What does the current show. Voltage and current

Definition 1

Current is a process during which (under the direct influence of an electric field) the movement of some charged particles begins to take place.

Such charged particles can be different elements (everything will depend on the situation). In the case of conductors, for example, electrons will act as such particles.

The concept of current strength

The strength of the electric current will be a quantity that characterizes the order of movement of electric charges, numerically equal to the amount of charge $\delta q$, which in this case flows through a certain surface $S$ (representing the cross section of the conductor) per unit time:

$I=\frac(\delta q)(\delta t)$

In order to determine the current strength $I$, it is required to divide the electric charge $\delta q$ passing through the cross section of the conductor during the time $\delta t$ by this time.

The strength of the current will depend on the charge carried by all particles, the speed of their movement oriented in a particular direction, and the cross-sectional area of the conductor.

Consider a conductor with a cross-sectional area $S$. We denote the charge of all particles by $q_o$. The volume of a conductor bounded by two sections contains $nS\delta l$ particles, where $n$ represents their concentration. Their total charge will be:

$q=(q_o)(nS\delta I)$

Under the condition of particle motion with an average velocity $v$, during the time $\delta t=\frac(\delta I)(v)$ all the particles contained in the considered volume will have time to pass through the second cross section, which means that the current strength corresponds to the calculations according to this formula:

$I=(q_o)(nvS)$, where:

$I$ - designation of the strength of electricity, measured in Amperes (A) or Coulombs / second;
$q$ - charge passing through the conductor, unit Coulomb (C);

In SI, the unit of current is considered the main unit, and it is called the ampere (A). The measuring device is an ammeter, whose principle of operation is based on the magnetic action of the current.

Remark 1

When assessing the speed of the ordered movement of electrons inside the conductor, performed according to the formula for a copper conductor with a cross-sectional area of \u200b\u200bone square millimeter, we get an insignificant value (0.1 mm / s).

The difference between current and voltage

In physics, there are such concepts as "current" and "voltage". There are some differences between them, the consideration of which is important for understanding the principle of the current strength.

Under the "strength" is understood a certain amount of electricity, "voltage", at the same time, a measure of potential energy is considered. At the same time, these concepts are quite strongly interdependent. The most important factors influencing them are:

conductor material;
temperature;
external conditions.

Differences can also be observed in the way they are obtained. If, in the case of action on electric charges, a voltage is created, the current will already arise due to the action of the voltage between the points of the circuit. There is also a difference in comparison with such a concept as "energy consumption". It will be in terms of power. So, if voltage is required to characterize potential energy, then current will already characterize kinetic energy.

Methods for determining the current strength

The current strength is calculated in practice using special measuring instruments or using separate formulas (subject to the availability of initial data). The basic formula according to which the current strength is calculated is as follows:

The existence of electricity can be constant (for example, the current contained in the battery), as well as variable (current in the socket). The lighting of the premises and the operation of all electric-type devices occurs precisely through the action of alternating electricity. The main difference between alternating current and direct current is its stronger tendency to transform.

A good example of the action of alternating current can also serve as the effect of turning on fluorescent lamps. So in the process of turning on such a lamp, the movement of charged particles begins to move forward and backward, which explains the effect of alternating current. It is this type of electricity that is considered the most common in everyday life. According to Ohm's law, the current strength is calculated by the formula (for a section of the electrical circuit):

The current strength, therefore, is directly proportional to the voltage $U$, measured in Volts, to the circuit section and inversely proportional to the $R$-resistance of the conductor of the specified section, expressed in Ohms. The calculation of the strength of electricity in a complete circuit is calculated as follows:

$I=\frac(E)(R+r)$, where:

$E$ - electromotive force, EMF, Volt;
$R$ - external resistance, Ohm;
$r$ - internal resistance, Ohm.

The main methods for determining the current strength through instrument systems in practice are as follows:

Magnetoelectric measuring method. Its advantages are high sensitivity and accuracy of readings with low power consumption. This method is applicable only when determining the magnitude of the direct current.
The electromagnetic method consists in finding the strength of currents of alternating and constant types by the process of transformation from an electromagnetic field into a signal of a magnetic modular sensor.
The indirect method is aimed at determining the voltage at a certain resistance using a voltmeter.

Remark 2

In order to find the current strength, in practice, a special device ammeter is often used. Such a device is included in the electrical circuit breaks at the required point for measuring the strength of the electric charge that has passed through the wire section for some time.

When determining the magnitude of the strength of small electricity, milliammeters, microammeters, and also galvanometers are used, which are also connected to a specific place in the circuit where it is necessary to find the current strength. The connection can be made in two ways:

consistent;
parallel.

Determining the current strength that is consumed is considered not as often in demand as measuring voltage or resistance. At the same time, without calculating the physical value of the current strength, it becomes impossible to calculate the power consumption.

Impossible. The concept of current is the basis on which, like a house on a solid foundation, further calculations of electrical circuits are built and new and new definitions are given. The current strength is one of the international quantities; therefore, the universal unit of measurement is Ampere (A).

The physical meaning of this unit is explained as follows: a current of one ampere arises when particles with a charge move along two conductors of infinite length, between which there is a gap of one meter. In this case, arising on each meter section of the conductors is numerically equal to 2 * 10 to the power of -7 Newton. It is usually added that the conductors are located in a vacuum (which allows leveling the influence of the intermediate medium), and their cross section tends to zero (at the same time, the conductivity is maximum).

However, as it usually happens, classical definitions are clear only to specialists who, in fact, are no longer interested in the basics. But a person unfamiliar with electricity will “get confused” even more. Therefore, let us explain what current strength is, literally “on the fingers”. Imagine an ordinary battery, from the poles of which two insulated wires go to the light bulb. A switch is connected to the break of one wire. As is known from the initial physics course, electric current is the movement of particles that have their own. Usually, they are considered to be electrons (indeed, it is the electron that has a unit negative charge), although in reality everything is a little more complicated. These particles are typical for conductive materials (metals), but in gaseous media, ions additionally transfer charge (recall the terms “ionization” and “breakdown of the air gap”); in semiconductors, conductivity is not only electronic, but also hole (positive charge); in electrolytic solutions, the conductivity is purely ionic (for example, car batteries). But back to our example. In it, the current forms the movement of free electrons. Until the switch is turned on, the circuit is open, the particles have nowhere to move, therefore, the current strength is zero. But it is worth "assembling the circuit" as the electrons rush from the negative pole of the battery to the positive, passing through the light bulb and causing it to glow. The force that makes them move comes from the electric field created by the battery (emf - field - current).

Current is the ratio of charge to time. That is, in fact, we are talking about the amount of electricity passing through the conductor per conventional unit of time. You can draw an analogy with water: the more the tap is open, the more water will pass through the pipeline. But if water is measured in liters (cubic meters), then current is measured in the number of charge carriers or, which is also true, in amperes. It's that simple. It is easy to understand that there are two ways to increase the current strength: by removing a light bulb from the circuit (resistance, an obstacle to movement), and also by increasing the electric field created by the battery.

Actually, we have come to how, in the general case, the calculation of the current strength is performed. There are many formulas: for example, for a complete circuit, taking into account the influence of the characteristics of the power supply; for alternating and for multiphase systems, etc. However, all of them are united by a single rule - the famous Ohm's law. Therefore, we present its general (universal) form:

where I is the current, in Amperes; U - voltage at the power supply terminals, in Volts; R is the resistance of the circuit or section, in ohms. This dependence only confirms all of the above: an increase in current can be achieved in two ways, through resistance (our light bulb) and voltage (source parameter).

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 still cannot fully characterize the electric current. 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 a 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. shows a conditional graphical representation of an ammeter and a milliammeter on electrical circuits.

Rice. 1. Symbols of the 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) 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 used up 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.

Do-it-yourself repair of household appliances and electrical wiring requires an understanding of the physical processes of electricity from the home master. But among practitioners there is a category of “forgetful” people.

Especially to remind them, and not just schoolchildren, I have prepared a material on how the current strength is created in a conductor and other various media.

I tried to present it in a slightly simplified and understandable language without complex formulas and conclusions, but in detail. Read, meet, remember.

Under what conditions does an electric current arise and what is the strength of the current in simple words

I draw your attention right away: the definition of electric current does not apply to static, frozen phenomena. It is directly related to the movement, the dynamic state.

It is created not by neutral, but by active particles of a positive or negative electric charge.

And they should not move randomly, like residents of a metropolis during rush hour, but in a directed way. Example: the movement of a mass of cars on a multi-lane road in one direction of a large city.

Have you submitted a picture? Inside the continuous stream, cars are added from the side, some drivers leave the highway for other roads. But these processes do not particularly affect the general movement: the direction remains one-way.

The same goes for the movement of electric charges. Inside metal conductors, current is created by electrons. In their normal state, they move there quite chaotically in all directions.

But it is worth attaching to them an external one with positive and negative potentials at opposite ends of the conductor, as the directed movement of charges begins.

It is the electric current. I pay attention to the last word. It characterizes the flow, movement, movement, dynamics and related processes, but not statics.

It is the magnitude of the applied external force that determines the quality of the directed flow of electrons in one direction. The higher its value, the more current begins to flow through the conductor.

However, here it is necessary to take into account several features related to:

accepted scientific conventions;
the intensity of movement of charges;
Counteraction of the internal environment of the conductor.

In the first case we have to overcome the prevailing historical stereotypes when people mix up the general direction of electrons and electric current.

All scientific calculations are based on the fact that the direction of the current is taken as the movement of charged particles from the plus of the voltage source to its minus.

Electric current inside metals
is created by moving electrons in the opposite direction: they are repelled from the negative pole of the same name and move towards the positive.

Failure to understand this provision can lead to errors. But they are easy to avoid: you just need to remember this feature and use it in calculations or analyzing the actions of electrical circuits.

The intensity of movement of charged particles characterize the amount of their charge flowing through a given area for a certain period of time.

It is called the current strength, denoted by the Latin letter I, calculated by the ratio ∆Q / ∆t.

Here ∆Q is the number of charges passing through a conductor with area S and length ∆L, and ∆t is the calibrated time span.

To increase the current strength, we need to increase the number of charges passing through the conductor per unit time, and to reduce it, we need to decrease it.

Again, look at the term “current strength”, or rather, its first word. I specifically showed a powerful biceps and a smoldering light bulb in the very top picture for comparison.

The power reserve of the energy source can vary from excessive to insufficient for the consumer. And we always need to feed the load optimally. For this, the concept of current strength was introduced.

To evaluate it, the unit of the measurement system is used: ampere, denoted by the Latin letter A.

Theoretically, to evaluate 1 ampere it is necessary:

take two very thin, infinitely long and perfectly even conductors;
place them on a plane strictly parallel to each other at a distance of 1 meter;
pass the same current through them, gradually increasing its value;
measure the force of attraction of the wires and fix the moment when it reaches the value of 2 × 10-7 Newtons.

That's when 1 ampere will begin to flow in the wires.

In practice, no one does this. For measurement, special devices have been created: ammeters. Their designs work in the dimensions of fractional and multiplicity: mi-, micro- and kilo-.

Another definition of the ampere is related to the unit of quantity of electricity: the coulomb (C), which passes through the cross section of a wire in 1 second.

The current strength in any place of a closed electrical circuit where it flows is always the same, and when it breaks, wherever it is, it disappears.

This phenomenon allows you to take measurements in the most convenient places of any electrical circuit.

When a complex branched circuit is created for the flow of several currents, the latter also remain constant in all individual sections.

The third case of environmental opposition is also important. Electrons in the process of movement collide with obstacles in the form of positively and negatively charged particles.

Such collisions are associated with the cost of energy spent on the release of heat. They were generalized by the term and described by physical laws in mathematical form.

The internal structure of each metal has a different resistance to the flow of current. Science has long studied these properties and reduced them to tables, graphs and formulas for electrical resistivity.

When making calculations, we can only use the already verified and prepared information. They can be performed on the basis of the formulas presented by the well-known electrician's cheat sheet.

But it's much easier to use an online Ohm's Law calculator. It will avoid making typical mathematical mistakes.

The most important conclusions from the current strength formulas for the home master

Of practical use is only a complete understanding of the processes of current flow through conductors. At home, we must:

Foresee the current loads on the wiring. This information will help to correctly design it for laying inside your apartment. And if it has already been laid, then it will be necessary to take into account and not exceed the connected capacities.

Eliminate typical errors in the installation of wires and equipment, on which there is a useless loss of electricity energy, excessive heat is created, and damage occurs.

Proper wiring.

Provide a protection system that will automatically protect the household network from accidental damage both inside the circuit and coming from the supply side.

Now I will not go into more detail to decipher each of these four points. I plan to paint them for you in more detail in a series of articles, publish them in the headings of the site. Follow the information or subscribe to the newsletter in order to be aware.

What are the types of electric current in everyday life

The waveform of the currents depends on the operation of the voltage source and the resistance of the medium through which the signal passes. Most often, in practice, the home master has to deal with the following types:

a constant signal generated from batteries or galvanic cells;
sinusoidal, created by industrial generators with a frequency of 50 hertz;
pulsating, formed due to the transformation of various power supplies;
impulse, penetrating into the household network due to lightning discharge into overhead power lines;
arbitrary.

Most often there is a sinusoidal or alternating current: all our devices are powered by it.

Electric current in various environments: what an electrician needs to know

Charged particles under the action of an applied voltage move not only inside metals, as we discussed above using the example of electrons, but also in:

transition layer of semiconductor elements;
liquids of various compositions;
gas environment;
and even inside a vacuum.

All these media are evaluated by the ability to pass current by a term called conductivity. This is the reciprocal of resistance. It is denoted by the letter G, evaluated through the conductivity, which can be found in the tables.

Conductivity is calculated by the formulas:

Current strength in a metal conductor: how it is used in a domestic environment

The ability of the internal structure of metals to influence the conditions of movement of directed charges in different ways is used to implement specific tasks.

Transportation of electrical power

To transmit electrical energy over a long distance, metal conductors of increased cross-section with high conductivity are used: copper or aluminum. The more expensive metals silver and gold work inside complex electronic circuits.

All kinds of designs of wires, cords and cables based on them are reliably operated in home wiring.

heating elements

For heating devices, tungsten and nichrome are used, which have high resistance. It allows you to heat the conductor to high temperatures with the correct selection of the applied power.

This principle was embodied in numerous designs of electric heaters - TEN-ah.

Safety devices

The overestimated current strength in a metal conductor with good conductivity, but a thin section allows you to create fuses used as current protection.

They work normally in the optimal load mode, but quickly burn out during voltage surges, short circuits or overloads.

For several decades, fuses have massively served as the main protection for home wiring. Now they have been replaced by automatic switches. But inside all power supplies, they continue to work reliably.

Current in semiconductors and its characteristics

The electrical properties of semiconductors are highly dependent on external conditions: temperature, light irradiation.

To increase their own conductivity, special impurities are added to the composition of the structure.

Therefore, inside the semiconductor, the current is created due to the intrinsic and impurity conduction of the internal p-n junction.

The charge carriers of a semiconductor are electrons and holes. If the positive potential of the voltage source is applied to the p pole, and the negative potential to n, then the current will flow through the p-n junction due to the movement created by them.

With the reverse application of polarity, the p-n junction remains closed. Therefore, in the picture above, in the first case, a luminous light bulb is shown, and in the second, it is extinguished.

Similar p-n junctions work in other semiconductor designs: transistors, zener diodes, thyristors ...

All of them are designed for the nominal current flow. To do this, marking is applied directly to their body. According to it, they enter the tables of technical reference books and evaluate the semiconductor in terms of electrical characteristics.

Current in liquids: 3 methods of application

If metals have good conductivity, then the medium of liquids can act as a dielectric, conductor, and even a semiconductor. But, the latter case is not for home use.

Insulating properties

Mineral oil of high degree of purification and low viscosity, designed to work inside industrial transformers, has high dielectric properties.

Distilled water also has high insulating properties.

Batteries and Electroplating

If a little salt, acid or alkali is added to distilled water, then it, due to the occurrence of electrolytic dissociation, will become a conductive medium - an electrolyte.

However, one must understand here: the current flowing in metals does not violate the structure of their substance. Destructive chemical processes take place in liquids.

Current in liquids is also created under the action of an applied voltage. For example, when positive and negative potentials from a battery or accumulator are connected to two electrodes dipped in an aqueous solution of some kind of salt.

The solution molecules form positively and negatively charged particles - ions. According to the charge sign, they are called anions (+) and cations (-).

Under the action of an applied electric field, anions and cations begin to move towards the electrodes of opposite signs: the cathode and the anode.

This counter movement of charged particles forms an electric current in liquids. In this case, the ions, having reached their electrode, are discharged on it and form a precipitate.

A good example can be galvanic processes taking place in a solution of copper sulphate CuSO4 with copper electrodes lowered into it.

Copper ions Cu are positively charged - they are anions. At the cathode, they lose their charge and settle in a thin metal layer.

The acid residue SO4 acts as a cation. They come to the anode, are discharged, enter into a chemical reaction with the copper of the electrode, form molecules of copper sulphate, and come back into the solution.

According to this principle, all electrolytes in electroforming work due to ionic conductivity, when the structure of the electrodes changes, and the composition of the liquid does not change.

With this method, thin coatings of precious metals are created on jewelry or a protective layer of various parts against corrosion. The current strength is selected according to the rate of the chemical reaction, depending on the specific environmental conditions.

All batteries work in the same way. Only they still have the ability to accumulate a charge from the applied energy of the generator and give off electricity when discharged to the consumer.

The operation of a nickel-cadmium battery in the mode of charging from an external generator and discharging to an applied load is demonstrated by a simple diagram.

Current in gases: dielectric properties of the medium and conditions for the flow of discharges

An ordinary gas medium has good dielectric properties: it consists of neutral molecules and atoms.

An example is the air atmosphere. It is used as an insulating material even on high-voltage power lines that transmit very high powers.

Bare metal wires are fixed on a support through insulators and separated from the ground loop by their high electrical resistance, and from each other by ordinary air. This is how overhead lines of all voltages work, including 1150 kV.

However, the dielectric properties of gases can be violated due to the influence of external energy: heating to a high temperature or applying an increased potential difference. Only then does the ionization of their molecules occur.

It differs from those processes that occur inside liquids. In electrolytes, molecules are split into two parts: anions and cations. A gas molecule releases an electron during ionization and remains in the form of a positively charged ion.

As soon as the external forces that create the ionization of gases cease to act, the conductivity of the gaseous medium immediately disappears. The discharge of lightning in the air is a short-term phenomenon confirming this position.

The current in gases, in addition to the lightning discharge, can be created by maintaining an electric arc. Spotlights and projectors of bright light, as well as industrial arc furnaces, work on this principle.

Neon and fluorescent lamps use the glow of a glow discharge flowing in a gas medium.

Another type of discharge in gases used in technology is spark. It is created by gas dischargers for measuring the magnitude of large potentials.

Current in a vacuum: how it is used in electronic devices

The Latin word vacuum is interpreted in Russian as emptiness. It is created in a practical way by pumping gases from an enclosed space with vacuum pumps.

There are no carriers of electric charges in a vacuum. They must be introduced into this environment in order to create a current. It uses the phenomenon of thermionic emission, which occurs when the metal is heated.

Electronic lamps work in this way, in which the cathode is heated by a filament. The electrons released from it, under the action of the applied voltage, move towards the anode, form a current in vacuum.

According to the same principle, a cathode ray tube of a kinescope TV, monitor, and oscilloscope was created.

It just added control electrodes to deflect the beam and a screen indicating its position.

In all of the listed devices, the current strength in the conductor of the medium must be calculated, controlled and maintained at a certain level of the optimal mode.

I end with this. A comment section has been made especially for you. It allows you to simply express your own opinion about the article you read.

In previous lessons, we talked about the current in the metal, also discussed the electrical circuit and its components, talked about the direction of the current. However, we did not touch upon such a question as the characteristics with which you can describe the electric current. Probably, all of you have heard about the expression "power surge" and watched the flashing of a light bulb. That is, we understand that electric currents are different, but how can electric currents be compared? What characteristics of the current make it possible to estimate its magnitude and its other parameters? Today we will begin to study the quantities that characterize the electric current, and we will start with such a characteristic as the current strength.

You already know that a metal rod contains a sufficiently large number of electric charge carriers - electrons. It is clear that when no electric current flows through the rod, these electrons move randomly, that is, we can assume that the number of electrons that pass through the section of the rod from left to right is approximately equal to the number of electrons that pass through the same section of the rod from right to left for one and the same time. same time. If we pass an electric current through the rod, then the movement of electrons becomes ordered and the number of electrons that pass through the section of the rod over a period of time increases significantly (meaning the number of electrons that pass in one direction).

Current strength- This is a physical quantity that characterizes the electric current and is numerically equal to the charge passing through the cross section of the conductor per unit time. The current strength is denoted by a symbol and is determined by the formula: , where is the charge passing through the cross section of the conductor in time.

To better understand the essence of the introduced value, let's turn to the mechanical model of the electrical circuit. If you consider the plumbing system in your apartment, it can be strikingly similar to an electrical circuit. Indeed, a pump acts as an analogue of the current source, which creates pressure and supplies water to apartments (see Fig. 1).

Rice. 1. Plumbing system

As soon as it stops working, the water in the taps will disappear. Faucets act as keys to an electrical circuit: when the faucet is open, water flows; when it is closed, it does not. Water molecules act as charged particles (see Fig. 2).

Rice. 2. Movement of water molecules in the system

If we now introduce a value similar to the current strength just introduced, that is, the number of water molecules through the pipe cross section per unit time, then we will actually get the amount of water passing through the pipe cross section in one second - what is often called pressure in everyday life. Accordingly, the greater the pressure, the more water flows out of the tap, similarly: the greater the current strength, the stronger the current and its action.

The unit of current is the ampere. This value is named after the French scientist André-Marie Ampère. Ampere is one of the units of the international system. Knowing the units of current, it is easy to get the definition of the unit of electric charge in SI. Because , then .

Hence, . That is, 1 C is a charge passing through the cross section of the conductor in 1 s at a current strength of 1 A in the conductor. In addition to the ampere, quantities such as milliamp (), microampere ( ), kiloampere (). To imagine what is small and what is high current, we present the following data: for a person, a current of less than 1 mA is considered safe, and a current of more than 100 mA can lead to significant health problems.

Some current values

To understand the magnitude of such a current as 1A, let's look at the following table.

X-ray medical device (see Fig. 3) - 0.1 A

Rice. 3. X-ray medical machine

Pocket flashlight bulb - 0.1-0.3 A

Portable tape recorder - 0.3 A

Bulb in the classroom - 0.5 A

Mobile phone in operation - 0.53 A

TV - 1 A

Washing machine - 2 A

Electric iron - 3 A

Electric milking machine - 10 A

Trolleybus engine - 160-220 A

Lightning - more than 1000 A

In addition, consider the effects of the action of the current that it has on the human body, depending on the strength of the current (the table shows the strength of the current at a frequency of 50 Hz and the effect of the current on the human body).

0-0.5 mA None

0.5-2 mA Loss of sensitivity

2-10 mA Pain, muscle contractions

10-20 mA Increasing effect on muscles, some damage

16 mA Current above which a person can no longer get rid of the electrodes

20-100 mA Respiratory paralysis

100 mA - 3 A Fatal ventricular fibrillation (requires urgent resuscitation)

More than 3 A Cardiac arrest, severe burns (if the shock was brief, then the heart can be resuscitated)

However, most devices are designed for a much higher current strength, so when working with them it is very important to follow some rules. Let us dwell on the main points that need to be remembered by everyone who deals with electricity.

It is forbidden:

1) Touch a bare wire, especially while standing on the ground, damp floor, etc.

2) Use faulty electrical devices.

Collect, fix, disassemble electrical devices without disconnecting them from the power source.

An ammeter is used to measure current. It is indicated by the letter A in a circle in a schematic representation in an electrical circuit. Like any device, the ammeter should not affect the value of the measured value, so it is designed in such a way that it practically does not change the value of the current in the circuit.

Rules to follow when measuring current with an ammeter

1) The ammeter is connected to the circuit in series with the conductor in which it is necessary to measure the current strength (see Fig. 4).

2) The ammeter terminal, near which there is a + sign, must be connected to the wire coming from the positive pole of the current source; a terminal with a minus sign - with a wire coming from the negative pole of the current source (see Fig. 5).

3) Do not connect the ammeter to a circuit where there is no current consumer (see Fig. 6).

Rice. 4. Serial connection of the ammeter

Rice. 5. Correctly connected terminal +

Rice. 6. Incorrectly connected ammeter

Let's look at the operation of the ammeter live. Before us is an electrical circuit, which consists of a current source, an ammeter, which is connected in series, and a light bulb, which is also connected in series (see Fig. 7).

Rice. 7. Electrical circuit

If we turn on the current source now, we can observe the strength in the circuit using an ammeter. At first, it indicates 0 (that is, there is no current in the circuit), and now we see that the current strength has become almost 0.2 A (see Fig. 8).

Rice. 8. The flow of current in the circuit

If we change the current in the circuit, we will see that the current strength will increase (it will become approximately 0.26 A), and at the same time the light will light up brighter (see Fig. 9), that is, the greater the current in the circuit, the brighter the light will burn .

Rice. 9. The current in the circuit is greater - the bulb burns brighter

Types of ammeters

Electromagnetic, magnetoelectric, electrodynamic, thermal and induction ammeters have become widespread.

AT electromagnetic ammeters ( see fig. ten ) the measured current, passing through the coil, draws in its soft iron core with a force that increases with increasing current strength; at the same time, the arrow mounted on the same axis with the core rotates and indicates the current strength in amperes on a graduated scale.

Rice. 10. Electromagnetic ammeter

AT thermal ammeters(see Fig. 11), the measured current is passed through a stretched metal thread, which, due to heating by the current, lengthens and sags, while turning the arrow indicating the current strength on the scale.

Rice. 11. Thermal ammeter

AT magnetoelectric ammeter(see Fig. 12) under the influence of the interaction of the measured current passed through the wire wound on a light aluminum frame and the magnetic field of a permanent horseshoe magnet, the frame, together with the pointer, rotates by a larger or smaller angle depending on the magnitude of the current.

Rice. 12. Magnetoelectric ammeter

AT electrodynamic ammeters(without iron) (see Fig. 13) the measured current is passed in series through the winding of the fixed and moving coils; the latter, due to the interaction of the current passing through it with the current in the fixed coil, rotates along with the arrow indicating the strength of the current.

Rice. 13. Electrodynamic ammeter

AT induction appliances(see Fig. 14) a movable metal disk or cylinder is subjected to a traveling or rotating field created by fixed coils connected by a magnetic system.

Rice. 14. Induction ammeter

Thermal and electrodynamic ammeters are suitable for measuring both direct and alternating currents, electromagnetic - for direct current and induction - for alternating

Problem solving

Consider the solution of several typical problems on this topic.

Task 1

How many electrons pass through the cross section of the conductor every second if a current of 0.32 A flows through it?

Decision

We know not only the current strength I = 0.32 A, the time t = 1 s, but also the charge of one electron: .

Let's use the definition of the current strength: , and the charge that passes per unit time modulo is equal to the sum of the modules of the charges of electrons that pass through the section in 1 s. We get . Where .

We check the units of the desired value: .

Answer:.

Task 2

Why does an ammeter, which shows the strength of the current flowing through the wire that connects the car battery to the on-board electrical network, have both positive and negative values \u200b\u200bon the scale?

Decision

The fact is that two processes take place in a car battery: sometimes it charges (see Fig. 15), that is, it receives a charge (charges move in one direction), and sometimes it feeds the on-board network, that is, it gives off a charge (respectively, charges move in the other direction) (see Fig. 16). In these two cases, the current strength will differ in sign.

Rice. 15. Charging the battery