Gentlemen, hello everyone!
Today we will talk about this fundamental concept physics in general and electronics in particular, as current strength. Each of you has probably heard this term more than once. Today we will try to understand it a little better.
Today we will primarily talk about DC
. That is, about something whose magnitude is constant in strength and direction all the time. Dear gentlemen, bores may begin to dig into the matter - what does “all the time” mean? There is no such term. To this we can answer that the current value should not change throughout the entire time observations.
So, current. Current strength. What is this? Everything is quite simple. Current is the directional movement of charged particles. Please note, gentlemen, that directed. The random - thermal - movement, from which electrons in a metal or ions in a liquid/gas rush back and forth, is of little interest to us. But if you superimpose on this random movement the movement of all particles in one direction, then this is a completely different calico.
What kinds of charged particles can there be? In general, it doesn’t matter what it is, it doesn’t matter. Positive ions, negative ions, electrons - doesn't matter. If we have a directed movement of these respected comrades, it means that there is an electric current.
Obviously, the current has some direction. For direction of current accepted to accept movement positive particles. That is, although the electrons run from minus to plus, it is believed that the direction of the current in this case is the opposite - from plus to minus. This is how everything is twisted. What can you do - a tribute to tradition.
A schematic representation of a current-carrying conductor is shown in Figure 1.
Figure 1 - Schematic representation of a current-carrying conductor
Let's imagine a cloud with mosquitoes. Yes, I know, vile creatures, and the cloud is generally some kind of horror. But still, suppressing disgust, we will try to imagine them. So, in this cloud, every nasty mosquito flies by itself. This is a disorderly movement. Now let’s imagine a saving breeze. He simultaneously carries away this entire horde of mosquitoes in one direction, hopefully away from us. This is a directed movement. Replacing mosquitoes with electrons, and the breeze with some mysterious driving force we get, in general, some analogy with electric shock.
Most often, there is a current caused by the movement of electrons. Yes, friends, throughout our lives we are surrounded by poor little electronics, forced to move directionally, one might say in formation, under the influence of coercive force. They run along power lines, in all our sockets, in all our smart devices - computers, laptops, smartphones and work just like Papa Carlo to facilitate our hard life and fill it with pleasures.
Mosquitoes are mosquitoes, that's all cool, but it's time for formal definitions.
So, gentlemen, the current strength is the ratio of the charge Δq, which is transferred through a certain cross-section of the conductor S during the time ∆t. 
Current strength is measured, as many already know, in Amperes. So - the current in a conductor is equal to 1 Ampere if 1 Coulomb passes through this conductor in 1 second.
"Great!" - dear reader will exclaim. And what should I do with this formula?!! Well, okay, I have a stopwatch on my iPhone, I’ll time it. What about the charge? Should I count the number of electrons in the wire and then multiply by the charge of one electron, fortunately this is a known quantity, in order to determine the current?!
Calm, gentlemen! Everything will happen. Don't rush. For now, just remember that there was some kind of formula. Then it turns out that with its help you can calculate some cool things like charging capacitors and much more.
Well, for now... For now, you can take an ammeter, measure the current in the circuit with a light bulb and find out how much charge flows every second through the cross-section of the conductor q = I t = I 1c= I.
Yes, every second a charge flows through the cross-section of a conductor, equal to strength current in it. You can now multiply this value by the charge of the electron (for those who have forgotten, I remind you that it is equal) and find out how many electrons are running in the circuit. Questions may arise - why? The author's answer is just for fun. Practical benefits you're unlikely to make the most of it. If only you please your teacher. This problem is purely academic.
The question may arise - how does an ammeter measure current? Is he counting electrons? Of course not, gentlemen. Here we have indirect measurements. They are based on magnetic action current in old-fashioned analog dial ammeters or on Ohm's law - by converting the flowing current through a known resistance into voltage and its subsequent processing - in all modern multimeters. But more on that a little later.
Now I will give this calculation. It is quite simple and should be digested even by humanists. If you have an individual intolerance to matan, well, you can just look at the result.
Let's remember our charge ∆q that passes during time ∆t through the conductor cross section ∆S which we talked about a little higher. Like true mathematicians, we will complicate it to the point of outrageousness, so that only after straining the brain it will be clear that we have written an identity.
Gentlemen, honestly, no deception. e − electron charge, n − electron concentration, that is, the number of pieces in one cubic meter, v − speed of electron movement. It's obvious that v∙∆t∙∆S − this is essentially the volume that the electrons will travel through. We multiply the concentration by the volume - we get pieces, how many pieces of electrons have passed. We multiply the pieces by the charge of one electron - we get the total charge passing through the cross section. I told you everything was fair!
Let us introduce the concept of current density. Bores who have already read something about this will now exclaim - yeah, this vector quantity! I don’t argue, gentlemen, it’s vector. But to simplify an already difficult life, we will assume that the direction of the current density vector coincides with the axis of the conductor, which is what happens in most cases. Therefore, vectors immediately become scalars. Roughly speaking, current density is how many amperes are there per square meter conductor cross sections. Obviously, to do this, you need to divide the current by the area. We have
Now, I hope, it’s clear why we transformed the formula this way? To cut down on a bunch of stuff!
We remember the main thing - we are looking for speed. Let's express it:
Everything would be fine, but we don’t know the concentration yet. Let's remember chemistry. There was such a formula
Where ρ=8900 kg/m 3- density of copper, N A =6·10 23 Avogadro's number M=0.0635 kg/mol- molar mass.
Gentlemen, I hope there will be no need to explain where this formula came from. I'm not very good with chemistry, to be honest. Although I studied at school for 11 years with an in-depth study of chemistry, however, in the 8th grade I entered a physics and mathematics class, became interested in physics, especially the part that talks about electricity, and, one might say, gave up on chemistry. Actually, they didn’t ask us deeply about it; we were physicists. However, if the need suddenly arises, I am still ready to delve into this chemical jungle and tell you what’s what.
Thus, the speed of movement of electrons in a conductor with current is equal to
Let's substitute specific numbers. For definiteness, let us set a current density of 5 A/mm 2.
We already have all the other numbers. The question may arise - why exactly 5 A/mm 2.
It's simple, gentlemen. This is not the first time that people have been involved in electronics. Some experience has been accumulated in this area, or, in scientific terms, empirical data. So, these empirical data say that the permissible current density in copper wires usually amounts to 5-10 A/mm 2. At higher density current, unacceptable overheating of the conductor is possible. However, for tracks on a printed circuit board this value is much higher and amounts to 20 A/mm 2 or even more. However, this is a topic for a completely different conversation. Let's return to our task, namely, calculating the speed of electrons in a conductor. Substituting the numbers, we get that
Gentlemen, the calculation irrefutably shows that electrons in a current-carrying conductor move at only a speed of 0.37 millimeters per second! Very slow. However, it should be remembered that this is not thermal movement, but directional. The thermal movement is much, much greater, on the order of 100 km/s. A reasonable question - why does the light flash instantly when I turn the switch? Remember what I said about some kind of coercive force? It's about her! But more on this in the next article. Great good luck to you all, and see you again!
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Content:
The movement of charged particles in a conductor in electrical engineering is called electric current. Electric current is not characterized only by the quantity passing through the conductor electrical energy, since in 60 minutes electricity equal to 1 Coulomb can pass through it, but the same amount of electricity can be passed through a conductor in one second.
What is current strength
When the amount of electricity flowing through a conductor over different time intervals is considered, it is clear that over a shorter period of time the current flows more intensely, so another definition is introduced into the characteristics of electric current - this is the current strength, which is characterized by the current flowing in the conductor per second of time. The unit of measurement for the magnitude of passing current in electrical engineering is the ampere.
In other words, the strength of the electric current in a conductor is the amount of electricity that has passed through its cross-section in a second of time, marked with the letter I. The strength of the current is measured in amperes - this is a unit of measurement that is equal to the strength of a constant current passing through infinite parallel wires with the smallest circular cross-section separated by 100 cm and located in a vacuum, which causes an interaction on a meter of length of the conductor with a force = 2 * 10 minus 7 powers of Newton for every 100 cm of length.
Experts often determine the magnitude of the passing current; in Ukraine (strum power) it is equal to 1 ampere, when 1 coulomb of electricity passes through the cross-section of the conductor every second.
In electrical engineering, you can see the frequent use of other quantities in determining the value of the passing current: 1 milliampere, which equal to one/ Ampere, 10 to the minus third power Ampere, one microampere is ten to the minus sixth power Ampere.
Knowing the amount of electricity passing through a conductor over a certain period of time, you can calculate the current strength (as they say in Ukraine - strumu force) using the formula:
When an electrical circuit is closed and has no branches, then the same amount of electricity flows per second at each place in its cross section. Theoretically, this is explained by the impossibility of accumulating electrical charges in any place in the circuit; for this reason, the current strength is the same everywhere.
This rule is also true in complex circuits when there are branches, but applies to some sections complex chain, which can be considered as a simple electrical circuit.
How is current measured?
The magnitude of the current is measured with a device called an ammeter, and also for small values - a milliammeter and a microammeter, which can be seen in the photo below:
There is an opinion among people that when the current strength in a conductor is measured before the load (consumer), the value will be higher than after it. This is an erroneous opinion based on the fact that supposedly some amount of force will be expended to bring the consumer into action. Electric current in a conductor is an electromagnetic process in which charged electrons participate; they move in a direction, but it is not the electrons that transmit energy, but the electromagnetic field that surrounds the conductor.
The number of electrons leaving the beginning of the chain will be equal to the number of electrons after the consumer at the end of the chain, they cannot be used up.
What types of conductors are there? Experts define the concept of “conductor” as a material in which particles with a charge can move freely. Almost all metals, acids and saline solutions have such properties in practice. A material or substance in which the movement of charged particles is difficult or even impossible is called insulators (dielectrics). Common dielectric materials are quartz or ebonite, an artificial insulator.
Conclusion
In practice modern equipment works with large quantities current, up to hundreds, or even thousands of amperes, as well as with small values. Example in everyday life The magnitude of the current in different devices can be an electric stove, where it reaches a value of 5 A, and a simple incandescent lamp can have a value of 0.4 A; in a photocell, the value of the passing current is measured in microamps. In the lines of the city public transport(trolleybus, tram) the value of the passing current reaches 1000 A.
Probably everyone has felt the effects of electric current at least once in their life. An ordinary battery tingles barely perceptibly when you put it on your tongue. The current in the apartment socket hits quite strongly if you touch the bare wires. But electric chair and power lines can take lives.
In all cases we are talking about the action of electric current. How is one current so different from another that the difference in its impact is so significant? Obviously, there is some quantitative characteristic that can explain this difference. Current, as is known, is electrons moving along a conductor. It can be assumed that the more electrons pass through the cross section of the conductor, the greater action will produce a current.
Current formula
In order to characterize the charge passing through a conductor, we introduced physical quantity, called the strength of the electric current. Current in a conductor is the amount of electricity passing through cross section conductor per unit of time. The current strength is equal to the ratio of the electric charge to the time it travels. To calculate the current strength, use the formula:
where I is the current strength,
q - electric charge,
t - time.
The unit of current in a circuit is 1 Ampere (1 A) in honor of the French scientist Andre Ampere. In practice, multiple units are often used: milliamps, microamps and kiloamps.
Measuring current with an ammeter
Ammeters are used to measure current. Ammeters vary depending on the measurements they are designed for. Accordingly, the instrument scale is calibrated in the required values. The ammeter is connected anywhere in the network in series. Where the ammeter is connected does not matter, since the amount of electricity passing through the circuit will be the same at any location. Electrons cannot accumulate in any place in the circuit; they flow evenly through all wires and elements. When an ammeter is connected before and after the load, it will show the same values.
The first scientists who studied electricity did not have instruments to measure current and charge. They checked the presence of current with their own sensations, passing it through their body. Quite a nasty way. At that time, the current strengths with which they worked were not very high, so most researchers got away with only unpleasant sensations. However, in our time, even in everyday life, not to mention industry, very high currents are used.
You should know that for human body A current value of up to 1 mA is considered safe. Current values greater than 100 mA can cause serious damage to the body. A current of several amperes can kill a person. At the same time, it is still necessary to take into account the individual susceptibility of the body, which is different for each person. Therefore, you should remember the main requirement when operating electrical appliances - safety.
Before talking about current strength, it is necessary, in general outline, imagine what it is - electric current?
According to classical definitions- this is the directed movement of charged particles (electrons) in a conductor. In order for it to occur, it is necessary pre-creation electric field, which will set charged particles in motion.
The occurrence of current strength
All material substances They are made up of molecules that are divided into atoms. Atoms are also divided into components: nuclei and electrons. During the period of occurrence chemical reaction, electrons transfer from one atom to another. The reason here is that some atoms have a lack of electrons, while others have an excess of them. This, first of all, is the concept of “opposite charges”. In the case of contact of such substances, electrons move, which, in fact, is an electric current. The current will continue to flow until the charges of the two substances are equalized.
Even in ancient times, people noticed that amber, which was rubbed on wool, became capable of attracting various light objects. It later turned out that other substances have the same properties. They began to be called electrified, from Greek word"electron" meaning amber.
The force of electricity can be strong or weak. Depends on the amount of charge flowing through electrical circuit for a certain period of time. The more electrons are moved from pole to pole, the higher the value of charge transferred by the electrons. Total quantity charge is also called the amount of electricity passing through a conductor.
The first definition of current strength was given by Andre-Marie Ampère (1775-1836), a French scientist, 
physicist and mathematician. His definition formed the basis of the concept of current strength that we use today.
Unit of measurement
Current strength is the quantity equal to the ratio the amount of charge passing through the cross-section of a conductor to the time of its passage. The charge passing through the conductor is measured in coulombs (C), the transit time is measured in seconds (s). For the unit of current, the value is (C/s). In honor of the French scientist, this unit was named (A) and is currently the main unit of measurement of current.
To measure current strength, a special measuring device is used. It turns on directly at the break in the circuit in the place where it is necessary to measure the force. Instruments that measure small currents are called milliammeter or microammeter.
Types of conductors
Substances in which charged particles (electrons) move freely among themselves are called conductors. These include almost all metals, solutions of acids and salts. In other substances, electrons move very weakly among themselves or do not move at all. This group of substances is called dielectrics or insulators. These include ebonite, amber, quartz, and gases without an altered state. Currently exists large number artificial materials, acting as insulators and widely used in electrical engineering.
Many of us, even from school, cannot understand what aspects distinguish current from voltage. Of course, teachers constantly argued that the difference between these two concepts is simply enormous. However, only some adults have the opportunity to boast of having the relevant knowledge, and if you are not one of them, then it’s time for you to pay attention to our review today.
What is current and voltage?
In order to talk about what current strength is and what nuances may be associated with it, we consider it necessary to draw your attention to what it is in itself. Current is a process during which, under the direct influence of an electric field, the movement of certain charged particles begins to occur. The latter may be a whole list of various elements; in this regard, it all depends on specific situation. So, for example, if we are talking about conductors, then in this case, electrons will act as the above-mentioned particles. 
Perhaps some of you did not know this, but current is actively used in modern medicine and in particular in order to save a person from a whole list of all kinds of diseases, the same epilepsy, for example. Current is also indispensable in everyday life, because with its help, the lights are on in your home and some electrical appliances work. Current strength, in turn, implies a certain physical quantity. It is designated by the symbol I. 
In the case of voltage, everything is much more complicated, even if you compare it with such a concept as “current strength”. There are single positive charges, which must move from different points. In addition, voltage is the energy through which the above-mentioned movement occurs. In schools, to understand this concept, they often give the example of the flow of water that occurs between two banks. In this situation, the current will be the flow of water itself, while the voltage will be able to show the difference in levels in these two banks. Therefore, the flow will be observed until both levels in the banks are equal.
What is the difference between current and voltage?
We dare to suggest that the main difference between these two concepts is their immediate definition:
- The words “current strength” and “current”, in particular, represent a certain amount of electricity, while voltage is usually considered a measure potential energy. In simple words, these two concepts depend quite strongly on each other, retaining some distinctive features, with all this. Their resistance is affected huge amount a wide variety of factors. The most important of them is the material from which a particular conductor is made, external conditions, as well as temperature.
- There is also some difference in receiving them. So, if the impact on electric charges, creates a voltage, then the current is obtained by applying voltage between the points of the circuit. By the way, such devices can be ordinary batteries or more advanced and convenient generators. For this reason, we can say that the main differences between these two concepts come down to their definition, as well as the fact that they are obtained as a result of completely different processes.
Current should not be confused with energy consumption. These concepts are completely different and their main difference should be perceived precisely power. So, in the event that the voltage is intended for that. to characterize potential energy, then in the case of current, this energy will already be kinetic. In ours, modern realities, the vast majority of pipes correspond to analogies from the world of electricity. It's about about the load that is created when connecting a light bulb or the same TV to the network. During this, a consumption of electricity is created, which ultimately leads to the appearance of current.
Of course, if you do not connect any electrical appliances to the outlet, the voltage will remain unchanged, while the current will be zero. Well, if there is no provision for flow, then how can we even talk about current and any of its strength? Therefore, current is just a certain amount of electricity, while voltage is considered a measure of the potential energy of a certain source of electricity.
