Constant electric current is the movement of charged particles in a certain direction. That is, its stress or force (characterizing quantities) have the same meaning and direction. This is how direct current differs from alternating current. But let's look at everything in order.

The history of the emergence and "war of currents"

Direct current used to be called galvanic due to the fact that it was discovered as a result of a galvanic reaction. tried to transmit it through electric transmission lines. At that time, there were serious disputes between scientists on this issue. They even got the name "war of currents". The question of the choice as the main, variable or constant was decided. The "fight" was won by an alternate form, since the permanent one suffers significant losses, being transmitted at a distance. But transforming the alternating form is not difficult, this is how direct current differs from alternating current. Therefore, the latter is easy to transmit even over great distances.

Sources of direct electric current

Batteries or other devices can serve as a source, where it occurs through a chemical reaction.

These are generators, where it is obtained as a result, and after that it is rectified at the expense of the collector.

Application

In various devices, direct current is used quite often. For example, many household appliances, chargers and car generators work with it. Any portable apparatus is powered by a source that produces a permanent view.

It is used commercially in motors and batteries. And in some countries, they are equipped with high-voltage power lines.

In medicine, health procedures are carried out using direct electric current.

On the railway (for transport), both variable and permanent types are used.

Alternating current

Most often, however, it is used. Here the average value of force and stress for a certain period is equal to zero. In magnitude and direction, it is constantly changing, and with equal intervals of time.

To induce alternating current, generators are used in which, during electromagnetic induction, this occurs.This is done with the help of a magnet rotating in a cylinder (rotor) and a stator made in the form of a fixed core with a winding.

Alternating current is used in radio, television, telephony and many other systems due to the fact that its voltage and power can be converted, almost without losing energy.

It is widely used in industry, as well as for lighting purposes.

It can be single-phase and multi-phase.

Which changes according to the sinusoidal law, is single-phase. It changes over a certain period of time (period) in magnitude and direction. The AC frequency is the number of cycles per second.

In the second case, the most widespread is the three-phase variant. This is a system of three electrical circuits that have the same frequency and EMF, out of phase by 120 degrees. It is used to power electric motors, ovens, lighting fixtures.

Many developments in the field of electricity and their practical application, as well as the impact on high frequency alternating current, humanity owes to the great scientist Nikola Tesla. Until now, not all of his works, left to posterity, are known.

How does direct current differ from alternating current and what is its path from source to consumer?

So, an alternating current is called a current that can change in direction and magnitude for a certain time. The parameters that are paid attention to are frequency and voltage. In Russia, in household electrical networks, alternating current is supplied with a voltage of 220 V and a frequency of 50 Hz. The frequency of an alternating current is the number of changes in the direction of particles of a given charge per second. It turns out that at 50 Hz it changes its direction fifty times, in what way the direct current differs from the alternating one.

Its source is sockets to which household appliances are connected under different voltages.

Alternating current begins its movement from power plants, where there are powerful generators, from where it comes out with a voltage of 220 to 330 kV. Then it goes into which are located near houses, businesses and other structures.

In the substation, the current flows under a voltage of 10 kV. There it is converted into a three-phase voltage of 380 V. Sometimes with such an indicator, the current goes directly to objects (where powerful production is organized). But basically it is reduced to 220 V.

Transformation

It is clear that we receive alternating current in the sockets. But often electrical appliances need a constant look. For this purpose, special rectifiers are used. The process consists of the following steps:

• connecting a bridge with four diodes with the required power;
• connecting a filter or capacitor to the output from the bridge;
• connection of voltage stabilizers to reduce ripple.

Conversion can occur both from AC to DC, and vice versa. But the latter case will be much more difficult to implement. You will need inverters, which, among other things, are not cheap at all.

A long time ago, electric current was invented by scientists. The first invention was permanent. But later, conducting experiments in his laboratory, Nikola Tesla invented alternating current. There were and still are many differences between them, according to which one of them is used in low-current equipment, and the other has the ability to overcome various distances with little losses. But much depends on the magnitude of the currents.

AC and DC current: difference and features

The difference between alternating current and direct current can be understood from the definitions. In order to better understand the principle of operation and features, it is necessary to know the following factors.

The main differences are:

• Movement of charged particles;
• Mode of production.

Variable is a current in which charged particles are able to change the direction of movement and the value in certain time... The main parameters of alternating current include its voltage and frequency.

Currently, public electrical networks and various objects use alternating current, with a specific voltage and frequency. These parameters are determined by equipment and devices.

Note! In household electrical networks, a current of 220 volts and a clock frequency of 50 Hz is used.

The direction of motion and the frequency of charged particles in direct current are unchanged. This current is used for power supply by various household devices such as TVs and computers.

Due to the fact that alternating current is simpler and more economical in terms of the method of production and transmission over various distances, it has become the basis for the electrification of facilities. Alternating current is produced at various power plants, from which, through conductors, it is supplied to the consumer.

Direct current, obtained by converting alternating current or by chemical reactions (for example, an alkaline battery). For conversion, current transformers are used.

What level of stress is acceptable for a person: features

In order to know what values ​​of electric current are permissible for a person, corresponding tables have been compiled, which indicate the values ​​of alternating and direct current and time.

Electric current exposure parameters:

• Power;
• Frequency;
• Time;
• Relative humidity.

The permissible touch voltage and current that flow through the human body in various modes of electrical installations do not exceed the following values.

Alternating current 50 Hz, should be no more than 2.0 Volts and a current strength of 0.3 mA. A current with a frequency of 400 Hz, a voltage of 3.0 Volts and a current strength of 0.4 mA. Constant current with voltage 8 and current strength 1 mA. Safe exposure to current with such indicators, up to 10 minutes.

Note! If electrical work is carried out at elevated temperatures and high relative humidity, these values ​​are reduced by three times.

In electrical installations with voltages up to 100 Volts, which are solidly grounded, or the neutral is insulated, safe touch currents are as follows.

Alternating current 50 Hz with voltage spread from 550 to 20 Volts and current from 650 to 6 mA, alternating current 400 Hz with voltage from 650 to 36 Volts, and direct current from 650 to 40 Volts, should not affect the human body in the range from 0.01 to 1 second.

Dangerous alternating current for humans

It is believed that alternating electric current is the most dangerous for human life. But this is on condition, if you do not go into details. Much depends on various quantities and factors.

Factors influencing hazardous exposure:

• Duration of contact;
• The path of the electric current;
• Current and voltage;
• What body resistance.

According to the rules of the PUE, the most dangerous current for a person is alternating with a frequency that varies from 50 to 500 Hz.

It is worth noting that provided that the current strength does not exceed 9 mA, then anyone can get rid of the current-carrying part of the electrical installation himself.

If this value is exceeded, then in order to get rid of the effects of electric current, a person needs strong help. This is due to the fact that the alternating current is much more capable of exciting nerve endings and causing involuntary muscle cramps.

For example, when touching a live part of the device with the inside of the palm, the muscle spasm will clench the fist more over time.

Why else is alternating current more dangerous? With the same values ​​of the current strength, the alternating one has several times stronger effect on the body.

Since the alternating current affects the nerve endings and muscles, it is worthwhile to understand that this also affects the work of the heart muscle. From which it follows that upon contact with alternating current, the risk of death increases.

An important indicator is the resistance of the human body. But with an AC shock at high frequencies, the body's resistance is significantly reduced.

What is the amount of direct current dangerous to humans?

Dangerous to humans, direct current can also be. Of course variable, ten times more dangerous. But if we consider currents in different quantities, then a constant can be much more dangerous than an alternating one.

The impact of direct current on a person is shared by:

• 1 threshold;
• 2 threshold;
• 3 threshold.

When exposed to direct current of the first threshold (tangible current), hands begin to tremble slightly, and a slight tingling sensation appears.

The second threshold (not letting out current), in the range from 5 to 7 mA, is the lowest value at which a person cannot free himself from the conductor on his own.

This current is considered not dangerous, since the resistance of the human body is higher than its values.

The third threshold (fibrillation), at values ​​from 100 mA and above, the current strongly affects the body and internal organs. In this case, the current, at these values, can cause a chaotic contraction of the heart muscle and lead to its stopping.

The strength of the impact is influenced by other factors. For example, dry human skin has a resistance of 10 to 100 kOhm. But if the touch occurs with a wet surface of the skin, then the resistance is significantly reduced.

There are two kinds of currents in electricity - direct and alternating. Devices also require one or the other type of current to supply power. The possibility of their work depends on this, and sometimes their integrity after being connected to the wrong power supply. We will describe the difference between alternating current and direct current in this article, giving a short answer in the most simple words.

Definition

Electric current is the directional movement of charged particles. This is the definition from a physics textbook. In simple words, it can be translated so that its components always have some direction. Actually, this direction is the defining one in today's conversation.

Alternating current (AC) differs from direct current (DC) in that the latter has electrons (charge carriers) always moving in the same direction. Accordingly, the difference between alternating current is that the direction of movement and its strength depend on time. For example, in an outlet, the direction and magnitude of the voltage, respectively, the current strength, changes according to a sinusoidal law with a frequency of 50 Hz (the polarity between the wires changes 50 times per second).

For dummies in electrics, so to speak, we will depict this on a graph, where polarity and voltage are shown along the vertical axis, and time along the horizontal:

The red line shows a constant voltage, it remains unchanged over time, except that it changes when switching a powerful load or short circuit. Green waves show sinusoidal current. You can see that it flows in one direction or the other, in contrast to direct current, where electrons always flow from minus to plus, and the path from plus to minus is chosen as the direction of movement of the electric current.

Simply put, the difference in these two examples is that the constant always has plus and minus on the same wires. If we talk about a variable, then the concepts of phase and zero are used in power supply. If we consider by analogy with a constant, then the phase and zero are a plus and a minus, only the polarity changes 50 times per second (in the USA and a number of other countries 60 times per second, and in airplanes more than 400 times).

Origin

The difference between AC and DC lies in their origin. DC current can be obtained from galvanic cells such as batteries and accumulators.

It can also be obtained using a dynamo - this is an outdated name for a DC generator. By the way, with their help, energy was generated for the first power grids. We talked about this in the article about, in the notes about the war of ideas between Tesla and Edison. Later, this was the name of small generators for powering bicycle headlights.

Alternating current is also produced with the help of generators, nowadays mainly three-phase.

Also, both voltages can be obtained using semiconductor converters and rectifiers. So you can rectify the alternating current or get it by converting the direct current.

DC Formulas

The difference between change and constant are also formulas for calculating the processes occurring in the chain. So the resistance is calculated for a section of a circuit or for a complete circuit:

E = I / (R + r)

Power is also easy to calculate:

AC formulas

In the calculations of alternating current circuits, the difference in the formulas is due to the difference in the processes taking place in the capacitors and inductors. Then the Ohm's law formula will be for active resistance.

Constant and variable then To

What is the difference between direct current from alternating

In a previous article, what is electric current you learned how the ordered movement of electrons in a closed circuit occurs. Now, I will tell you what an electric current is. Electric current is constant and alternating. What is the difference between alternating current and direct current? DC characteristics.

D.C

Direct Current or DC so in English means an electric current that does not change the direction of movement for any length of time and always moves from plus to minus. In the diagram, it is indicated as plus (+) and minus (-), on the case of a device operating on direct current, a designation is applied in the form of one (-) or (=) stripes. An important feature of direct electric current is the ability to accumulate it, i.e. accumulation in batteries or receiving it through a chemical reaction in batteries. Many modern portable electrical devices work using the accumulated direct current electric charge, which is located in the accumulators or batteries of these very devices.

Alternating current

(Alternating Current) or AC an English abbreviation denoting a current that changes its direction and magnitude over a time interval. On electrical circuits and housings of electrical devices operating from alternating current, the alternating current symbol is designated as a segment of a sinusoid "~". If we talk about alternating current in simple words, then we can say that if a light bulb is connected to an alternating current network, the plus and minus on its contacts will change places with a certain frequency or otherwise, the current will change its direction from direct to reverse. In the figure, the opposite direction is the area of ​​the chart below zero.

Now let's figure out what frequency is. Frequency is the period of time during which the current performs one complete oscillation, the number of complete oscillations in 1 s is called the frequency of the current and is denoted by the letter f. Frequency is measured in hertz (Hz). In industry and everyday life in most countries, alternating current with a frequency of 50 Hz is used.This value shows the number of changes in the direction of the current in one second to the opposite and return to its original state. In other words, in an electrical outlet that is in every home and where we turn on irons and vacuum cleaners, plus and minus on the right and left terminals of the outlet will change places with a frequency of 50 times per second - this is the frequency of the alternating current. Why do you need such a “changeable” alternating current, why not use only direct current? This is done in order to be able to obtain the required voltage in any quantity by using transformers without any special losses. The use of alternating current allows the transmission of electricity on an industrial scale over long distances with minimal losses.

The voltage supplied by powerful generators of power plants is about 330,000-220,000 volts. Such voltage cannot be applied to houses and apartments, it is very dangerous and difficult from the technical point of view. Therefore, alternating electric current from power plants is fed to electrical substations, where there is a transformation from high voltage to the lower one that we use.

AC to DC conversion

From alternating current, it is possible to obtain direct current, for this it is enough to connect the alternating current network to a diode bridge or, as it is also called, a “rectifier”. From the name "rectifier" it is perfectly clear what the diode bridge does, it rectifies the sinusoid of the alternating current in a straight line, thereby forcing the electrons to move in one direction.

what is a diode and how does a diode bridge work, you can find out in my next articles.

Electricity is a type of energy transmitted by the movement of electrons through a conductive material. For example, metals are highly electrically conductive materials and allow electrons to move easily. Inside a conductive material, electrons can move in one or more directions.

The concept of direct and alternating current

What is direct current is determined from the nature of the movement of electric charges. Similarly, you can establish what an alternating current is.

1. When the flow of electric charges is set in one direction, it is considered constant current;
2. When an electron stream changes direction and intensity over time, it is called alternating current. Moreover, the changes are cyclical, according to a sinusoidal law.

Most modern power grids use alternating electrical current, which is generated in power plants by appropriate generators.

Direct current (DC) is generated by batteries, fuel cells and photovoltaic modules. There are also DC generators. Another way to get it is conversion from single-phase and three-phase alternating current (AC) using rectifiers.

Otherwise, AC can be obtained from DC using inverters, although the technology is somewhat more complicated.

Story

In nature, electricity is relatively rare: it is generated by only a few animals and exists in some natural phenomena. In their quest to artificially generate a stream of electrons, scientists realized that it was possible to force electrons to pass through a metal wire or other conductive material, but only in one direction, since they are repelled from one pole and attracted to the other. This is how batteries and DC generators were born. The invention is attributed mainly to Thomas Edison.

At the end of the 19th century, another famous scientist, Nikola Tesla, was developing methods for producing alternating current. The main reasons for work in this area were the discovered shortcomings of direct current when transmitting electricity over long distances. It turned out that for alternating current it is much easier to increase the voltage of the transmission lines, thereby reducing losses and making it possible to transport large amounts of electrical energy, and it was not feasible to effectively increase the voltage on direct current lines in those days.

To generate alternating current, Tesla used a rotating magnetic field. If the MF changes direction, the direction of the electron flow also changes, and an alternating current is generated.

The change in direction in the electron flow occurs very quickly, many times per second. Frequency measurements are made in hertz (equal to cycles per second). Thus, an alternating current with a frequency of 50 Hz can be represented as performing 50 cycles per second. In each cycle, the electrons change direction and return to their original direction, so the flow of electrons changes direction 100 times per second.

Comparative characteristics of direct and alternating currents

The difference between the two types of currents lies in their nature and the resulting properties.

The difference between direct current and alternating current:

1. With an alternating current, the direction and intensity of the electron flow change, with a constant current, it is unchanged;
2. DC frequency cannot exist. This concept applies only to alternating current;
3. The poles (plus and minus) are always the same in a DC circuit. In an alternating current circuit, the positive and negative poles change at periodic intervals;
4. In AC transmission, the voltage is easily converted and transported with an acceptable level of loss.

Reversing the polarity of the DC connection can cause permanent damage to the devices. To avoid this, pole markings are usually put on equipment. Similarly, the contacts differ in the traditional use of a metal spring for the negative pole and a plate for the positive. In devices with rechargeable batteries, the rectifier transformer has an output so that the connection is made only one way, which prevents polarity reversal.

In large-scale installations, such as telephone exchanges and other telecommunications equipment, where there is a centralized DC distribution, special connecting and protective elements are used,

Direct and alternating current have their own advantages and disadvantages, which are reflected in their area of ​​application. For the most part, the breadth of use of alternating current is due to the ease of its conversion.

Differences in transportation

When current flows, some of the electron energy is converted into heat due to the resistance of the wires. Electric heaters are also based on this effect. At the end of the line, less energy is transferred to the consumer. The power dissipated is called loss. To reduce losses, an increase in the voltage during transportation is applied. These physical relationships apply to both direct and alternating currents, however, differences arise in the implementation of transmission schemes.

Advantages and disadvantages of alternating current

At the beginning of the construction of transmission power grids, the use of transformers was the only way to receive high voltages and then reduce them to the required level when distributing to consumers. This technology was called transformer technology, and so far the structure of electricity transportation has not changed. Alternating current is almost universally used, which is a three-phase system.

Later, direct current lines began to be designed, which have been used more and more in recent years. The increased interest in their use is explained by the significant disadvantages of alternating current systems: in long lines, electricity losses are significant. Their reasons are the presence of capacitive and inductive resistance.

1. With a rapid change in the direction of the flow of electrons, an effect similar to the recharging of capacitors is observed. Additional capacitive currents appear. This is especially true for terrestrial and submarine cables, the insulating layer of which has a high capacitor effect;
2. The inductive resistance of the lines appears because electric currents generate magnetic fields that change with the frequency of the current. Inductive currents appear.

Important! Both types of reactance increase with increasing line length.

Advantages of AC:

• easy transformation of tension;
• the possibility of combining various transmission systems;
• the ability to use the system-wide frequency.

Disadvantages of AC:

• the need to compensate for reactive power during transportation over long distances;
• relatively high losses.

DC advantages and disadvantages

First of all, what distinguishes alternating current from direct current is the presence of sources of reactive energy losses. However, direct electric current implies heating losses. Their exact definition depends on technology and voltage level. For high voltages - about 3% per 1000 km.

Another source of losses in DC transmission systems are substations for converting AC to DC and vice versa. The total losses are much lower than for alternating current, but the material costs for the construction of these substations are significant.

Important! To increase the profitability of DC transmission lines, long-distance power lines are used.

DC power transmission has recently undergone technical development, thanks to the development of new electronic components for creating high levels of DC voltage - high-performance thyristors or bipolar transistors.

Interesting. Today, DC transmission systems with voltages of up to 800 kV and a transmission capacity of up to 8000 mW are possible over a distance of more than 2000 km.

Advantages of high-voltage DC transmission lines:

• the ability to transmit power over submarine, terrestrial and underground cable lines over long distances;
• no losses due to reactive power;
• better use of cable insulation.

Disadvantages of high-voltage DC transmission lines:

• insufficiently fast switching of existing DC channels;
• little standardized electrical engineering;
• distribution networks for the transmission of electricity are not developed, transportation is carried out from point to point.

Other DC and AC Applications

1. DC is ideal for charging rechargeable batteries and battery cells. They need this kind of power because the charging power must always go in the same direction. Accordingly, battery-powered devices also require DC power, such as a flashlight or laptop;
2. Television, radio, computer technology use DC;
3. Electric motors used in industry and in everyday life operate on both AC and DC. The same applies to stoves, irons, kettles and incandescent lamps;
4. DC is needed for electrolysis installations, where the presence of constant poles is important. Only sometimes it is not necessary to observe the polarity, in particular when electrolysis of gases. Then an alternating electric current can be applied;
5. About half of the world's railroad overhead contact networks use DC. At the beginning of the development of electrified railways, there were attempts to use three-phase motors, but the creation of a contact network for them ran into problems. DC operates city electric transport: trams, trolleybuses, metro. Another method for the construction of railway contact networks is the use of one phase of alternating current;