asynchronous and synchronous machines. The selection of the type of generator will depend on the application and the origin of the rotation. For example, burning coal that generates heat is transmitted to a fluid such as water, that evaporates, and with high pressure induces forces to the blades of a turbine, that is coupled to the shaft (rotor) of the generator.

Induction machine inside a wind turbine

In wind energy applications, a synchronous generator is preferable, and more specifically, a PMSIG (Permanent Magnet Synchronous Generator), which incorporates Neodymium magnets for increasing the magnetic flux density, and then, having a better performance. Also, these generators are prepared for rotating at lower speeds, which is desirable due to the velocity of the blades of a wind turbine. The coupling between the wind and the generator shaft does not need any regulation (e.g. a gearbox), because they operate in the same velocity range. This does not happen with asynchronous generators, that need to adjust their rotational velocity to match the wind shaft velocity range.

However, every induction machine operates with the same principle: The spinning of the rotor produces a rotating magnetic field that creates an electromotive force (or EMF), which is induced voltage, and then, an alternating current is created at the output of the generator. But what happened here?

First, let's get deeply into physics. If we want to create electromotive forces by magnetic coupling, Faraday's law tells us that magnetic fields must depend on time, and changing their value accordingly. This would not happen if a static magnetic field is applied, for example, a magnet placed in a certain position without movement. This could indicate that the energy is transmitted by magnetic fields, but it cannot generate energy on its own. A restless magnet is not enough. But a spinning magnet can allow us to extract energy from a source: Burning coal or gas, wind currents, water flows, and so on. In short terms: We need movement, kinetic energy.

An example of the application of Faraday's law

We have learned now that alternating magnetic fields are not something strange. It is a magnetic perturbation of the space that transfers energy when oscillates and it is coupled to a metal, usually called the inductor. An inductor is only a metal structure that can store energy in the form of magnetic fields, even if they are static or dynamic. And Faraday's law is applied to inductors, so this metal structure can use the energy contained in the magnetic fields, and then transfer it to the electrons. This is the point of interest. A good start for explaining what electricity is.

The first mechanism that is important to explain is how an electromagnetic field can create electric currents flowing through a conductor. Faraday's law does not dive inside the process, just states the relation between the magnitudes, and the variables of interest. Because of this, a good approach is first needed.

Electromagnetic waves: Maxwell's equations

What is an electromagnetic wave? This question was asked in the classical physics spectrum, by the well-known James Clerk Maxwell, a physicist who stated the relations between electric and magnetic fields. And that is, the explanation of how an electromagnetic field works. The electromagnetic (EM) wave is a composition of two different waves, that oscillate in different dimensions, and these two directions of space are perpendicular to each other. The EM wave is a transverse wave, which means that the direction of propagation is perpendicular to the oscillations. We will discuss this as we develop the technical framework needed for debating the true nature of electricity. For this, I will present the conception of electricity postulated by Nikola Tesla, the Serbian inventor and electrical engineer, who stablished the definition of electric propagation as a longitudinal wave traveling through different mediums (solid, liquid or gas). In the scientific realm, this idea opposes Maxwell's equations and its theory, which is the basis of nearly all electricity phenomenon. However, we will assume that Maxwell was entirely right, as one can comprehend electricity with this vision.

Maxwell's equations

Maxwell's equations state that magnetic fields (H) are created with electric currents (I). The movement of electrons implies the formation of a magnetic perturbation (Ampere's law). Also, a time varying magnetic flux (phi) can induce electromotive forces in a conductor (Faraday's law). These magnetic field lines "born" and "die", they cannot just travel infinitely through space. We can say then, that the divergence of the magnetic flux density (B) is zero. With this, one can conclude that magnetic monopoles do not exist, and magnetic fields are always polarized in two different directions (North and South).

An electromagnetic wave propagates at a constant speed, if the medium does not change. Its velocity in vacuum corresponds to the light speed in vacuum, which is 300.000 km/s. The interesting thing about it is that the speed can be calculated as a function of the electric permittivity and the magnetic permeability of the medium. These are electromagnetic properties of space. That being said, what if one of these constants is lower than in vacuum? And what happens if these values are zero? Well, the light speed will be infinite. The conclusion then, is that permittivity and permeability values influence photons and slow them down. That is why the speed of light is limited and is constant, whoever measures it.

It is interesting how Maxwell demonstrated that light interacts with electric and magnetic entities of nature. Also, photons have a spin, that is, they are polarized, and fit well with the description of light as an electromagnetic wave. Because of this, an induced voltage is created. Photons interact with the free electrons of the metal, producing Coulomb electric forces, and forming high concentrations of charges on two sides of the inductance. Then, the electric current flows through the inductance in a closed loop due to the E.M.F.

Galileo Ferraris

This is the complex process of how electricity is produced in a generator with inertia. However, there are more curious phenomenon behind the induction machine. For example, Galileo Ferraris demonstrated in 1885 the concept of a rotating magnetic field with the development of MMF (Magnetomotive Forces) in a three-phase induction generator. The resultant MMF had an amplitud of 1.5 times the MMF amplitude of a single phase. But I will not show this here. The main physical principles have been described before, which is sufficient.

Electricity generation with the photovoltaic effect

You may wonder how electricity is produced with a Photovoltaic (PV) system. The concept is analogous, but the physics are far from equal. The main principle that leads all behavior is the photovoltaic effect applied in a semiconductor.

I want to clarify the difference between the photovoltaic and the photoelectric effect. These are not really the same. Photoelectric phenomenon stands for the exit of an electron from a metal that has absorbed an amount of energy from a photon at a certain frequency. In the photovoltaic effect, this process is almost the same, but electrons do not go out of the material, they flow in a closed loop. Nonetheless, the entire process is far more complicated. 

The work function and the kinetic energy

Photovoltaic semiconductors use light as an energy source, producing electricity with the photovoltaic effect. These photons reach the surface of the material, and then transmit its energy to the electrons through momentum, that is proportional to the frequency of the wave of the photon. However, transmitting energy with momentum implies a frequency threshold that, if the light does not surpass it, the electrons will not suffer any change in their kinetic energy level. Nonetheless, if the frequency of the photon is higher than the threshold, an energy transference happens. The state of the electron, however, is subjected to two parameters: The work function and the kinetic energy. 

Energy band gap in semiconductors

An electron requires a certain level of energy for escaping from the material due to electrostatic forces that restrain him. With this achievement, the electron is completely free from his electric bound, but in order to be displaced and move through other physical mediums, more energy is needed. This additional portion is invested in kinetic energy for the electron, which forms an electrical flow in the material. Then, two objectives must be fulfilled to produce the photovoltaic effect: Energy for liberation, and energy for movement. 

In a PV Solar cell, this work function is the energy band gap that separates electrons and forms two different bands: The valence band and the conduction band. The valence band is filled with electrons in its last layer, that could jump to the conduction if they have an energy higher than the work function. This energy band gap Eg in a silicon crystal is very large (7 eV), due to the strong covalent bond. Others, such as silicon (0,55 eV) or germanium (0,36 eV) have a much lower energy gap. Then, these two materials are very suitable for industrial applications, but not yet, their electrical properties must be improved. And that is the doping of a semiconductor.

Conduction and valence bands of an atom

Doping and the P-N junction: The constitution of the solar cell

For creating a solar cell, two different structures are needed, that have an electric polarity. The negative material (n-type) is formed by using a negative dopant (atom with an excess of electrons), and the positive (p-type) uses a positive dopant (atom that demands electrons). In both of them, these dopants diffuse through the material, and replace some of the atoms of the structure, creating a completely new material. These two structures, then, are combined to form the doped semiconducting material and, when it happens, the electrons from the n-type travel to the positive charges (or holes) of the p-type, resulting in a recombination of the electric charges. This union creates a region between the two materials, also known as P-N junction. Because of the electric potential difference between the n-type and the p-type structures, an energy band gap is created. The electrons require a certain level of energy for jumping to the other side of the semiconductor. But this band gap demands less energy than before, compared to the pure silicon and germanium materials. Then, the process is complete, we have a functional solar cell, enhanced and prepared for industrial needs. 

Common arrangement of a PV Solar Cell (with the P-N junction)

Direct current and alternating current

Now, we have generated electricity with the photons that constitute the sunlight. But this phenomenon results in a very different scenario compared to the electric generator. First of all, the flow of electrons is constant in time, it does not oscillate as in the case of the induction machine. When the electric wave that propagates through the circuit has the same magnitude over time, we say that it is DC (Direct-Current). The output of an electric generator produces an oscillating flow. The electrons travel back and forth, transmitting variable electric forces through the cable. The magnitude of these forces depend on the time. This is what we call AC (Alternating Current). 

DC and AC waveforms represented over time

A solar cell generates a DC current at the output. Then, it is injected in a closed circuit, and generates voltage drops in the components that interact with it. These electric and magnetic interactions will be explained in the next chapters of the WOE saga. 

The momentum of the photon

We have assumed in this explanation that photons create momentum and is transmitted to electrons. But if the photons are particles with zero mass, how is that possible? Can we apply the Newton laws in this case?

Equations of the momentum of a photon

The answer to this question can be represented mathematically, but also deduced from Maxwell's equations and the photon spin. The interaction between the photon and the electron is derived from its electric and magnetic properties. An electron surrounded by a magnetic field suffers a magnetic force (Lorentz force). An electric field can also create electric forces in charges. So, due to the spin of the photon, this development is possible, and if electric and magnetic forces appear, then, the momentum also does. Indeed, this phenomenon does not imply the use of a mass, but classical physics have always dictated this particular fact, it is nothing new. But it is important to notice, since there is a lot of misconceptions about the interactions between photons and electrons. 

This does not end here. There are a lot of questions related to the electromagnetic nature of light, but we will end this ride here. 

I hope you understand now one piece of this enigmatic puzzle. We have a very long journey from this point. Electricity is one of the most interesting and mysterious phenomena ever known. But do not worry, I am here to cover the issues. Also, if you have questions or even suggestions, please, let me know them. You can write here about all of these topics. Education and knowledge is for everyone.