What kind of current is direct or alternating? What is the difference between direct current and alternating current?

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Electric current is the transfer of charge or the movement of charged particles between points with different electrical potentials. Electrical charge can be carried by ions, protons and/or electrons. IN Everyday life Almost everywhere, the movement of electrons through conductors is used. There are usually two types of electricity - alternating and direct. It's important to know what D.C. different from variable.

Direct and alternating current

Any phenomenon that cannot be seen or “touched” directly is easier to understand using analogies. In the case of electricity, we can consider water in a pipe as the closest example. Water and electricity flow through their conductors - wires and pipes.

The volume of flowing water is the current strength.
The pressure in the pipe is tension.
Pipe diameter is conductivity, the reciprocal of resistance.
Volume per pressure - power.

The pressure in the pipe is created by the pump - the pump pumps harder, the pressure is higher, more water flows. The diameter of the pipe is larger - the resistance is less, more water flows. The source produces more voltage - more electricity flows. Wires are thicker - less resistance, higher current.

For example, you can take any chemical source power - battery or accumulator. Its terminals have pole designations: plus or minus. If you connect a corresponding light bulb to the battery, through the wires and the switch, it will light up. What happens? The negative terminal of the source emits electrons - elementary particles carrying a negative charge. Along the wires, through the switch connectors and the lamp spiral, they move towards the positive terminal, trying to equalize the potential of the terminals. As long as the circuit is closed across the switch connectors and the battery is not dead, electrons flow in a spiral and the light bulb is on.

The direction of movement of charges remains unchanged all the time - from minus to plus. This is direct current, it can be pulsating - weaken or increase.

For many reasons Using only constant voltage is inappropriate: Take, for example, the inability to use transformers. Therefore, by now a system of supply and consumption has developed AC voltage nutrition, for which they are created Appliances.

There is a simple answer to what is the difference between direct and alternating current. In this light bulb example, the voltage on one terminal of the power supply will always be zero. This is the neutral wire, but on the other - the phase wire - the voltage changes. And not only in size, but also in direction - from plus to minus. Electrons do not flow in orderly rows in one direction, on the contrary, they rush back and forth, the same particles run back and forth along the incandescent spiral and do all the work. Changing the direction of electricity and gives the very concept of “variable”.

Advanced network settings

In addition to voltage, force, power and resistance/conductivity, two new features appear that describe processes. These parameters are required, just like the first four. When any of them changes, the properties of the entire chain change.

Form.
Frequency.

The type of voltage change graph plays a big role. Ideally, it has the form of a sinusoid with smooth transitions from value to value. Deviations from the sinusoidal shape can lead to poor energy quality.

Frequency is the number of transitions from one extreme state to another in a certain time. The European standard of 50 Hz (hertz) means that the voltage changes plus and minus 50 times per second, and the electrons change direction a hundred times. For reference: doubling the frequency leads to a fourfold reduction in device dimensions.

If the outlet has an alternating current of 50 Hz and 220 V (volts), then this means that maximum voltage The power supply in the network reaches 380 V. Where does this come from? IN permanent network The voltage value is constant, but when there is a change, it either falls or rises. This 220 V is the value effective voltage sinusoidal current with an amplitude of 380 V. Therefore, the form of change in values is so important that if it differs greatly from the sinusoid, the effective voltage will also change greatly.

Practical significance of the differences

This is what it is, alternating and direct current. It's not that difficult to figure out what the difference is. There is a difference and a very big one. A DC source will not allow you to connect a welding, or any other, transformer. When calculating insulation or capacitors, it is not the current that is taken for breakdown, but maximum value voltage. After all, the thought may certainly arise: “why do you need 400-volt capacitors in a 220-volt network?” Here is the answer, in a 220 V network the voltage reaches 380 V at normal operation, and in the event of a minor failure, 400 V is not the limit.

Another "paradox". A capacitor has infinite resistance in a DC network, and conductivity in an AC network; the higher the frequency, the lower the resistance of the capacitor. With coils it’s different - an increase in frequency causes an increase inductive reactance. This property is used in oscillatory circuit- the basis of all communication.

A very long time ago, scientists invented electricity. 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 overcome different distances with minor 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 in 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.

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

The direction of movement and frequency of charged particles in direct current are unchanged. Given current Various household devices such as televisions and computers are used for power supply.

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:

Force;
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.

Note! If electric installation work produced 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 live part of the electrical installation.

If given 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 identical values current, 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, 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.

Instructions

First, let's figure out what electric current is. The directed movement () of charged particles is called electric current. In the alternating current of a conductor, different numbers of charged particles pass through in equal periods of time. In a constant, the number of these particles for the same time is always equivalent.

Alternating current constantly changes its strength, magnitude or direction. And these changes are always periodic, that is, they are repeated at regular intervals. For example, using the variable current the battery cannot be charged or cannot be used for such technical purposes.

Unlike permanent current, the variable has several additional meanings: - period - the temporary value of completing a full cycle of variable indicators current; half-cycle and frequency (number of cycles for a specific period of time); - amplitude - the highest value of the variable current;- instantaneous value – value current V this moment time.

Alternating current is more common and widely used. It is easier to convert it into alternating current of a different voltage, to change the voltage in the networks depending on the necessary needs. This can be done using a transformer. Transformer - a device that converts alternating current of one voltage into the same current, but of a different voltage at the same frequency current.

Lobar pneumonia begins acutely, most often after severe hypothermia. The temperature reaches 39-40 degrees, the patient has severe chills. Pain immediately appears when breathing and from the affected lung. The cough is accompanied by the release of purulent, viscous sputum from the blood. The patient's condition is serious. Breathing is shallow, rapid, with flaring of the wings of the nose. Affected side chest significantly lags behind healthy breathing when breathing.

On planet Earth today, 98% of all electricity is generated by alternating current generators. Such a current is quite easy to produce and transmit to long distances. In this case, the current and voltage can be repeatedly increased and decreased - transformed. Work is done not by voltage, but by current. Therefore, the lower its value, the lower the losses in the wires.

Many users believe that only alternating current with a voltage of 220V and a frequency of 50Hz is used. This is only true for incandescent lamps, electric motors in vacuum cleaners, and refrigerators.

In any difficult household device powered by AC mains, there are units that operate at constant voltage With different meanings. It is virtually impossible to predict what these values might be. Therefore, all consumers in the outlet have alternating current of the same frequency and voltage.

D.C

Although the share of DC generation is only 2%, its value is quite large. Direct current is generated galvanic cells, batteries, thermocouples, solar panels.

Solar panels are becoming a very promising area of energy today, when the issue of using renewable energy sources is urgently raised.

Direct current powers locomotive engines in railway transport and is used in the on-board network of aircraft and automobiles.

There are more and more electric and hybrid cars on the roads of modern cities. To recharge their batteries, stations are built that provide their needs for direct current.

What kind of sockets should there be?

The dimensions of the sockets, their type, the material from which they are made depend primarily on the purpose of the sockets, the currents and voltages for which they are designed. Devices operating at constant voltage have polarized plugs. Therefore, the sockets for them must be polarized. Then even an inexperienced user will not be able to confuse where “+” and “–” are.

Alternating current in a circuit is an electric flow of charged particles, the direction and speed of which periodically changes over time according to a certain law.

Instructions

Refer to the general electrical circuit described in the school textbook. There you will see that alternating current is an electric current, the value of which varies according to a sinusoidal or cosine law. This means that the magnitude of the current in an alternating current network varies according to the law of sine or cosine. Strictly speaking, this corresponds to the current that flows in the household electrical network. However, the sinusoidality of the current is not general definition alternating current and does not fully explain the nature of its flow.

Draw a sine wave graph on a piece of paper. This graph shows that the value of the function itself, expressed by the current strength in this context, changes from a positive value to a negative value. Moreover, the time after which the sign changes is always the same. This time is called the period of current oscillations, and the reciprocal of the time is called the frequency of alternating current. For example, AC frequency household network is 50 Hz.

Please note that the sign of the function changes physically. In fact, this only means that at some point in time the current begins to flow in the opposite direction. Moreover, if the law of change is sinusoidal, then the change in direction of movement does not occur abruptly, but with gradual braking. Hence the concept of alternating current, and its main difference from direct current, which always flows in the same direction and has a constant value. As is known, the direction of the current is determined by the direction of positively charged particles in the circuit. Thus, in an alternating current circuit, charged particles after a certain time change the direction of their movement to the opposite.

Initially, people did not know what current was. Static charge was known, but no one understood or understood the nature of electricity. It took many centuries until Coulomb developed his own theory, and the German priest von Klein discovered that the jar was capable of storing energy. By the time Van de Graaff created the first generator, everyone already knew the difference between direct current and alternating current.

History of alternating and direct electric current

For a long time, for example, people have seen that a tourmaline crystal attracts ashes. By the way, the properties of piezoelectricity were first described using the example of tourmaline.

In the early 19th century, it was shown that a heated crystal acquires an electrical charge. Due to deformation, two poles were formed:

Southern (analogous).
Northern (antilogical).

Moreover, if the temperature remains constant after heating, the electricity disappears. Then the appearance of the poles is noted during cooling. It turns out that a tourmaline crystal produces electricity when the temperature changes. Further research showed that the size of the potential depends on:

Cross section of the crystal (cut across the poles).
Temperature differences.

Other factors do not influence the amount of charge. This phenomenon is called pyroelectricity. The dielectric tourmaline was slowly charged from the current flowing inside. And the charge remained in place (certain areas of the surface) due to its insulating properties. Until the tourmaline poles are short-circuited with a conductor, the crystal will continue to accumulate charge as the temperature changes. The line connecting the poles was called the pyroelectric axis.

Piezoelectricity was discovered by the famous Curie pair based on tourmaline in 1880. It was realized that when the size of the crystal changes, charges will begin to be generated; all that remained was to come up with a technique for carrying out the experiment. Curie used for this static pressure normal weight. The experiment is carried out on an insulating surface. For example, a mass of 1 kg causes the appearance of tourmaline in a crystal electric charge within five hundredths of static units.

How does electric current appear?

It is curious that a coherent theory on the described phenomenon has not yet been created. It is important to note that in nature there are charges obtained various methods. During a thunderstorm, this occurs due to the frictional forces of air masses, moisture molecules and other phenomena. The earth is negatively charged, current constantly flows upward through the atmosphere. Current is the movement of charge carriers due to certain reasons. For example, potential differences are differences in carrier levels between two points in space.

Let's compare it with water pressure. When the obstruction is removed, the flow will rush in the direction of lower pressure. Now let's take the analogy of a tourmaline crystal. Let's say charges appear at its ends. Next you will need to cause movement, for example, with a copper wire. Let's connect the poles and electric current will flow. The movement of carriers will continue until the potential is equalized. In this case, the crystal is discharged.

It is impossible to say about the variability or constancy of the current during the indicated process. Alternating and direct current are physical ideals, and are used due to the relative ease of obtaining mathematical models and using them to control technological equipment.

Electric current in reality

In practice, the current shape (charge density versus time) is not sinusoidal. By various reasons the graph view is distorted. This, for example, happens when equipment starts and stops due to induced interference of various natures. The shape of alternating and direct current is distorted. Moreover, it has long been established that this harms the equipment. To combat such a scourge, methods were required, and mathematicians came up with spectral analysis.

An oscillation of any shape can be represented as a sum with different specific gravity of the simplest sinusoids different frequencies. It turns out that a mass of components move along the circuit simultaneously, collectively producing current. Moreover, not all components necessarily move at the same time as the main mass. Imagine the elements as a group of ants, each pulling in its own direction, and the resulting effect causes the load to move in only one direction. Let us mention that in addition to the coefficient (amplitude), each component has a phase (direction), and is called a harmonic.

The equipment cascades are designed so that useful frequencies (mainly 50 Hz) pass inside the device, and the rest goes to the ground. A sign is indicated to solve the difficulty mentioned at the beginning. Any oscillation is represented as a set of useful and harmful signals, based on this, the equipment must be designed appropriately. For example, all receivers operate on the described principle: they selectively pass current of the desired frequency. This way it is possible to cut off interference, and the wave is transmitted with minimal distortion long distances.

Examples of using AC and DC current

The discharge current of a car battery is considered approximately constant. The voltage here gradually drops, and therefore, even with the same load, the effect varies chronometrically. In general, this happens smoothly. The current flows in one direction and exhibits approximately constant density. They work similarly:

Cell phone battery.
Any type of battery.
Laptop battery.

In nature, there are no direct current sources (generators), with the exception of Mother Earth. It is much more convenient for a person to create rotors that, rotating at a specific frequency, create conditions for the formation of alternating electric current in the stator coils. Then the industrial frequency of 50 Hz passes through the wires and is supplied to the consumer through the substation.

Adapters can be considered a DC source. These are devices that convert alternating current into direct current. Let's say cell phones this is +5 V, and for mobile radios characterized by a large scatter. A DC device can only operate at the rating for which it is designed. Otherwise, either the performance is impaired, or, with large deviations, complete failure is possible.

This applies to both alternating and direct current. Now the time has come to say that in industry the conversion of direct current to alternating current and vice versa is not practiced. For reasons of economy, the motors operate on three phases. Each is considered an alternating current with a frequency of 50 Hz. We said above that any harmonic has a phase. In the case under consideration, the phase is 120 degrees. A circle is formed by 360 degrees. It turns out that the three phases are equally spaced from each other. In this situation, it is easier for hydroelectric power station generators to produce energy that enters homes unchanged. But the only phase of alternating current enters the apartment.

Therefore, household appliances internal structure very different from industrial ones. AC parameters are considered important. In any state they are standardized and strictly adhered to. AC parameters include:

Effective value voltage - causing a constant of identical rating in an ordinary conductor. The effective value is below the amplitude by the root of two times or close to the specified one. The requirements for the Russian Federation are 220-230 V plus or minus 10% of the nominal value.
The frequency of alternating current is subject to increased stringent requirements. The limit of deviations from 50 Hz is measured in tenths of a percent. That is why so much attention is paid to stabilizing the movement of the shaft at hydroelectric power plants. The parameter depends on the speed of its rotation.
Nonlinear distortions are considered a separate topic. There are many requirements, it’s not easy to decide. The harmonics of the fundamental frequency are especially strictly standardized, for example: 100, 150, 200, 250 Hz.

Similar requirements apply to direct current parameters. Let's say known car batteries in fact, they include in the arsenal not 12, but 14 V. As the discharge progresses, the voltage drops. If a voltage of 11.9 V is registered on the battery, the bank is considered faulty. We suggest you read the instructions carefully. Let's add: in separate laptops There is a charge to conserve battery power. In this case, the level is maintained within two-thirds of the full level. It is believed that then the battery will last longer.

So, the requirements are aimed at maintaining long-term and proper functioning of the equipment. The parameters of direct and alternating current are considered a factor determining the reliability and performance of the system.

Electric current itself is nothing more than the orderly movement of all charged particles in gases, electrolytes and metal objects. These elements that carry a certain charge include ions and electrons. Today we will try to clarify what alternating current differs from direct current, because in practice one often encounters both types.

DC Characteristics

Direct Current or DC in English means a similar variety, which has the inherent property of not changing its parameters over any period of time. A small horizontal line or two parallel lines with a line design of one of them - graphic image direct current.

Scope of application – most and electronic devices, including computer equipment, TVs and gadgets, use in home networks and cars. To convert alternating current into direct current in the outlet area, voltage transformers with rectifiers or specialized power supplies are used.

A common example of DC consumption is almost all power tools that operate with batteries. The battery device remains in any case a constant power source. Conversion to variable is achieved, if necessary, with the help of inverters - special elements.

What is the working principle of alternating current

The English abbreviation AC (Alternating Current) denotes a current that changes its direction and magnitude over time periods. The sinusoid segment “~” is its conventional marking on devices. Applying after this icon and other characteristics is also used.

Below is a figure with the main characteristics of this type of current - nominal frequency and operating voltage.

It should be noted the features of the change in the left graph, made for a single-phase current, in the magnitude and direction of the voltage with the transition to zero over a certain period of time T. For one third of the period, three sinusoids are shifted for a three-phase current on another graph.

The marks “a” and “b” indicate the phases. Any of us has an idea of the presence of 220V in a regular outlet. But for many it will be a discovery that the maximum or otherwise called amplitude value more than the current by an amount equal to the root of two and is 311 Volts.

Obviously, in the case of current permanent type The parameters of direction and voltage remain unchanged, but for a variable, a transformation of these quantities is observed. In the figure, the opposite direction is the area of the graph below zero.

Let's move on to frequency. This concept means the ratio of periods ( full cycles) to a conventional unit of time interval. This indicator is measured in Hertz. The standard European frequency is 50, in the USA the applicable standard is 60G.

This value shows the number of changes in the direction of the current in one second to the opposite and return to the original state.

Alternating current is present at direct current and in outlets. For what reason is there no direct current here? This is done in order to be able to obtain the required voltage in any quantity by using transformers without any significant losses. This technique remains the best way transmit electricity on an industrial scale over long distances with minimal losses.

Rated voltage supplied powerful generators power plants, the output is about 330,000-220,000 Volts. At a substation located in the consumption area, this value is transformed to 10,000V with a transition to a three-phase version of 380 Volts. and your apartment receives single-phase voltage. The voltage between zero and phase will be 220 V, and in the shield between different phases this figure is 380 Volts.

Asynchronous motors operating with alternating current are much more reliable and have a simpler design than their direct current counterparts.

Converting AC to DC

For a variant of such a transformation the best way– use of rectifiers:

Connection diode bridge is the first step in this procedure. The design of 4 diodes with the required power contributes to the process of peculiarly cutting off the upper boundaries of the sinusoids already familiar to us variable type. In this way, a unidirectional current is achieved.

Changes resulting from decreased ripple are shown in blue.

are installed to reduce the operating level of pulsation if necessary.

DC to AC converter

IN in this case The process looks quite complicated. An inverter is a standard technique in everyday life; it is a generator of a periodic voltage obtained from a constant voltage close to a sine wave.

The high prices for such a device are due to the complexity of the design. The cost is largely determined by the maximum current output.

Used in rather rare situations. For example, if it is necessary to connect some kind of tool or devices to the car’s electrical network.