This site could not do without an article about resistance. Well, no way! There is the most fundamental concept in electronics, which is also physical property. You probably already know these friends:
Resistance is the ability of a material to interfere with the flow of electrons. The material seems to resist, impede this flow, like the sails of a frigate against a strong wind!
In the world, almost everything 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 handle this kind of flow very well.
Only superconductors have no resistance, but that’s another story, since since they have no resistance, we are not interested in them today.
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 such interaction is measured in Volts and is called voltage. Can you tell me what sounds strange? Nothing strange. The electrons are strained and move other electrons with force. A little rustic, but the basic principle is clear.
It remains to mention power. Power is when current, voltage and resistance come together at one table and start working. Then power appears - the energy that electrons lose when passing through resistance. By the way:
I = U/R P = U * I
For example, you have a 60W light bulb with a wire. You plug it into a 220V socket. What's next? The light bulb provides some resistance to the flow of electrons with a potential of 220V. If there is too little resistance, boom, it burns out. If it is too large, the filament will glow very faintly, if at all. But if it is “just right,” then the light bulb will consume 60W and turn this energy into light and heat.
It's warm by-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 flow of electrons 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 - resistivity conductor substances, Ohm m, l— conductor length, m, a S— cross-sectional area, m².
A few animations to complete the picture
And it is clear how the flow of electrons varies depending on the temperature of the conductor and its thickness
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Or an electric circuit to an electric current.
Electrical resistance is defined as a proportionality coefficient R between voltage U and DC power I in Ohm's law for a section of a circuit.
The unit of resistance is called ohm(Ohm) in honor of the German scientist G. Ohm, who introduced this concept into physics. One ohm (1 Ohm) is the resistance of such a conductor in which, at voltage 1 IN the current is equal to 1 A.
Resistivity.
The resistance of a homogeneous conductor of constant cross-section depends on the material of the conductor, its length l And cross section S and can be determined by the formula:
Where ρ - specific resistance of the substance from which the conductor is made.
Specific resistance of a substance- this is a physical quantity that shows what resistance a conductor made from this substance of unit length and unit cross-sectional area has.
From the formula it follows that
Reciprocal value ρ , called conductivity σ :
Since the SI unit of resistance is 1 ohm. unit of area is 1 m 2, and unit of length is 1 m, then the unit resistivity in SI it will be 1 ohm · m 2 /m, or 1 Ohm m. The SI unit of conductivity is Ohm -1 m -1 .
In practice, the cross-sectional area of thin wires is often expressed in square millimeters (mm2). In this case, a more convenient unit of resistivity is Ohm mm 2 /m. Since 1 mm 2 = 0.000001 m 2, then 1 Ohm mm 2 /m = 10 -6 Ohm m. Metals have a very low resistivity - about (1·10 -2) Ohm·mm 2 /m, dielectrics - 10 15 -10 20 greater.
Dependence of resistance on temperature.
As the temperature rises, the resistance of metals increases. However, there are alloys whose resistance almost does not change with increasing temperature (for example, constantan, manganin, etc.). The resistance of electrolytes decreases with increasing temperature.
Temperature coefficient of resistance of a conductor is the ratio of the change in resistance of the conductor when heated by 1 °C to the value of its resistance at 0 ºC:
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The dependence of the resistivity of conductors on temperature is expressed by the formula:
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IN general case α depends on temperature, but if the temperature range is small, then the temperature coefficient can be considered constant. For pure metals α = (1/273)K -1. For electrolyte solutions α < 0 . For example, for a 10% solution table salt α = -0.02 K -1. For constantan (copper-nickel alloy) α = 10 -5 K -1.
The dependence of conductor resistance on temperature is used in resistance thermometers.
§ 15. Electrical resistance
The directional movement of electric charges in any conductor is prevented by the molecules and atoms of this conductor. Therefore, both the external section of the circuit and the internal one (inside the energy source itself) interfere with the passage of current. The quantity characterizing the resistance of an electrical circuit to the passage of electric current is called electrical resistance.
Source electrical energy, included in a closed electrical circuit, expends energy to overcome the resistance of the external and internal circuits.
Electrical resistance is indicated by the letter r and is depicted on the diagrams as shown in Fig. 14, a.
The unit of resistance is ohm. Ohm is the electrical resistance of a linear conductor in which, with a constant potential difference of one volt, a current of one ampere flows, i.e.
When measuring large resistances, units of a thousand and a million times the ohm are used. They are called kilo-ohms ( com) and megohm ( Mom), 1 com = 1000 ohm; 1 Mom = 1 000 000 ohm.
IN various substances contained different quantities free electrons, and the atoms between which these electrons move have different locations. Therefore, the resistance of conductors to electric current depends on the material from which they are made, the length and cross-sectional area of the conductor. If you compare two conductors of the same material, the longer conductor has greater resistance at equal areas cross sections, and a conductor with a large cross section has less resistance for equal lengths.
For relative valuation electrical properties The material of a conductor is its resistivity. Resistivity is the resistance of a metal conductor of length 1 m and cross-sectional area 1 mm 2 ; denoted by the letter ρ, and is measured in
If a conductor made of a material with resistivity ρ has a length l meters and cross-sectional area q square millimeters, then the resistance of this conductor
Formula (18) shows that the resistance of a conductor is directly proportional to the resistivity of the material from which it is made, as well as its length, and inversely proportional to the cross-sectional area.
The resistance of conductors depends on temperature. The resistance of metal conductors increases with increasing temperature. This dependence is quite complex, but within a relatively narrow range of temperature changes (up to approximately 200 ° C) we can assume that for each metal there is a certain, so-called temperature resistance coefficient (alpha), which expresses the increase in conductor resistance Δ r when the temperature changes by 1° C, referred to 1 ohm initial resistance.
Thus, the temperature coefficient of resistance
and increase in resistance
Δ r = r 2 - r 1 = α r 2 (T 2 - T 1) (20)
Where r 1 - conductor resistance at temperature T 1 ;
r 2 - resistance of the same conductor at temperature T 2 .
Let us explain the expression for the temperature coefficient of resistance using an example. Let us assume that a copper linear wire at a temperature T 1 = 15° has resistance r 1 = 50 ohm, and at temperature T 2 = 75° - r 2 - 62 ohm. Therefore, the increase in resistance when the temperature changes by 75 - 15 = 60° is 62 - 50 = 12 ohm. Thus, the increase in resistance corresponding to a temperature change of 1° is equal to:
Temperature coefficient resistance for copper is equal to the increase in resistance divided by 1 ohm initial resistance, i.e. divided by 50:
Based on formula (20), it is possible to establish the relationship between the resistances r 2 and r 1:
(21)It should be borne in mind that this formula is only an approximate expression of the dependence of resistance on temperature and cannot be used for measuring resistance at temperatures exceeding 100 ° C.
Adjustable resistances are called rheostats(Fig. 14, b). Rheostats are made from wire with high resistivity, for example nichrome. The resistance of rheostats can vary uniformly or in steps. Liquid rheostats are also used, which are a metal vessel filled with some solution that conducts electric current, for example a solution of soda in water.
The ability of a conductor to pass electric current is characterized by conductivity, which is the reciprocal of resistance and is denoted by the letter g. The SI unit of conductivity is (Siemens).
Thus, the relationship between resistance and conductivity of a conductor is as follows.
Concept of electrical resistance and conductivity
Any body through which electric current flows exhibits a certain resistance to it. The property of a conductor material to prevent electric current from passing through it is called electrical resistance.
Electronic theory This explains the essence of the electrical resistance of metal conductors. Free electrons, when moving along a conductor, encounter atoms and other electrons on their way countless times and, interacting with them, inevitably lose part of their energy. Electrons experience a kind of resistance to their movement. Various metal conductors having different atomic structure, have different resistance to electric current.
The same thing explains the resistance of liquid conductors and gases to the passage of electric current. However, we should not forget that in these substances it is not electrons, but charged particles of molecules that encounter resistance during their movement.
Resistance is denoted by the Latin letters R or r.
The unit of electrical resistance is the ohm.
Ohm is the resistance of a column of mercury 106.3 cm high with a cross section of 1 mm2 at a temperature of 0° C.
If, for example, the electrical resistance of a conductor is 4 ohms, then it is written like this: R = 4 ohms or r = 4 ohms.
For measuring resistance large size The unit adopted is called the megom.
One megohm is equal to one million ohms.
The greater the resistance of a conductor, the worse it conducts electric current, and, conversely, the less less resistance conductor, the easier it is for electric current to pass through that conductor.
Consequently, to characterize a conductor (from the point of view of the passage of electric current through it), one can consider not only its resistance, but also the reciprocal of the resistance and called conductivity.
Electrical conductivity is the ability of a material to pass electric current through itself.
Since conductivity is the reciprocal of resistance, it is expressed as 1/R, denoted conductivity Latin letter g.
The influence of conductor material, its dimensions and ambient temperature on the value of electrical resistance
The resistance of various conductors depends on the material from which they are made. To characterize electrical resistance various materials the concept of so-called resistivity was introduced.
Resistivity is the resistance of a conductor with a length of 1 m and a cross-sectional area of 1 mm2. Resistivity is denoted by the letter p of the Greek alphabet. Each material from which a conductor is made has its own resistivity.
For example, the resistivity of copper is 0.017, i.e. a copper conductor 1 m long and 1 mm2 cross-section has a resistance of 0.017 ohms. The resistivity of aluminum is 0.03, the resistivity of iron is 0.12, the resistivity of constantan is 0.48, the resistivity of nichrome is 1-1.1.
The resistance of a conductor is directly proportional to its length, i.e. the longer the conductor, the greater its electrical resistance.
The resistance of a conductor is inversely proportional to its cross-sectional area, i.e. the thicker the conductor, the lower its resistance, and, conversely, the thinner the conductor, the greater its resistance.
To better understand this relationship, imagine two pairs of communicating vessels, with one pair of vessels having a thin connecting tube, and the other having a thick one. It is clear that when one of the vessels (each pair) is filled with water, its transfer to the other vessel through a thick tube will occur much faster than through a thin tube, i.e., a thick tube will have less resistance to the flow of water. In the same way, it is easier for electric current to pass through a thick conductor than through a thin one, i.e., the first offers it less resistance than the second.
The electrical resistance of a conductor is equal to the resistivity of the material from which the conductor is made, multiplied by the length of the conductor and divided by the cross-sectional area of the conductor:
R = р l/S,
Where - R is the resistance of the conductor, ohm, l is the length of the conductor in m, S is the cross-sectional area of the conductor, mm 2.
Cross-sectional area of a round conductor calculated by the formula:
S = π d 2 / 4
Where π - constant, equal to 3.14; d is the diameter of the conductor.
And this is how the length of the conductor is determined:
l = S R / p,
This formula makes it possible to determine the length of the conductor, its cross-section and resistivity, if the other quantities included in the formula are known.
If it is necessary to determine the cross-sectional area of the conductor, then the formula takes the following form:
S = р l / R
Transforming the same formula and solving the equality with respect to p, we find the resistivity of the conductor:
R = R S / l
The last formula must be used in cases where the resistance and dimensions of the conductor are known, but its material is unknown and, moreover, difficult to determine by appearance. To do this, you need to determine the resistivity of the conductor and, using the table, find a material that has such a resistivity.
Another reason that affects the resistance of conductors is temperature.
It has been established that with increasing temperature the resistance of metal conductors increases, and with decreasing temperature it decreases. This increase or decrease in resistance for pure metal conductors is almost the same and averages 0.4% per 1°C. The resistance of liquid conductors and carbon decreases with increasing temperature.
The electronic theory of the structure of matter provides the following explanation for the increase in resistance of metal conductors with increasing temperature. When heated, the conductor receives thermal energy, which is inevitably transmitted to all atoms of the substance, as a result of which the intensity of their movement increases. The increased movement of atoms creates greater resistance to the directional movement of free electrons, which is why the resistance of the conductor increases. With a decrease in temperature, Better conditions for the directional movement of electrons, and the resistance of the conductor decreases. This explains interesting phenomenon - superconductivity of metals.
Superconductivity, i.e., a decrease in the resistance of metals to zero, occurs at a huge negative temperature - 273 ° C, called absolute zero. At a temperature absolute zero the metal atoms seem to freeze in place, without at all interfering with the movement of electrons.
Without having certain basic knowledge about electricity, it’s hard to imagine how they work electrical devices, why do they work at all, why do you have to plug in the TV for it to work, but a flashlight only needs a small battery to shine in the dark.
And so we will understand everything in order.
Electricity
Electricity- This a natural phenomenon, confirming the existence, interaction and movement of electric charges. Electricity was first discovered back in the 7th century BC. Greek philosopher Thales. Thales noticed that if a piece of amber is rubbed on wool, it begins to attract light objects. 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 he watches as hair, scraps of thread, feathers and scraps of paper are attracted to the amber.
This phenomenon is called static electricity . You can repeat this experience. To do this, rub a regular plastic ruler thoroughly with a woolen cloth and bring it to the 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, over the course of three centuries, the most advanced world scientists They study electricity, write treatises, formulate laws, invent electrical machines, and only in 1897 Joseph Thomson discovers the first material carrier of electricity - the electron, a particle that makes electrical processes in substances possible.
Electron- This elementary particle, It has negative charge approximately equal -1.602·10 -19 Cl (Pendant). Designated 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 (IN or V). In formulas and calculations, voltage is denoted by the letter V . To obtain a voltage of 1 V, you need to transfer a charge of 1 C between the poles, while doing 1 J (Joule) of work.
For clarity, imagine a water tank 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 speed of water flow is electricity.
Thus, than more water in the tank, the higher the pressure. Similarly from an electrical point of view, the greater the charge, the higher the voltage.
Let's start draining the water, the pressure will decrease. Those. The charge level drops - the voltage decreases. This phenomenon can be observed in a flashlight; the light bulb becomes dimmer as the batteries are discharged. Please note that the lower the water pressure (voltage), the lower the water flow (current).
Electricity
Electricity- This physical process directional movement of charged particles under the influence electromagnetic field from one pole of a closed electrical circuit to the other. Charge-carrying particles can include electrons, protons, ions and holes. Without a closed circuit, no current is possible. Particles capable of transporting 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.
Current unit – Ampere (A). In formulas and calculations, current strength is indicated by the letter I . A current of 1 Ampere is generated when a charge of 1 Coulomb (6.241·10 18 electrons) passes through a point in an electrical circuit in 1 second.
Let's look again at our water-electricity analogy. Only now let’s take two tanks and fill them equal amount water. The difference between the tanks is 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 diameter of the pipe is larger) than from the right. This experience is clear evidence of the dependence of flow speed on pipe diameter. Now let's try to equalize the two flows. To do this, add water (charge) to the right tank. This will give more pressure (voltage) and increase flow rate (current). In an electrical circuit, the pipe diameter is played by resistance.
The experiments carried out clearly demonstrate the relationship between voltage, electric shock And resistance. We'll talk more about resistance a little later, but 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 first in one direction, then in the other, this is already alternating current And AC voltage . Maximum and minimum values(indicated on the graph as Io ) - This amplitude or peak values current strength. In home sockets, the voltage changes its polarity 50 times per second, i.e. the current oscillates here and there, it turns out that the frequency of these oscillations is 50 Hertz, or 50 Hz for short. In some countries, for example in the USA, the frequency is 60 Hz.
Resistance
Electrical resistance – physical quantity, which determines the property of a conductor to impede (resist) the passage of current. Resistance unit – Ohm(denoted Ohm or 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. Their 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 concept of conductivity.
Using the plumbing model as an example, 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 a conductor manifests itself, for example, in the heating of the conductor when current flows through 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 is so many watts.” This is the power consumed by the light bulb per unit of time during operation, i.e. converting one type of energy into another at a certain speed.
Sources of electricity, such as generators, are also characterized by power, but already generated per unit of time.
Power unit – Watt(denoted W or W). In formulas and calculations, power is indicated by the letter P . For chains alternating current term used Full power , unit - Volt-amps (VA or V·A), denoted by the letter S .
And finally about Electric circuit . This chain represents a certain set of electrical components capable of conducting electric current and interconnected accordingly.
What we see in this image is a basic electrical device (flashlight). Under voltage U(B) a source of electricity (batteries) through conductors and other components having different resistances 4.59 (220 Votes)