The difference between direct current and alternating current and their features. Alternating current and direct current: the difference

What is the difference between AC and DC current

The general concept of electric current can be expressed as the movement of various charged particles (electrons, ions) in a certain direction. And its value can be characterized by the number of charged particles that passed through the conductor in a certain period of time.

If the value of charged particles of 1 coulomb passes through a certain cross-section of a conductor in a time of 1 second, then we can talk about a current strength of 1 ampere flowing through the conductor. This determines the number of amperes or current. This is the general concept of current. Now let's look at the concept of alternating and direct current and their differences.

A direct electric current, by definition, is a current that flows in only one direction and does not change over time. Alternating current is characterized by the fact that it changes its direction and magnitude over time. If direct current is graphically displayed as a straight line, then alternating current flows through the conductor according to the sine law and is graphically displayed as a sine wave.

Since alternating current varies according to the law of a sinusoid, it has such parameters as the period of a complete cycle, the time of which is denoted by the letter T. The frequency of alternating current is the inverse of the period of a complete cycle. The frequency of alternating current is expressed by the number of complete periods in a certain period of time (1 sec).

There are 50 such periods in our AC power network, which corresponds to a frequency of 50 Hz. F = 1/T, where the period for 50 Hz is 0.02 sec. F =1/0.02 = 50 Hz. Alternating current is denoted by the English letters AC and the sign “~”. Direct current is designated DC and has a “-” symbol. In addition, alternating current can be single-phase or multiphase. A three-phase network is mainly used.

Why is there alternating voltage in the network and not constant?

Alternating current has many advantages over direct current. Low losses during the transmission of alternating current in power lines (power lines) compared to direct current. Alternators are simple and cheap. When transmitted over long distances along power lines, high voltage reaches 330 thousand volts with minimal current.

The lower the current in the power line, the lower the losses. Transmission of direct current over long distances will incur considerable losses. Also, high-voltage alternators are much simpler and cheaper. It is easy to get lower voltage from AC voltage through simple transformers.

Also, it is much cheaper to obtain DC voltage from AC voltage than, on the contrary, to use expensive DC-AC voltage converters. Such converters have low efficiency and high losses. Double conversion is used along the AC transmission path.

First, it receives 220 - 330 kV from the generator, and transmits it over long distances to transformers, which lower the high voltage to 10 kV, and then there are substations that lower the high voltage to 380 V. From these substations, the electricity is distributed to consumers and supplied to homes and electrical panels apartment building.

Three phases of three-phase current shifted by 120 degrees

Single-phase voltage is characterized by one sinusoid, and three-phase voltage is characterized by three sinusoids, shifted by 120 degrees relative to each other. A three-phase network also has its advantages over single-phase networks. These are smaller dimensions of transformers, electric motors are also structurally smaller.

It is possible to change the direction of rotation of the rotor of an asynchronous electric motor. In a three-phase network, you can get 2 voltages - 380 V and 220 V, which are used to change the engine power and adjust the temperature of the heating elements. Using three-phase voltage in lighting, it is possible to eliminate the flickering of fluorescent lamps, for which they are connected to different phases.

Direct current is used in electronics and in all household appliances, since it is easily converted from alternating current by dividing it on a transformer to the required value and further straightening it. The source of direct current is batteries, batteries, direct current generators, LED panels. As you can see, the difference in alternating and direct current is considerable. Now we have learned - Why does our socket flow alternating current and not direct current?

Children are taught that they should not stick their fingers into electrical sockets! Why? Because it will be bad. There are often problems with a more detailed explanation: there is some kind of voltage, current, something is flowing somewhere. So that in the future you can explain to your children what’s what, we will now explain to you. This article is about alternating and direct currents, their differences, applications and the history of electricity in general. Science needs to be made interesting, and we modestly try to do this to the best of our ability.

For example: what current is in our sockets? Variable, of course! Voltage 220 Volts and frequency 50 Hertz. And the network through which the current is transmitted is three-phase. By the way, if at the words “phase” and “zero” you fall into a stupor, read what it is, and the day will be lived doubly not in vain! But let's not get ahead of ourselves. First things first.

A Brief History of Electricity

Who invented electricity? And no one! People gradually understood what it was and how to use it.

It all started in the 7th century BC, on one sunny (or maybe rainy, who knows) day. Then the Greek philosopher Thales noticed that if you rub amber on wool, it will attract light objects.

Then there were Alexander the Great, wars, Christianity, the fall of the Roman Empire, wars, the fall of Byzantium, wars, the Middle Ages, the Crusades, epidemics, the Inquisition and more wars. As you understand, people had no time for any electricity or ebonite sticks rubbed with wool.

In what year was the word “electricity” invented? In 1600, the English naturalist William Gilbert decided to write the work “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth.” It was then that the term appeared "electricity".

One hundred and fifty years later, in 1747, Benjamin Franklin, whom we all love very much, created the first theory of electricity. He viewed this phenomenon as a fluid or immaterial liquid.

It was Franklin who introduced the concept positive And negative charges (previously separated glass And resin electricity), invented the lightning rod and proved that lightning is electrical in nature.

Everyone loves Benjamin, because his portrait is on every hundred dollar bill. In addition to his work in the exact sciences, he was a prominent political figure. But contrary to popular belief, Franklin was not the President of the United States.

1785 - Coulomb finds out with what force opposite charges attract and like charges repel.

1791 - Luigi Galvani accidentally noticed that the legs of a dead frog contracted under the influence of electricity.

The operating principle of the battery is based on galvanic cells. But who created the first galvanic cell? Based on Galvani's discovery, another Italian physicist Alessandro Volta created the Volta column in 1800, the prototype of the modern battery.

At excavations near Baghdad, they found a battery more than two thousand years old. What ancient iPhone was recharged with its help remains a mystery. But we know for sure that the battery has already run out. This case seems to say: maybe people knew about electricity much earlier, but then something went wrong.

Already in the 19th century, Oersted, Ampere, Ohm, Thomson and Maxwell made a real revolution. Electromagnetism was discovered, induced emf, electrical and magnetic phenomena were linked into a single system and described by fundamental equations.

By the way! If you don't have time to deal with all this yourself, our readers are currently offering a 10% discount on

The 20th century brought quantum electrodynamics and the theory of weak interactions, as well as electric cars and ubiquitous power lines. By the way, the famous Tesla electric car runs on direct current.

Of course, this is a very brief history of electricity, and we have not mentioned very many names that influenced progress in this field. Otherwise, a whole multi-volume reference book would have to be written.

First, recall that current is the movement of charged particles.

Direct current is current that flows in one direction.

A typical DC source is a galvanic cell. Simply put, a battery or accumulator. One of the oldest artifacts related to electricity is the Baghdad battery, which is 2000 years old. It is believed that it provided a current of 2-4 Volts.

Where is DC used:

in powering most household appliances;
in batteries and accumulators for autonomous power supply of devices;
for powering car electronics;
on ships and submarines;
V public transport(trolleybuses, trams).

The easiest way to represent direct current is visually, on a graph. Here's what it looks like:

Household appliances operate on direct current, but alternating current comes into the network sockets in the apartment. Almost everywhere, direct current is obtained by rectifying alternating current.

Alternating current is a current that changes magnitude and direction. Moreover, it changes at equal intervals.

Alternating current is used in industry and power supply. This is what is received at stations and sent to consumers. Already on site, the conversion of alternating electric current into direct current occurs with the help of inverters.

Alternating current - alternating current (AC). Direct current - direct current (DC). The abbreviation AC/DC can be seen on the transformer boxes where the conversion takes place. It's also the name of a great Australian rock band.

And here is a visual representation of alternating current.

Alternating current flows in a circuit in two directions: back and forth. One of them is considered positive, and the second - negative.

Since the magnitude of the current changes not only in direction, but also in magnitude, do not think that there is always 220 volts in your outlet. 220 is the effective voltage value, which occurs 50 times per second. By the way, in America they use a different standard for alternating current in the network: 110 Volts and 60 Hertz.

War of Currents

Active use of direct current began at the end of the 19th century. Then Edison perfected the light bulb (1890) and founded the first power plants in New York that produced 110 volt direct current.

The use of direct current was associated with significant losses when transmitting it over long distances. Alternating current could not be used due to the lack of adequate meters and motors that operated on alternating current. The process of converting direct current into alternating current was also difficult. At the same time, alternating current could be transmitted over long distances without loss.

At that time, Nikola Tesla came to America from Serbia and got a job at Edison’s company. Tesla invented the alternating current electric motor, realized all the benefits and suggested its use to Edison.

Edison did not listen to Tesla and also did not pay him his salary. This is how the famous confrontation between inventors began - the war of currents.

It lasted more than a hundred years and ended in 2007. Then New York completely switched to alternating current electricity.

Why is alternating current more dangerous than direct current?

In the war of currents, in order not to suffer losses and financial collapse from the introduction and use of Tesla's ideas, Edison publicly demonstrated how alternating current kills animals. The case where an American citizen died from an alternating current shock was very detailed and widely covered in the press.

For humans, alternating current is, in general, actually more dangerous than direct current. Although you always need to take into account the magnitude of the current, its frequency, voltage, and the resistance of the person being shocked. Let's consider these nuances:

Alternating current with a frequency of 50 Hertz is three to four times more dangerous to life than direct current. If the frequency of the current is more than 1000 Hertz, then it is considered less dangerous.
At voltages of about 400-600 Volts, alternating and direct currents are considered equally dangerous. At voltages greater than 600 volts, direct current is more dangerous.
Alternating current, due to its nature and frequency, excites the nerves more strongly, stimulating the muscles and heart. That is why it poses a great danger to life.

Whatever current you work with, be careful and vigilant! Take care of yourself and your nerves, and also remember: a professional student service with the best experts will help you do this effectively.

At the very beginning, let's give a short definition of electric current. Electric current is the 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 let's look at concepts such as direct and alternating current and identify their fundamental differences.

The difference between direct current and alternating current

The main feature of constant voltage is that it is constant both in magnitude and sign. Direct current “flows” in one direction all the time. For example, along 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 assume that the current flows from plus to minus and still adhere to this rule in calculations.

How does alternating current (voltage) differ from direct current? From the name itself it follows that it changes. But - how exactly? Alternating current changes over a period both its magnitude and the direction of movement of electrons. 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 a light bulb as an example, we get the following:

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

The alternating voltage in our household outlet varies according to a sinusoidal law. What does it mean? The voltage from zero increases 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 100% again. It turns out that in a second, electrons change the direction of their 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. There were several problems 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 with high voltage and low current, or vice versa: with low voltage and high current. The greater the current, the larger the wire cross-section required, i.e. the wire should be thicker. For voltage, the thickness of the wire is not important, as long as the insulators are good. Alternating current (as opposed to direct current) 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 losses. Then, for example, a transformer at a 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 district 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 diagram is simplified and refers to the entire set of district substations in the city, and not any one in particular.

A personal computer (PC) works on a similar principle, but in the opposite direction. It converts alternating current into direct current and then, using , reduces its voltage to the values necessary for the operation of all components inside.

At the end of the 19th century, worldwide electrification could well have taken a different path. Thomas Edison (who is believed to have invented one of the first commercially successful incandescent light bulbs) actively promoted his idea of direct current. And if it were not for the research of another outstanding person who proved the effectiveness of alternating current, then everything could have been different.

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

Now let's look at the "habitats" of direct and alternating current. The permanent one, for example, is found in our telephone battery or batteries. Chargers transform alternating current from the network into direct current, and in this form it ends up in places where it is stored (batteries).

DC voltage sources are:

ordinary batteries used in various devices (flashlights, players, watches, testers, etc.)
various batteries (alkaline, acid, etc.)
DC generators
other special devices, for example: rectifiers, converters
emergency energy sources (lighting)

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

AC voltage sources:

generators
various converters (transformers)
household electrical networks (household sockets)

We talked about how and with what to measure direct and alternating voltage, and finally (to all those who read the article to the end) I want to tell a short story. My boss voiced it to me, and I will retell it from his words. It really suits our topic today!

He once went on a business trip with our directors to a neighboring city. Establish friendly relations with the IT people there :) And right next to the highway there is such a wonderful place: a spring with clean water. Everyone stops near it and gets water. This is, in a way, already a tradition.

Local authorities, having decided to improve this place, did everything with the latest technology: they dug a large rectangular hole right under the spring, lined it with bright tiles, installed an overflow, LED lighting, and it turned out to be a pool. Further - more! The spring itself was “packed” in speckled granite chips, given a noble shape, an icon above the vent was embedded under glass - a holy place, it seems!

And the final touch - we installed a water supply system based on a photocell. It turns out that the pool is always full and “gurgles” in it, but to draw water directly from the spring, 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 had the opposite opinion. And so, they drive up to the spring, put their hands where they should be, 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”! I made 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 a canister straight from the pool and went to the car!

So it turns out that we can set up anything, “raise” a sophisticated server, provide the best and most popular service, but, all the same, the most important person is Uncle Vasya the electrician in a quilted jacket, who with one movement of the hand can organize a complete skipped of all this technical power and grace :)

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

That’s all for today and until the next articles. Take care of yourself! Below is a short video on the topic of the article.

A long time ago, scientists invented electric current. The first invention was the permanent one. But later, while conducting experiments in his laboratory, Nikola Tesla invented alternating current. There were and are many differences between them, according to which one of them is used in low-current equipment, and the other has the ability to cover various distances with small losses. But a lot 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 based on the definitions. In order to better understand the operating principle and features, you need to know the following factors.

Main differences:

Movement of charged particles;
Mode of production.

Variable current is a current in which charged particles are capable of changing the direction of movement and magnitude at a certain time. The main parameters of alternating current include its voltage and frequency.

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

Pay attention! In household electrical networks, a current of 220 Volts and a clock frequency of 50 Hz is used.

The direction of movement and frequency of charged particles in direct current are unchanged. This current is used to power various household devices, such as televisions and computers.

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

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

What voltage level is acceptable for a person: features

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

Electric current exposure parameters:

Strength;
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 of 0.3 mA. Current with a frequency of 400 Hz with a voltage of 3.0 Volts and a current strength of 0.4 mA. Direct current with a voltage of 8 and a current of 1 mA. Safe exposure to current with such indicators is up to 10 minutes.

Pay attention! If electrical installation 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, the safe touch currents are as follows.

50 Hz alternating current with a voltage range from 550 to 20 Volts and a current strength from 650 to 6 mA, 400 Hz alternating current with a voltage from 650 to 36 Volts, and direct current from 650 to 40 Volts, should not affect the human body within the range of 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 provided, if you do not go into details. Much depends on various quantities and factors.

Factors influencing hazardous exposure:

Duration of contact;
The path of electric current;
Current and voltage;
What is the resistance of the body?

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

It is worth noting that, provided that the current does not exceed 9 mA, then anyone can free themselves from the current-carrying part of the electrical installation.

If this value is exceeded, then in order to free yourself from the effects of electric current, a person needs strong help. This is due to the fact that alternating current is much more capable of stimulating nerve endings and causing involuntary muscle spasms.

For example, when you touch the live part of the device with the inside of your palm, the muscle spasm will cause the fist to clench more strongly over time.

Why is alternating current more dangerous? At the same current values, the alternating current has a several times stronger effect on the body.

Since alternating current affects nerve endings and muscles, it is worth understanding that this also affects the functioning of the heart muscle. From which it follows that when contacting alternating current, the risk of death increases.

An important indicator is the resistance of the human body. But when struck by alternating current with high frequencies, the body resistance is significantly reduced.

What magnitude is direct current dangerous for humans?

Direct current can also be dangerous for humans. Of course, variable, ten times more dangerous. But if we consider currents in different quantities, then constant can be much more dangerous than alternating.

The effects of direct current on humans are divided into:

1 threshold;
2 threshold;
3 threshold.

When exposed to direct current at the feather threshold (the current is noticeable), your hands begin to tremble a little and a slight tingling sensation appears.

The second threshold (not releasing current), ranging 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 value.

The third threshold (fibrillation), with values of 100 mA and above, the current has a strong effect on the body and internal organs. In this case, the current at these values can cause chaotic contraction of the heart muscle and lead to its stop.

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

Electric current- movement of charged particles along a conductor in a certain direction. More precisely, this is a value that shows how many charged particles passed through the conductor per unit of time. If in one second a number of charged particles the size of one coulomb passed through the cross-section of a conductor, then a current of one ampere (current designation in accordance with the international SI system) flows through this conductor. The amount of electric current (number of amperes) is called current strength. Depending on the change in value over time, current can be constant or variable.

D.C is an electric current that does not change its direction over time. AC- over time, in a certain pattern, changes both its magnitude and direction. Moreover, these changes are repeated at certain intervals - that is, they are periodic.

Alternating and direct current in electrical installations

A three-phase electrical network is characterized by AC. The flow of alternating current through conductors is determined by the presence of a source of alternating electromotive force (EMF), which changes its value, both in magnitude and in direction. In this case, the change in the magnitude and direction of the EMF is carried out according to the sine law, that is, the graph of changes in alternating current over time is a sinusoid. The source of sinusoidal EMF is an alternating current generator.

Almost all electrical equipment of electrical installations and industrial enterprises is powered from an alternating current network, since this is the most appropriate and has many advantages. But there is also some equipment that operates from a direct current network (or some of its parts): synchronous motor, electromagnetic, DC motor and others. In order to convert alternating current into direct current (necessary to power the above electrical equipment), rectifiers are used.

In addition, direct current is used to transmit large amounts of electrical energy through high-voltage lines. In this case, when transmitting electrical energy over long distances, electrical losses are significantly less than during the same transmission using alternating current.