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Electricity surrounds us although it does not always manifest itself. In fact, all physical bodies (objects) are formed with molecules of different materials that in turn are built with one of the 92 different atoms that exist in nature. And in each atom, there is a positive nucleus and a negative electron cloud that perfectly compensate for the atom to be neutral. And if it is neutral it cannot manifest itself electrically.
We do not have easy access to the nucleus that allows us to remove protons, but reaching the electrons in higher orbits is very easy and all it takes is rubbing materials with a cloth to remove or add electrons and generate fixed electrical charges in the material used. Adding or removing depends on the material being rubbed on the cloth. Some materials are givers and others are acceptors.
This is how we can have an object with an excess of electrons (negative) and another with a lack of electrons (positive). As long as the objects are separated (isolated) they will remain permanently charged. If you approach them until they touch, electric charges (electrons) will immediately circulate between them until they are neutralized so that each body is neutral.
As the reader can observe, all these actions occur in an instant of time and then cease as soon as the bodies are neutralized. There is no permanent circulation of electricity. An instant after the bodies touch, the electrical phenomena cease. For these reasons, these phenomena are included among static or electrostatic electricity. They serve to establish the principles of our specialty, but they are not the phenomena that normally occur within an electronic device, where electron currents circulate permanently.
The most important concept of electronics is that of the circulation of electric current, which can be clearly explained by static electricity.
So far we have two electrically charged bodies. One is made of donor material (positive) and the other is made of acceptor material (negative).
1.If we join them with a glass bar, the bodies will remain charged and then we say that the glass bar is an insulator.
2.If we join them with a copper bar, the bodies will discharge and then we say that the copper bar is conductive.
It is a mistake to consider that the same electron that leaves the body with an excess of electrons and penetrates the conducting bar, reaches the one that has a lack of electrons. In effect, the phenomenon that occurs is a displacement of electrons from atom to atom so that an electron enters through one end of the bar but what leaves is another electron that was located at the other end. The charge moves practically at the speed of light, the corpuscle (electron) does so much more slowly.
1.In an insulating body the electrons are tightly bound to its nucleus and it is difficult or impossible to remove them from their orbits.
2.In a conducting body, the electrons are loosely attached to its nucleus, and they often move and change nuclei casually; although whenever an atom acquires an electron it gives up another to maintain neutrality.
Now it is easy to understand that if a body with excess electrons is attached to a copper bar , this body transfers some of its electrons so that the new body with the addition of the copper bar has negative characteristics distributed evenly throughout the body. compound. That is to say, the copper bar is also negative and therefore, by bringing it closer to the positive body, it will establish the circulation of electrons.
On the other hand, the glass rod does not accept its electrons moving from atom to atom and therefore the body with excess electrons cannot influence it. These two concepts of insulating and conductive bodies are fundamental in our specialty.
1. Examples of conductive bodies are metals such as copper, aluminum, silver, gold, etc. But we must clarify that not only metals are conductors; some liquids are too. Let's leave the obvious case of liquid metals at room temperature like mercury. Some compound liquids such as acids, bases, and dissolved salts (such as salt water) are conductors, although not as good as metals. There are also conductive solids such as graphite (a state of carbon aggregation).2. As examples of insulators we can indicate glass, plastic materials and distilled water. They are actually insulators to some extent. In fact, if a body is very charged with electricity and the insulating bar is not very long, a disruptive circulation effect can occur that pierces the insulator and makes it conductive. In general, this circulation occurs with the presence of noise, lighting and thermal effects, giving rise to what is called an electric discharge and in many cases the insulating body is definitively transformed into a conductor.
This effect does not actually require the insulating bar; The air itself between the two charged bodies can act as a conductor if the charge of the bodies is high enough. In this case, arcs are produced through the air of which lightning is a natural manifestation that occurs due to the electrical charge of storm clouds.
You can even form an arc in a vacuum. In fact, a very negatively charged body can reject its excess electrons so much that they are capable of acquiring enough energy to jump through empty space. The arc that is visually observed as a luminous line and the noise that is produced are caused by electrons circulating at high speed and in large quantities.
The electric current
The electrons that circulate between two charged bodies with opposite charges, when joined with a conductor, form what is classically known as electric current. That is to say, circulation of electrons and electric current are synonyms. Generally, when it comes to electrostatic phenomena, we talk about the circulation of charges or electrons and when the processes are continuous, we talk about electric current.
The current of water that circulates through a pipe is measured in liters/Sec. What is electric current measured in? It is evident that it could be measured in electrons/Sec. but the charge of an electron is so small that the numbers would be very high, that is, the unit electrons/Sec. It is not practical. Even the unit of electrical charge of a body charged by friction measured in electrons is already a very high number.
For all this, practical units were devised for both the amount of electricity or electric charge and for the electric current, giving these units the name of different scientists who worked with electrical phenomena.
The practical unit of electric current is the Coulomb (coulomb) and is equal to 6.28 1018 electrons (6 trillion 228,000 electrons) or 6,280,000,000,000,000,000 electrons.
The practical unit of electric current is the Amper and is equal to one Coulomb per second.
To simplify the notation, letters are used to represent the different concepts and units. For example, charge is always represented by the letter Q and its practical unit by the letters Cb. The electric current is represented by an I and its unit by an A. The unit of time is represented with the lowercase “t” (because the uppercase T is reserved for temperature). With these representations it can be written that the electric current
I = Q/t
measured in Cb/Sec or the equivalent unit A.
Units always involve multiples and submultiples of them. In electronics, the submultiples of A, that is, the mA (milliamper) and the uA (microamper), are generally used. These equivalences can be seen in the following table.
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Dynamic electricity occurs when there is a permanent source of electricity that causes the permanent circulation of electrons through a conductor.
Permanent sources of electricity are divided into chemical and electromechanical.
An electric battery is a chemical source of electricity. Chemical reactions are generated inside the battery, the result of which is the production of electrons. These electrons are available to circulate, for example, through a conductor, but unlike a charged body, this source of electrons is not exhausted. When they are used, the battery generates more electrons to replace those taken. It could be considered that the battery has inside both a body with an excess of electrons (the negative terminal) and a body with a lack of electrons (the positive terminal) and that the battery transforms chemical energy into electrical energy to take an electron from the negative terminal. and raise it to positive.
A dynamo is an electromechanical machine that transforms mechanical rotation energy into electrical energy. It does the same thing as the battery, that is, we can assimilate it to two charged bodies with different polarities where the charges that circulate are replaced as they are taken. In this case the energy necessary to restore the charges is derived from a magnetic interaction between the electrons and the rotating magnetic field of the dynamo.
With dynamic electricity, another concept is reached, which is the ability of a generator to produce a permanent current circulation. What does the electric current that circulates between two charged bodies depend on? It depends on the difference in load between these bodies and the type of bar with which we interconnect them. It is not actually necessary for one of the bodies to be negative and the other positive. If one is very full of electrons and the other only has a small excess of electrons and they are connected with a conductive bar, it will balance the charges so that both bodies will have an average number of electrons after a while. It can be said therefore that the circulation of current depends on the difference in electrical potential between the two bodies (the more charged one body is than the other) and the type of bar used to establish the union between the bodies. We talk about potential because a charged body has potential energy, in the sense that if we do not place the bar there is no circulation and therefore electricity cannot generate work of any kind.
The different types of bars used to circulate the charges and the different types of sources generate the concept of the electrical potential difference and the resistance of the bar that we will analyze below.
Electrical resistance
The most important characteristic of what until now we call a bar is its ability to level the loads of the bodies with greater or lesser speed. Intuitively we know that if I place a copper bar the charges level out quickly; On the other hand, if I place a graphite bar, the loads may take much longer to level out (depending on the type of graphite). In the first case we say that the copper bar has very little resistance to the circulation of electric current and the second that graphite presents more resistance to the circulation of electrons.
How would you compare the resistance to electronic circulation of different materials? The logical thing would be to make identical specimens and operate by comparison. Deep down what it does is very similar but more scientific.
A test piece of the material is defined as a 1 meter long wire with a section of 1 mm2 and the specific resistance of that material is said to be unitary when the resistor has a resistance of 1 Ohms. The letter chosen to name the resistance is R. The formula that gives the resistance based on the specific resistance of the material and its dimensions is the following:
R = Re.L/S
where Re is the specific resistance of the material
In the following table we express the specific resistance of the most common materials.
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| Specific resistance table |
In electronics, enormous use is made of bars of different resistance. So much so that a component called a resistor is actually defined, which can have specific resistance values that differ from each other by 1%, 5% or 10% according to their quality. These resistors are built with graphite and have copper terminals for soldering into printed circuits with copper on an insulating sheet.
The unit Ohm represented by the Greek letter Omega of course has multiples and submultiples like the Amper. The following equalities indicate the most used multiples and submultiples:
milliohm
1000 mΩ = 1 Ω
kiloohm 1 KΩ = 1,000 Ωmegaohm 1 MΩ =
1,000,000 Ω
The electrical tension
It is said that a source has a potential or voltage difference of 1 Volt when, when a 1 Ohm resistor is connected to it, 1 A of electric current flows through it. The voltage of a source is identified by the letter E and its unit the Volt by the letter V. The following equalities indicate the most used multiples and submultiples:
microvolt
1,000,000 uV = 1 V
millivolt 1,000 mV = 1 VKilovolt 1 KV =
1,000 V
In reality, the voltage of a source and the potential difference do not obey the same concept. There is a small difference between both characteristics that we will explain.
Every source of electricity has an associated internal resistance that cannot be avoided. Take for example a type A battery (the largest used in flashlights). If we measure the voltage delivered by a new battery without placing any load resistor, we will measure a voltage of exactly 1.52V (the voltage depends on the materials used for its construction, the most common batteries use graphite and zinc as electrodes and they are the ones that give exactly that tension). But graphite and the rest of the materials that are part of the battery have a certain resistance that must be considered. On the other hand, if we place a 1 Ohm load resistor, the battery voltage is reduced to approximately 1.3 V. This means that this battery has an internal resistance that we will learn to calculate later.
For now we can say that the potential difference of the battery (or the no-load voltage, which is the same thing) is 1.52V and that the loaded voltage depends on the connected load, but for a 1 Ohm load it is 1. 3 V.
Electromechanical generators (dynamos) also have a potential difference and a working voltage with load. In this case, the internal resistance of the source is formed by the resistance of the device windings.
Ohm's Law
One of the most important laws of electronics is Ohm's law. The student's knowledge of this law is essential and its application should not present any type of doubt. Doubting the application of Ohm's law implies that all knowledge subsequently acquired will be tainted with nullity. That is why we ask you to pay the greatest attention and practice with Ohm's law until you have no doubt. In the next lesson we are going to insist on the topic but using an invaluable tool for this; the Live Wire virtual laboratory. But first we must grasp the concept the old-fashioned way, that is, using only our reasoning ability.
Ohm's
law is very logical and intuitive and the student will surely
understand it with complete ease. In figure 1 you can see what in
electronics is called a circuit.
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| Fig. 1 Circuit of a charged battery with a resistor |
Obviously this is a schematic drawing of reality. Instead of drawing an actual battery, an actual resistor and the wires connecting those components are replaced with an easy-to-draw schematic. On the right you can see the symbol of a resistor and on the left the symbol of a battery joined by lines that represent the circuit cables or the tracks of the copper printed circuit. In fact, the battery is always drawn so that the longest line is the positive terminal of the battery.
In this circuit two most important parameters are clearly determined. The voltage of the battery and the resistance of the resistor connected to it and which of course has the same voltage applied to the battery. We ignore the current that circulates through the circuit. Ohm's law allows us to calculate it using an equation.
Ohm says that I = E / R and this formula is totally logical because as the resistance R increases, the circulating current I is reduced and vice versa. It also tells us that as we increase the voltage E, the current will increase and vice versa.
Let's calculate the circulating current in our simple circuit:
I = 1.5V / 1 Kohm or I = 1.5V / 1,000 Ohms = 0.0015 A = 1.5 mA
Ohm's law is not only used to calculate the current through the circuit. It could happen that we actually know the battery voltage and we want a certain current to flow through the circuit (for example 2 mA), leaving the value of the resistor unknown. Performing a transposition of terms we can say that:
R = E/I
and replacing
R = 1.5V / 2 mA => R = 0.75 Koms = 750 Ohms
Finally, it could happen that we know the value of R and I and need to calculate the value of the battery voltage. For example, if R = 2K and I = 2 mA, it can be calculated that:
E = R x I
and replacing
E = 2K x 2 mA = 2000 x 0.002 = 4 V
We ask the student to apply Ohm's law for different values of E, R and I so that they gain confidence with the topic. And above all we ask that you try to use scientific notation to solve the circuits more quickly and safely. We advise you to purchase a scientific calculator to carry out the practices within your means. Or if you have a PC that uses the scientific calculator that comes with Windows.
Conclusions
And so we get into the topic. In this lesson we learned the principles of electronics in a practical and simple way. The student may encounter difficulties performing Ohm's law calculations; If that is your problem, don't worry, in the next lesson we are going to teach you how to use the Live Wire virtual laboratory that will allow you to verify the calculations made.
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