The lesson is devoted to the concept electrical voltage, its designation and units of measurement. The second part of the lesson is devoted primarily to demonstrating voltage measuring devices on a section of a circuit and their features.
If we give a standard example about the meaning of the well-known inscription on any household appliances “220 V”, then it means that 220 J of work is done on a section of the circuit to move a charge of 1 C.
Formula for calculating voltage:
Electric field work on charge transfer, J;
Charge, Cl.
Therefore, the voltage unit can be represented as follows:
There is a relationship between the formulas for calculating voltage and current that you should pay attention to: and. Both formulas contain the quantity electric charge, which may be useful in solving some problems.
To measure voltage, a device called voltmeter(Fig. 2).
Rice. 2. Voltmeter ()
There are various voltmeters according to the features of their application, but the principle of their operation is based on the electromagnetic effect of current. All voltmeters are designated by a Latin letter, which is applied to the instrument dial and is used in a schematic representation of the device.
In school settings, for example, voltmeters are used, shown in Figure 3. They are used to measure voltage in electrical circuits during laboratory work.
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Rice. 3. Voltmeters
The main elements of a demonstration voltmeter are the body, scale, pointer and terminals. The terminals are usually labeled plus or minus and are highlighted in different colors for clarity: red - plus, black (blue) - minus. This was done in order to ensure that the terminals of the device are obviously correctly connected to the corresponding wires connected to the source. Unlike an ammeter, which is connected to the open circuit in series, a voltmeter is connected to the circuit in parallel.
Of course, any electrical measuring device should have minimal influence on the circuit under study, therefore the voltmeter has such design features that a minimum current flows through it. This effect is ensured by the selection of special materials that contribute to minimal charge flow through the device.
Schematic representation of a voltmeter (Fig. 4):
Rice. 4.
Let us draw, for example, an electrical circuit (Fig. 5) in which a voltmeter is connected.
Rice. 5.
The circuit contains an almost minimal set of elements: a current source, an incandescent lamp, a switch, an ammeter connected in series, and a voltmeter connected in parallel to the light bulb.
Comment. Better start assembling electrical circuit from all elements except the voltmeter, and connect it at the end.
There are many various types voltmeters with different scales. Therefore, the question of calculating the price of the device in this case is very relevant. Microvoltmeters, millivoltmeters, simply voltmeters, etc. are very common. Their names make it clear with what frequency the measurements are made.
In addition, voltmeters are divided into direct current and alternating current devices. Although there is alternating current in the city network, at this stage of studying physics we are dealing with direct current, which is supplied by all galvanic elements, so we will be interested in the corresponding voltmeters. The fact that the device is intended for alternating current circuits is usually depicted on the dial as a wavy line (Fig. 6).
Rice. 6. AC voltmeter ()
Comment. If we talk about voltage values, then, for example, a voltage of 1 V is a small value. Industry uses much higher voltages, measured in hundreds of volts, kilovolts and even megavolts. In everyday life, a voltage of 220 V or less is used.
In the next lesson we will learn what the electrical resistance of a conductor is.
Bibliography
- Gendenshtein L. E., Kaidalov A. B., Kozhevnikov V. B. Physics 8 / Ed. Orlova V. A., Roizena I. I. - M.: Mnemosyne.
- Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
- Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M.: Education.
Additional precommended links to Internet resources
- Cool physics ().
- YouTube().
- YouTube().
Homework
That is, the electric field had to “pull” electrons through the load, and the energy that was consumed in this case is characterized by a quantity called electrical voltage. The same energy was spent on some change in the state of the load substance. Energy, as we know, does not disappear into nowhere and does not appear from nowhere. This is what it says Law of energy conservation. That is, if the current spent energy passing through the load, the load acquired this energy and, for example, heated up.
That is, we come to the definition: electric current voltage is a quantity that shows how much work the field did when moving a charge from one point to another. The voltage in different parts of the circuit will be different. The voltage on a section of an empty wire will be very small, and the voltage on a section with any load will be much greater, and the magnitude of the voltage will depend on the amount of work done by the current. Voltage is measured in volts (1 V). To determine the voltage there is a formula:
where U is the voltage, A is the work done by the current to move charge q to a certain section of the circuit.
Voltage at the poles of the current source
As for the voltage on the circuit section, everything is clear. What then does the voltage at the poles mean? current source? In this case, this voltage means the potential amount of energy that the source can impart to the current. It's like water pressure in pipes. This is the amount of energy that will be consumed if a certain load is connected to the source. Therefore, the higher the voltage at the current source, the more work the current can do.
2) Dielectrics in an electric field
Unlike conductors, dielectrics have no free charges. All charges are
connected: electrons belong to their atoms, and ions of solid dielectrics vibrate
near the nodes of the crystal lattice.
Accordingly, when a dielectric is placed in an electric field, no directional movement of charges occurs
Therefore, our proofs of properties do not pass for dielectrics
conductors - after all, all these arguments were based on the possibility of the appearance of current. Indeed, none of the four properties of conductors formulated in the previous article
does not apply to dielectrics.
2. The volumetric charge density in a dielectric can be different from zero.
3. Tension lines may not be perpendicular to the surface of the dielectric.
4. Different points of the dielectric may have different potentials. Therefore, talk about
“dielectric potential” is not necessary.
Polarization of dielectrics- a phenomenon associated with a limited displacement of bound charges in a dielectric or rotation of electric dipoles, usually under the influence of an external electric field, sometimes under the influence of other external forces or spontaneously.
The polarization of dielectrics is characterized by electric polarization vector. The physical meaning of the electric polarization vector is the dipole moment per unit volume of the dielectric. Sometimes the polarization vector is briefly called simply polarization.
The polarization vector is applicable to describe the macroscopic state of polarization not only of ordinary dielectrics, but also of ferroelectrics, and, in principle, any media with similar properties. It is applicable not only to describe induced polarization, but also spontaneous polarization (in ferroelectrics).
Polarization is a state of a dielectric, which is characterized by the presence of an electric dipole moment in any (or almost any) element of its volume.
A distinction is made between polarization induced in a dielectric under the influence of an external electric field and spontaneous (spontaneous) polarization, which occurs in ferroelectrics in the absence of an external field. In some cases, polarization of a dielectric (ferroelectric) occurs under the influence of mechanical stress, frictional forces, or due to temperature changes.
Polarization does not change the net charge in any macroscopic volume within a homogeneous dielectric. However, it is accompanied by the appearance on its surface of bound electric charges with a certain surface density σ. These bound charges create in the dielectric an additional macroscopic field with intensity , directed against the external field with intensity . As a result, the field strength inside the dielectric will be expressed by the equality:
Depending on the polarization mechanism, the polarization of dielectrics can be divided into the following types:
Electronic - displacement of the electron shells of atoms under the influence of an external electric field. The fastest polarization (up to 10−15 s). Not associated with losses.
Ionic - displacement of nodes of a crystal structure under the influence of an external electric field, and the displacement is by an amount less than the lattice constant. Flow time 10−13 s, without losses.
Dipole (Orientation) - occurs with losses in overcoming coupling forces and internal friction. Associated with the orientation of dipoles in an external electric field.
Electron relaxation - orientation of defect electrons in an external electric field.
Ion-relaxation - displacement of ions that are weakly fixed in the nodes of the crystal structure, or located in the interstice.
Structural - orientation of impurities and inhomogeneous macroscopic inclusions in the dielectric. The slowest type.
Spontaneous (spontaneous) - due to this type of polarization, in dielectrics in which it is observed, the polarization exhibits significantly nonlinear properties even at low values of the external field, and the phenomenon of hysteresis is observed. Such dielectrics (ferroelectrics) are characterized by very high dielectric constants (from 900 to 7500 for some types of capacitor ceramics). The introduction of spontaneous polarization, as a rule, increases the loss tangent of the material (up to 10 −2)
Resonant - the orientation of particles whose natural frequencies coincide with the frequencies of the external electric field.
Migration polarization is caused by the presence in the material of layers with different conductivities, the formation of space charges, especially at high voltage gradients, has large losses and is a slow-acting polarization.
Polarization of dielectrics (except for resonant polarization) is maximum in static electric fields. In alternating fields, due to the presence of inertia of electrons, ions and electric dipoles, the electric polarization vector depends on frequency.
Surely, each of us, at least once in our lives, has had questions about what current is, voltage, charge, etc. All these are components of one large physical concept - electricity. Let's try to study the basic patterns of electrical phenomena using simple examples.
What is electricity?
Electricity is a set of physical phenomena associated with the emergence, accumulation, interaction and transfer of electric charge. According to most historians of science, the first electrical phenomena were discovered by the ancient Greek philosopher Thales in the seventh century BC. Thales observed the effect of static electricity: the attraction of light objects and particles to amber rubbed with wool. To repeat this experiment yourself, you need to rub any plastic object (for example, a pen or ruler) on a wool or cotton fabric and bring it to finely cut pieces of paper.
The first serious scientific work, which describes the study of electrical phenomena, was the treatise of the English scientist William Gilbert “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth,” published in 1600. In this work, the author described the results of his experiments with magnets and electrified bodies. The term electricity is also mentioned here for the first time.
W. Gilbert's research gave a serious impetus to the development of the science of electricity and magnetism: during the period from the beginning of the 17th to the end of the 19th century, a large number of experiments and formulated the basic laws describing electromagnetic phenomena. And in 1897, the English physicist Joseph Thomson discovered the electron, an elementary charged particle that determines electrical and magnetic properties substances. An electron (in ancient Greek, electron is amber) has a negative charge approximately equal to 1.602 * 10-19 C (Coulomb) and a mass equal to 9.109 * 10-31 kg. Thanks to electrons and other charged particles, electrical and magnetic processes occur in substances.
What is tension?
There are direct and alternating electric currents. If charged particles constantly move in one direction, then there is a direct current in the circuit and, accordingly, constant voltage. If the direction of movement of particles periodically changes (they move in one direction or another), then this is an alternating current and it arises, accordingly, in the presence of an alternating voltage (i.e., when the potential difference changes its polarity). Alternating current is characterized by a periodic change in the current strength: it takes on a maximum and then a minimum value. These current values are amplitude, or peak. The frequency of voltage polarity changes may vary. For example, in our country this frequency is 50 Hertz (that is, the voltage changes its polarity 50 times per second), and in the USA the frequency of alternating current is 60 Hz (Hertz).
The basic unit of measurement for electrical voltage is the volt. Depending on the magnitude, voltage can be measured in volts(IN), kilovolts(1 kV = 1000 V), millivolts(1 mV = 0.001 V), microvolts(1 µV = 0.001 mV = 0.000001 V). In practice, most often you have to deal with volts and millivolts.
There are two main types of stress - permanent And variable. Batteries and accumulators serve as a source of constant voltage. The source of alternating voltage can be, for example, the voltage in the electrical network of an apartment or house.
To measure voltage use voltmeter. There are voltmeters switches(analog) and digital.
Today, pointer voltmeters are inferior to digital ones, since the latter are more convenient to use. If, when measuring with a pointer voltmeter, the voltage readings have to be calculated on a scale, then with a digital one, the measurement result is immediately displayed on the indicator. And in terms of dimensions, a pointer instrument is inferior to a digital one.
But this does not mean that pointer instruments are not used at all. There are some processes that cannot be seen with a digital instrument, so arrows are more often used in industrial enterprises, laboratories, repair shops, etc.
On electrical circuit diagrams, a voltmeter is indicated by a circle with a capital Latin letter “ V" inside. Near symbol voltmeter is indicated by its letter designation “ P.U." and the serial number in the diagram. For example. If there are two voltmeters in the circuit, then next to the first one they write “ PU 1", and about the second " PU 2».
When measuring direct voltage, the diagram indicates the polarity of the voltmeter connection, but if alternating voltage is measured, the polarity of the connection is not indicated.
Voltage is measured between two points circuits: in electronic circuits between positive And minus poles, in electrical diagrams between phase And zero. Voltmeter connected parallel to the voltage source or parallel to the chain section- a resistor, lamp or other load on which the voltage needs to be measured:
Let's consider connecting a voltmeter: in the upper diagram, the voltage is measured across the lamp HL1 and simultaneously on the power source GB1. In the diagram below, the voltage is measured across the lamp HL1 and resistor R1.
Before measuring the voltage, determine it view and approximate size. The fact is that the measuring part of voltmeters is designed for only one type of voltage, and this results in different measurement results. A voltmeter for measuring direct voltage does not see alternating voltage, but a voltmeter for alternating voltage, on the contrary, can measure direct voltage, but its readings will not be accurate.
It is also necessary to know the approximate value of the measured voltage, since voltmeters operate in a strictly defined voltage range, and if you make a mistake with the choice of range or value, the device can be damaged. For example. The measurement range of a voltmeter is 0...100 Volts, which means that voltage can only be measured within these limits, since if a voltage is measured above 100 Volts, the device will fail.
In addition to devices that measure only one parameter (voltage, current, resistance, capacitance, frequency), there are multifunctional ones that measure all these parameters in one device. Such a device is called tester(mostly pointer measuring instruments) or digital multimeter.
We won’t dwell on the tester, that’s the topic of another article, but let’s move straight to the digital multimeter. For the most part, multimeters can measure two types of voltage within the range of 0...1000 Volts. For ease of measurement, both voltages are divided into two sectors, and within the sectors into subranges: DC voltage has five subranges, AC voltage has two.
Each subrange has its own maximum measurement limit, which is indicated by a digital value: 200m, 2V, 20V, 200V, 600V. For example. At the “200V” limit, voltage is measured in the range of 0...200 Volts.
Now the measurement process itself.
1. DC voltage measurement.
First we decide on view measured voltage (DC or AC) and move the switch to the desired sector. For example, let's take a AA battery, the constant voltage of which is 1.5 Volts. We select the constant voltage sector, and in it the measurement limit is “2V”, the measurement range of which is 0...2 Volts.
The test leads must be inserted into the sockets as shown in the figure below:
red the dipstick is usually called positive, and it is inserted into the socket, opposite which there are icons of the measured parameters: “VΩmA”;
black the dipstick is called minus or general and it is inserted into the socket opposite which there is a “COM” icon. All measurements are made relative to this probe.
We touch the positive pole of the battery with the positive probe, and the negative pole with the negative one. The measurement result of 1.59 Volts is immediately visible on the multimeter indicator. As you can see, everything is very simple.
Now there's another nuance. If the probes on the battery are swapped, a minus sign will appear in front of the one, indicating that the polarity of the multimeter connection is reversed. The minus sign can be very convenient in the process of setting up electronic circuits, when you need to determine the positive or negative buses on the board.
Well, now let’s consider the option when the voltage value is unknown. We will use a AA battery as a voltage source.
Let’s say we don’t know the battery voltage, and in order not to burn the device, we start measuring from the maximum limit “600V”, which corresponds to the measurement range of 0...600 Volts. Using the multimeter probes, we touch the poles of the battery and on the indicator we see the measurement result equal to “ 001 " These numbers indicate that there is no voltage or its value is too small, or the measurement range is too large.
Let's go lower. We move the switch to the “200V” position, which corresponds to the range of 0...200 Volts, and touch the battery poles with the probes. The indicator showed readings equal to “ 01,5 " In principle, these readings are already enough to say that the voltage of the AA battery is 1.5 Volts.
However, the zero in front suggests going even lower and measuring the voltage more accurately. We go down to the “20V” limit, which corresponds to the range of 0...20 Volts, and take the measurement again. The indicator showed “ 1,58 " Now we can say with certainty that the voltage of a AA battery is 1.58 Volts.
In this way, without knowing the voltage value, they find it, gradually decreasing from a high measurement limit to a low one.
There are also situations when, when taking measurements, the unit "" is displayed in the left corner of the indicator. 1 " A unit indicates that the measured voltage or current is higher than the selected measurement limit. For example. If you measure a voltage of 3 Volts at the “2V” limit, then a unit will appear on the indicator, since the measurement range of this limit is only 0...2 Volts.
There remains one more limit “200m” with a measurement range of 0...200 mV. This limit is intended to measure very small voltages (millivolts), which are sometimes encountered when setting up some amateur radio design.
2. AC voltage measurement.
The process of measuring alternating voltage is no different from measuring direct voltage. The only difference is that for alternating voltage the polarity of the probes is not required.
The AC voltage sector is divided into two subranges 200V And 600V.
At the “200V” limit, you can measure, for example, the output voltage of the secondary windings of step-down transformers, or any other voltage in the range of 0...200 Volts. At the “600V” limit, you can measure voltages of 220 V, 380 V, 440 V or any other voltage in the range of 0...600 Volts.
As an example, let's measure the voltage of a 220 Volt home network.
We move the switch to the “600V” position and insert the multimeter probes into the socket. The measurement result of 229 Volts immediately appeared on the indicator. As you can see, everything is very simple.
And one moment.
Before measuring high voltages, ALWAYS double check that the insulation of the probes and wires of the voltmeter or multimeter is in good condition. and also additionally check the selected measurement limit. And only after all these operations take measurements. This way you will protect yourself and the device from unexpected surprises.
And if anything remains unclear, then watch the video, which shows how to measure voltage and current using a multimeter.
Electrical voltage refers to the work done by an electric field to move a charge of 1 C (coulomb) from one point of a conductor to another.
How does tension arise?
All substances consist of atoms, which are a positively charged nucleus around which smaller negative electrons circle at high speed. In general, atoms are neutral because the number of electrons matches the number of protons in the nucleus.
However, if a certain number of electrons are taken away from the atoms, they will tend to attract the same number, forming a positive field around themselves. If you add electrons, then an excess of them will appear, and a negative field will appear. Potentials are formed - positive and negative.
When they interact, there will be mutual attraction.
The greater the difference - the potential difference - the stronger the electrons from the material with their excess content will be drawn to the material with their deficiency. The stronger the electric field and its voltage will be.
If you connect potentials with different charges of conductors, then electric will arise - a directed movement of charge carriers, seeking to eliminate the difference in potentials. To move charges along a conductor, the electric field forces perform work, which is characterized by the concept of electric voltage. 
What is it measured in?
Temperatures;
Types of voltage
Constant pressure
The voltage in the electrical network is constant when there is always a positive potential on one side and a negative potential on the other. Electric in this case has one direction and is constant.
The voltage in a direct current circuit is defined as the potential difference at its ends.
When connecting a load to a DC circuit, it is important not to mix up the contacts, otherwise the device may fail. Classic example DC voltage sources are batteries. Networks are used when there is no need to transmit energy over long distances: in all types of transport - from motorcycles to spacecraft, in military equipment, electric power industry and telecommunications, during emergency power supply, in industry (electrolysis, smelting in electric arc furnaces, etc.).
AC voltage
If you periodically change the polarity of the potentials, or move them in space, then the electric one will rush in the opposite direction. Number of such direction changes per certain time shows a characteristic called frequency. For example, standard 50 means that the polarity of the voltage in the network changes 50 times per second.
Voltage in electrical networks AC current is a time function.
The law of sinusoidal oscillations is most often used.
This happens due to what appears in the coil asynchronous motors due to the rotation of an electromagnet around it. If you expand the rotation in time, you get a sinusoid.
Consists of four wires - three phase and one neutral. the voltage between the neutral and phase wires is 220 V and is called phase. Between phase voltages also exist, called linear and equal to 380 V (potential difference between two phase wires). Depending on the type of connection in a three-phase network, you can get either phase voltage or linear voltage.
