Electrical resistance - Hypermarket of knowledge. What is resistance

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 this phenomenon has not been studied for a long time. 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 (IN 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 speed of the water flow 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 (A). 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 tension, 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 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. 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 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(B) a source of electricity (batteries) through conductors and other components with different resistances 4.59 (220 Votes)

Electrical resistance is understood as any resistance that detects current when passing through a closed circuit, weakening or inhibiting the free flow of electrical charges.

Jpg?x15027" alt=" Measuring resistance with a multimeter" width="600" height="490">!}

Measuring resistance with a multimeter

The physical concept of resistance

Electrons, when passing current, circulate in a conductor in an organized manner according to the resistance they encounter along the way. The lower this resistance, the greater the existing order in the microcosm of electrons. But when the resistance is high, they begin to collide with each other and release heat energy. In this regard, the temperature of the conductor always rises slightly, by a larger amount, the higher the electrons find resistance to their movement.

Materials used

All known metals are more or less resistant to the passage of current, including the best conductors. Gold and silver have the least resistance, but they are expensive, so the most commonly used material is copper, which has a high electrical conductivity. Aluminum is used on a smaller scale.

Nichrome wire has the highest resistance to the passage of current (an alloy of nickel (80%) and chromium (20%)). It is widely used in resistors.

Another widely used resistor material is carbon. From it, fixed resistances and rheostats are made for use in electronic circuits. Fixed resistors and potentiometers are used to control current and voltage values, for example, when controlling the volume and tone of audio amplifiers.

Resistance calculation

To calculate the value of the load resistance, the formula derived from Ohm's law is used as the main one if the values ​​​​of current and voltage are known:

The unit of measure is Ohm.

For a series connection of resistors, the total resistance is found by summing the individual values:

R = R1 + R2 + R3 + …..

When connecting in parallel, the expression is used:

1/R = 1/R1 + 1/R2 + 1/R3 + …

And how to find the electrical resistance for a wire, given its parameters and material of manufacture? There is another resistance formula for this:

R \u003d ρ x l / S, where:

• l is the length of the wire,
• S are the dimensions of its cross section,
• ρ is the specific volume resistance of the wire material.

Data-lazy-type="image" data-src="http://elquanta.ru/wp-content/uploads/2018/03/2-1-600x417.png?.png 600w, https://elquanta.ru/wp-content/uploads/2018/03/2-1-768x533..png 792w" sizes="(max-wid th: 600px) 100vw, 600px">

Resistance Formula

The geometric dimensions of the wire can be measured. But in order to calculate the resistance using this formula, you need to know the coefficient ρ.

Important! beat values volume resistance has already been calculated for different materials and summarized in special tables.

The value of the coefficient allows you to compare the resistance of different types of conductors at a given temperature in accordance with their physical properties without regard to size. This can be illustrated with examples.

An example of calculating the electrical resistance of a copper wire, 500 m long:

1. If the dimensions of the wire section are unknown, you can measure its diameter with a caliper. Let's say it's 1.6mm;
2. When calculating the cross-sectional area, the formula is used:

Then S = 3.14 x (1.6 / 2)² = 2 mm²;

1. According to the table, we found the value of ρ for copper, equal to 0.0172 Ohm x m / mm²;
2. Now the electrical resistance of the calculated conductor will be:

R \u003d ρ x l / S \u003d 0.0172 x 500/2 \u003d 4.3 ohms.

Another example– nichrome wire with a cross section of 0.1 mm², length 1 m:

1. The ρ index for nichrome is 1.1 Ohm x m / mm²;
2. R \u003d ρ x l / S \u003d 1.1 x 1 / 0.1 \u003d 11 ohms.

Two examples clearly show that a meter-long nichrome wire with a cross section 20 times smaller has an electrical resistance 2.5 times greater than 500 meters of copper wire.

Jpg?.jpg 600w, https://elquanta.ru/wp-content/uploads/2018/03/3-6-768x381..jpg 960w

Resistivity of some metals

Important! The resistance is influenced by temperature, with the increase of which it increases and, conversely, decreases with a decrease.

Impedance

Impedance is a more general term for resistance that takes into account a reactive load. The calculation of resistance in an AC circuit is to calculate the impedance.

While a resistor provides resistance for a specific purpose, reactive is an unfortunate by-product of some electrical circuit components.

Two types of reactance:

1. Inductive. Created by coils. Calculation formula:

X (L) = 2π x f x L, where:

• f is the current frequency (Hz),
• L - inductance (H);

1. Capacitive. Created by capacitors. Calculated according to the formula:

X (C) = 1/(2π x f x C),

where C is the capacitance (F).

Like its active counterpart, reactance is expressed in ohms and also limits the flow of current through the loop. If there is both a capacitance and an inductor in the circuit, then the total resistance is:

X = X (L) - X (C).

Jpg?.jpg 600w, https://elquanta.ru/wp-content/uploads/2018/03/4-3.jpg 622w

Active, inductive and capacitive reactance

Important! Interesting features follow from the reactive load formulas. With an increase in the frequency of the alternating current and inductance, X (L) increases. Conversely, the higher the frequency and capacitance, the smaller X (C).

Finding the impedance (Z) is not a simple addition of the active and reactive components:

Z = √ (R² + X²).

Example 1

A coil in a circuit with a power frequency current has an active resistance of 25 Ohms and an inductance of 0.7 H. You can calculate the impedance:

1. X (L) \u003d 2π x f x L \u003d 2 x 3.14 x 50 x 0.7 \u003d 218.45 ohms;
2. Z = √ (R² + X (L)²) = √ (25² + 218.45²) = 219.9 ohms.

tg φ \u003d X (L) / R \u003d 218.45 / 25 \u003d 8.7.

The angle φ is approximately equal to 83 degrees.

Example 2

There is a capacitor with a capacity of 100 microfarads and an internal resistance of 12 ohms. You can calculate the impedance:

1. X (C) \u003d 1 / (2π x f x C) \u003d 1 / 2 x 3.14 x 50 x 0, 0001 \u003d 31.8 ohms;
2. Z \u003d √ (R² + X (C)²) \u003d √ (12² + 31.8²) \u003d 34 ohms.

On the Internet, you can find an online calculator to simplify the calculation of the resistance and impedance of the entire electrical circuit or its sections. There you just need to keep your calculated data and record the results of the calculation.

Video

When the electrical circuit is closed, in the presence of a potential difference on the terminals, then, in this case, the action of an electric current occurs. The strength of the electric field affects the free electrons, causing them to move along the conductor. During the movement, the electrons collide with the atoms of the conductor, giving off the available kinetic energy. All electrons move at a continuously changing speed.

The decrease in speed occurs when the electrons collide with other electrons and atoms in their path. In the future, under the influence of electric, the speed of the electrons increases again until a new collision.

This process is continuous, as a result of which, the flow of electrons in the conductor moves evenly. At the same time, electrons, while moving, constantly meet resistance. This ultimately leads to heating of the conductor.

What is conductor resistance

Resistance is a property of a medium or body, which contributes to the conversion of electrical energy into heat, at a time when an electric current passes through it. You can change the value of the current in the circuit using a variable electrical resistance, called a rheostat. The required resistance is entered using a special slider set in a certain position.

A conductor with a long length and a small cross section has a higher resistance. And, conversely, a short conductor with a large cross section is able to provide very little resistance to the current.

Two conductors having the same section and length, but made of different materials, conduct electricity in completely different ways. It follows that the material directly affects the resistance.

Influence of additional factors

Additional factors influence the value and intrinsic temperature of the conductor. As the temperature rises, there is an increase in resistance in various metals. In liquids and coal, on the contrary, resistance decreases. There are certain types of alloys in which, with increasing temperature, the resistance practically does not change.

Thus, the resistance of a conductor depends on factors such as its length and cross-section, as well as on temperature and the material from which it is made. The resistance of all conductors is measured in ohms.

With high resistance, such a conductor has, accordingly, less conductivity, and vice versa, low resistance contributes to much better conductivity of electric current. Therefore, the values ​​of conductivity and resistance are reversed.

§ 15. Electrical resistance

The directed movement of electric charges in any conductor is hindered 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 value characterizing the resistance of an electric circuit to the passage of electric current is called electrical resistance.
The source of electrical energy, included in a closed electrical circuit, consumes energy to overcome the resistance of the external and internal circuits.
Electrical resistance is denoted by the letter r and is depicted in the diagrams as shown in Fig. 14, a.

The unit of resistance is the ohm. Ohm called the electrical resistance of such a linear conductor in which, with a constant potential difference of one volt, a current of one ampere flows, i.e.

When measuring high resistances, units of a thousand and a million times more ohms are used. They are called kiloohm ( com) and megohm ( Mom), 1 com = 1000 ohm; 1 Mom = 1 000 000 ohm.
Different substances contain different numbers of free electrons, and the atoms between which these electrons move have a different arrangement. Therefore, the resistance of conductors to electric current depends on the material from which they are made, on the length and cross-sectional area of ​​\u200b\u200bthe conductor. If two conductors of the same material are compared, then a longer conductor has more resistance for equal cross-sectional areas, and a conductor with a larger cross-section has less resistance for equal lengths.
For a relative assessment of the electrical properties of the conductor material, its resistivity serves. Resistivity is the resistance of a metal conductor with a length of 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 the 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 complicated, but within a relatively narrow range of temperature changes (up to about 200 ° C), we can assume that for each metal there is a certain, so-called temperature, resistance coefficient (alpha), which expresses the increase in the resistance of the conductor 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 a temperature T 2 .
Let us explain the expression for the temperature coefficient of resistance with an example. Let us assume that a copper linear wire at a temperature T 1 = 15° has resistance r 1 = 50 ohm, and at a temperature T 2 = 75° - r 2 - 62 ohm. Therefore, the increase in resistance when the temperature changes by 75 - 15 \u003d 60 ° is 62 - 50 \u003d 12 ohm. Thus, the increase in resistance corresponding to a change in temperature by 1 ° is equal to:

The temperature coefficient of 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 resistances at temperatures exceeding 100 ° C.
Adjustable resistances are called rheostats(Fig. 14, b). Rheostats are made of wire with high resistivity, such as nichrome. The resistance of rheostats can vary evenly or in steps. Liquid rheostats are also used, which are a metal vessel filled with some kind of 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 indicated by the letter g. The SI unit for conductivity is (siemens).

Thus, the relationship between the resistance and conductivity of a conductor is as follows.

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 the electric current. 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