Basic quantities of electric current. Scheme, description

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Basic quantities of electric current. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electricity for beginners

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The amount of electricity and current strength. The effects of electric current can be strong or weak. The strength of the electric current depends on the amount of charge that flows through the circuit in a certain unit of time.

The more electrons moved from one pole of the source to the other, the greater the total charge carried by the electrons. This total charge is called the amount of electricity passing through the conductor.

In particular, the chemical effect of the electric current depends on the amount of electricity, i.e., the more charge passed through the electrolyte solution, the more substance will settle on the cathode and anode. In this regard, the amount of electricity can be calculated by weighing the mass of the substance deposited on the electrode and knowing the mass and charge of one ion of this substance.

The current strength is a quantity that is equal to the ratio of the electric charge that has passed through the cross section of the conductor to the time of its flow. The unit of charge is the coulomb (C), time is measured in seconds (s). In this case, the unit of current strength is expressed in C/s. This unit is called the ampere (A).

In order to measure the current strength in a circuit, an electrical measuring device called an ammeter is used. For inclusion in the circuit, the ammeter is equipped with two terminals. It is included in the circuit in series.

Voltage. We already know that electric current is an ordered movement of charged particles - electrons. This is movement. generated by an electric field that does a certain amount of work. This phenomenon is called the work of an electric current.

In order to move more charge through an electric circuit in 1 second, the electric field must do more work. Based on this, it turns out that the work of an electric current should depend on the strength of the current. But there is another value on which the work of the current depends. This value is called voltage.

Voltage is the ratio of the work of the current in a certain section of the electrical circuit to the charge flowing through the same section of the circuit. The current work is measured in joules (J), the charge is measured in pendants (C). In this regard, the unit of voltage measurement will be 1 J/C. This unit is called the volt (V).

In order for a voltage to appear in an electrical circuit, a current source is needed. In an open circuit, voltage is present only at the current source terminals. If this current source is included in the circuit, voltage will also appear in certain sections of the circuit.

In this regard, there will also be a current in the circuit. That is, briefly we can say the following: if there is no voltage in the circuit, there is no current.

In order to measure voltage, an electrical measuring device called a voltmeter is used. In its appearance, it resembles the previously mentioned ammeter, with the only difference being that the letter V is on the voltmeter scale (instead of A on the ammeter). The voltmeter has two terminals, with the help of which it is connected in parallel to the electrical circuit.

Electrical resistance. After connecting all kinds of conductors and an ammeter to an electrical circuit, you can notice that when using different conductors, the ammeter gives different readings, that is, in this case, the current strength available in the electrical circuit is different.

This phenomenon can be explained by the fact that different conductors have different electrical resistance, which is a physical quantity. In honor of the German physicist, she was named Ohm. As a rule, larger units are used in physics: kiloohm, megaohm, etc.

The resistance of the conductor is usually denoted by the letter R, the length of the conductor is L, the cross-sectional area is S. In this case, the resistance can be written as a formula:

R = R * L/S,

where the coefficient p is called resistivity. This coefficient expresses the resistance of a conductor 1 m long with a cross-sectional area equal to 1 m2. Resistivity is expressed in Ohm x m.

Since the wires, as a rule, have a rather small cross section, their areas are usually expressed in square millimeters. In this case, the unit of resistivity will be Ohm x mm²/ m. In the table below. 1 shows the resistivity of some materials.

According to Table. 1, it becomes clear that copper has the smallest electrical resistivity, and an alloy of metals has the largest. In addition, dielectrics (insulators) have high resistivity.

Electric capacity. We already know that two conductors isolated from each other can accumulate electric charges. This phenomenon is characterized by a physical quantity, which is called electrical capacitance.

The electrical capacitance of two conductors is nothing more than the ratio of the charge of one of them to the potential difference between this conductor and the neighboring one. The lower the voltage when the conductors receive a charge, the greater their capacitance. The farad (F) is taken as the unit of electrical capacitance. In practice, fractions of this unit are used: microfarad (µF) and picofarad (pF).

If you take two conductors isolated from each other, place them at a small distance from one another, you get a capacitor.

The capacitance of a capacitor depends on the thickness of its plates and the thickness of the dielectric and its permeability. By reducing the thickness of the dielectric between the plates of the capacitor, it is possible to greatly increase the capacitance of the latter.

On all capacitors, in addition to their capacitance, the voltage for which these devices are designed must be indicated.

Table 1. Electrical resistivity of some materials
The main quantities of electric current

Operation and power of electric current. From the foregoing, it is clear that the electric current does a certain amount of work. When electric motors are connected, the electric current makes all kinds of equipment work, moves trains along the rails, illuminates the streets, heats the home, and also produces a chemical effect, i.e., allows electrolysis, etc.

We can say that the work of the current in a certain section of the circuit is equal to the product of the current strength, voltage and time during which the work was done. Work is measured in joules, voltage in volts, current in amperes, and time in seconds. In this regard, 1 J = 1 V x 1 A x 1 s. From this it turns out that in order to measure the work of an electric current, three devices should be used at once: an ammeter, a voltmeter and a clock. But this is cumbersome and inefficient. Therefore, usually, the work of electric current is measured by electric meters. The device of this device contains all of the above devices.

The power of an electric current is equal to the ratio of the work of the current to the time during which it was performed. Power is denoted by the letter "P" and is expressed in watts (W). In practice, kilowatts, megawatts, hectowatts, etc. are used. In order to measure the power of the circuit, you need to take a wattmeter. Electrical engineering work current, expressed in kilowatt-hours (kWh).

Author: Smirnova L.N.

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