The formula by which the value of electrical resistance is determined. Resistance unit
Electric current (I) is the directed movement of electric charges (ions - in electrolytes, conduction electrons in metals).
A necessary condition for the flow of electric current is the closure of the electrical circuit.
Electric current is measured in amperes (A).
The derived units of current are:
1 kiloampere (kA) = 1000 A;
1 milliamp (mA) 0.001 A;
1 microamp (µA) = 0.000001 A.
A person begins to feel a current of 0.005 A passing through his body. A current of more than 0.05 A is dangerous for human life.
Electrical voltage (U) called the potential difference between two points of the electric field.
unit electrical potential differences is the volt (V).
1 V = (1 W): (1 A).
The derived units of voltage are:
1 kilovolt (kV) = 1000 V;
1 millivolt (mV) = 0.001 V;
1 microvolt (µV) = 0.00000 1 V.
The resistance of the section of the electrical circuit called a value that depends on the material of the conductor, its length and cross section.
Electrical resistance is measured in ohms (Ohm).
1 Ohm = (1 V): (1 A).
The derived units of resistance are:
1 kiloOhm (kOhm) = 1000 Ohm;
1 megaohm (MΩ) = 1,000,000 ohms;
1 milliOhm (mOhm) = 0.001 Ohm;
1 microohm (µohm) = 0.00000 1 ohm.
The electrical resistance of the human body, depending on a number of conditions, ranges from 2,000 to 10,000 ohms.
Specific electrical resistance (ρ) is the resistance of a wire with a length of 1 m and a cross section of 1 mm2 at a temperature of 20 ° C.
The reciprocal of resistivity is called electrical conductivity (γ).
Power (R) is a quantity that characterizes the rate at which energy is converted, or the rate at which work is done.
The power of a generator is a quantity that characterizes the rate at which mechanical or other energy is converted into electrical energy in the generator.
Consumer power is a value that characterizes the speed with which the conversion of electrical energy in certain sections of the circuit into other useful forms of energy.
The SI system unit for power is the watt (W). It is equal to the power at which 1 joule of work is done in 1 second:
1W = 1J/1sec
The derived units of measurement of electrical power are:
1 kilowatt (kW) = 1000 W;
1 megawatt (MW) = 1000 kW = 1,000,000 W;
1 milliwatt (mW) = 0.001 W; o1i
1 horsepower (hp) \u003d 736 W \u003d 0.736 kW.
Units of measurement of electrical energy are:
1 watt second (W sec) = 1 J = (1 N) (1 m);
1 kilowatt hour (kWh) = 3.6 106 W sec.
Example. The current consumed by the electric motor connected to the 220 V network was 10 A for 15 minutes. Determine the energy consumed by the motor.
W * sec, or by dividing this value by 1000 and 3600, we get the energy in kilowatt-hours:
W \u003d 1980000 / (1000 * 3600) \u003d 0.55 kW * h
Table 1. Electrical quantities and units
- an electrical quantity that characterizes the property of a material to prevent the flow of electric current. Depending on the type of material, the resistance can tend to zero - be minimal (miles / micro ohms - conductors, metals), or be very large (giga ohms - insulation, dielectrics). The reciprocal of electrical resistance is .
unit of measurement electrical resistance - Ohm. It is denoted by the letter R. The dependence of resistance on current and in closed circuit is determined.
Ohmmeter- a device for direct measurement of circuit resistance. Depending on the range of the measured value, they are divided into gigaohmmeters (for large resistance - when measuring insulation), and into micro / milliohmmeters (for small resistances - when measuring transient resistance of contacts, motor windings, etc.).
There is a wide variety of ohmmeters by design. different manufacturers, from electromechanical to microelectronic. It is worth noting that a classic ohmmeter measures the active part of the resistance (the so-called ohms).
Any resistance (metal or semiconductor) in an AC circuit has an active and a reactive component. The sum of active and reactance is AC circuit impedance and is calculated by the formula:
where, Z is the total resistance of the AC circuit;
R is the active resistance of the AC circuit;
Xc is the capacitive reactance of the AC circuit; 
(C is the capacitance, w is the angular velocity of the alternating current)
Xl is the inductive reactance of the AC circuit; 
(L is the inductance, w is the angular velocity of the alternating current).
Active resistance- this is part of the impedance of the electrical circuit, the energy of which is completely converted into other types of energy (mechanical, chemical, thermal). Distinctive property the active component is the total consumption of all electricity (energy is not returned to the network back to the network), and the reactance returns part of the energy back to the network (a negative property of the reactive component).
The physical meaning of active resistance
Every environment where electric charges, creates obstacles on their way (it is believed that these are the nodes of the crystal lattice), into which they seem to hit and lose their energy, which is released in the form of heat.
Thus, there is a drop (loss of electrical energy), part of which is lost due to internal resistance conducting medium.
The numerical value characterizing the ability of a material to prevent the passage of charges is called resistance. It is measured in Ohms (Ohm) and is inversely proportional to the electrical conductivity.
Miscellaneous elements periodic system Mendeleev have different electrical resistivity (p), for example, the smallest sp. silver (0.016 Ohm * mm2 / m), copper (0.0175 Ohm * mm2 / m), gold (0.023) and aluminum (0.029) have resistance. They are used in industry as the main materials on which all electrical engineering and energy are built. Dielectrics, on the other hand, have a high sp. resistance and used for insulation.
The resistance of a conducting medium can vary significantly depending on the cross section, temperature, magnitude and frequency of the current. In addition, different media have different charge carriers (free electrons in metals, ions in electrolytes, "holes" in semiconductors), which are the determining factors of resistance.
The physical meaning of reactance
In coils and capacitors, when applied, energy is accumulated in the form of magnetic and electric fields, which requires some time.
Magnetic fields in alternating current networks change following the changing direction of movement of charges, while providing additional resistance.
In addition, there is a stable phase shift and current strength, and this leads to additional losses of electricity.
Resistivity
How to find out the resistance of a material if it does not flow through it and we do not have an ohmmeter? There is a special value for this - electrical resistivity of the material in
(these are tabular values that are determined empirically for most metals). With this value and the physical quantities of the material, we can calculate the resistance using the formula:
where, p- resistivity (units of measurement ohm * m / mm 2);
l is the length of the conductor (m);
S - cross section (mm 2).
This site could not do without an article about resistance. Well, no way! There is the most fundamental concept in electronics, which is also a physical property. You probably already know these friends:
Resistance is the property of a material to interfere with the flow of electrons. The material, as it were, resists, impedes this flow, like the sails of a frigate against a strong wind!
Almost everything in the world has the ability to resist: air resists the flow of electrons, water also resists the flow of electrons, but they still slip through. copper wires also resist the flow of electrons, but lazily. So they pass such a stream very well.
Only superconductors have no resistance, but this is another story, since since they have no resistance, today they are not of interest to us.
By the way, the flow of electrons is electricity. The formal definition is more pedantic, so look for it yourself in the same dry book.
And yes, electrons interact with each other. The strength of this interaction is measured in Volts and is called voltage. You say that sounds strange? Yes, nothing strange. The electrons tense up and move other electrons with force. Somewhat rustic, but the basic principle is clear.
It remains to mention the power. Power is when current, voltage and resistance gather at the same table and start working. Then the power appears - the energy that the electrons lose when passing through the resistance. By the way:
I = U/R P = U * I
Do you have, for example, a 60W light bulb with a wire. You plug it into a 220V outlet. What's next? The light bulb provides some resistance to the flow of electrons with a potential of 220V. If the resistance is too low - boom, burned out. If too large, the filament will glow very little, if at all. But if it is "just right", then the light bulb eats 60W and turns this energy into light and heat.
Heat in this case is a side effect and is called "loss" of energy, since instead of shining brighter, the light bulb spends energy on heating. Use energy-saving lamps! By the way, the wire also has resistance, and if the electron flow is too large, it will also heat up to a noticeable temperature. Here you can suggest reading a note about why high-voltage lines are used.
I'm sure you understand more about resistance now. At the same time, we did not fall into details like the resistivity of the material and formulas like
where ρ is resistivity conductor substances, Ohm m, l— conductor length, m, a S— cross-sectional area, m².
A few animations to complete the picture
And clearly about how the electron flow changes from depending on the temperature of the conductor and its thickness
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- this is measuring device, which serves to determine the value of resistance in electrical circuits. Resistance is measured in Omaha and is denoted by the Latin letter R. About what Om is in a popular form, see the site article "Law of current strength".
Structural diagram and designation on the Ohmmeter diagrams
Measuring device Ohmmeter is structurally a pointer or digital indicator with a battery or power supply connected in series, as shown in the photo.
All combined instruments have the function of measuring resistance - pointer testers and digital multimeters.
In practice, a meter that only measures resistance is used for special cases, such as measuring insulation resistance at elevated voltage, ground loop resistance, or as a reference instrument for checking other low-precision Bosch ohmmeters.
On electrical measuring circuits, an ohmmeter is indicated by the Greek letter omega enclosed in a circle, as shown in the photograph.
Preparing the Ohmmeter for Measurements
Repair of electrical wiring, electrical and radio engineering products consists in checking the integrity of the wires and in searching for contact violations in their connections.
In some cases, the resistance must be equal to infinity, for example, insulation resistance. And in others it is equal to zero, for example, the resistance of wires and their connections. And in some cases it is equal to a certain value, for example, the resistance of the filament of a light bulb or a heating element.
Attention! It is allowed to measure the resistance of circuits, in order to avoid failure of the Ohmmeter, only when they are completely de-energized. It is necessary to remove the plug from the socket or remove the batteries from the compartment. If the circuit contains electrolytic capacitors of a larger capacity, then they must be discharged by shorting the capacitor leads through a resistance of about 100 kOhm for a few seconds.
As with voltage measurements, before measuring resistance, it is necessary to prepare the device. To do this, you need to set the switch of the device to the position corresponding to the minimum measurement of the resistance value.
Before measurements, you should check the operability of the device, as there may be bad batteries and the Ohmmeter may not work. To do this, connect the ends of the probes together.
At the same time, the arrow of the tester should be set exactly to zero, if it has not been set, then you can turn the “Set. 0". If it doesn't work, you need to replace the batteries.
For continuity of electrical circuits, for example, when checking light bulb incandescent, you can use a device in which the batteries are dead and the arrow is not set to 0, but at least reacts a little when the probes are connected. It will be possible to judge the integrity of the circuit by the fact of the deviation of the arrow. Digital instruments should also show zero readings, a deviation in tenths of ohms is possible, due to the resistance of the probes and the transient resistance in the contacts for connecting them to the terminals of the device.
With the ends of the probes open, the arrow of the tester should be set to the point indicated on the scale ∞, and in digital devices, the overload will flash or the number will be displayed 1 on the indicator on the left side.
The ohmmeter is ready to go. If you touch the ends of the probes to the conductor, then in case of its integrity, the device will show zero resistance, in otherwise, the readings will not change.
In expensive models of multimeters, there is a circuit continuity function with sound indication, indicated in the resistance measurement sector with a diode symbol. It is very convenient for continuity of low-resistance circuits, such as cable wires. twisted pair for internet or household electrical wiring. If the wire is intact, then the dialing is accompanied sound signal, which eliminates the need to read the readings from the multimeter indicator.
Examples from the practice of measuring the resistance of products
Theoretically, everything is usually clear, but in practice questions often arise that are best answered by examples of checking the most common products with an ohmmeter.
Checking incandescent lamps
The incandescent bulb in the lamp or in the car on-board instruments has stopped shining, how to find out the reason? The switch, electrical cartridge, or wiring may be faulty. With the help of a tester, any incandescent lamp from a home lamp or car headlight, filament of fluorescent lamps and energy-saving lamps is easily checked. To check, it is enough to set the switch of the device to the position of measuring the minimum resistance and touch the ends of the probes to the terminals of the light bulb base.
The resistance of the filament of the light bulb was 51 ohms, which indicates its serviceability. If the thread were broken, then the device would show infinite resistance. The resistance of a 220 V halogen bulb with a power of 50 watts when lit is about 968 ohms, a 12 volt car bulb with a power of 100 watts, about 1.44 ohms.
It is worth noting that the resistance of the filament of an incandescent lamp in a cold state (when the bulb is not lit) is several times less than in a heated one. This is due to the physical property of tungsten. Its resistance increases non-linearly with heating. Therefore, incandescent lamps, as a rule, burn out at the moment of switching on.
Checking sound-reproducing headphones
It happens that the headphones in one of the emitters, or in both at once, the sound is distorted, periodically disappears or is absent. Two options are possible here, either the headphones are faulty, or the device from which the signal is received. Using an ohmmeter, it is easy to find the cause of their breakdown and repair the headphones.
To test the headphones, you need to connect the ends of the probes to their connector. Headphones are usually connected to the equipment using the 3.5 mm jack connector shown in the photo.
One end of the probe is touched to the common output, and the second in turn to the outputs of the right and left channels. The resistance should be the same and be about 40 ohms. Usually, the impedance is indicated in the passport for headphones.
If the resistance of the channels is very different, then there may be a short circuit or a wire break in the wires. It is easy to verify this, it is enough to connect the ends of the probes to the outputs of the right and left channels. The resistance should be twice as much as one earphone, that is, already 80 ohms. In practice, the total resistance of series-connected emitters is measured.
If the resistance changes when the conductors are moved during measurements, then the wire is frayed in some place. Usually the wires are frayed at the exit from the Jack or emitters.
To localize the place of a wire break, it is necessary during measurements to bend the wire locally, fixing the rest of it. By the instability of the ohmmeter readings, you will determine the location of the defect. If Jack has it, then you need to purchase a collapsible connector, bite off the old one with a section of bad wire and solder the wire to the contacts of the new Jack.
If the break is at the entrance to the headphones, then you need to disassemble them, remove the defective part of the wire, strip the ends and solder to the same contacts to which the wires were soldered before. In the site article "How to solder with a soldering iron" you can learn about the art of soldering.
Measuring the value of the resistor (resistance)
Resistors (resistances) are widely used in electrical diagrams. Therefore, when repairing electronic devices there is a need to check the health of the resistor or determine its value.
On electrical circuits, a resistor is indicated in the form of a rectangle, inside which its power is sometimes written in Roman numerals. I - one watt, II - two watts, IV - four watts, V - five watts.
You can check the resistor (resistance) and determine its value using a multimeter included in the resistance measurement mode. In the sector of the resistance measurement mode, there are several switch positions. This is done in order to improve the accuracy of the measurement results.
For example, position 200 allows you to measure resistances up to 200 ohms. 2k - up to 2000 Ohm (up to 2 kOhm). 2M - up to 2000000 Ohm. (up to 2 MΩ). The letter k after the numbers indicates the prefix kilo - the need to multiply the number by 1000, M stands for Mega, and the number must be multiplied by 1,000,000.
If the switch is set to the 2k position, then when measuring a 300 kΩ resistor, the device will show an overload. It is necessary to switch it to position 2M. In contrast to measuring voltage, it does not matter what position the switch is in, you can always switch it during the measurement process.
Online calculators for determining the value of resistors
by color coding
Sometimes when checking a resistor, an ohmmeter shows some kind of resistance, but if the resistor has changed its resistance as a result of overloads and it no longer matches the marking, then such a resistor should not be used. Modern resistors are marked with colored rings. It is most convenient to determine the value of a resistor marked with colored rings using an online calculator.
marked with 4 colored rings
Online calculator for determining the resistance of resistors
marked with 5 colored rings
Checking diodes with a multimeter or tester
Semiconductor diodes are widely used in electrical circuits to convert AC to D.C., and usually when repairing products, after an external examination printed circuit board Diodes are checked first. Diodes are made from germanium, silicon and other semiconductor materials.
By appearance diodes come in various shapes, transparent and colored, in a metal, glass or plastic case. But they always have two conclusions and immediately catch the eye. The circuits mainly use rectifier diodes, zener diodes and LEDs.
The symbol for diodes in the diagram is an arrow resting on a straight line segment. The diode is denoted by the Latin letters VD, with the exception of LEDs, which are denoted by the letters HL. Depending on the purpose of the diodes, additional elements are introduced into the designation scheme, which is reflected in the drawing above. Since there is more than one diode in the circuit, for convenience, a serial number is added after the letters VD or HL.
Testing a diode is much easier if you understand how it works. And the diode works like a nipple. When you inflate a ball, a rubber boat or a car wheel, the air enters them, but the nipple does not let it back out.
The diode works the same way. Only passes in one direction not air, but electric current. Therefore, to test the diode, you need a constant current source, which can be a multimeter or a pointer tester, since they have a battery installed.
Above is a block diagram of a multimeter or tester in resistance measurement mode. As you can see, a DC voltage of a certain polarity is applied to the terminals. Plus is usually applied to the red terminal, and minus to the black. When you touch the diode terminals in such a way that the positive output of the device is at the anode terminal of the diode, and the negative output is at the cathode of the diode, then the current will flow through the diode. If the probes are interchanged, then the diode will not pass current.
The diode can usually have three states - to be serviceable, broken or open. Upon breakdown, the diode turns into a piece of wire, it will pass current in any order the probes are touched. With a break, on the contrary, the current will never flow. Rarely, but there is another condition when the transition resistance changes. Such a malfunction can be identified from the indications on the display.
According to the above instructions, you can check rectifier diodes, zener diodes, Schottky diodes and LEDs, both with leads and in SMD versions. Consider how to test diodes in practice.
First of all, it is necessary, observing the color marking, to insert the probes into the multimeter. Usually, a black wire is inserted into COM, and a red wire is inserted into V / R / f (this is the positive terminal of the battery). Next, you need to set the operation mode switch to the dialing position (if there is such a measurement function), as in the photo or to the 2kOm position. Turn on the device, close the ends of the probes and make sure that it works.
Let's start the practice by checking the ancient germanium diode D7, this instance is already 53 years old. Diodes based on germanium are now practically not produced due to high cost germanium itself and a low limiting operating temperature, only 80-100 ° C. But these diodes have the smallest voltage drop and self-noise level. They are very much appreciated by assemblers of tube sound amplifiers. In direct connection, the voltage drop across the germanium diode is only 0.129 V. The pointer tester will show approximately 130 ohms. When changing the polarity, the multimeter shows 1, the dial tester shows infinity, which means a very large resistance. This diode is correct.
The procedure for testing silicon diodes is no different from testing those made of germanium. On the body of the diode, as a rule, the cathode terminal is marked, it can be a circle, a line or a dot. In direct connection, the drop at the diode junction is about 0.5 V. For powerful diodes, the drop voltage is less, and is about 0.4 V. Zener diodes and Schottky diodes are checked in the same way. The voltage drop of Schottky diodes is about 0.2 V.
At powerful LEDs on the direct transition drops more than 2 V and the device can show 1. But here the LED itself is a health indicator. If even the faintest glow of the LED is visible during direct connection, then it is working.
It should be noted that some types of powerful LEDs consist of a chain of several LEDs connected in series and this is not noticeable externally. Such LEDs sometimes have a voltage drop of up to 30 V, and it is possible to check them only from a power supply with an output voltage of more than 30 V and a current limiting resistor connected in series with the LED.
Checking electrolytic capacitors
There are two main types of capacitors, simple and electrolytic. Simple capacitors can be included in the circuit as you like, and electrolytic ones only with polarity, otherwise the capacitor will fail.
In electrical diagrams, a capacitor is represented by two parallel lines. When designating an electrolytic capacitor, its polarity of connection is necessarily indicated with a “+” sign.
Electrolytic capacitors are of low reliability, and are the most common cause of electronic component failure in products. A swollen capacitor in the power supply of a computer or other device is not a rare sight.
With a tester or multimeter in resistance measurement mode, you can successfully check the health of electrolytic capacitors, or, as they say, ring. The capacitor must be unsoldered from the printed circuit board and must be discharged so as not to damage the device. To do this, you need to short-circuit its conclusions with a metal object, such as tweezers. To check the capacitor, the switch on the device must be set to the resistance measurement mode in the range of hundreds of kilo-ohms or mega-ohms.
Next, you need to touch the probes to the terminals of the capacitor. At the moment of contact, the arrow of the device should deviate sharply along the scale and slowly return to the position of infinite resistance. The rate of deflection of the arrow depends on the value of the capacitance of the capacitor. The larger the capacitance of the capacitor, the slower the arrow will return to its place. A digital device (multimeter), when the probes touch the terminals of the capacitor, will first show a small resistance, and then increasing up to hundreds of megohms.
If the behavior of the devices differs from that described above, for example, the resistance of the capacitor is zero ohms or infinity, then in the first case there is a breakdown between the capacitor windings, and in the second, a break. Such a capacitor is defective and cannot be used.
Without a certain initial knowledge of electricity, it’s hard to imagine how electrical appliances work, why they work at all, why you need to plug in the TV to make it work, and a small battery is enough for a flashlight to shine in the dark.
And so we will understand everything in order.
Electricity
Electricity is a natural phenomenon that confirms the existence, interaction and movement of electric charges. Electricity was first discovered as early as the 7th century BC. Greek philosopher Thales. Thales drew attention to the fact that if a piece of amber is rubbed against wool, it begins to attract light objects to itself. 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, looks at how hair, scraps of thread, feathers and scraps of paper are attracted to amber.
This phenomenon is called static electricity. You can repeat this experience. To do this, thoroughly rub a regular plastic ruler with a woolen cloth and bring it to small pieces of paper.
It should be noted that for a long time this phenomenon has not been studied. 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 of the world have been exploring electricity, writing treatises, formulating laws, inventing electrical machines, and only in 1897, Joseph Thomson discovers the first material carrier of electricity - an electron, a particle, due to which electrical processes in substances are possible.
Electron is an elementary particle, has a negative charge approximately equal to -1.602 10 -19 Cl (Pendant). Denoted 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 (AT or V). In formulas and calculations, stress is indicated by the letter V . To get a voltage of 1 V, you need to transfer a charge of 1 C between the poles, while doing work of 1 J (Joule).
For clarity, imagine a tank of water 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 water flow rate is electricity.
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.
We begin to drain the water, while the pressure will decrease. Those. the charge level drops - the voltage value decreases. This phenomenon can be observed in a flashlight, the light bulb shines dimmer as the batteries run out. Note that the lower the water pressure (voltage), the lower the water flow (current).
Electricity
Electricity- this is a physical process of directed movement of charged particles under the influence of an electromagnetic field from one pole of a closed electrical circuit to another. Charge-transporting particles can be electrons, protons, ions, and holes. In the absence of a closed circuit, current is not possible. Particles capable of carrying 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.
Unit of measurement of current strength - Ampere (BUT). In formulas and calculations, the current strength is indicated by the letter I . A current of 1 Ampere is formed when a charge of 1 Coulomb (6.241 10 18 electrons) passes through a point in the electrical circuit in 1 second.
Let's go back to 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 in 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 pipe diameter is larger) than from the right one. This experience is a clear proof of the dependence of the flow rate on the diameter of the pipe. Now let's try to equalize the two streams. To do this, add water to the right tank (charge). This will give more pressure (voltage) and increase the flow rate (current). In an electrical circuit, the pipe diameter is resistance.
The conducted experiments clearly demonstrate the relationship between voltage, current and resistance. We'll talk more about resistance a little later, and 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 correspondingly constant pressure . If the voltage source changes its polarity and the current flows in one direction, then in the other - this is already alternating current and AC voltage. Maximum and minimum values (marked on the graph as io ) - this is amplitude or peak currents. In household outlets, the voltage changes its polarity 50 times per second, i.e. the current oscillates back and forth, it turns out that the frequency of these oscillations is 50 Hertz, or 50 Hz for short. In some countries, such as the USA, the frequency is 60 Hz.
Resistance
Electrical resistance- a physical quantity that determines the property of the conductor to prevent (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. Them 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 of conduction.
On the example of a plumbing model, the 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 the conductor is manifested, for example, in the heating of the conductor when current flows in it. Moreover, the greater the current and the smaller the cross section of the conductor, the stronger the heating.
Power
Electric power is a physical quantity that determines the rate of electricity conversion. For example, you have heard more than once: "a light bulb for so many watts." This is the power consumed by the light bulb per unit of time during operation, i.e. converting one form of energy into another at a certain rate.
Sources of electricity, such as generators, are also characterized by power, but already generated per unit of time.
Power unit - Watt(denoted Tue or W). In formulas and calculations, power is indicated by the letter P . For AC circuits, the term is used Full power, unit - Volt-ampere (V A or VA), denoted by the letter S .
And finally about electrical circuit . This circuit is a set of electrical components capable of conducting electric current and connected to each other in an appropriate way.
What we see in this image is an elementary electrical appliance (flashlight). under tension U(C) a source of electricity (batteries) through conductors and other components with different resistances 4.59 (237 Votes)
