What is current strength definition and formula. What is current strength
Definition
electric shock called the ordered motion of charge carriers. In metals, these are electrons, negatively charged particles with a charge equal to the elementary charge. The direction of current is the direction of movement of positively charged particles.
The current strength (current) through a certain surface S is called a scalar physical quantity, which is denoted by I, equal to:
where q is the charge passing through the surface S, t is the charge transit time. Expression (1) determines the value of the current strength at time t (instantaneous value of the current strength).
Some types of current
The current is called constant, if its strength and direction do not change over time, then:
Formula (2) shows that the direct current is equal to the charge that passes through the surface S per unit time.
If the current is variable, then the instantaneous current strength (1), the amplitude current strength and the effective current strength are distinguished. The effective value of the alternating current (I eff) is such a direct current that will do work equal to the work of the alternating current during one period (T):
If alternating current can be represented as a sinusoidal:
then I m is the amplitude of the current strength ( is the frequency of the alternating current strength).
current density
The distribution of electric current over the cross section of the conductor is characterized using the current density vector (). Wherein:
where is the angle between the vectors and ( is the normal to the surface element dS), j n is the projection of the current density vector onto the direction of the normal ().
The current strength in the conductor is determined using the formula:
where integration in expression (6) is carried out over the entire cross section of the conductor S
For direct current we have:
If we consider two conductors with sections S 1 and S 2 and direct currents, then the relation is fulfilled:
Current strength in conductor connections
When the conductors are connected in series, the current strength in each of them is the same:
With a parallel connection of conductors, the current strength (I) is calculated as the sum of the currents in each conductor (I i):
Ohm's law
The current strength is included in one of the basic laws of direct current - Ohm's law (for a circuit section):
where - is the potential difference at the ends of the section under consideration, is the EMF of the source that enters the section of the circuit, R is the resistance of the section of the circuit.
Current strength - physical quantity, equal to the ratio of the amount of charge passed through a certain surface in time to the value of this time interval:
The cross section of the conductor is often used as the surface under consideration.
The current strength in the International System of Units (SI) is measured in amperes (Russian designation: A; international: A), the ampere is one of the seven basic SI units. 1 A = 1 C/s.
According to Ohm's law, the current strength for a circuit section is directly proportional to the applied voltage to the circuit section and inversely proportional to the conductor resistance of this circuit section:
Charge carriers, whose movement leads to the appearance of a current, are charged particles, which are usually electrons, ions or holes. The strength of the current depends on the charge of these particles, their concentration, the average speed of the ordered movement of particles, as well as the area and shape of the surface through which the current flows.
If and are constant over the volume of the conductor, and the surface of interest is flat, then the expression for the current strength can be represented as
where is the angle between the particle velocity and the normal vector to the surface.
In a more general case, when the above restrictions are not met, a similar expression can be written only for the current flowing through a small surface element with area:
Then the expression for the current flowing through the entire surface is written as an integral over the surface
In metals, charge is carried by electrons, respectively, in this case, the expression for the current strength has the form
where e is the elementary electric charge.
The vector is called the electric current density. As follows from the above, its value is equal to the strength of the current flowing through a small surface element of unit area, located perpendicular to the velocity , and the direction coincides with the direction of the ordered motion of charged particles .
To measure the current strength, a special device is used - an ammeter (for devices designed to measure small currents, the names milliammeter, microammeter, galvanometer are also used). It is included in the open circuit in the place where you need to measure the current strength. The main methods for measuring current strength are: magnetoelectric, electromagnetic and indirect (by measuring voltage with a voltmeter at a known resistance).
In the case of alternating current, instantaneous current strength, amplitude (peak) current strength and effective current strength (equal to the strength of direct current, which allocates the same power) are distinguished.
15. Ohm's law for a chain section;
Ohm's law - an empirical physical law that determines the relationship of the electromotive force of a source or electrical voltage with the current strength and resistance of the conductor was established in 1826, and named after its discoverer George Ohm.
In its original form, it was written by its author as: 
Here X is the readings of the galvanometer, that is, in modern notation, the current strength I, a is a value that characterizes the properties of the voltage source, which is constant over a wide range and does not depend on the magnitude of the current, that is, in modern terminology, the electromotive force (EMF), l is the value , determined by the length of the connecting wires, which in modern concepts corresponds to the resistance of the external circuit R and, finally, b is a parameter characterizing the properties of the entire installation, which now can be seen as taking into account the internal resistance of the current source r.
In this case, in modern terms and in accordance with the author of the note, Ohm's formulation (1) expresses
Ohm's law for a complete circuit:
- EMF of the voltage source,
- current strength in the circuit,
- resistance of all external elements of the circuit,
· - internal resistance of the voltage source.
From Ohm's law for a complete circuit, the following consequences follow:
For r< · When r>>R, the current strength does not depend on the properties of the external circuit (on the magnitude of the load). And the source can be called a current source. Often expression where there is voltage or voltage drop, (or, what is the same, the potential difference between the beginning and end of the conductor section) is also called "Ohm's Law". Thus, the electromotive force in a closed circuit, through which current flows in accordance with (2) and (3), is: That is, the sum of the voltage drops on the internal resistance of the current source and on the external circuit is equal to the EMF of the source. The last term in this equation is called by experts the “terminal voltage”, since it is the voltmeter that shows the voltage of the source between the beginning and the end of the closed circuit attached to it. In this case, it is always less than the EMF. To another entry of formula (3), namely: Electric current is a directed movement of electric charges. The magnitude of the current is determined by the amount of electricity passing through the cross section of the conductor per unit time. By one amount of electricity passing through a conductor, we cannot yet fully characterize electricity. Indeed, an amount of electricity equal to one pendant can pass through a conductor in one hour, and the same amount of electricity can be passed through it in one second. The intensity of the electric current in the second case will be much greater than in the first, since the same amount of electricity passes in a much shorter period of time. To characterize the intensity of the electric current, the amount of electricity passing through the conductor is usually referred to as a unit of time (second). The amount of electricity passing through a conductor in one second is called current. The unit of current in the system is the ampere (a). Current strength - the amount of electricity passing through the cross section of the conductor in one second. The current strength is indicated by the English letter I. Ampere - a unit of electric current strength (one of), denoted by A. 1 A is equal to the strength of an unchanging current, which, when passing through two parallel straight conductors of infinite length and negligible circular cross-sectional area, located at a distance of 1 m from one another in a vacuum, would cause on a section of the conductor 1 m long an interaction force equal to 2 10 -7 N for each meter of length. The current strength in a conductor is equal to one ampere if one pendant of electricity passes through its cross section every second. Ampere - the strength of the electric current at which an amount of electricity equal to one pendant passes through the cross section of the conductor every second: 1 ampere \u003d 1 coulomb / 1 second. Auxiliary units are often used: 1 milliamp (mA) \u003d 1/1000 amperes \u003d 10 -3 amperes, 1 microamp (mA) \u003d 1/1000000 amperes \u003d 10 -6 amperes. If you know the amount of electricity that has passed through the cross section of the conductor for a certain period of time, then the current strength can be found by the formula: I \u003d q / t If an electric current passes in a closed circuit without branches, then the same amount of electricity passes through any cross section (anywhere in the circuit) per second, regardless of the thickness of the conductors. This is because charges cannot accumulate anywhere in the conductor. Hence, the current strength is the same anywhere in the circuit. In complex electrical circuits with various branches, this rule (the constancy of the current at all points of a closed circuit) remains, of course, valid, but it applies only to individual sections of the general circuit, which can be considered simple. Current measurement A device called an ammeter is used to measure current. To measure very small currents, milliammeters and microammeters, or galvanometers, are used. On fig. 1. A conditional graphic representation of an ammeter and a milliammeter is shown on electrical circuits.
Rice. 1. Conventions ammeter and milliammeter
Rice. 2. Ammeter In order to measure the current strength, you need to turn on the ammeter in the open circuit (see Fig. 3). The measured current passes from the source through the ammeter and receiver. The ammeter needle shows the current in the circuit. Where exactly to turn on the ammeter, i.e. before the consumer (counting in the direction of the current) or after it, is completely indifferent, since the current strength in a simple closed circuit (without branches) will be the same at all points in the circuit.
Rice. 3. Turn on the ammeter Sometimes it is mistakenly believed that an ammeter connected before the consumer will show a greater current strength than one switched on after the consumer. In this case, it is believed that "part of the current" is spent in the consumer to drive it. This, of course, is not true, and here's why. An electric current in a metal conductor is an electromagnetic process accompanied by an orderly movement of electrons along the conductor. However, the energy is carried not by electrons, but by the electromagnetic field surrounding the conductor. Through any cross-section of the conductors of a simple electrical circuit passes exactly the same number of electrons. How many electrons came out of one pole of the source of electrical energy, the same number will pass through the consumer and, of course, will go to the other pole, the source, because electrons, as material particles, cannot be consumed during their movement. Rice. 4. Measuring current with a multimeter In technology, there are very large currents (thousands of amperes) and very small ones (millionths of an ampere). For example, the current strength of an electric stove is approximately 4 - 5 amperes, incandescent lamps - from 0.3 to 4 amperes (and more). The current passing through the photocells is only a few microamperes. In the main wires of substations that provide electricity for the tram network, the current reaches thousands of amperes. The strength of the struma is a whole bunch of charged particles in one straight. You can know the strength of the strum in practice with the use of special devices for wiping, or you can develop it for help by using ready-made formulas, which means that you have to give it away. The physical quantity, which shows the charge that passes through the conductor in just one hour, is called the strength of the struma. The basic formula, zgіdno z kakoyu can razrahuvat tsyu force: I = q / t. That way the installation passed through the transverse section of the charge up to the interval of an hour, with the help of which the electrician went through, so very shukanoї the value of I. Significance decoding: Electricity can be installed - ce strum, which is to replace the battery, or in the form of a mobile phone robot, and change - what is in the rosette. Illumination and operation of all electrical appliances are the most important electricity. The importance of the changeable strum lies in the fact that it is easier for the vin to undergo transformation, the lower is constant. The first butt on the robotic strum can be tested when the fluorescent lamps are turned on: while the lamp is turned on, the particles move forward - backward - forward. In tsomu i є the main essence of the snake struma. For zamovchuvannyam Ide about vimiryuvannya the most tsgo type of electricity, so because of the greatest extensions in pobutі. Depending on Ohm's law, the strength of the strum can be developed using the formula (for an electric stake): I \u003d U / R єї dilyanki , expressed in Ohms. Vihodyachi іz Ohm's law, the power of electricity in the new lance looks like this: I \u003d E / R + r, de Ohm's law of zastosovnі for the calculation of the constant strum, as if it were necessary to recognize the magnitude of the power of the replaceable electrician, then you should find the value of the following to add to the root of the two. The main ways of assigning the power of the strum for the auxiliary systems of fittings in practice: In order to know the strength of the strum on the ground, the most common vicorist is a special accessory for this - an ammeter. Ce attachments are included in the expansion of the electric stake at the necessary points for mitigating the strength of the electric charge, which passes through the crossbar in an hour. To know the magnitude of the strength of a small electrician, a milliammeter, a microammeter and a galvanometry are used, which are also connected before that time in the lance, where it is necessary to recognize the strength of the struma. Connection can be done in two ways: in series and in parallel. The appointment of the strength of a relaxed strum is not so often requested, as it is the support or support of the tension, but without the importance of the physical value of the strength of the struma of the impossibly roiling of the stunted tension. All cicava Electric strum - tse ruh (ordered) charged particles. Vinikaє elektrichny strum when moved vіlnyh ionіv аbo elektronіv. Zero additional charge, patience through the wire conductor. So like with the same average swedishness ... A miraculous physical phenomenon, named in honor of Yogo Vidkrivach, was revealed by the famous French physicist A. M. Ampère. In 1820 André Marie experimented with the presence of a kind of kinetic force, like it poured into an electric conductor ...
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