Direct or alternating current. Alternating electric current

Instructions

First, let's figure out what the electric current is. Directional movement () of charged particles is called electric current. In the alternating current of a conductor, a different number of charged particles pass through at equal intervals. In a constant, the amount 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 it is impossible to charge the battery or it cannot be used for such technical purposes.

Unlike constant current, the variable has several additional values: - period - the time value of completing a full cycle of indicators of the variable current; half-cycle and frequency (number of cycles in a specific period of time); - amplitude - the highest value of the variable current; - instantaneous value - value current at this point in time.

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

Croupous pneumonia begins acutely, most often, after severe hypothermia. The temperature is up to 39-40 degrees, the patient has a strong chill. Pain immediately occurs when breathing and from the side of the affected lung. The cough is accompanied by the discharge of purulent viscous sputum from the blood. The patient's condition is grave. Breathing is shallow, quickened, with swelling of the wings of the nose. The affected side of the chest noticeably lags behind the healthy side when breathing.

On planet Earth today, 98% of all electricity is generated by alternators. Such a current is fairly easy to generate and transmit over long distances. In this case, the current and voltage can repeatedly rise and fall - transform. The 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, refrigerators.

In any complex household device powered by an alternating current network, there are nodes that operate at constant voltage with different values. It is virtually impossible to predict what these values ​​might be. Therefore, all consumers in the socket have alternating current of the same frequency and voltage.

D.C

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

Solar batteries are becoming a very promising area of ​​energy today, when the question of using renewable energy sources is sharply raised.

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

There are more and more electric and hybrid vehicles on the roads of modern cities. To recharge their batteries, stations are being built to meet their DC needs.

What should be the sockets

The dimensions of the sockets, their type, the material from which they are made, depend primarily on the purpose of the outlets, currents and voltages for which they are designed. Constant voltage devices have polarized plugs. Therefore, the sockets for them must be polarized. Then even an inexperienced user will not be able to confuse where "+" and "-".

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

Instructions

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

Draw a sinusoid graph on a piece of paper. It can be seen from this graph 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 change occurs is always the same. This time is called the period of current fluctuations, and the value inverse to time is called the frequency of the alternating current. For example, the AC frequency of a household network is 50 Hz.

Notice the sign reversal function 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 the direction of movement occurs not abruptly, but with gradual deceleration. 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 you know, the direction of the current is set by the direction of the positively charged particles in the circuit. Thus, in an alternating current circuit, charged particles change their direction of motion to the opposite after a certain time.

Electricity- it is the directed or ordered movement of charged particles: electrons in metals, in electrolytes - ions, and in gases - electrons and ions. Electric current can be either constant or alternating.

Determination of direct electric current, its sources

D.C(DC, in English Direct Current) is an electric current whose properties and direction do not change over time. Direct current and voltage are indicated in the form of a short horizontal line or two parallel lines, one of which is dashed.

Constant current used in cars and in homes, in numerous electronic devices: laptops, computers, televisions, etc. The measured electrical current from the outlet is converted into direct current using a power supply unit or a voltage transformer with a rectifier.

Any power tool, device or device powered by batteries is also a DC consumer, because a battery or accumulator is exclusively a direct current source, which, if necessary, is converted into alternating current using special converters (inverters).

How AC works

Alternating current(AC in English Alternating Current) is an electric current that changes in magnitude and direction over time. On electrical appliances, it is conventionally designated by a segment of a sinusoid "~".
Sometimes after the sinusoid, the characteristics of the alternating current can be indicated - frequency, voltage, number of phases.

Alternating current can be either one- or three-phase, for which the instantaneous values ​​of current and voltage vary according to a harmonic law.

Main characteristics alternating current - rms voltage and frequency.

note, as in the left graph for a single-phase current, the direction and magnitude of the voltage changes with a transition to zero over a period of time T, and in the second graph for a three-phase current there is a shift of three sinusoids by one third of the period. On the right graph, phase 1 is designated by the letter "a", and the second by the letter "b". It is well known that there is 220 volts in a home socket. But few people know that this is the effective value of the alternating voltage, but the peak or maximum value will be greater by the root of two, that is, it will be equal to 311 Volts.

Thus, if for a direct current the voltage value and direction do not change over time, then for an alternating current, the voltage constantly changes in magnitude and direction (a graph below zero is the opposite direction).

And so we came to the concept of frequency Is the ratio of the number of complete cycles (periods) to a unit of time of a periodically varying electric current. Measured in Hertz. In our country and in Europe, the frequency is 50 Hertz, in the USA - 60 Hz.

What does 50 Hertz mean? It means that our alternating current changes its direction to the opposite and vice versa (segment T- on the graph) 50 times per second!

AC sources are all the sockets in the house and everything that is connected directly with wires or cables to the electrical panel. Many people have a question: why is there no direct current in the outlet? The answer is simple. In AC networks, the voltage value is easily and with minimal losses converted to the required level using a transformer in any volume. The voltage must be increased in order to be able to transmit electricity over long distances with the lowest losses on an industrial scale.
From the power plant where there are powerful electric generators, a voltage of 330,000-220,000 comes out, then near our house at a transformer substation it is converted from 10,000 Volts into a three-phase voltage of 380 Volts, which comes to an apartment building, and a single-phase voltage comes to our apartment , because between the voltage is 220 V, and between opposite phases in the electrical panel is 380 volts.

And one more of the important advantages of AC voltage is that asynchronous AC motors are structurally simpler and work much more reliably than DC motors.

How to make alternating current constant

For consumers operating on direct current, the alternating current is converted using rectifiers.

DC to AC converter

If there are no difficulties with the conversion of alternating current into direct current, then with the reverse conversion everything is much more complicated. At home for this inverter used- This is a generator of periodic voltage from a constant, in shape close to a sinusoid.

Despite the fact that electric current is an indispensable part of modern life, many users do not even know basic information about it. In this article, omitting the course of basic physics, we will consider how direct current differs from alternating current, as well as how it is used in modern domestic and industrial conditions.

In contact with

Difference of current types

What is current, we will not consider here, but immediately move on to the main topic of the article. Alternating current differs from constant current in that it continuously changes in the direction of movement and its magnitude.

These changes are carried out in periods at equal time intervals. To create such a current, special sources or generators are used that produce an alternating EMF (electromotive force), which changes regularly.

The basic circuit of the aforementioned AC generating device is quite simple. This is a rectangular frame made of copper wires, which is fixed to an axle, and then rotates in the field of a magnet using a belt drive. The ends of this frame are soldered to copper contact rings that slide directly over the contact plates, rotating in sync with the frame.

Under the condition of a uniform rhythm of rotation, EMF begins to be induced, which periodically changes. Measure the EMF generated in the frame, possibly with a special device. Thanks to the appearance, it is really possible to determine the alternating EMF and, along with it, the alternating current.

In graphic design, these values ​​are characteristically depicted in the form of a wavy sinusoid... The concept of sinusoidal current often refers to alternating current, since this type of current change is the most common.

Alternating current is an algebraic quantity, and its value at a specific time moment is called an instantaneous value. The sign of the alternating current itself is determined by the direction in which the current flows at a given time. Therefore, the sign is positive and negative.

Current characteristics

For a comparative assessment of all kinds of alternating currents, criteria are used, called AC parameters, among which:

• period;
• amplitude;
• frequency;
• circular frequency.

Period - the period of time when a complete cycle of current change is performed. The maximum value is called the amplitude. The number of completed periods in 1 second was called the frequency of alternating current.

The above parameters make it possible to distinguish between different types of alternating currents, voltages and EMF.

When calculating the resistance of different circuits to the action of alternating current, it is permissible to connect another characteristic parameter, called angular or circular frequency... This parameter is determined by the speed of rotation of the above frame at a certain angle in one second.

Important! It should be understood how the current differs from the voltage. The fundamental difference is known: the current is the amount of energy, and the voltage is called the measure.

The alternating current gets its name because the direction of motion of electrons is constantly changing, like the charge. It has different frequencies and electrical voltages.

This is what distinguishes it from direct current, where the direction of movement of electrons is unchanged... If the resistance, voltage and current strength are unchanged, and the current flows in only one direction, then such a current is constant.

For the passage of direct current in metals, it will be required that the constant voltage source is closed to itself with the help of a conductor, which is the metal. In some situations, a chemical energy source called a galvanic cell is used to generate direct current.

Current transmission

AC power sources are conventional wall outlets. They are located in objects of various purposes and in residential premises. Various electrical devices are connected to them, which receive the voltage necessary for their operation.

The use of alternating current in electrical networks is economically feasible, since the magnitude of its voltage can be transformed to the level of required values. This is done with the help of transformer equipment with allowable insignificant losses. Transportation from power supply sources to end users is cheaper and easier.

The transmission of current to consumers begins directly at the power plant, which uses a variety of extremely powerful electrical generators. From them, electric current is obtained, which is directed through cables to transformer substations. Substations are often located near industrial or residential electrical consumption facilities. The current received by the substations is converted into three-phase alternating voltage.

Batteries and accumulators contain constant current, which is characterized by the stability of properties, i.e. they do not change over time. It is used in all modern electrical products, as well as in cars.

Current conversion

Let us consider separately the process of converting alternating current into direct current. This process is carried out using specialized rectifiers and includes three steps:

1. The first step is to connect a four-diode bridge of a given power. This, in turn, makes it possible to set the motion of a unidirectional type for charged particles. In addition, it lowers the upper values ​​of sinusoids inherent in alternating current.
2. Next, a smoothing filter or a specialized capacitor is connected. This is done from the diode bridge to the output. The filter itself helps to correct the troughs between the peak values ​​of the sinusoids. And connecting a capacitor significantly reduces ripple and brings them to minimum values.
3. Then the voltage stabilizing devices are connected in order to reduce the ripple.

This process, if necessary, can be carried out in two directions, converting direct and alternating current.

Another distinctive feature is the propagation of electromagnetic waves in relation to space. It has been proven that a constant type of current does not allow electromagnetic waves to propagate in space, and an alternating current can cause their propagation. In addition, when transporting AC through wires, induction losses are significantly less than when transmitting DC.

Justification for the choice of current

The variety of currents and the lack of a single standard is determined not only by the need for different characteristics in each individual situation. In solving most of the issues, the preponderance is in favor of alternating current. This difference between the types of currents is due to the following aspects:

• The ability to transmit alternating current over long distances. The ability to convert in dissimilar electrical circuits with an ambiguous level of consumption.
• Maintaining a constant voltage for alternating current turns out to be twice as cheap as for constant voltage.
• The process of converting electrical energy directly into mechanical force is carried out at a significantly lower cost in mechanisms and AC motors.

The movement of electrons in a conductor

To understand what current is and where it comes from, you need to have a little knowledge about the structure of atoms and the laws of their behavior. Atoms are made up of neutrons (with a neutral charge), protons (a positive charge), and electrons (a negative charge).

Electric current arises as a result of the directed movement of protons and electrons, as well as ions. How can the movement of these particles be directed? During any chemical operation, electrons are “torn off” and transferred from one atom to another.

Those atoms from which the electron is "torn off" become positively charged (anions), and those to which it has joined - negatively charged and are called cations. As a result of these "skips" of electrons, an electric current is generated.

Naturally, this process cannot go on forever, the electric current will disappear when all the atoms of the system are stabilized and have a neutral charge (a great everyday example is an ordinary battery that "sits down" as a result of the end of a chemical reaction).

Study history

The ancient Greeks were the first to notice an interesting phenomenon: if you rub an amber stone on a woolen fabric, then it begins to attract small objects. The next steps were taken by scientists and inventors of the Renaissance, who built several interesting devices that demonstrate this phenomenon.

A new stage in the study of electricity was the work of the American Benjamin Franklin, in particular his experiments with the Leiden Bank - the world's first electric capacitor.

It was Franklin who introduced the concepts of positive and negative charges, and he also invented a lightning rod. Finally, the study of electric current became an exact science after the description of Coulomb's law.

Basic patterns and forces in electric current

Ohm's Law - his formula describes the relationship of force, tension and resistance. Discovered in the 19th century by the German scientist Georg Simon Ohm. The unit for measuring electrical resistance is named after him. His discoveries were very useful directly for practical use.

The Joule-Lenz law says that work is done on any part of the electrical circuit. As a result of this work, the conductor heats up. This thermal effect is often used in practice in engineering and technology (an excellent example is an incandescent lamp).

This pattern received this name because 2 scientists at once, approximately simultaneously and independently, deduced it through experiments
.

At the beginning of the 19th century, the British scientist Faraday realized that by changing the number of induction lines that penetrate a surface bounded by a closed circuit, it was possible to make an induction current. Extraneous forces acting on free particles are called electromotive force (EMF of induction).

Varieties, characteristics and units of measurement

The electric current can be or variable, or permanent.

A constant electric current is a current that does not change its direction and sign over time, but it can change its value. Constant electric current most often uses galvanic cells as a source.

A variable is one that changes direction and sign according to the cosine law. Its characteristic is frequency. The SI units are Hertz (Hz).

In recent decades, it has become very widespread. This is a type of alternating current that includes 3 circuits. In these circuits, variable EMF of the same frequency acts, but unfolded in phase one relative to the other for a third of the period. Each individual electrical circuit is called a phase.

Almost all modern generators produce three-phase electrical current.

• Strength and amount of current

The strength of the current depends on the amount of charge flowing in the electrical circuit per unit of time. The strength of the current is the ratio of the electric charge passing through the cross-section of the conductor to the time of its passage.

In the SI system, the unit for measuring the force of the charge is the coulomb (C), the time is the second (s). As a result, we get C / s, this unit is called Ampere (A). The strength of the electric current is measured using an instrument - an ammeter.

• Voltage

Voltage is the ratio of work to charge. Work is measured in joules (J), charge in pendants. This unit is called Volt (V).

• Electrical resistance

Ammeter readings on different conductors give different values. And in order to measure the power of the electrical circuit, 3 devices would have to be used. The phenomenon is explained by the fact that each conductor has different conductivity. The unit of measurement is called Ohm and is denoted by the Latin letter R. The resistance also depends on the length of the conductor.

• Electrical capacity

Two conductors, one insulated from the other, can store electrical charges. This phenomenon is characterized by physical. a quantity called electrical capacity. Its unit of measurement is farad (F).

• Power and work of electric current

The work of the electric current at a specific section of the circuit is equal to the multiplication of the voltage by the strength and time. Voltage is measured in volts, power in amperes, time in seconds. The unit of measure for work was the joule (J).

The power of the electric current is the ratio of the work to the time of its completion. Power is denoted with the letter P and is measured in watts (W). The power formula is very simple: Amperage multiplied by voltage.

There is also a unit called a watt hour. It should not be confused with watts, these are 2 different physical quantities. Watts measure power (the rate at which energy is consumed or transmitted), while watt-hours are used to express the energy produced over a specific time. This measurement is often applied to household electrical appliances.

For example, a lamp with a power of 100 W worked for one hour, then it consumed 100 W * h, and a lamp with a power of 40 watts will consume the same amount of electricity in 2.5 hours.

In order to measure the power of the electrical circuit, use a wattmeter

What type of current is more efficient and what is the difference between them?

Constant electric current is easy to use in the case of parallel connection of generators; for alternating current, synchronization of the generator and the power system is necessary.

An event called "War of currents" took place in history. This "war" took place between two brilliant inventors - Thomas Edison and Nikola Tesla. The first supported and actively promoted constant electric current, and the second variable. The "war" ended with Tesla's victory in 2007, when New York finally switched to alternating.

The difference in the efficiency of power transmission over a distance turned out to be huge in favor of alternating current. Constant electric current cannot be used if the station is far from the consumer.

But the permanent one still found a scope: it is widely used in electrical engineering, galvanization, and some types of welding. Also, constant electric current has become very widespread in the field of urban transport (trolleybuses, trams, metro).

Naturally, there are no good or bad currents, each type has its own advantages and disadvantages, the most important thing is to use them correctly.

Speaking about direct current (see the section "About current"), we found out that it flows in one direction - from the plus of the source to the minus (this was accepted, although in fact it was the other way around). However, in most cases, you have to deal with alternating current. With alternating current, electrons move not in one direction, but alternately in one direction, then in another, changing their direction. Therefore, when the lamp is on, the electrons in its incandescent filament (and in the wires too) move in one direction or the other. This movement is conventionally shown in Fig. 1 and Fig. 2. Try to run one way or the other. It is easy to guess that with such a movement, before changing the direction of movement, you must first slow it down, then freeze in place, and only then rush in the other direction. What is the relationship with current? Before changing the motion, the electrons must slow down (we are considering all this in slow motion). This means the current will decrease, and the lamp should decrease the brightness. And when they stop before changing the movement, it should go out altogether. But we do not see this. Why? Because the incandescent thread has thermal inertness and cannot cool down in a split second. Therefore, we do not see blinking. However, each of us heard the hum of a working transformer, which is associated with the alternating direction of the current flow.

Now it's worth considering. Does this mean that in a split second the electrons from the power plant reach the house, and in the next split second - back? Earlier, in the section "About current", we found out that the electric field in conductors propagates at a speed of 300000 km / s, and the electrons themselves move in the conductors at a speed of about 0.1 mm / s. But in 1/100 of a second (this is how long one half-period lasts, during which the electrons move in one direction), the electrons only have time to move in one direction, as the electric field begins to act in the opposite direction. That is why the electrons are deflected in one direction or the other and do not leave, so to speak, the limit of our dwellings. That is, you have your own "home" electrons in your house (apartment). If we could slow down the time and plug in a voltmeter parallel to the load, i.e. lamp (Fig. 3) or an ammeter in series through the load (Fig. 4), you would see how the arrow of the device smoothly changes its reading from zero to the maximum value when measuring voltage (Fig. 3) or current (Fig. 4). The figure next to it is demonstrated. In reality, of course, we will not see this. The reason is the inertness of the arrow, because of which it cannot produce a hundred in a second. By the way, in Fig. 3 and Fig. 4 there is an explanatory Fig. 5, where you can certainly see without much effort how a voltmeter and an ammeter are connected when measuring voltage and current in an electrical circuit. Where is the voltmeter, and where is the ammeter, I think, you can easily guess. In the diagrams, they are designated as V and A, respectively.

So, the first thing you need to know is that changes in current and voltage in an electrical circuit occur according to the so-called sinusoidal law. Second, any sinusoidal oscillation (current or voltage) is characterized by the following important quantities:

Period T- the time of one complete oscillation. Half of this time is called half cycle. It is obvious that in one half-period the current flows (well, or as we stipulated - electrons move) in one direction, which we can conventionally take as positive, and in the other half-period it flows in another direction, which we can take as negative. On the charts, a positive half-period will be represented by the upper half-wave above the X-axis, and a negative half-period - by the lower one. Speaking about our network, we can indicate that the period of the alternating current T = 1 / 50sec - 0.02sec.

Frequency f is the number of vibrations per second. Now let's count. If we have one oscillation during the period T, which is 0.02 sec, then in one second we will have 50 oscillations (1 / 0.02 = 50). And one vibration is the movement of electrons, first in one direction, then in the other (two half periods). Those. in 1 second the electrons will move alternately in one direction or the other 50 times. So much for our current frequency in the network, which is 50 Hz (Hertz).

Amplitude- the highest value of current (Imax) or voltage (Umax = 310V) during the period T. It is obvious that in one period the sinusoidal current and voltage reach twice their maximum value.

Instant value - we already know that alternating current continuously changes its direction and magnitude. The magnitude of the voltage at the moment is called instantaneous value voltage. The same applies to the magnitude of the current.

As an illustration, Fig. 6 shows several instantaneous values ​​(200V, 300V, 310V, - 150V, - 310V, - 100V) of the voltage in the electrical circuit during one period. It can be seen that at the initial moment the voltage is equal to zero, after which it gradually increases to 100V, 200V, etc. Having reached the maximum value of 310V, the voltage begins to gradually decrease to zero, after which it changes its direction and increases again, reaching a value of minus 310V (- 310V), etc. If someone can hardly imagine what a change of direction is, they can imagine that the plus and minus in the socket are reversed - i.e. if we conditionally take zero (ground) as minus, and phase as plus. And this happens 50 times a second. Well, something like this ...

Effective value

So, let's ask ourselves the question - what constant voltage is equal in its action to our alternating voltage in the network, shown in Fig. 6? Theory and practice show that it is equal to a constant voltage of 220V - Fig. 7. Taking this on faith is not so difficult, since it is easy to see that the voltage considered during one period has a value of 310V only at two moments, and the rest of the time it is less. Since our sinusoidal voltage changes continuously, it was advisable to introduce such a concept as -effective voltage ... Indeed, it is for any specific voltage (or current) value, and not its changing value, that we can "estimate" its strength. So, by the effective value of an alternating current (or voltage), we mean such a direct current that, in the same time, does the same work (or emits the same amount of heat) as this alternating current.

Therefore, our ordinary light bulb (or, for example, a heating device) will work in the same way both with an alternating voltage ranging from zero to 310V, and with a constant voltage of 220V. A 12-volt light bulb will shine equally both from a 12V alternating voltage source (varying from zero to 16.8V) and from any battery or accumulator (and they are, as you know, constant voltage sources).

So remember !!!

An electric current (voltage) that periodically changes its direction and magnitude is called alternating current. Any alternating current is characterized mainly by its frequency, amplitude and effective value;

Devices designed to measure alternating current show its effective value;

The voltage is measured with a voltmeter (or a combined device - an avometer), the current - with an ammeter (or a combined device - an avometer). Also, the current can be measured with the so-called current clamp.... They serve for non-contact current measurement - the working part of the device forms a ring around the measured wire and, according to the magnitude of the electromagnetic field acting on the working part of the device, information is displayed on its small display about the value of the flowing current. An autometer is a combined device (in the common people it is also called simply a tester), which is called an ampere-voltmeter in its data sheet completely and serves to measure both current, voltage, and resistance. And digital models can measure both the frequency of the voltage (current), and the capacitance of capacitors and other things - that's how the developer wants it;

Knowing the value of the (effective) alternating voltage, you can always find out its maximum value (do not forget - it changes according to a sinusoidal law). And the connection here is -Umax = 1.4U, where U is the effective value, and Umax is the maximum value (amplitude).