What is voltage and current?

Today we will talk about the most basic concepts current strength, voltage, without a general understanding of which it is impossible to build any electrical device.

So what is tension?

Simply put voltage- potential difference between two points in an electrical circuit, measured in volts. It is worth noting that voltage is always measured between two points! That is, when they say that the voltage on the controller leg is 3 Volts, it means that the potential difference between the controller leg and the ground is the same 3 Volts.

Earth(Mass, Zero) is a point electrical circuit with a potential of 0 Volt. However, it is worth noting that voltage is not always measured relative to ground. For example, by measuring the voltage between the two terminals of the controller, we will get the difference in the electrical potentials of these circuit points. That is, if there are 3 Volts on one leg (That is given point has a potential of 3 Volts relative to the ground), and on the second 5 Volt (Again, the potential relative to the ground), we will get a voltage value equal to 2 volts, which is equal to the potential difference between points 5 and 3 Volts.

From the concept of voltage follows the next concept - electric current. From the course of general physics, we remember that electric current is the directed movement of charged particles along a conductor, measured in amperes. Charged particles move due to the potential difference between the points. It is generally accepted that current flows from a point with a large charge to a point with a smaller charge. That is, it is the voltage (potential difference) that creates the conditions for the flow of current. In the absence of voltage, current is impossible, that is, between points with equal potential there is no current.

On its way, the current encounters an obstacle in the form of resistance, which prevents its flow. Resistance is measured in ohms. We'll talk more about it in the next lesson. However, the following relationship has long been derived between current, voltage and resistance:

Where I - Current in Amps, U - Voltage in Volts, R - Resistance in Ohms.

This relationship is called Ohm's law. The following conclusions from Ohm's law are also valid:

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That's all, in the next lesson we'll talk about resistance.

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During this lesson, the phenomenon of electric current will be defined, considered various situations its course and its various effects on bodies. We will also characterize the current using the magnitude of the current, give its definition, and also consider its relationship with other physical quantities.

From this lesson, we begin to repeat the knowledge we received in the eighth grade about electric current, as well as deepen this knowledge.

Definition.Electricity– directed ordered motion of charged particles (Fig. 1).

Rice. 1. Movement of charged particles

The mentioned particles can be completely different: electrons, ions (both positive and negative). Even an ordinary macrobody (for example, a ball), which is given a certain charge and a certain speed, produces a current by its movement.

It is also important to understand that ordered motion does not have to be extended to all particles. Each particle can move randomly, however, in general, the entire mass of these particles is displaced in a certain direction, and it is this displacement that causes the presence of current (Fig. 2).

Rice. 2. Orderly movement

For simplicity, we will study the so-called D.C., that is, the current at which the average velocity of charged particles does not change either its value or direction.

The main physical quantity characterizing the current is the current strength.

The current has three main actions (properties).

• Thermal. When current is passed through a conductor, heat is actively released (Fig. 3).

Rice. 3. Thermal effect of current

• Chemical. The flow of current can affect the chemical structure of substances (Fig. 4).

Rice. four. Chemical action current

• Magnetic. The presence of current initiates the presence magnetic field(Fig. 5).

Rice. 5. Magnetic effect of current

The current strength is determined by the ratio of the charge that has passed through the cross section per unit of time (per time interval) (Fig. 6).

Definition.Current strength– physical quantity, equal to the ratio of the charge that has passed through the cross section of the conductor, to the time interval for which this charge has passed.

Unit of measurement: A - ampere (in honor of the French physicist André-Marie Ampère (Fig. 7).

Rice. 7. André-Marie Ampère (1775-1836)

The device for measuring the current strength is an ammeter (Fig. 8, 9). it electrical appliance, which must be connected in series to the section where the current strength must be measured (Fig. 10).

Rice. eight. Appearance ammeter

Rice. 9. Designation of the ammeter on the electrical diagram

Rice. 10. The ammeter is connected to the circuit in series

An electric current can be compared to the movement of water through a pipe, and an ammeter is a device that measures the speed of this movement.

Consider the case of flow direct current in a cylindrical conductor and derive a formula that determines the speed of the ordered movement of electrons in metals.

Rice. 11. Scheme of current flow in a conductor

Let's write down the definition of current strength:

During the time, the cross section managed to cross all those electrons located in the space of the conductor, limited by the length (the distance that the electrons traveled in time). Therefore, it can be calculated as:

Here: - charge of one particle; is the concentration of electrons in the conductor.

We substitute this equality into the definition of the current strength, and taking into account the fact that is the modulus of the value of the electron charge:

The average speed of the ordered movement of charges.

We get the formula:

That is, the current strength and the speed of the directed movement of electrons are directly proportional.

To determine the electron concentration, it is necessary to apply the formulas from the course of molecular physics. If we make the assumption that there is one electron for each atom of the substance of the conductor, then it is true:

Knowing that , we get:

Substitute and , where is the molar mass (the mass of one mole of a substance); - Avogadro's number (the number of molecules in one mole of a substance). We get:

That is, under our assumption, the concentration of free electrons depends only on the material of the conductor (density and molar mass).

Rice. 12. All electrons throughout the volume of the conductor begin to move almost simultaneously

In the next lesson, we will consider the conditions that must be present for the existence of a current.

Bibliography

1. Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemozina, 2012.
2. Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.
3. Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.

1. Internet portal "Physics.ru" ().
2. Internet portal "Mugo.narod.ru" ().
3. Internet portal “Electric current. The strength and density of the current "().

Homework

1. Page 101: No. 775. Physics. Task book. 10-11 grades. Rymkevich A.P. - M.: Bustard, 2013. ()
2. Do charged particles move in a conductor through which no current flows?
3. What effects of current can be observed by passing current through sea water?
4. At what current strength does 32 C pass through the cross section of the conductor in 4 s?
5. *Is electric current possible in the absence of electric field?

Probably, everyone at least once in their life felt the effect of current. An ordinary battery barely perceptibly pinches when applied to the tongue. The current in the apartment socket beats quite strongly if you touch the bare wires. But the electric chair and power lines can take lives.

In all cases, we are talking about the action of electric current. How is one current so different from another that the difference in its effect is so significant? Obviously, there is some quantitative characteristic that can explain such a difference. Current, as you know, is electrons moving along a conductor. It can be assumed that the more electrons run through the cross section of the conductor, the greater action will produce a current.

Current formula

In order to characterize the charge passing through the conductor, a physical quantity called the strength of the electric current was introduced. The current strength in a conductor is the amount of electricity passing through the cross section of the conductor per unit of time. The current strength is equal to the ratio of the electric charge to the time of its passage. To calculate the current strength, the formula is used:

where I is the current strength,
q - electric charge,
t - time.

The unit of current in a circuit is 1 ampere (1 A) in honor of the French scientist André Ampère. In practice, multiple units are often used: milliamps, microamps and kiloamps.

Current measurement with an ammeter

Ammeters are used to measure current. Ammeters are different depending on what measurements they are designed for. Accordingly, the scale of the instrument is calibrated in the required values. The ammeter is connected anywhere in the network in series. Where the ammeter is connected does not matter, since the amount of electricity passing through the circuit will be the same in any place. Electrons cannot accumulate in any places in the circuit, they flow evenly through all the wires and elements. When connecting an ammeter before and after the load, it will show the same values.

The first scientists who studied electricity did not have instruments for measuring current strength and charge magnitude. They checked the presence of current with their own sensations, passing it through their body. Enough unpleasant way. At that time, the strength of the currents with which they worked were not very high, so most researchers got off with only unpleasant sensations. However, in our time, even in everyday life, not to mention industry, very large currents are used.

You should know that for human body current values ​​up to 1 mA are recognized as safe. A current greater than 100 mA can cause serious damage to the body. A current of a few amperes can kill a person. At the same time, it is also necessary to take into account the individual susceptibility of the body, which is different for each person. Therefore, one should remember the main requirement when operating electrical appliances - safety.

We are starting to publish the materials of the new heading "" and in today's article we will talk about the fundamental concepts, without which there is no discussion of any electronic device or schemas. As you may have guessed, I mean current, voltage and resistance😉 In addition, we will not bypass the law that determines the relationship of these quantities, but I will not get ahead of myself, let's move gradually.

So let's start with the concept voltage.

Voltage.

By definition voltage- this is the energy (or work) that is spent on moving a single positive charge from a point with a low potential to a point with a high potential (i.e., the first point has a more negative potential compared to the second). From the course of physics, we remember that the potential of an electrostatic field is a scalar quantity equal to the ratio of the potential energy of a charge in the field to this charge. Let's look at a small example:

There is a constant in space electric field, the intensity of which is equal to E. Consider two points located at a distance d from each other. So the voltage between two points is nothing more than the potential difference at these points:

At the same time, do not forget about the relationship between the strength of the electrostatic field and the potential difference between two points:

And as a result, we get a formula linking stress and tension:

In electronics, when considering various schemes, the voltage is still considered to be the potential difference between the points. Accordingly, it becomes clear that the voltage in the circuit is a concept associated with two points in the circuit. That is, to say, for example, “voltage in the resistor” is not entirely correct. And if they talk about voltage at some point, then they mean the potential difference between this point and "earth". So smoothly we came to another important concept in the study of electronics, namely the concept "Earth"🙂 So "earth" in electrical circuits, it is most often customary to consider the point of zero potential (that is, the potential of this point is 0).

Let's say a few more words about the units that help characterize the quantity voltage. The unit of measurement is Volt (V). Looking at the definition of voltage, we can easily understand that to move a charge of magnitude 1 Pendant between points having a potential difference 1 Volt, it is necessary to do work equal to 1 Joule. With this, everything seems to be clear and you can move on 😉

And next in line we have one more concept, namely current.

Current, current in the circuit.

What is electricity?

Let's think about what will happen if charged particles, for example, electrons, fall under the influence of an electric field ... Let's consider a conductor to which a certain voltage:

From the direction of the electric field strength ( E) we can deduce that title="(!LANG:Rendered by QuickLaTeX.com" height="16" width="60" style="vertical-align: -4px;"> (вектор напряженности всегда направлен в сторону уменьшения потенциала). На каждый электрон начинает действовать сила:!}

Where e is the charge of the electron.

And since the electron is a negatively charged particle, the force vector will be directed in the direction opposite to the direction of the field strength vector. Thus, under the action of a force, particles along with chaotic motion acquire a directed motion (velocity vector V in the figure). As a result, there is electricity 🙂

Current is the ordered movement of charged particles under the influence of an electric field.

An important nuance is that it is generally accepted that the current flows from a point with a more positive potential to a point with a more negative potential, despite the fact that the electron moves in the opposite direction.

Charge carriers can be not only electrons. For example, in electrolytes and ionized gases, the flow of current is primarily associated with the movement of ions, which are positively charged particles. Accordingly, the direction of the force vector acting on them (and at the same time the velocity vector) will coincide with the direction of the vector E. And in this case, there will be no contradiction, because the current will flow exactly in the direction in which the particles move 🙂

In order to estimate the current in the circuit, they came up with such a value as the current strength. So, current strength (I) is a value that characterizes the speed of movement of an electric charge at a point. The unit of current strength is Ampere. The current strength in the conductor is 1 ampere if for 1 second charge passes through the cross section of the conductor 1 Pendant.

We have already considered the concepts current and voltage, now let's see how these quantities are related. And for this we have to study what it is conductor resistance.

Conductor/circuit resistance.

The term " resistance” already speaks for itself 😉

So, resistance- physical quantity characterizing the properties of the conductor to prevent ( resist) the passage of an electric current.

Consider a copper conductor with a length l with a cross-sectional area equal to S:

The resistance of a conductor depends on several factors:

Resistivity is a tabular value.

The formula by which you can calculate the resistance of a conductor is as follows:

For our case, it will be 0.0175 (ohm * sq. mm / m) – resistivity copper. Let the length of the conductor be 0.5 m, and the cross-sectional area is 0.2 sq. mm. Then:

As you already understood from the example, the unit of measure resistance is Ohm 😉

FROM conductor resistance everything is clear, it is time to study the relationship voltage, current and circuit resistance.

And here the fundamental law of all electronics comes to our aid - Ohm's law:

The strength of the current in the circuit is directly proportional to the voltage and inversely proportional to the resistance of the section of the circuit under consideration.

Consider the simplest electrical circuit:

As follows from Ohm's law, the voltage and current in the circuit are related as follows:

Let the voltage be 10 V and the circuit resistance be 200 ohms. Then the current strength in the circuit is calculated as follows:

As you can see, everything is easy 🙂

Perhaps this is where we will end today's article, thanks for your attention and see you soon! 🙂

In § 8-i we considered an experiment with a lamp and two coils (resistors). We noted that by a change in current strength we mean a change in the flow of electrons passing through a conductor. This phrase refers to solid metal conductors. In liquid metals (for example, in mercury), in molten or dissolved substances (for example, in salts, acids and alkalis), as well as in gases, the current is created by electrons and ions (see § 8th). All of them are carriers of electric charge.
Therefore, it is more convenient to understand the current strength not as the number of various charged particles (electrons and / or ions) passing through the conductor in a certain time, but the total charge transferred through the conductor per unit time. In formula form, it looks like this:

So, current strength - a physical quantity showing the charge passing through the conductor per unit of time.

A device is used to measure the current ammeter. It is connected in series with the section of the circuit in which the current is to be measured. Unit of current strength - 1 amp(1 A). It is set by measuring the force of interaction (attraction or repulsion) of conductors with current. For an explanation, see the picture with foil strips posted at the very beginning of this topic.
For 1 ampere, the strength of such a current is taken that, when passing through two parallel straight conductors of infinite length and small diameter, located at a distance of 1 m from each other in vacuum, causes an interaction force equal to 0.0000002 H on a section of the conductor 1 m long.
Let's get acquainted with current distribution laws in circuits with different connection of conductors. In diagrams "a", "b", "c" the lamp and the rheostat are connected sequentially. In the schemes "g", "d", "e" the lamps are connected parallel. Take an ammeter and measure the current strength in the places marked with red dots.
First, turn on the ammeter between the rheostat and the lamp (diagram "a"), measure the current strength and denote it with the symbol Icommon. Then we place the ammeter to the left of the rheostat (diagram "b"). Let's measure the current strength, denoting it with the symbol I1 . Then we place the ammeter to the left of the lamp, we denote the current strength I2 (scheme "c").


in all sections of the circuit with a series connection of conductors, the current strength is the same:

Let us now measure the current in different areas chains with parallel connection two lamps. In the "d" scheme, the ammeter measures the total current strength; on diagrams "d" and "e" - the strength of the currents passing through the upper and lower lamps.


Numerous measurements show that current strength in the unbranched part of the circuit with parallel connection of conductors ( total strength current) is equal to the sum of the currents in all branches of this circuit.