ELECTRIC CHARGE, CURRENT AND VOLTAGE
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In this lesson we deal with some main concepts regarding the electrical engineering: electric charge, current and voltage.
Although electrical systems are a branch of electrical engineering, which is the technical discipline concerning the production, transmission and distribution of electrical energy, we want to introduce this lesson with the definition of “electronics” which gives us the opportunity to introduce the fundamental concepts of electric charge, electric energy, voltage and current which are then the basis not only of electronics but also of electrical engineering.
In particular, “electronics” is the branch of physics that concerns the emission and propagation of electrons in vacuum or in matter.
At the basis of this definition there are therefore two very important concepts: the concept of electron and the concept of motion or propagation of such electrons.
As known, electrons are negatively charged particles of the atom that move around a central nucleus along paths that are called orbits (Figure 1). Inside the nucleus there are neutral particles, i.e. without charge, called neutrons and positively charged particles called protons that attract electrons; it is thanks to this attraction that the electrons remain in their orbit and do not move away from the atom itself.
The number of electrons equals the number of protons of the atom and therefore overall the atom is neutral, i.e. it is neither positively nor negatively charged.
Figure 1: Structure of the atom.
However, in particular circumstances electrons can escape the attraction of the nucleus of their atom (Figure 2) and therefore under the action of appropriate forces they can start moving away from their atom. This consideration is directly related to the concept of electric current which represents an orderly movement of electric charges.
Figure 2: Generation of the electric current as an orderly movement of electrons.
Therefore, whenever a movement of electric charges occurs in a material, we will say that in this material is flowing an electric current and, for that reason, we will say that this material is an electrical conductor.
In particular circumstances, as in the case of propagation in gases or liquids, in addition to electrons that have a negative charge, we can also have the motion of positively charged particles. The latter are made up of atoms which lose one or more electrons and consequently exhibit an overall positive charge; these positively charged atoms are called positive ions. In these circumstances, the electric current can therefore also be generated by the motion of positive charges.
Most electronic circuits are based on currents that circulate in solid conductive materials (Figure 3) and therefore the electric currents generated in these circuits are due to the orderly movement of electrons which as mentioned are particles with negative charge. In the course of this lesson, we will therefore refer to currents generated by electrons, however the same concepts also apply to currents generated by positive charges or even by both positive and negative charges.
Figure 3: Orderly motion of electrons in a solid conductive material.
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It is known from physics that the electrons inside the conductive materials are in continuous random movement (Figure 4). This chaotic motion of negative electric charges therefore leads us to say that electrons are unable to spontaneously produce an electric current, i.e. a net flow of electric charge through a region.
Figure 4: Random movement of electrons in a solid conductive material.
In order to force electrons to move in a given direction, we must apply forces capable of acting on these negative charges (Figure 5). It is evident that given the very small size of the electrons, we certainly cannot think of applying mechanical forces to push them in the desired direction!
Figure 5: Force to be applied to electrons to generate an orderly movement of charges, i.e. an electric current.
Instead, what can be done is to exploit the fact that electric charges exert forces upon themselves called electric forces (Figure 6). For example, we know from physics that two charges of equal sign (both with positive or both with negative sign) repel each other, while if we take into consideration two charges of opposite sign, that is, a negative and a positive one, these attract each other.
Figure 6: Forces of attraction and repulsion between electric charges.
This explains why electrons remain confined within the atom (Figure 7): since the atom contains positive charges in its nucleus, these attract electrons and (within certain limits) prevent them from escaping.
Figure 7: Attraction forces between electrons and protons inside the atom that prevent electrons from escaping.
The attraction and repulsion electrical forces can be used to generate a motion of electrons in the desired direction, as indicated in Figure 8.
Figure 8: Electric forces applied to electrons to generate an orderly movement of charges within an electrical conductor.
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In order to generate these electrical forces, devices called electromotive force generators are used. These generators are also commonly known as electric generators and without their presence within a circuit, it would not be possible to activate the various devices that make up the circuit itself.
Therefore, the electric generator, through the electric forces that it is capable of producing, supplies energy to the electric charges that are available in a circuit so that they can keep moving. We come therefore to the concept of electrical energy: electric generators are devices capable of producing electrical energy by transferring it to the charges of the circuit to start them up and keep them moving. Since nothing is created and nothing is destroyed, to produce electrical energy to be transferred to the electrical charges of the circuit, the electromotive force generator converts another form of energy into electrical energy (Figure 9). Photovoltaic panels, for example, convert solar energy (i.e. the light composed of photons) into electrical energy, wind turbines, set in motion by the wind, convert mechanical energy into electrical energy, while common batteries convert chemical energy into electrical energy.
Figure 9: Examples of electric generators obtained by converting other forms of energy into electrical energy.
It is well understood that to have a continuous flow of charges within a circuit, the electric generator must have two terminals (Figure 10): a first terminal from which electrons are emitted with sufficient energy to make them able to move in an orderly manner in the circuit and a second terminal from which to collect the electrons that have passed through the circuit, and finally transferring these electrons to the first terminal in order to give continuity to this process.
Figure 10: Continuous and orderly movement of electrons generated by a battery.
Regardless of the type of energy used by the electromotive force generator to produce electrical energy, it is understood that the main parameter of an electric generator is the energy it can supply to the electrical charges of a circuit.
An electric generator can therefore be characterized by the value of the electrical energy that each charge receives when passing internally from one terminal to the other of the electric generator. Without going into more complex concepts, we can say that the energy that the generator is able to supply to an electric charge of unit value in its transfer from one terminal to the other is defined as potential difference (Figure 11) and represents the main parameter of an electric generator. This electrical quantity is measured in Volt and very often rather than referring to it as a potential difference, we also speak of voltage and therefore an electric generator, in addition to being called a potential difference generator, is also called a voltage generator.
In addition to knowing the value of the amount of energy that the voltage generator supplies to electric charges, it is very important in electronics to know what are the electron emission and collection terminals since this information tells us the direction of the electrons motion inside the circuit, or in other words the direction of the electric current.
By convention, the terminal from which the electrons are emitted is called the negative pole and is indicated with the “-” sign, while the terminal that collects the electrons is called the positive pole and is indicated with the “+” sign.
By convention the red color is associated with the positive pole while the black color is associated with the negative pole.
Figure 11: Main parameters and characteristics of a voltage generator.
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It should be noted here that there are cases where the positive and negative poles of a voltage generator do not always remain indefinitely at the same terminals. In the case of generators operating in alternating current, used for example to supply electricity to our homes (Figure 12), the positive and negative poles invert their position several times per second and this is why we speak of alternating current, whose symbol is “~”. We will have the opportunity to deepen the functioning of alternating current in a subsequent lesson that will also allow us to better understand the choice of the symbol associated with it.
Figure 12: example of wall sockets (electrical outlets) used in civil electrical systems operating in alternating current with voltage values that generally are in the ranges 110¸120V or 220¸240V, depending on the country considered. The wall sockets showed in the figure are just one the many different models available; again, they depend on the country considered.
Up to now we have thought of electric current as an orderly movement of electrons which, as said, are negative electric charges. On the other hand, this is what really happens in solid conductive materials. However, nobody forbids us to think ideally of an electric current as an orderly movement of positive electric charges. In this case, it should be noted that these positive charges subjected to the action of an electric generator would move in the opposite direction with respect to the direction of the electrons motion; in other words, the positive electric charges would move from the positive pole to the negative pole of the electric generator. This observation is not a simple detail but it is very important because in electronics when we talk about electric current in a circuit we always think of a flow of positive charges even if in reality we know well that it is the electrons and therefore the negative charges to generate currents flowing in the circuit. As a result, when we have to draw the direction of a current flowing in a conductor we will draw it indicating a movement of charges from the positive pole to the negative pole as shown in Figure 13.
Figure 13: Electric current generated by a flow of positive charges. Unlike negative charges, these charges flow from the positive pole to the negative pole. By convention, in electrical circuits the direction of current is always indicated thinking about the movement of positive charges.
A similar consideration applies to the potential difference. We know that the electrons on the negative terminal have more potential energy than the electrons on the positive terminal of the electric generator. In the case of positive charges, we have a reversed situation, i.e. these charges have more potential energy at the positive pole with respect to charges at the negative pole. In electronics, when we have to express a measure of potential difference, by convention we refer to positive charges. Therefore, by convention, the difference between the potential of the positive terminal and the potential of the negative terminal generates a positive value of potential difference, on the contrary the difference between the potential of the negative terminal and the potential of the positive terminal generates a negative value of potential difference. For example, as shown in Figure 14, in the case of a 1.5 V battery, if we measure the difference between the potential of the positive pole and the potential of the negative pole, we will obtain +1.5 V. On the contrary, if we measure the difference between the potential of the negative pole and the potential of the positive pole we will obtain -1.5 V.
Figure 14: Convention used for the calculation of the potential difference (or voltage) measured across the terminals of a battery. V+ indicates the potential measured at the positive pole, while V– indicates the potential measured at the negative pole.
In summary, by convention, it is assumed that in a generic circuit we have positive electric charges that can propagate and therefore their motion direction goes from the positive pole to the negative pole of the electric generators. The rate of flow of these positive electric charges is defined as electric current and the motion direction of these positive charges represents the direction of the electric current generated by them. The energy supplied by the generator to the positive charge unit, in order to put it in a condition to be able to move from the positive pole to the negative pole, is directly related to the difference between the potential of the positive pole and the potential of the negative pole of the electric generator.
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