The concept of current strength is the basis of modern electrical engineering. Without this basic knowledge, it is impossible to make calculations for diagrams, perform electrical actions, prevent, identify and eliminate damage in a circuit.
How does
To understand what the current strength is, one should know the condition for its occurrence - the existence of particles with a free charge. It moves through the conductor (its cross section) from one point to another. The physics of current strength consists in the orderly movement of electrons, which are acted upon by an electric field from a power source. The more charged particles are transported, and the faster they move in one direction, the more charge will reach its destination.
In addition to the power source, the elements of a closed circuit are connecting wires through which electricity passes and energy consumers (installations, resistors).
Additional Information. In metal conductors, electrons act as a transmitter of charges, gaseous - ions, liquid - the transfer of charged particles is performed using both types of particles. Violation of the order of passage indicates a chaotic movement of charges, during which the circuit will become de-energized.
Definition
The current in a conductor is the amount of electricity that travels through the cross-section in a unit time interval. To increase this value, you need to remove the lamp from the circuit or increase the magnetic field created by the battery.
The unit of measurement of the strength of the electric current by the international system SI (Systеme International) is considered to be ampere (A), named after the surname of the outstanding French scientist of the XIX century André-Marie Ampere.
Additional Information. Ampere is a fairly impressive electrical measure. For human life it represents mortal danger current value up to 0.1A. A burning household light bulb of 100 watts allows electricity to pass at about 0.5 A. In a room heater, this value reaches 10 A, one thousandth of an ampere will suffice for a portable calculator.
In electrical engineering, measurements of small quantities can be expressed in micro- and milliamperes.
The current strength is found with a measuring device (ampere - or a galvanometer), sequentially including it in the desired section of the circuit. Small values are measured with a micro, - or milliammeter. The main methods for finding the amount of electricity using devices are:
- Magnetoelectric - at a constant current value. This method is distinguished by increased accuracy and low energy consumption;
- Electromagnetic - for stationary and variable quantities. With this method, the current in the circuit is the result of the transformation magnetic field in the output signal of the modulation sensor;
- Indirect - based on measuring voltage at a known resistance. Next, calculate the desired value according to Ohm's law, shown below.
According to the definition, the current strength (I) can be found by the formula:
I = q / t, where:
- q is the charge going across the conductor (C);
- t is the duration of the time spent on the movement of particles (s).
The formula for the current is read as follows: the required value of I is the ratio of the charge passed through the conductor to the time used.
Note! The strength of the current is determined not only through the charge, but also by calculation formulas based on Ohm's law, which says: the strength of electricity is directly proportional to the voltage of the conductor and inversely proportional to its resistance.
Ohm's law formula will help you find the current strength, which looks like a ratio:
I = U / R, here:
- U - voltage (V);
- R - resistance (Ohm).
This established connection physical quantities are used for various calculations:
- taking into account the characteristics of the power supply;
- for calculations in circuits of currents of any direction;
- for polyphase circuits.
Note! If the conductors are connected in a sequential manner, then the electricity of each of them is equal. Parallel connection provides for the number of amperes, which is the sum of the current values of each conductor.
How do I find the power (transfer rate or power conversion rate) using the current value? To do this, you need to use the formula:
P = U * I, where the values to be multiplied are mentioned above.
Views
With constant and alternating electricity, its strength is of different nature... For a chain with particles moving in a constant direction, all parameters remain unchanged. A variable view is capable of changing its value in the same or changing direction. The amount of electricity in this case is:
- instantaneous, depending on the amplitude and vibration frequency associated with the angular frequency;
- amplitude - the maximum value of the instantaneous current strength for a certain period;
- efficient - when converting energy, the amount of heat from both types of current is the same.
Household electrical networks pass alternating current, which is converted into direct current when passing through the power supply unit of an electrical appliance (computer, TV).
The magnitude of the current is a concept closely related to electrical energy, which is of great importance for the sphere of everyday life, the national economy, objects strategic purpose... Moreover, the electric power industry is the economic basis of the state and the defining vector of development within the country and at the international level.
Video
Current strength
The characteristic of the current in the circuit is a quantity called the current strength ( I ). Current strength- physical quantity characterizing the rate of passage of the charge through the conductor and equal to the charge ratio q passed through the cross-section of the conductor in a period of time t , by this time interval: I = q / t ... Current unit - 1 ampere(1 A).
The definition of the unit of current strength is based on the magnetic action of the current, in particular on the interaction of parallel conductors through which an electric current flows. Such conductors attract if the current flows through them in one direction, and repel if the direction of the current in them is opposite.
For a unit of current strength, such a current strength is taken at which segments of parallel conductors 1 m long, located at a distance of 1 m from each other, interact with the force 2 * 10 -7 N... This unit is called ampere(1 A).
Knowing the formula for the current strength, you can get the unit of electric charge: 1 Cl = 1A * 1s.
Ammeter
The device with which the current in the circuit is measured is called ammeter... Its work is based on the magnetic action of the current. The main parts of the ammeter magnet and coil ... When an electric current passes through the coil, as a result of interaction with a magnet, it turns and rotates the arrow connected to it. The greater the strength of the current passing through the coil, the stronger it interacts with the magnet, the greater the angle of rotation of the arrow. Ammeter is included in the circuit consistently with the device, the current strength in which you want to measure, and therefore it has a small internal resistance, which practically does not affect the resistance of the circuit and the current strength in the circuit.
The ammeter terminals have signs «+» and «-» , when the ammeter is connected to the circuit, the terminal with the sign «+» connects to the positive pole of the current source, and the terminal with the sign «-» to the negative pole of the current source.
Voltage
The current source creates an electric field that sets electric charges in motion. The characteristic of the current source is a quantity called tension... The larger it is, the stronger the field created by it. Voltage characterizes the work that an electric field does to move an electric charge.
Voltage (U) Is a physical quantity equal to the ratio of work ( A) of the electric field according to the movement of electric charge to charge (q): U = A / q .
Another definition of voltage is possible. If the numerator and denominator in the voltage formula are multiplied by the time the charge moves ( t ), we get: U = At / qt ... The numerator of this fraction contains the current power ( R), and the denominator is the current strength ( I ). It turns out the formula: U = P / I , i.e. voltage is a physical quantity equal to the ratio of the power of the electric current to the strength of the current in the circuit.
Voltage unit: [ U ] = 1 J / 1 C = 1 in(one volt).
Voltmeter
The voltage is measured with a voltmeter. It has the same device as the ammeter and the same operating principle, but it connects parallel that section of the circuit, the voltage on which they want. The internal resistance of the voltmeter is large enough, respectively, the current passing through it is small compared to the current in the circuit.
The voltmeter terminals have signs «+» and «-» , when the voltmeter is connected to the terminal circuit with the sign «+» connects to the positive pole of the current source, and the terminal with the sign «-» to the negative pole of the current source.
Formulas and definitions.
1. All conductors used in electrical circuits have legend for representation on diagrams and can form serial, parallel and mixed connections.
2. Power current- a physical quantity that characterizes the rate of conversion of electrical energy into its other types. Measurement unit - 1 watt(1 W). The measuring device is a wattmeter.
3. Current strength Is a physical quantity that characterizes the rate of passage of a charge through a conductor and is equal to the ratio of the charge that has passed through the cross-section of the conductor to the time of movement. Unit - 1 ampere(1 A). Measuring device - ammeter(connected in series).
4. Electrical voltage - a physical quantity that characterizes the electric field that creates a current, and is equal to the ratio of the power of the current to its strength. Unit - 1 volt(1 B). Measuring device - voltmeter(connected in parallel)
In electrical engineering, it is generally accepted that a simple circuit is a circuit that is reduced to a circuit with one source and one equivalent resistance. You can collapse the chain using the equivalent serial, parallel, and mixed conversions. An exception is chains containing more complex star and triangle connections. Calculation of DC circuits produced using Ohm's and Kirchhoff's law.
Example 1
Two resistors are connected to a 50 V constant voltage source, with an internal resistance r = 0.5 ohm. Resistor resistances R 1 = 20 and R 2 = 32 Ohm. Determine the current in the circuit and the voltage across the resistors.
Since the resistors are connected in series, the equivalent resistance will be equal to their sum. Knowing it, let's use Ohm's law for a complete circuit to find the current in the circuit.
Now, knowing the current in the circuit, you can determine the voltage drops across each of the resistors.
There are several ways to check the correctness of the solution. For example, using Kirchhoff's law, which states that the sum of the EMF in the circuit is equal to the sum of the voltages in it.
But using Kirchhoff's law it is convenient to check simple circuits with one circuit. A more convenient way to check is the power balance.
The power balance must be observed in the circuit, that is, the energy given by the sources must be equal to the energy received by the receivers.
The power of the source is defined as the product of EMF and current, and the power received by the receiver as the product of voltage drop and current.
The advantage of checking the power balance is that you do not need to draw up complex cumbersome equations based on Kirchhoff's laws, it is enough to know the EMF, voltages and currents in the circuit.
Example 2
Total current of a circuit containing two resistors connected in parallel R 1 = 70 Ohm and R 2 = 90 Ohm, equal to 500 mA. Determine the currents in each of the resistors.
Two series-connected resistors are nothing more than a current divider. It is possible to determine the currents flowing through each resistor using the divider formula, while we do not need to know the voltage in the circuit, we only need the total current and the resistances of the resistors.
Currents in resistors 
In this case, it is convenient to check the problem using the first Kirchhoff's law, according to which the sum of the converging currents at the node is equal to zero.
If you do not remember the current divider formula, then you can solve the problem in another way. To do this, you need to find the voltage in the circuit, which will be common for both resistors, since the connection is parallel. In order to find it, you must first calculate the resistance of the circuit
And then the tension
Knowing the voltages, we find the currents flowing through the resistors
As you can see, the currents are the same.
Example 3
In the electrical circuit shown in the diagram R 1 = 50 Ohm, R 2 = 180 Ohm, R 3 = 220 ohms. Find the power dissipated across a resistor R 1, the current through the resistor R 2, the voltage across the resistor R 3, if it is known that the voltage at the terminals of the circuit is 100 V.
To calculate the DC power dissipated across the resistor R 1, it is necessary to determine the current I 1, which is common to the entire circuit. Knowing the voltage at the terminals and the equivalent resistance of the circuit, you can find it.
Equivalent resistance and current in the circuit
Hence the power allocated to R 1
Gentlemen, hello everyone!
Today we will talk about such a fundamental concept of physics in general and electronics in particular, as amperage... Each of you, for sure, has heard this term more than once. Today we will try to understand it a little better.
Today, we will primarily focus on direct current... That is, about such, the value of which is constant all the time in strength and direction. Dear sirs, bores can start digging - and what does "all the time" mean? There is no such term. To this we can answer that the value of the current should not change throughout the entire time observation.
So the current. Current strength. What is it? Everything is simple enough. The directional movement of charged particles is called current. Notice, gentlemen, it is directed... The disorderly - 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 the movement of all particles in one direction on this disorderly movement, then this is a completely different calico.
What charged particles can there be? In general, do not care what, no difference. Positive ions, negative ions, electrons - it doesn't matter. If we have a directed movement of these respected comrades, then there is an electric current.
Obviously, the current has some direction. Per direction of current it is customary to take the movement of 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 conductor with current is shown in Figure 1.
Figure 1 - Schematic representation of a conductor with current
Imagine a mosquito cloud. Yes, I know, disgusting creatures, and the cloud is in general 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 carries away at the same time all this horde of mosquitoes in one direction, hopefully, away from us. This is a directional movement. Replacing mosquitoes with electrons, and the breeze with some mysterious driving force, we get, in general, some kind of analogy with an electric current.
Most often, there is a current caused by the movement of electrons. Yes, friends, in our whole life we are surrounded by poor electronics, forced to move directionally, one might say in formation, under the influence of a compelling force. They run along the wires of power lines, in all our sockets, in all our smart devices - computers, laptops, smartphones, and they just work like Dad Carlo to make our hard life and fill it with pleasures.
Mosquitoes are mosquitoes, this is all cool, but the time has come for formal definitions.
So, gentlemen, the current strength is the ratio of the charge Δq, which is transferred through a certain section of the conductor S during the time Δt. 
The current strength is measured, as many already know, in Amperes. So - the current in the conductor is equal to 1 Ampere, if 1 Coulomb passes through this conductor in 1 second.
"Fine!" - the dear reader will exclaim. And what should I do with this formula ?! Well, time is fine, I have a stopwatch in my iPhone, I will mark it. And what about the charge? Should I count the number of electrons in the wire and then multiply by the charge of one electron, since this is a known quantity to determine the current ?!
Calm, gentlemen! All will be. Do not hurry. For now, just remember that there was some kind of formula. Then it turns out that with its help you can count some cool things such as charging capacitors and whatnot.
In the meantime ... While you can take an ammeter, measure the current in the circuit with a light bulb and find out what charge flows every second through the conductor cross-section q = I t = I 1c = I.
Yes, every second a charge flows through the cross-section of the conductor equal to the current in it. You can now multiply this value by the electron charge (for those who have forgotten, I remind you that it is equal) and find out how many electrons run in the circuit. Voros may arise - what for? The author's answer is just for fun. You are unlikely to squeeze practical benefits out of this. If only you will 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 the magnetic action of the current in old-fashioned analog dial ammeters or on the Ohm law - by converting the flowing current through a known resistance into voltage and its subsequent processing - in all modern multimeters. But more on that later.
Now I will give this calculation. It is quite simple and should be digested even by the humanities. If you have an individual intolerance to matan, well, you can just look at the result.
Let's remember our charge ∆q that passes over time ∆t through conductor cross-section ∆S which we talked about just above. As true mathematicians, let us complicate it to the point of disgrace, so that only after straining the brain it is clear that we have written an identity.
Gentlemen, honestly, no deception. e - electron charge, n - the concentration of electrons, that is, the number of pieces in one cubic meter, v - the speed of movement of electrons. It's obvious that v ∙ ∆t ∙ ∆S Is essentially the volume that the electrons will pass. 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 that has passed through the cross section. Well I said that everything is fair!
Let us introduce the concept of current density. The bores who have already read something about this will now exclaim - aha, this is a vector quantity! I do not argue, gentlemen, vector. But we, to simplify the already difficult life, will assume that the direction of the current density vector coincides with the axis of the conductor, which happens in most cases. Therefore, vectors immediately become scalars. Roughly speaking, current density is how many amperes are there per square meter of conductor cross-section. Obviously, for this it is necessary to divide the current strength by the area. We have
Now, I hope, it is clear why we transformed the formula like this? To cut down on a bunch of everything!
We remember the main thing - we are looking for speed. We express it:
Everything would be fine, but we do not know the concentration yet. Remembering chemistry. There was such a formula
Where ρ = 8900 kg / m 3- copper density, 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. Honestly, I'm not very friendly with chemistry. Although I studied all 11 years at a school with an in-depth study of chemistry, however, in the 8th grade I entered the physics and mathematics class, became interested in physics, especially that part of it, which talks about electricity, and, one might say, forgot about chemistry. Actually, they did not ask her deeply, we were physicists. However, if all of a sudden the need arises, I am 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
Let's substitute specific numbers. Let us set for definiteness 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. People are not in the first year engaged in electronics. Some experience has been accumulated in this area, or, in the language of science, empirical data. So, these empirical data say that the permissible current density in copper wires is, usually 5-10 A / mm 2... With a higher current density, 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 and even more. However, this is a topic for a completely different conversation. Let's return to our problem, namely, to calculate the speed of electrons in a conductor. Substituting the numbers, we find that
Gentlemen, the calculation irrefutably shows that electrons in a conductor with current move only at a speed of 0.37 millimeters per second! So slow. True, it should be remembered that this is not a thermal movement, but a directional one. Thermal motion is much, much more, on the order of 100 km / s. A reasonable question - why does the light flash instantly when I turn the switch? Do you remember when I talked about some coercive force? It's about her! But more on that in the next article. Great luck to you all, and see you soon!
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Many of us, even from school, cannot understand what aspects distinguish current strength from voltage. Of course, teachers have consistently argued that the difference between the two is enormous. Nevertheless, only a few adults have the opportunity to boast of having the relevant knowledge, and if you are not one of those, then it's time for you to pay attention to our today's review.
What are current and voltage?
In order to talk about what the current strength is and what nuances can 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, some charged particles begin to move. As the latter, there can be a whole list of all kinds of elements, in this regard, it all depends on the specific situation. So, for example, if we are talking about conductors, then in this case, electrons will act as the aforementioned particles. 
Perhaps some of you did not know this, but the 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. The current is also irreplaceable in everyday life, because with its help, the light is on in your house and some electrical appliances are working. The strength of the current, 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 we compare it with such a concept as "current strength". There, single positive charges are provided, which must move from different points. In addition, voltage is called such energy, through which the above-mentioned movement occurs. In schools, to understand this concept, they often cite the example of the flow of water that occurs between two banks. In this situation, the water flow itself will act as a current, while the voltage will be able to show the difference in levels in these two banks. Therefore, the current will be observed until both levels in the banks are equal.
What distinguishes amperage from voltage?
We dare to suggest that the main difference between these two concepts is their immediate definition:
- Under the words "current" and "current", in particular, they represent a certain amount of electricity, while voltage is usually considered a measure of potential energy. In simple words, these two concepts are quite dependent on each other, while retaining some distinctive features, with all this. Their resistance is influenced by a huge variety of factors. The most important of them is the material from which this or that conductor is made, external conditions as well as temperature.
- There is also a certain difference. to receive them... So, if the impact on electric charges creates a voltage, then the current is obtained by applying a voltage between the points of the circuit. By the way, ordinary batteries or more advanced and convenient generators can act as such devices. For this reason, we can say that the main differences between these two concepts are reduced to their definition, as well as the fact that they are obtained as a result of completely different processes.
The current should not be confused also together 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 a current, this energy will already be kinetic. In our modern realities, the vast majority of pipes correspond to analogies from the world of electricity. It is about the load that is created when a light bulb or the same TV is connected to the network. During this, a consumption of electricity is created, which ultimately leads to the appearance of current.
Of course, in the event that you do not connect any electrical appliances to the outlet, the voltage will remain unchanged, at the same time as the current will be zero. Well, if no consumption is provided, then what kind of current can we talk about and any of its strength? Therefore, current is just a certain amount of electricity, while voltage is a measure of the potential energy of a particular source of electricity.
