What is current in physics. What is electric current

Electric current is the ordered movement of charged particles.

2. Under what conditions does electric current occur?

Electric current occurs if there are free charges, as well as as a result of the action of an external electric field. To obtain an electric field, it is enough to create a potential difference between some two points of the conductor.

3. Why is the movement of charged particles in a conductor in the absence of an external electric field chaotic?

If there is no external electric field, then there is also no additional velocity component directed along the electric field strength, which means that all directions of particle motion are equal.

4. How does the movement of charged particles in a conductor differ in the absence and presence of an external electric field?

In the absence of an electric field, the movement of charged particles is chaotic, and in its presence, the movement of particles is the result of chaotic and translational movements.

5. How is the direction of electric current selected? In what direction do electrons move in a metal conductor carrying electric current?

The direction of the electric current is taken to be the direction of movement of positively charged particles. In a metal conductor, electrons move in the direction opposite to the direction of the current.

Without some basic knowledge about electricity, it is difficult to imagine how electrical appliances work, why they work at all, why you need to plug in the TV to make it work, and why a flashlight only needs a small battery to shine in the dark.

And so we will understand everything in order.

Electricity

Electricity- This natural phenomenon, confirming the existence, interaction and movement of electric charges. Electricity was first discovered back in the 7th century BC. Greek philosopher Thales. Thales noticed that if a piece of amber is rubbed on wool, it begins to attract light objects. Amber in ancient Greek is electron.

This is how I imagine Thales sitting, rubbing a piece of amber on his himation (this is the woolen outerwear of the ancient Greeks), and then with a puzzled look he watches as hair, scraps of thread, feathers and scraps of paper are attracted to the amber.

This phenomenon is called static electricity. You can repeat this experience. To do this, rub a regular plastic ruler thoroughly with a woolen cloth and bring it to the small pieces of paper.

It should be noted that this phenomenon has not been studied for a long time. And only in 1600, in his essay “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth,” the English naturalist William Gilbert introduced the term electricity. In his work, he described his experiments with electrified objects, and also established that other substances can become electrified.

Then, for three centuries, the most advanced scientists in the world research electricity, write treatises, formulate laws, invent electrical machines, and only in 1897 Joseph Thomson discovers the first material carrier of electricity - the electron, a particle that makes electrical processes in substances possible.

Electron– this is an elementary particle, has a negative charge approximately equal to -1.602·10 -19 Cl (Pendant). Designated e or e –.

Voltage

To make charged particles move from one pole to another, it is necessary to create between the poles potential difference or - Voltage. Voltage unit – Volt (IN or V). In formulas and calculations, voltage is denoted by the letter V . To obtain a voltage of 1 V, you need to transfer a charge of 1 C between the poles, while doing 1 J (Joule) of work.

For clarity, imagine a water tank located at a certain height. A pipe comes out of the tank. Water under natural pressure leaves the tank through a pipe. Let's agree that water is electric charge, the height of the water column (pressure) is voltage, and the speed of water flow is electric current .

Thus, the more water in the tank, the higher the pressure. Similarly from an electrical point of view, the greater the charge, the higher the voltage.

Let's start draining the water, the pressure will decrease. Those. The charge level drops - the voltage decreases. This phenomenon can be observed in a flashlight; the light bulb becomes dimmer as the batteries are discharged. Please note that the lower the water pressure (voltage), the lower the water flow (current).

Electric current

Electric current- This physical process directional movement of charged particles under the influence of an electromagnetic field from one pole of a closed electrical circuit to the other. Charge-carrying particles can include electrons, protons, ions and holes. Without a closed circuit, no current is possible. Particles capable of transporting electric charges do not exist in all substances, those in which they exist are called conductors And semiconductors. And substances in which there are no such particles - dielectrics.

Current unit – Ampere (A). In formulas and calculations, current strength is indicated by the letter I . A current of 1 Ampere is generated when a charge of 1 Coulomb (6.241·10 18 electrons) passes through a point in an electrical circuit in 1 second.

Let's look again at our water-electricity analogy. Only now let’s take two tanks and fill them with an equal amount of water. The difference between the tanks is the diameter of the outlet pipe.

Let's open the taps and make sure that the flow of water from the left tank is greater (the diameter of the pipe is larger) than from the right. This experience is clear evidence of the dependence of flow speed on pipe diameter. Now let's try to equalize the two flows. To do this, add water (charge) to the right tank. This will give more pressure (voltage) and increase flow rate (current). In an electrical circuit, the pipe diameter is played by resistance.

The experiments carried out clearly demonstrate the relationship between voltage, electric shock And resistance. We'll talk more about resistance a little later, but now a few more words about the properties of electric current.

If the voltage does not change its polarity, plus to minus, and the current flows in one direction, then this is D.C. and accordingly constant voltage. If the voltage source changes its polarity and the current flows first in one direction, then in the other, this is already AC And alternating voltage. Maximum and minimum values ​​(indicated on the graph as Io ) - This amplitude or peak current values. In home sockets, the voltage changes its polarity 50 times per second, i.e. the current oscillates here and there, it turns out that the frequency of these oscillations is 50 Hertz, or 50 Hz for short. In some countries, for example in the USA, the frequency is 60 Hz.

Resistance

Electrical resistance– a physical quantity that determines the property of a conductor to impede (resist) the passage of current. Resistance unit – Ohm(denoted Ohm or the Greek letter omega Ω ). In formulas and calculations, resistance is indicated by the letter R . A conductor has a resistance of 1 ohm to the poles of which a voltage of 1 V is applied and a current of 1 A flows.

Conductors conduct current differently. Their conductivity depends, first of all, on the material of the conductor, as well as on the cross-section and length. The larger the cross-section, the higher the conductivity, but the longer the length, the lower the conductivity. Resistance is the inverse concept of conductivity.

Using the plumbing model as an example, resistance can be represented as the diameter of the pipe. The smaller it is, the worse the conductivity and the higher the resistance.

The resistance of a conductor manifests itself, for example, in the heating of the conductor when current flows through it. Moreover, the greater the current and the smaller the cross-section of the conductor, the stronger the heating.

Power

Electrical power is a physical quantity that determines the rate of electricity conversion. For example, you have heard more than once: “a light bulb is so many watts.” This is the power consumed by the light bulb per unit of time during operation, i.e. converting one type of energy into another at a certain speed.

Sources of electricity, such as generators, are also characterized by power, but already generated per unit of time.

Power unit – Watt(denoted W or W). In formulas and calculations, power is indicated by the letter P . For alternating current circuits the term is used Full power, unit of measurement – Volt-amps (VA or V·A), denoted by the letter S .

And finally about Electric circuit. This circuit is a certain set of electrical components capable of conducting electric current and interconnected accordingly.

What we see in this image is a basic electrical device (flashlight). Under voltage U(B) a source of electricity (batteries) through conductors and other components with different resistances 4.59 (220 Votes)

What is current strength called? This question has arisen in our minds more than once or twice in the process of discussing various issues. Therefore, we decided to deal with it in more detail, and we will try to make it as accessible as possible without huge amount formulas and unclear terms.

So, what is electric current? This is a directed flow of charged particles. But what are these particles, why are they suddenly moving, and where? This is all not very clear. Therefore, let's look at this issue in more detail.

• Let's start with the question about charged particles, which, in fact, are carriers of electric current. IN different substances they are different. For example, what is electric current in metals? These are electrons. In gases there are electrons and ions; in semiconductors - holes; and in electrolytes these are cations and anions.

• These particles have a certain charge. It can be positive or negative. The definition of positive and negative charge is given conditionally. Particles that have the same charge repel, and particles that have the same charge attract.

• Based on this, it turns out to be logical that the movement will occur from the positive pole to the negative one. And the greater the number of charged particles present at one charged pole, the greater their number will move to the pole with a different sign.
• But this is all deep theory, so let's take a concrete example. Let's say we have an outlet to which no appliance is connected. Is there current there?
• To answer this question we need to know what voltage and current are. To make this clearer, let's look at this using the example of a pipe with water. To put it simply, the pipe is our wire. The cross-section of this pipe is the voltage of the electrical network, and the flow speed is our electric current.
• Let's return to our outlet. If we draw an analogy with a pipe, then a socket without electrical appliances connected to it is a pipe closed with a valve. That is, there is no electric current there.

• But there is tension there. And if in a pipe, in order for a flow to appear, it is necessary to open the valve, then in order to create an electric current in the conductor, you need to connect a load. This can be done by plugging the plug into the outlet.
• Of course, this is a very simplified presentation of the issue, and some professionals will criticize me and point out inaccuracies. But it gives an idea of ​​what is called electric current.

Direct and alternating current

The next question we propose to understand is: what is alternating current and direct current. After all, many do not quite correctly understand these concepts.

Constant is a current that does not change its magnitude and direction over time. Quite often, pulsating current is also considered constant, but let’s talk about everything in order.

• Direct current is characterized by the fact that the same number of electrical charges constantly replace each other in one direction. Direction is from one pole to the other.
• It turns out that a conductor always has either a positive or a negative charge. And over time this remains unchanged.

Pay attention! When determining the direction of direct current, there may be disagreements. If the current is generated by the movement of positively charged particles, then its direction corresponds to the movement of the particles. If the current is formed by the movement of negatively charged particles, then its direction is considered to be opposite to the movement of the particles.

• But the concept of direct current often includes the so-called pulsating current. It differs from a constant only in that its value changes over time, but at the same time it does not change its sign.
• Let's say we have a current of 5A. For direct current, this value will remain unchanged throughout the entire period of time. For pulsating current, in one period of time it will be 5, in another 4, and in the third 4.5. But at the same time, it in no case drops below zero and does not change its sign.

• This ripple current is very common when converting AC to DC. This is exactly the pulsating current produced by your inverter or diode bridge in electronics.
• One of the main advantages of direct current is that it can be stored. You can do this yourself, using batteries or capacitors.

AC

To understand what alternating current is, we need to imagine a sine wave. It is this flat curve that best characterizes the change in direct current and is the standard.

	Like a sine wave, alternating current with a constant frequency changes its polarity. In one period of time it is positive, and in another period of time it is negative.
	Therefore, there are no charge carriers, as such, directly in the conductor of movement. To understand this, imagine a wave rushing onto the shore. It moves in one direction and then in the opposite direction. As a result, the water seems to move, but remains in place.
	Based on this, for alternating current it is very important factor becomes its rate of polarity change. This factor is called frequency. The higher this frequency, the more often per second the polarity of the alternating current changes. In our country there is a standard for this value - it is equal to 50Hz. That is, alternating current changes its value from extremely positive to extremely negative 50 times per second.
	But there is not only alternating current with a frequency of 50 Hz. Many equipment operates on alternating current of different frequencies. Indeed, by changing the frequency of the alternating current, you can change the rotation speed of the motors. You can also get higher data processing performance - like in the chipsets of your computers, and much more.

Pay attention! You can clearly see what alternating and direct current is using the example of an ordinary light bulb. This is especially clearly visible on low-quality diode lamps, but if you look closely, you can also see it on a regular incandescent lamp. When operating on direct current, they glow with an even light, and when operating on alternating current, they flicker barely noticeably.

What is power and current density?

Well, we found out what constant current is and what alternating current is. But you probably still have a lot of questions. We will try to consider them in this section of our article.

From this video you can learn more about what power is.

• And the first of these questions will be: what is electric voltage? Voltage is the potential difference between two points.

• The question immediately arises, what is potential? Now professionals will criticize me again, but let’s say this: this is an excess of charged particles. That is, there is one point at which there is an excess of charged particles - and there is a second point where there are either more or fewer of these charged particles. This difference is called voltage. It is measured in volts (V).

• Let's take a regular outlet as an example. You all probably know that its voltage is 220V. We have two wires in the socket, and a voltage of 220V means that the potential of one wire is greater than the potential of the second wire by exactly these 220V.
• We need to understand the concept of voltage in order to understand what electric current power is. Although from a professional point of view, this statement is not entirely correct. Electric current does not have power, but is its derivative.

• To understand this point, let's go back to our water pipe analogy. As you remember, the cross-section of this pipe is the voltage, and the flow rate in the pipe is the current. So: power is the amount of water that flows through this pipe.
• It is logical to assume that with equal cross sections, that is, voltages, the stronger the flow, that is, the electric current, the greater the flow of water moves through the pipe. Accordingly, the more power will be transferred to the consumer.
• But if, in the analogy with water, we can transmit a strictly defined amount of water through a pipe of a certain cross-section, since water is not compressed, then with electric current everything is different. We can theoretically transmit any current through any conductor. But practically, a conductor with a small cross-section at a high current density will simply burn out.

• In this regard, we need to understand what current density is. Roughly speaking, this is the number of electrons that moves through a certain cross-section of a conductor per unit time.
• This number should be optimal. After all, if we take a conductor of large cross-section and transmit a small current through it, then the price of such an electrical installation will be high. At the same time, if we take a conductor of small cross-section, then due to the high current density it will overheat and quickly burn out.
• In this regard, the PUE has a corresponding section that allows you to select conductors based on the economic current density.

• But let's return to the concept of what is current power? As we understood from our analogy, with the same cross-section of the pipe, the transmitted power depends only on the current strength. But if the cross-section of our pipe is increased, that is, the voltage is increased, in this case, at the same flow rates, completely different volumes of water will be transmitted. It's the same in electrics.

• The higher the voltage, the less current is needed to transmit the same power. That is why high-voltage power lines are used to transmit large amounts of power over long distances.

After all, a line with a wire cross-section of 120 mm 2 for a voltage of 330 kV is capable of transmitting many times more power in comparison with a line of the same cross-section, but with a voltage of 35 kV. Although what is called the current strength will be the same in them.

Methods of transmitting electric current

We figured out what current and voltage are. It's time to figure out how to distribute electric current. This will allow you to feel more confident in dealing with electrical appliances in the future.

As we have already said, current can be alternating and constant. In industry, and in your sockets, alternating current is used. It is more common because it is easier to transmit over wires. The fact is that changing DC voltage is quite difficult and expensive, but changing AC voltage can be done using ordinary transformers.

Pay attention! No AC transformer will operate on DC current. Since the properties that it uses are inherent only to alternating current.

• But this does not mean at all that direct current is not used anywhere. He has another useful property, which is not inherent in the variable. It can be accumulated and stored.
• In this regard, direct current is used in all portable electrical appliances, including railway transport, as well as at some industrial facilities where it is necessary to maintain functionality even after a complete loss of power supply.

• The most common method of storing electrical energy is batteries. They have special chemical properties, allowing you to accumulate and then, if necessary, release direct current.
• Each battery has a strictly limited amount of accumulated energy. This is called the battery capacity, and is determined in part by the battery's inrush current.
• What is battery starting current? This is the amount of energy that the battery is capable of delivering at the very initial moment the load is connected. The fact is that, depending on their physical and chemical properties, batteries differ in the way they release the accumulated energy.

• Some people can give a lot at once. Because of this, they, of course, will quickly discharge. And the latter give for a long time, but a little at a time. Besides, important aspect The battery is able to maintain voltage.
• The fact is that, as the instructions say, for some batteries, as their capacity is released, their voltage gradually decreases. And other batteries are capable of delivering almost the entire capacity with the same voltage. Based on these basic properties, these storage facilities for electricity are chosen.
• To transmit direct current, two wires are used in all cases. This is a positive and negative vein. Red and blue.

AC

But with alternating current everything is much more complicated. It can be transmitted over one, two, three or four wires. To explain this, we need to understand the question: what is three-phase current?

• Our alternating current is produced by a generator. Typically, almost all of them have a three-phase structure. This means that the generator has three terminals and an electric current is supplied to each of these terminals, differing from the previous ones by an angle of 120⁰.

• To understand this, let's remember our sinusoid, which is a model for describing alternating current, and according to the laws of which it changes. Let's take three phases - “A”, “B” and “C”, and take a certain point in time. At this point, the sine wave of phase “A” is at the zero point, the sine wave of phase “B” is at the extreme positive point, and the sine wave of phase “C” is at the extreme negative point.
• Each subsequent unit of time, the alternating current in these phases will change, but synchronously. That is, after a certain time, in phase “A” there will be a negative maximum. In phase “B” there will be a zero, and in phase “C” there will be a positive maximum. And after some time, they will change again.

• As a result, it turns out that each of these phases has its own potential, different from the potential of the neighboring phase. Therefore, there must be something between them that does not conduct electric current.
• This potential difference between two phases is called line voltage. In addition, they have a potential difference relative to the ground - this voltage is called phase voltage.
• And so, if the linear voltage between these phases is 380V, then the phase voltage is 220V. It differs by a value of √3. This rule always applies for any voltage.

• Based on this, if we need a voltage of 220V, then we can take one phase wire and a wire rigidly connected to the ground. And we will get a single-phase 220V network. If we need a 380V network, then we can take only any 2 phases and connect some kind of heating device as in the video.

But in most cases, all three phases are used. All powerful consumers are connected to a three-phase network.

Conclusion

What is induced current, capacitive current, starting current, no-load current, negative sequence currents, stray currents and much more, we simply cannot consider in one article.

After all, the issue of electric current is quite extensive, and for its consideration it was created whole science electrical engineering. But we really hope that we were able to explain the main aspects in an accessible language this issue, and now electric current will not be something scary and incomprehensible for you.

If an insulated conductor is placed in an electric field \(\overrightarrow(E)\), then the force \(\overrightarrow(F) = q\overrightarrow(E)\) will act on the free charges \(q\) in the conductor \(\overrightarrow(F) = q\overrightarrow(E)\) As a result, conductor there is a short-term movement of free charges. This process will end when the own electric field of the charges arising on the surface of the conductor completely compensates for the external field. The resulting electrostatic field inside the conductor will be zero.

However, in conductors, under certain conditions, continuous ordered movement of free electric charge carriers can occur.

The directed movement of charged particles is called electric current.

The direction of the electric current is taken to be the direction of movement of positive free charges. For an electric current to exist in a conductor, an electric field must be created in it.

A quantitative measure of electric current is current strength\(I\) is a scalar physical quantity, equal to the ratio charge \(\Delta q\) transferred through the cross section of the conductor (Fig. 1.8.1) during the time interval \(\Delta t\), to this time interval:

$$I = \frac(\Delta q)(\Delta t) $$

If the current strength and its direction do not change with time, then such a current is called permanent .

In the International System of Units (SI) current is measured in Amperes (A). The current unit of 1 A is determined by the magnetic interaction of two parallel conductors with current.

Direct electric current can only be created in closed circuit , in which free charge carriers circulate along closed trajectories. Electric field in different points such a chain is constant over time. Consequently, the electric field in a direct current circuit has the character of a frozen electrostatic field. But when an electric charge moves in an electrostatic field along a closed path, the work done by electric forces is zero. Therefore, for the existence of direct current, it is necessary to have a device in the electrical circuit that is capable of creating and maintaining potential differences in sections of the circuit due to the work of forces non-electrostatic origin. Such devices are called DC sources . Forces of non-electrostatic origin acting on free charge carriers from current sources are called outside forces .

The nature of external forces may vary. In galvanic cells or batteries they arise as a result electrochemical processes, in direct current generators, external forces arise when conductors move in a magnetic field. The current source in the electrical circuit plays the same role as the pump, which is necessary to pump fluid in a closed hydraulic system. Under the influence of external forces, electric charges move inside the current source against electrostatic field forces, due to which a constant electric current can be maintained in a closed circuit.

When electric charges move along a direct current circuit, external forces acting inside the current sources perform work.

A physical quantity equal to the ratio of the work \(A_(st)\) of external forces when moving a charge \(q\) from the negative pole of the current source to the positive one to the value of this charge is called electromotive force of the source (EMF):

$$EMF=\varepsilon=\frac(A_(st))(q). $$

Thus, the EMF is determined by the work done by external forces when moving a single positive charge. Electromotive force, like potential difference, is measured in Volts (V).

When a single positive charge moves along a closed direct current circuit, the work done by external forces is equal to the sum of the emf acting in this circuit, and the work done by the electrostatic field is zero.

A DC circuit can be divided into separate sections. Those areas where no external forces act (i.e. areas that do not contain current sources) are called homogeneous . Areas containing current sources are called heterogeneous .

When a single positive charge moves along a certain section of the circuit, work is performed by both electrostatic (Coulomb) and external forces. The work of electrostatic forces is equal to the potential difference \(\Delta \phi_(12) = \phi_(1) - \phi_(2)\) between the initial (1) and final (2) points of the inhomogeneous section. The work of external forces is equal, by definition, to the electromotive force \(\mathcal(E)\) acting in a given area. Therefore the total work is equal to

$$U_(12) = \phi_(1) - \phi_(2) + \mathcal(E)$$

Size U 12 is usually called voltage on chain section 1-2. In the case of a homogeneous area, the voltage is equal to the potential difference:

$$U_(12) = \phi_(1) - \phi_(2)$$

The German physicist G. Ohm experimentally established in 1826 that the current strength \(I\) flowing through a homogeneous metal conductor (i.e., a conductor in which no external forces act) is proportional to the voltage \(U\) at the ends of the conductor :

$$I = \frac(1)(R) U; \: U = IR$$

where \(R\) = const.

Size R usually called electrical resistance . A conductor with electrical resistance is called resistor . This ratio expresses Ohm's law for homogeneous section of the chain: The current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.

The SI unit of electrical resistance of conductors is Ohm (Ohm). A resistance of 1 ohm has a section of the circuit in which a current of 1 A occurs at a voltage of 1 V.

Conductors that obey Ohm's law are called linear . Graphical dependence of current \(I\) on voltage \(U\) (such graphs are called volt-ampere characteristics , abbreviated as CVC) is depicted by a straight line passing through the origin. It should be noted that there are many materials and devices that do not obey Ohm's law, for example, a semiconductor diode or a gas-discharge lamp. Even with metal conductors, at sufficiently high currents, a deviation from Ohm’s linear law is observed, since the electrical resistance of metal conductors increases with increasing temperature.

For a section of a circuit containing an emf, Ohm's law is written in the following form:

$$IR = U_(12) = \phi_(1) - \phi_(2) + \mathcal(E) = \Delta \phi_(12) + \mathcal(E)$$
$$\color(blue)(I = \frac(U)(R))$$

This ratio is usually called generalized Ohm's law or Ohm's law for a non-uniform section of the circuit.

In Fig. 1.8.2 shows a closed DC circuit. Chain section ( CD) is homogeneous.

Figure 1.8.2. DC circuit

According to Ohm's law

$$IR = \Delta\phi_(cd)$$

Plot ( ab) contains a current source with an emf equal to \(\mathcal(E)\).

According to Ohm's law for a heterogeneous area,

$$Ir = \Delta \phi_(ab) + \mathcal(E)$$

Adding both equalities, we get:

$$I(R+r) = \Delta\phi_(cd) + \Delta \phi_(ab) + \mathcal(E)$$

But \(\Delta\phi_(cd) = \Delta \phi_(ba) = -\Delta \phi_(ab)\).

$$\color(blue)(I=\frac(\mathcal(E))(R + r))$$

This formula expresses Ohm's law for a complete circuit : the current strength in the complete circuit is equal to the electromotive force of the source divided by the sum of the resistances of the homogeneous and inhomogeneous sections of the circuit (internal resistance of the source).

Resistance r heterogeneous area in Fig. 1.8.2 can be thought of as internal resistance of the current source . In this case, the area ( ab) in Fig. 1.8.2 is the internal portion of the source. If points a And b short with a conductor whose resistance is small compared to the internal resistance of the source (\(R\ \ll r\)), then the circuit will flow short circuit current

$$I_(kz)=\frac(\mathcal(E))(r)$$

Short circuit current is the maximum current that can be obtained from a given source with electromotive force \(\mathcal(E)\) and internal resistance \(r\). For sources with low internal resistance, the short circuit current can be very high and cause destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have short-circuit currents of several hundred amperes. Short circuits in lighting networks powered from substations (thousands of amperes) are especially dangerous. To avoid the destructive effects of such large currents, fuses or special circuit breakers are included in the circuit.

In some cases, to prevent dangerous values ​​of short circuit current, some external resistance is connected in series to the source. Then resistance r is equal to the sum of the internal resistance of the source and the external resistance, and during a short circuit the current strength will not be excessively large.

If the external circuit is open, then \(\Delta \phi_(ba) = -\Delta \phi_(ab) = \mathcal(E)\), i.e. the potential difference at the poles of an open battery is equal to its emf.

If the external load resistance R turned on and current is flowing through the battery I, the potential difference at its poles becomes equal

$$\Delta \phi_(ba) = \mathcal(E) - Ir$$

In Fig. 1.8.3 shows a schematic representation of a direct current source with an emf equal to \(\mathcal(E)\) and internal resistance r in three modes: “idling”, load operation and short circuit mode (short circuit). The intensity \(\overrightarrow(E)\) of the electric field inside the battery and the forces acting on positive charges are indicated:\(\overrightarrow(F)_(e)\) - electric force and \(\overrightarrow(F)_(st )\) is an outside force. In short circuit mode, the electric field inside the battery disappears.

To measure voltages and currents in DC electrical circuits, special instruments are used - voltmeters And ammeters.

Voltmeter designed to measure the potential difference applied to its terminals. He connects parallel the section of the circuit where the potential difference is measured. Any voltmeter has some internal resistance \(R_(V)\). In order for the voltmeter not to introduce a noticeable redistribution of currents when connected to the circuit being measured, its internal resistance must be large compared to the resistance of the section of the circuit to which it is connected. For the circuit shown in Fig. 1.8.4, this condition is written as:

$$R_(B)\gg R_(1)$$

This condition means that the current \(I_(V) = \Delta \phi_(cd) / R_(V)\) flowing through the voltmeter is much less than the current \(I = \Delta \phi_(cd) / R_(1 )\), which flows through the tested section of the circuit.

Since there are no external forces acting inside the voltmeter, the potential difference at its terminals coincides, by definition, with the voltage. Therefore, we can say that a voltmeter measures voltage.

Ammeter designed to measure current in a circuit. The ammeter is connected in series to the open circuit of the electrical circuit so that the entire measured current passes through it. The ammeter also has some internal resistance \(R_(A)\). Unlike a voltmeter, the internal resistance of an ammeter must be quite small compared to the total resistance of the entire circuit. For the circuit in Fig. 1.8.4 The resistance of the ammeter must satisfy the condition

$$R_(A) \ll (r + R_(1) + R(2))$$

so that when the ammeter is turned on, the current in the circuit does not change.

Measuring instruments - voltmeters and ammeters - come in two types: pointer (analog) and digital. Digital electrical meters are complex electronic devices. Typically, digital instruments provide higher measurement accuracy.

Current and voltage are quantitative parameters used in electrical diagrams. Most often, these quantities change over time, otherwise there would be no point in the operation of the electrical circuit.

Voltage

Conventionally, voltage is indicated by the letter "U". The work expended in moving a unit of charge from a point of low potential to a point of high potential is the voltage between these two points. In other words, it is the energy released after a unit of charge moves from high to low potential.

Voltage can also be called potential difference, as well as electromotive force. This parameter is measured in volts. To move 1 coulomb of charge between two points that have a voltage of 1 volt, 1 joule of work must be done. Coulombs measure electrical charges. 1 coulomb is equal to the charge of 6x10 18 electrons.

Voltage is divided into several types, depending on the types of current.

• Constant voltage . It is present in electrostatic and direct current circuits.
• AC voltage . This type of voltage is found in circuits with sinusoidal and alternating currents. In the case of sinusoidal current, the following voltage characteristics are considered:
  amplitude of voltage fluctuations– this is its maximum deviation from the x-axis;
  instantaneous voltage, which is expressed at a certain point in time;
  effective voltage, is determined by the active work performed in the 1st half-cycle;
  average rectified voltage, determined by the magnitude of the rectified voltage over one harmonic period.

When transmitting electricity through overhead lines, the design of supports and their dimensions depend on the magnitude of the applied voltage. The voltage between phases is called line voltage , and the voltage between the ground and each phase is phase voltage . This rule applies to all types of overhead lines. In Russia, in household electrical networks, the standard is three-phase voltage with a linear voltage of 380 volts and a phase voltage of 220 volts.

Electric current

Current in an electrical circuit is the speed of movement of electrons at a certain point, measured in amperes, and denoted in diagrams by the letter “ I" Derived units of ampere with the corresponding prefixes milli-, micro-, nano, etc. are also used. A current of 1 ampere is generated by moving a unit of charge of 1 coulomb in 1 second.

It is conventionally considered that the current flows in the direction from positive potential to negative. However, from the physics course we know that the electron moves in the opposite direction.

You need to know that voltage is measured between 2 points on the circuit, and current flows through one specific point in the circuit, or through its element. Therefore, if someone uses the expression “tension in resistance,” then this is incorrect and illiterate. But often we are talking about voltage at a certain point in the circuit. This refers to the voltage between the ground and this point.

Voltage is generated from exposure to electrical charges in generators and other devices. Current is created by applying a voltage to two points on a circuit.

To understand what current and voltage are, it would be more correct to use. On it you can see the current and voltage, which change their values ​​over time. In practice, the elements of an electrical circuit are connected by conductors. At certain points, the elements of the circuit have their own voltage value.

Current and voltage obey the rules:

• The sum of currents entering a point is equal to the sum of currents leaving the point (charge conservation rule). This rule is Kirchhoff's law for current. The point of entry and exit of the current in this case is called a node. A corollary of this law is the following statement: in a series electrical circuit of a group of elements, the current value is the same for all points.
• IN parallel circuit elements, the voltage on all elements is the same. In other words, the sum of the voltage drops in a closed circuit is zero. This Kirchhoff law applies to stresses.
• The work done per unit time by a circuit (power) is expressed as follows: P = U*I. Power is measured in watts. 1 joule of work done in 1 second is equal to 1 watt. Power is distributed in the form of heat, consumed to perform mechanical work (in electric motors), and converted into radiation various types, accumulates in containers or batteries. When designing complex electrical systems, one of the challenges is the thermal load of the system.

Characteristics of electric current

A prerequisite for the existence of current in an electrical circuit is a closed circuit. If the circuit is broken, the current stops.

Everyone in electrical engineering operates on this principle. They break the electrical circuit with movable mechanical contacts, and thereby stop the flow of current, turning off the device.

In the energy industry, electric current occurs inside current conductors, which are made in the form of busbars and other parts that conduct current.

There are also other ways to create internal current in:

• Liquids and gases due to the movement of charged ions.
• Vacuum, gas and air using thermionic emission.
• , due to the movement of charge carriers.

Conditions for the occurrence of electric current

• Heating of conductors (not superconductors).
• Application of potential difference to charge carriers.
• A chemical reaction that releases new substances.
• The effect of a magnetic field on a conductor.

Current Waveforms

• Straight line.
• Variable harmonic sine wave.
• A meander, similar to a sine wave, but having sharp corners(sometimes the corners may be smoothed).
• A pulsating form of one direction, with an amplitude varying from zero to the greatest value according to a certain law.

Types of work of electric current

• Light radiation created by lighting devices.
• Generating heat using heating elements.
• Mechanical work (rotation of electric motors, operation of other electrical devices).
• Creation of electromagnetic radiation.

Negative phenomena caused by electric current

• Overheating of contacts and live parts.
• The occurrence of eddy currents in the cores of electrical devices.
• Electromagnetic radiation into the external environment.

When designing, creators of electrical devices and various circuits must take into account the above properties of electric current in their designs. For example, the harmful effects of eddy currents in electric motors, transformers and generators are reduced by fusion of the cores used to pass magnetic fluxes. Lamination of the core is its production not from a single piece of metal, but from a set of individual thin plates of special electrical steel.

But, on the other hand, eddy currents are used to operate microwave ovens and ovens operating on the principle of magnetic induction. Therefore, we can say that eddy currents are not only harmful, but also beneficial.

Alternating current with a signal in the form of a sinusoid can differ in frequency of oscillations per unit time. In our country, the industrial frequency of electrical current is standard and equal to 50 hertz. In some countries, a current frequency of 60 hertz is used.

For various purposes in electrical engineering and radio engineering, other frequency values ​​are used:

• Low frequency signals with a lower current frequency.
• High frequency signals that are much higher than the frequency of industrial current.

It is believed that electric current arises from the movement of electrons within a conductor, which is why it is called conduction current. But there is another type of electric current, which is called convection. It occurs when charged macrobodies move, for example, raindrops.

Electric current in metals

The movement of electrons when subjected to a constant force is compared to a parachutist descending to the ground. In these two cases, uniform motion occurs. The force of gravity acts on the skydiver, and the force of air resistance opposes it. The movement of electrons is affected by the force of the electric field, and the ions of the crystal lattices resist this movement. Average speed electrons reaches a constant value, as does the speed of the parachutist.

In a metal conductor, the speed of movement of one electron is 0.1 mm per second, and the speed of electric current is about 300 thousand km per second. This is because electric current only flows where voltage is applied to charged particles. Therefore it is achieved high speed current flow.

When electrons move in a crystal lattice, the following pattern exists. Electrons do not collide with all oncoming ions, but only with every tenth of them. This is explained by the laws of quantum mechanics, which can be simplified as follows.

The movement of electrons is hampered by large ions that offer resistance. This is especially noticeable when metals are heated, when heavy ions “sway”, increase in size and reduce the electrical conductivity of the conductor crystal lattices. Therefore, when metals are heated, their resistance always increases. As the temperature decreases, the electrical conductivity increases. When the metal temperature drops to absolute zero superconductivity effect can be achieved.