What unit is used to measure tension? How is voltage measured? A unit of measurement for electrical voltage. What is electricity

How voltage differs from current, resistance and power can be used using this analogy. Imagine a pipe to which a certain gas or liquid pressure is applied. This pressure is tension. The amount of substance passing through the pipe per unit time will depend on the pressure. Here is an analogue of resistance, and the amount of substance passing through a pipe per unit of time is an analogue of current strength. At the same time, a certain power will be released in the form of heat on the pipe due to friction. This is an analogue of the thermal power released on a current-carrying conductor.

Voltage is measured in volts. This one is named after the Italian scientist Alessandro Volta, the inventor of one type of electrochemical current sources. A thousand volts is called a kilovolt, a million volts is called a kilovolt. A thousandth of a volt is called a millivolt, a millionth is called a microvolt.

Voltage can be constant or variable. In the second case, it periodically changes polarity with a certain frequency. AC voltage has two values: amplitude and effective. The first characterizes the amplitude of oscillations, and the second characterizes the equivalent constant voltage that would produce the same power at the same load. The relationship between the amplitude and effective voltage values ​​depends on its shape. The sine wave has one phase voltage the amplitude value exceeds the effective value by a number of times equal to the root of two.

The concept of “dangerous voltage” is not entirely correct. The danger of exposure to electricity to a person depends not on the voltage, but on the strength of the current. Another thing is that the skin has a certain resistance, and therefore a dangerous current can arise in it at a certain voltage value. The skin of different people has different resistance, and it also depends on the mental and physical state. Therefore, the threshold for dangerous voltage can vary even within the same person. At a certain voltage, the skin breaks through, and significantly less resistance from the subcutaneous layers is applied to the source, which is even more dangerous.

In addition to electrical stress, there is also mechanical stress. It occurs in structures to which external mechanical influences are applied. In addition, in some structures internal stresses may arise even at the manufacturing stage. If you make an object from a transparent material and place it between two polarizers, you can determine the presence of such voltages in them. And in a figurative sense, tension is the tense state of the human psyche.

Content:

Electric current is characterized by quantities such as current, voltage and resistance that are interconnected. Before considering the question of how voltage is measured, it is necessary to find out exactly what this quantity is and what its role is in the formation of current.

How does tension work?

The general concept of electric current is the directed movement of charged particles. These particles are electrons, the movement of which occurs under the influence of an electric field. The more charges you need to move, the big job performed by the field. This work is affected not only by current, but also by voltage.

The physical meaning of this value is that the work done by the current in any section of the circuit is correlated with the amount of charge that passes through this section. During this work positive charge moves from a point where there is a small potential to a point with a large potential value. Thus, voltage is defined as electromotive force, and work itself is energy.

The work done by electric current is measured in joules (J), and the magnitude electric charge is the pendant (Cl). As a result, the voltage is a ratio of 1 J/C. The resulting unit of voltage is called a volt.

To clearly explain the physical meaning of stress, you need to refer to the example of a hose filled with water. In this case, the volume of water will play the role of current strength, and its pressure will be equivalent to voltage. When water moves without a tip, it is free and in large quantities moves along the hose, creating low pressure. If you press the end of the hose with your finger, the volume will decrease while the water pressure increases. The jet itself will travel a much greater distance.

The same thing happens in electricity. The strength of the current is determined by the number or volume of electrons moving through the conductor. The voltage value is essentially the force with which those electrons are pushed through. It follows that, given the same voltage, a conductor that conducts a larger amount of current must also have a larger diameter.

Voltage unit

The voltage can be constant or variable, depending on the current. This value can be designated as the letter B (Russian designation) or V, corresponding to the international designation. To indicate alternating voltage, the “~” symbol is used, which is placed in front of the letter. For constant voltage there is a “-” sign, but in practice it is almost never used.

When considering the question of how voltage is measured, it should be remembered that there are not only volts for this. Larger quantities are measured in kilovolts (kV) and megavolts (mV), which means 1 thousand and 1 million volts, respectively.

How to measure voltage and current

The lesson is devoted to the concept of 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 home 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 value of 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's depict for example electrical diagram(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. It is better to start assembling an electrical circuit with all elements except the voltmeter, and connect it at the end.

There are many different types of 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 devices direct current and alternating current. 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

1. Gendenshtein L. E., Kaidalov A. B., Kozhevnikov V. B. Physics 8 / Ed. Orlova V. A., Roizena I. I. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M.: Education.

Additional precommended links to Internet resources

1. Cool physics ().
2. YouTube().
3. YouTube().

Homework

In practice, voltage measurements have to be performed quite often. Voltage is measured in radio engineering, electrical devices and circuits, etc. The type of alternating current can be pulsed or sinusoidal. Voltage sources are either current generators.

Pulse current voltage has amplitude and average voltage parameters. Sources of such voltage can be pulse generators. Voltage is measured in volts and is designated “V” or “V”. If the voltage is alternating, then the symbol “ ~ ", for constant voltage the symbol "-" is indicated. The alternating voltage in the home household network is marked ~220 V.

These are instruments designed to measure and control the characteristics of electrical signals. Oscilloscopes work on the principle of deflecting an electron beam, which produces an image of the values ​​of variable quantities on the display.

AC voltage measurement

According to regulatory documents, the voltage in a household network must be equal to 220 volts with a deviation accuracy of 10%, that is, the voltage can vary in the range of 198-242 volts. If the lighting in your home has become dimmer, lamps have begun to fail frequently, or household devices have become unstable, then to identify and eliminate these problems, you first need to measure the voltage in the network.

Before measuring, you should prepare your existing measuring device for use:

• Check the integrity of the insulation of control wires with probes and tips.
• Set the switch to AC voltage, with an upper limit of 250 volts or higher.
• Insert the tips of the control wires into the sockets measuring instrument, For example, . To avoid mistakes, it is better to look at the designations of the sockets on the case.
• Turn on the device.

The figure shows that the measurement limit of 300 volts is selected on the tester, and 700 volts on the multimeter. Some devices require that several different switches be set to the desired position in order to measure voltage: the type of current, the type of measurement, and also insert the wire tips into certain sockets. The end of the black tip in the multimeter is inserted into the COM socket (common socket), the red tip is inserted into the socket marked “V”. This socket is common for measuring any kind of voltage. The socket marked “ma” is used for measuring small currents. The socket marked “10 A” is used to measure a significant amount of current, which can reach 10 amperes.

If you measure the voltage with the wire inserted into the “10 A” socket, the device will fail or the fuse will blow. Therefore, you should be careful when performing measuring work. Most often, errors occur in cases where the resistance was first measured, and then, forgetting to switch to another mode, they begin to measure the voltage. In this case, a resistor responsible for measuring resistance burns out inside the device.

After preparing the device, you can begin measurements. If nothing appears on the indicator when you turn on the multimeter, this means that the battery located inside the device has expired and requires replacement. Most often, multimeters contain “Krona”, which produces a voltage of 9 volts. Its service life is about a year, depending on the manufacturer. If the multimeter has not been used for a long time, the crown may still be faulty. If the battery is good, the multimeter should show one.

The wire probes must be inserted into the socket or touched with bare wires.

The multimeter display will immediately show the value of the network voltage in digital form. On a dial gauge, the needle will deviate by a certain angle. The pointer tester has several graduated scales. If you look at them carefully, everything becomes clear. Each scale is designed for a specific measurement: current, voltage or resistance.

The measurement limit on the device was set to 300 volts, so you need to count on the second scale, which has a limit of 3, and the readings of the device must be multiplied by 100. The scale has a division value equal to 0.1 volts, so we get the result shown in the figure, about 235 volts. This result is within acceptable limits. If the meter readings constantly change during measurement, there may be poor contact in the electrical wiring connections, which can lead to arcing and network faults.

DC voltage measurement

Sources of constant voltage are batteries, low-voltage or batteries whose voltage does not exceed 24 volts. Therefore, touching the battery poles is not dangerous, and there is no need for special safety measures.

To assess the performance of a battery or other source, it is necessary to measure the voltage at its poles. For AA batteries, the power poles are located at the ends of the case. The positive pole is marked “+”.

Direct current is measured in the same way as alternating current. The only difference is in setting the device to the appropriate mode and observing the polarity of the terminals.

The battery voltage is usually marked on the case. But the measurement result does not yet indicate the health of the battery, since the electromotive force of the battery is measured. The duration of operation of the device in which the battery will be installed depends on its capacity.

To accurately assess the performance of the battery, it is necessary to measure the voltage with a connected load. For a AA battery as a load regular one will do 1.5 volt flashlight bulb. If the voltage decreases slightly when the light is on, that is, by no more than 15%, therefore, the battery is suitable for operation. If the voltage drops significantly more, then such a battery can only serve for wall clock, which consume very little energy.

Voltage unit

First, we'll briefly review the concept of voltage and the units of voltage. Electric current can be thought of as the directed movement of electrons caused by an electric field.

Voltage unit

The greater the number of moving electrons, the more work is done by the electric field. In addition to current, voltage also affects the operation of the electric field.

This work involves moving electrons from a point of low potential to a point where the charge on the electrons is greater. In other words, voltage can be considered as a potential difference, and it is determined by the ratio:

U = A/q where: A is expressed in joules as the work of the electric field, and q is the charge of electrons in coulombs.

Where does the voltage unit come from:

1B = 1 J/1C. That is, the unit of voltage measurement is 1 Volt.

In the electrical network of residential buildings, the standard phase voltage of 220 V or linear three-phase voltage of 380 V is adopted.

Measuring voltage with a multimeter

To measure voltage, you need a multimeter, tester or voltmeter. The multimeter is convenient to use when installing electrical wiring, testing cables, repairing sockets, chandeliers and switches. Thus, a multimeter has become a necessary device in every home.

There are three types of voltage - alternating voltage (ACV), direct voltage (DCV) and pulse voltage. Pulse voltage has several parameters and is best checked with an oscilloscope. You can check with a multimeter impulse voltage in the position of the DCV switch, but only purely conditionally. When repairing switching power supplies, use an oscilloscope.

In most apartments and houses, the electrical network has 220 V. When measuring alternating voltage, the measurement type switch is set to V ~. If the measured alternating voltage is known, then the measurement limit is set to the appropriate position, and if its value is not known, then the switch is set to the maximum limit of 750 V.

Switch position when measuring voltage

Before measuring voltage with a multimeter, the black probe is inserted into the COM socket, and the red socket into VΩmA. When measuring, do not touch the metal parts of the probes with your hands and short-circuit them, in order to avoid a short circuit. The 10A multimeter socket is designed to measure DC current up to 10A.

In this case, the red probe is inserted into the 10 A socket, the black one remains in the COM socket, and the switch is set to the 10 A position. When measuring direct voltage, the probes are placed in the same sockets as when measuring alternating voltage, and the choice of measurement mode is set to position V - the corresponding limit.

Voltage sockets used

In this case, the probes should be set to the appropriate polarity, the red probe to the plus (+) of the source being measured, and the black probe to the minus (-). If the probes are mixed up, then nothing bad will happen, only the multimeter will show a minus sign (-) in front of the number. For alternating voltage, the polarity of the probes does not matter. In everyday life, DC voltage measurements are carried out when checking batteries, accumulators, and repairing household appliances.

How to check the voltage in an outlet with a multimeter

To measure the voltage in an outlet, you need to perform the same operations with a multimeter as when measuring alternating voltage. Since an alternating voltage of 220 V is supplied to the socket, with some variation, the measurement limit is set to 750 V. The black probe should be in the COM socket, and the red one in VΩmA. Carefully, without touching the metal ends of the probes with your hands, insert them into the sockets of the socket. The display will show the mains voltage.

Measuring voltage in a socket

You can also use a multimeter to determine the phase in the socket. To do this, one probe is applied to the ground, on the third grounding contact of the socket, and the other probe is inserted in turn into the sockets of the socket until the mains voltage appears on the display. This socket will contain the phase, and the other will contain the neutral. It is possible that there will be no voltage in this outlet. This indicates a malfunction in the outlet itself or in the electrical wires connected to it.