This is direct current. D.C

Right at the start, let's give short definition electric current. An electric current is called an ordered (directed) movement of charged particles. Current is the movement of electrons in a conductor, voltage- this is what sets them (electrons) in motion.

Now consider such concepts as direct and alternating current and identify their fundamental differences.

The difference between direct current and alternating current

The main feature of a constant voltage is that it is constant both in magnitude and in sign. D.C, "flows" in one direction all the time. For example, by metal wires from the positive terminal of the voltage source to the negative one (in electrolytes it is created by positive and negative ions). The electrons themselves move from minus to plus, but even before the discovery of the electron, they agreed to consider that the current flows from plus to minus and still adhere to this rule in calculations.

What is the difference between alternating current (voltage) and direct current? From the name itself it follows that it changes. But - how exactly? An alternating current changes both its magnitude and the direction of electron movement over a period. In our household sockets, this is a current with sinusoidal (harmonic) oscillations with a frequency of 50 hertz (50 oscillations per second).

If we consider a closed circuit using the example of a light bulb, then we get the following:

with direct current, electrons will flow through the bulb always in the same direction from (-) minus to (+) plus
with a variable, the direction of electron movement will change depending on the frequency of the generator. i.e. if in our network the frequency alternating current 50 hertz (Hz), then the direction of movement of electrons will change 100 times in 1 second. Thus, + and - in our socket change places a hundred times per second (which is why we can plug electrical plug into the outlet "upside down" and everything will work).

Variable voltage in our household socket changes sinusoidally. What does it mean? The voltage increases from zero to a positive amplitude value (positive maximum), then decreases to zero and continues to decrease further - to a negative amplitude value (negative maximum), then increases again, passing through zero and returns to a positive amplitude value.

In other words, with alternating current, its charge is constantly changing. This means that the voltage is either 100%, then 0%, then again 100%. It turns out that in a second, electrons change their direction of movement and their polarity 100 times, from positive to negative (remember that their frequency is 50 hertz - 50 periods or oscillations per second?).

The first electrical networks were direct current. Several problems were associated with this, one of them was the complexity of the design of the generator itself. And the alternator has a simpler design, and therefore is simple and cheap to operate.

The fact is that the same power can be transmitted by high voltage and low current, or vice versa: low voltage and high current. How more current, the larger the wire cross-section is needed, i.e. the wire should be thicker. For voltage, the thickness of the wire is not important, if the insulators were good. AC (as opposed to DC) is simply easier to convert.

And this is convenient. So, through a wire of a relatively small cross section, a power plant can send five hundred thousand (and sometimes up to one and a half million) volts of energy at a current of 100 amperes with virtually no loss. Then, for example, the transformer of the city substation will "take" 500,000 volts at a current of 10 amperes and "give" 10,000 volts at 500 amperes to the city network. And regional substations are already converting this voltage into 220/380 volts at a current of about 10,000 amperes, for the needs of residential and industrial areas of the city.

Of course, the scheme is simplified and refers to the entire set of district substations in the city, and not any one in particular.

Personal Computer(PC) works on a similar principle, but in the opposite direction. It converts alternating current to direct current and then, using it, lowers its voltage to the values \u200b\u200bnecessary for the operation of all components inside.

At the end of the 19th century, worldwide electrification may well have gone the other way. Thomas Edison (it is believed that it was he who invented one of the first commercially successful incandescent lamps) actively promoted his idea of \u200b\u200bdirect current. And if not for the research of another outstanding person, which proved the effectiveness of alternating current, then everything could be different.

The Serbian genius Nikola Tesla (who worked for Edison for a time) was the first to design and build a polyphase alternating current generator, proving its efficiency and superiority over similar designs that worked with a constant power source.

Now let's look at the "habitats" of direct and alternating current. Permanent, for example, is in our phone battery or batteries. Chargers transform alternating current from the network into direct current, and already in this form it ends up in its storage places (batteries).

DC voltage sources are:

conventional batteries used in various devices (flashlights, players, watches, testers, etc.)
various batteries(alkaline, acid, etc.)
DC generators
other special devices, e.g. rectifiers, converters
emergency power sources (lighting)

For example, urban electric transport operates on a direct current of 600 volts (trams, trolleybuses). For the subway, it is higher - 750-825 Volts.

AC voltage sources:

generators
various converters (transformers)
domestic electrical networks(home sockets)

About how and with what to measure the constant and AC voltage we talked with you here, and finally (to all those who read the article to the end) I want to tell a little story. My boss voiced it to me, and I will retell it from his words. Painfully, it fits our topic today!

He once went on a business trip with our directors to a neighboring town. Establish friendly relations with local IT specialists :) And right next to the highway there is such a wonderful place: a spring with clean water. Near all be sure to stop and collect water. It's kind of a tradition.

The local authorities, having decided to ennoble this place, did everything with the latest technology: they dug a large rectangular hole right under the fontanel, lined it with bright tiles, made an overflow, LED lighting, and the pool turned out. Further more! The spring itself was "packed" in marked granite chips, given it a noble shape, the icon over the vent was walled up under glass - Holy place, it means!

And the final touch - we put a water supply system on a photocell. It turns out that the pool is always full and "gurgles" in it, and in order to draw water directly from the fontanel, you need to bring your hands with a vessel to the photocell and from there it "flows" :)

I must say that on the way to the source, our boss told one of the directors how cool it was: new technologies, Wi-Fi, photocells, retinal scanning, etc. The director was a classic technophobe, so he was of the opposite opinion. And so, they drive up to the fontanel, put their hands where they should, but the water does not flow!

They do this and that, but the result is zero! It turned out that there was stupidly no voltage in the electrical network that fed this shaitan system :) The director was "on horseback"! He released several "control" phrases about all these n ... x technologies, the same n ... x elements, all machines in general and this particular one in particular. I scooped up the canister directly from the pool and went to the car!

So it turns out, we can set up anything, "raise" a heaped server, provide the best and most demanded service, but, anyway, the most main man- this is Uncle Vasya, an electrician in a padded jacket, who with one movement of his hand can organize a complete skipped of all this technical power and grace :)

So remember: the main thing is high-quality power supply. Good (uninterruptible power supply) and stable voltage in sockets, and everything else will follow :)

For today, we have everything and until the next articles. Take care of yourself! Below - short video on the topic of the article.

D.C (direct current) –it is the ordered movement of charged particles in one direction. In other words
quantities characterizing electricity, such as voltage or current, are constant in both value and direction.

In a direct current source, for example in a conventional AA battery, electrons move from minus to plus. But historically, the direction from plus to minus is considered to be the technical direction of the current.

For direct current, all the basic laws of electrical engineering apply, such as Ohm's law and Kirchhoff's laws.

Story

Initially, direct current was called - galvanic current, since it was first obtained using a galvanic reaction. Then, at the end of the nineteenth century, Thomas Edison made attempts to organize the transmission of direct current through power lines. At the same time, the so-called "war of currents", in which there was a choice as the main current between alternating and direct. Unfortunately, direct current “lost” this “war” because, unlike alternating current, direct current carries big losses in power when transmitting over distances. Alternating current is easy to transform and therefore transmit over long distances.

DC sources

DC sources can be batteries, or other sources in which the current appears due to chemical reaction(for example, a finger battery).

Also, DC sources can be a DC generator, in which the current is generated due to
phenomenon of electromagnetic induction, and then rectified by means of a collector.

Direct current can be obtained by rectifying alternating current. For this, there are various rectifiers and converters.

Application

Direct current, widely used in electrical diagrams and devices. For example, at home, most appliances such as a modem or Charger for mobile, operate on direct current. The car's alternator generates and converts direct current to charge the battery. Any portable device is powered by a DC source.

In industry, DC is used in DC machines such as motors or generators. In some countries there are high voltage DC power lines.

Direct current has also found its use in medicine, for example in electrophoresis, a treatment procedure using electric current.

In railway transport, in addition to alternating current, direct current is also used. This is due to the fact that traction motors, which have more rigid mechanical characteristics than asynchronous, are DC motors.

Impact on the human body

Direct current, unlike alternating current, is safer for humans. For example, a lethal current for a person is 300 mA if it is a constant current, and if it is an alternating current with a frequency of 50 Hz, then 50-100 mA.

Direct current is a current that has one direction and one magnitude.

Graphically, direct current is a straight line.

The nature of the electric current

Copper, aluminum, steel, silver and other metals are called conductors. They have many free electrons. Therefore, they are good conductors of electricity. They are used as wires and are called conductors.

Conductors have many free electrons. If the electrical circuit is open, then the free electrons in the conductors are in chaotic motion.

Let's close the electrical circuit. The current source forms in electrical circuit electric field which interacts with electric fields every electron. As a result, free electrons will move in one direction.

Conclusion:Electric current in conductors is a directed flow of free electrons.

Direction of electric current

Electric current is a closed flow of electrons. It has neither beginning nor end.

The question arises from where to show the electric current circuit.

There can be many consumers in the circuit, and the current source is usually one; therefore, it is customary to show the current circuit from the output of the current source to another output.

There are two directions of electric current

1. True direction. This is the direction from minus the source to its plus. Electrons go in this direction, so the direction is called true.

2.Technical direction

The technical direction is the opposite of the true one. This is the direction from the plus of the source to its minus.

The technical direction arose historically. When people did not know the nature of the current, they set everything to show the same from plus to minus. When we learned that the current is a flow of electrons moving from minus to plus, we decided to leave this direction and call it technical and use it in technology.

The question arises when and which direction to use.

When it comes to the nature of the current, you need to use the true direction. In other cases, use the technical direction.

Will there be misunderstandings.

It will not be so, since in technology it is the electrical circuit that matters and not the direction of the current in it.

direct current called an electric current that does not change in time in direction and value.

DC sources are galvanic cells, batteries and DC generators.

Electric current has a certain direction. The direction of movement of positively charged particles is taken as the direction of the current. If the current is formed by the movement of negatively charged particles, the direction of the current is considered opposite to the direction of movement of these particles.

The concept of current strength is used to quantify the current in an electrical circuit. The current strength is the amount of electricity Q flowing through the cross section of the conductor per unit time.

If during the time t through the cross section of the conductor the amount of electricity Q has moved, then the current strength is I \u003d Q / t.

The unit of current strength is ampere (A).

The current density A / mm 2 is the ratio of the current strength I to the cross-sectional area F of the conductor:

In a closed electrical circuit, current arises under the action of a source electrical energy, which creates and maintains a potential difference on its clamps; measured in volts (V).

An important characteristic of an electrical circuit is resistance; the strength of the current in the conductor at a given voltage depends on this value. The resistance of a conductor is a kind of measure of the resistance of a conductor to the flow of electric current in it. Electrical resistance is measured in ohms (Ohm). Widely used and the reciprocal of the resistance (1 / Ohm), which is called conductivity.

The resistance depends on the material of the conductor, its length l and cross-sectional area F, i.e.

Where ρ is the resistivity of the conductor.

Resistivity in SI units is numerically equal to the resistance of a conductor having the shape of a cube with an edge of 1 m, if the current passes between two opposite faces of the cube.

The resistance of conductors changes as their temperature changes. As the temperature rises, the resistance of metal conductors increases. The resistance of coal, solutions and melts of salts and acids decreases with increasing temperature.

Denoting through R 0 the resistance of the conductor at a temperature of 0 ° C, we obtain for the resistance at any temperature the formula R \u003d R 0 (l + αt), where α is the thermal coefficient of resistance, showing the relative increment resistivity when the conductor is heated by 1 ° C.

This property is used in wire temperature sensors.

The relationship between the potential difference (voltage) at the terminals of an electrical circuit, resistance and current in the circuit is expressed by Ohm's law.

According to Ohm's law for a section of a homogeneous circuit, the current strength is directly proportional to the value of the applied voltage, i.e. I \u003d U / R, where U is the voltage at the circuit terminals B; R - resistance, Ohm; I - current strength, A.

In practice, parallel, series and mixed connections of electrical circuit elements are used. At parallel connection elements, such as resistors, their conclusions are connected to common nodal points and each resistor is turned on for a voltage applied to nodal points A and B (Fig. 1).

The total resistance of the circuit is determined by the formula: 1 / R 0 \u003d 1 / R 1 +1 / R 2 +1 / R 3

At serial connection elements of electrical targets are switched on one after another, i.e., the beginning of the next one is connected to the end of the previous one (Fig. 2).

The electric current in a circuit with a serial connection is common to all elements.

The total resistance of the circuit when resistors are connected in series is calculated by the formula R 0 \u003d R 1 + R 2 + R 3

The formulas above can be used to calculate the total resistance of any number of resistors connected in parallel or in series.

The work done by an electric current per unit of time (second) is called power and is denoted by the letter P. This value is characterized by the intensity of the work done by the current. Power is determined by the formula P=W/t=UIt/t=UI.

The unit of measure for power is the watt (W). A watt is the power at which one joule of work is done uniformly per second. Then the formula above can be written as follows: W=Pt.

Multiple units of power: kilowatt-1 kW = 1000 W and megawatt-1 MW = 1,000,000 W.

The unit of measurement of electrical energy - kilowatt-hour (kWh) is the work done when constant power at 1 kW for 1 hour.

The expression for the power of an electric current can be converted by replacing, based on Ohm's law, the voltage U = IR. As a result, we obtain three expressions for the power of the electric current

P=UI = I 2 R=U 2 /R

Of great practical importance is the fact that the same power of electric current can be obtained at low voltage and great strength current or high voltage and low current.

Electric current flowing through a conductor heats it. The amount of heat released in the conductor is determined by nj formula Q-I 2 Rt.

This relationship is called the Joule-Lenz law.

Wires are usually electrical insulation, which worsens the conditions for cooling the current-carrying core. In addition, the insulation, depending on the type of material from which it is made, can withstand a certain (permissible) heating temperature. The number of wires and the way they are laid also significantly affect the conditions for their cooling.

When designing electrical wires choose such sections and brands of wires so that their temperature does not exceed permissible values. The minimum wire cross-section for a given current strength is determined from the table of long-term permissible current loads on wires and cables. These tables are given in electrical reference books and in the "Electrical Installation Rules" (PUE).

Based on Ohm's law and the Joule-Lenz law, it is possible to analyze the phenomenon that occurs when conductors are directly connected to each other, supplying electric current to the load. Noteworthy is the phenomenon in which the current flows in a shorter way, bypassing the load (short circuit).

Figure 3 shows the connection diagram electric lamp incandescent in electrical network. If the resistance of this lamp is R \u003d 484 Ohm, and the mains voltage is U \u003d 220V, then the current in the lamp circuit in accordance with the equation

Consider the case where the wires leading to an incandescent lamp are connected through a very small resistance, such as a thick metal rod. In this case, the circuit current, passing to point A, branches along two paths: one, most of it, will go along the path with low resistance - to the metal rod, and the other, small part of the current - along the path with high resistance - to the incandescent lamp.

In reality, when short circuit the mains voltage will be less than 220 V, since a large current in the circuit will cause big fall voltage and therefore the current flowing through the metal rod will be somewhat less. But nevertheless, this current will be many times greater than the current that previously flowed through the circuit.

In accordance with the dependence Q=I 2 Rt, the current passing through the wires generates heat, and the wires heat up. In our example, the cross section of the wires was designed for a small current - 0.455 A. When connecting the wires in a shorter way, bypassing the load, a very large current flows through the circuit - 22,000 A. Such a current will cause a huge amount of heat to be released, which will lead to charring and ignition of the insulation wires, melting of the wire material, damage to electrical measuring instruments, melting of the contacts of switches, knife switches, etc. The source of electrical energy supplying such a circuit can also be damaged. Overheating wires can cause a fire.

Each electrical wiring designed for a specific current.

The emergency mode of operation of the circuit, when, due to a decrease in its resistance, the current in it increases sharply compared to the normal one is called a short circuit.

Due to the dangerous, destructive and sometimes irreparable consequences of a short circuit, certain conditions must be observed during installation and operation. electrical installations. The main ones are the following:

1. The insulation of the wires must be suitable for the mains voltage and working conditions.
2. The cross section of the wires must be such that their heating under normal load does not reach a dangerous value.
3. The laid wires must be protected from mechanical damage.
4. Connections and branches should be as well insulated as wires.
5. Wires must be laid through walls, ceilings and floors so that they are protected from mechanical and chemical damage, dampness and do not touch each other.

To avoid a sudden, dangerous increase in current in the electrical circuit during its short circuit, the circuit is protected by fuses or maximum current relays.