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. Power electric field affects 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 that contributes to the transformation electrical energy into the thermal, at the time when it passes through electricity. 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, completely different conduct electrical . 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. Exists certain types 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.
Now it's time to find out what resistance is. Imagine now an ordinary crystal lattice. So ... The denser the crystals are located to each other, the more charges will linger in them. So, saying plain language- the greater the resistance of the metal. By the way, the resistance of any ordinary metal can be temporarily increased by heating it. "Why?" - ask. Yes, because when heated, the metal atoms begin to vibrate vigorously near their position fixed by bonds. Therefore, moving charges will more often collide with atoms, which means they will linger more often and more in the nodes of the crystal lattice. Figure 1 shows a visual assembly diagram, so to speak for the "uninitiated", where you can immediately see how to measure the voltage across the resistance. In the same way, you can measure the voltage on a light bulb. By the way, if, as can be seen from the figure, our battery has a voltage of, say, 15V (Volt), and the resistance is such that 10V “settles” on it, then the remaining 5V will fall on the light bulb.
This is what Ohm's law looks like for a closed circuit.
Without going into details, this law says that the voltage of the power source is equal to the sum of the voltage drops in all its sections. Those. in our case, 15V = 10V + 5V. But ... if you nevertheless delve a little into the details, then you need to know that what we called the battery voltage is nothing more than its value when the consumer is connected (in our case, it is a light bulb + resistance). If you disconnect the light bulb with resistance and measure the voltage on the battery, then it will be slightly more than 15V. This will be the open circuit voltage and it is called the EMF of the battery - the electromotive force. In reality, the circuit will work as shown in Fig.2. In reality, the battery can be imagined as some other battery with a voltage of, say, 16V, which has its own internal resistance Rin. The value of this resistance is very small and is due to the technological features of manufacturing. It can be seen from the figure that when the load is connected, part of the battery voltage will “settle” on its internal resistance and at its output it will no longer be 16V, but 15V, i.e. 1B will be "absorbed" by its internal resistance. And Ohm's law for a closed circuit also works here. The sum of the voltages in all sections of the circuit will be equal to EMF batteries. 16V = 1V + 10V + 5V. The unit of measure for resistance is a quantity called an ohm. It is named so in honor of the German physicist Georg Simon Ohm, who was engaged in these works. 1 ohm is equal to the electrical resistance of the conductor (it can, for example, be a light bulb) between the ends of which a voltage of 1 volt arises at a direct current of 1 ampere. To determine the resistance of the lamp, it is necessary to measure the voltage on it and measure the current in the circuit (see Fig. 5). And then divide the resulting voltage value by the current value (R=U/I). Resistances in electrical circuits can be connected in series (the end of the first with the beginning of the second - in this case they can be designated arbitrarily) and in parallel (the beginning with the beginning, the end with the end - and in this case they can be designated arbitrarily). Consider both cases using light bulbs as an example - after all, their filaments are composed of tungsten, i.e. are resistance. The case of serial connection is shown in Fig.3.
It turned out to be known to everyone (and, therefore, we will consider it understandable - a garland). With such a connection, the current I will be the same everywhere, regardless of whether they are the same lamps for the same voltage or for different ones. We must immediately make a reservation that lamps are considered the same, on which:
- the same voltage and current are indicated (like light bulbs from a flashlight);
- the same voltage and power are indicated (like lighting lamps).
The voltage U of the power source in this case "scatters" over all the lamps, i.e. U = U1 + U2 + U3. At the same time, if the lamps are the same, the voltage will be the same on all of them. If the lamps are not the same, then depending on the resistance of each particular lamp. In the first case, the voltage across each lamp can be easily calculated by dividing the source voltage by the total number of lamps. In the second case, you need to delve into the calculations. We will cover all this in the tasks of this section. So, we found out that when the conductors (in this case, lamps) are connected in series, the voltage U at the ends of the entire circuit is equal to the sum of the voltages of the series-connected conductors (lamps) - U = U1 + U2 + U3. According to Omad's law for the circuit section: U1 = I*R1, U2 = I*R2, U3 = I*R3, U = I*R where R1 is the resistance of the filament of the first lamp (conductor), R2 is the second and R3 is the third, R is total resistance of all lamps. Replacing the value U with I*R, U1 with I*R1, U2 with I*R2, U3 with I*R3 in the expression “U = U1 + U2 +U”, we get I*R = I*(R1+R2+R3 ). Hence R \u003d R1 + R2 + R3. Conclusion: when the conductors are connected in series, their total resistance is equal to the sum of the resistances of all conductors. Let's conclude: series connection is used for several consumers (for example, New Year's garland lamps) with a supply voltage lower than the source voltage ..
The case of parallel connection of conductors is shown in Fig.4.
When conductors are connected in parallel, their beginnings and ends have common connection points to the source. At the same time, the voltage on all lamps (conductors) is the same, regardless of which one and what voltage it is designed for, as they are directly connected to the source. Naturally, if the lamp is at a lower voltage than the voltage source, it will burn out. But the current I will be equal to the sum of the currents in all lamps, i.e. I = I1 + I2 + I3. And lamps can be of different power - each will take the current for which it is designed. This can be understood if instead of a source we imagine a socket with a voltage of 220V, and instead of lamps - connected to it, for example, an iron, desk lamp and a phone charger. The resistance of each device in such a circuit is determined by dividing its voltage by the current that it consumes ... again, according to Ohm's law for a section of the circuit, i.e.
Let us immediately state the fact that there is a value reciprocal to resistance and it is called conductivity. It is designated Y. In the SI system, it is designated as CM (Siemens). Reciprocal resistance means that
Without going into mathematical conclusions, we will immediately say that when conductors are connected in parallel (whether it be lamps, irons, microwave ovens or televisions), the reciprocal of the total resistance is equal to the sum of the reciprocals of the resistances of all conductors connected in parallel, i.e.
Given that
Sometimes in tasks they write Y = Y1 + Y2 + Y3. This is the same. There is also a more convenient formula for finding the total resistance of two resistors connected in parallel. It looks like this:
Let's conclude: the parallel switching method is used to connect lighting lamps and household electrical appliances to the electrical network.
As we found out, collisions of free electrons in conductors with atoms of the crystal lattice slow down their forward movement ... This is a counteraction to the directed movement of free electrons, i.e. direct current, is the physical essence of the resistance of the conductor. The mechanism of direct current resistance in electrolytes and gases is similar. The conductive properties of a material determine its volume resistivity ρv, which is equal to the resistance between opposite sides of a cube with an edge of 1m, made from this material. The reciprocal of volume resistivity is called volume conductivity and is equal to γ = 1/ρv. The unit of volume resistance is 1 Ohm * m, volumetric conductivity - 1 Sm / m. The DC resistance of a conductor depends on the temperature. In the general case, a rather complex dependence is observed. But with temperature changes within relatively narrow limits (about 200 ° C), it can be expressed by the formula:
where R2 and R1 are resistances, respectively, at temperatures T1 and T2; α - temperature coefficient of resistance, equal to the relative change in resistance when the temperature changes by 1°C.
Important Concepts
An electrical device that has resistance and is used to limit current is called a resistor. An adjustable resistor (that is, it is possible to change its resistance) is called a rheostat.
Resistive elements are idealized models of resistors and any other electrical devices or their parts that resist direct current, regardless of the physical nature of this phenomenon. They are used in the preparation of circuit equivalent circuits and calculations of their modes. In idealization, currents through the insulating coatings of resistors, frames of wire rheostats, etc. are neglected.
A linear resistive element is an equivalent circuit for any part of an electrical device in which current is proportional to voltage. Its parameter is the resistance R = const. R = const means that the value of the resistance is constant (const means constant).
If the dependence of current on voltage is non-linear, then the equivalent circuit contains a non-linear resistive element, which is given by a non-linear current-voltage characteristic (volt-ampere characteristic) I (U) - read as "And from U". Figure 5 shows the current-voltage characteristics of linear (line a) and non-linear (line b) resistive elements, as well as their designations on the equivalent circuits.
Having assembled an electrical circuit consisting of a current source, a resistor, an ammeter, a voltmeter, a key, it can be shown that current strength (I ) flowing through the resistor is directly proportional to the voltage ( U ) at its ends: I - U . Voltage to current ratio U/I - there is a value constant.
Therefore, there is physical quantity characterizing the properties of a conductor (resistor) through which an electric current flows. This value is called electrical resistance conductor, or simply resistance. Resistance is denoted by the letter R .
(R) is a physical quantity equal to the voltage ratio ( U ) at the ends of the conductor to the current strength ( I ) in him. R = U/I . Resistance unit - Ohm (1 ohm).
one ohm- the resistance of such a conductor, in which the current strength is 1A at a voltage at its ends of 1V: 1 ohm = 1 V / 1 A.
The reason that the conductor has resistance is that the directional movement electric charges in him ions of the crystal lattice performing random motion. Accordingly, the speed of the directed movement of charges decreases.
Specific electrical resistance
R ) is directly proportional to the length of the conductor ( l ), inversely proportional to its cross-sectional area ( S ) and depends on the material of the conductor. This dependence is expressed by the formula: R = p*l/SR is a value that characterizes the material from which the conductor is made. It is called conductor resistivity, its value is equal to the resistance of a conductor with a length 1m and cross-sectional area 1 m 2.
The unit of resistivity of a conductor is: [p] \u003d 1 0m 1 m 2 / 1 m. Cross-sectional area is often measured in mm 2, therefore, in reference books, the values of the resistivity of the conductor are given as in Ohm m so in Ohm mm 2 / m.
By changing the length of the conductor, and therefore its resistance, it is possible to control the current strength in the circuit. The device with which this can be done is called rheostat.
Among other indicators characterizing the electrical circuit, the conductor, it is worth highlighting the electrical resistance. It determines the ability of the atoms of a material to prevent the directed passage of electrons. Assistance in determining this value can be provided both by a specialized device - an ohmmeter, and mathematical calculations based on knowledge of the relationship between the quantities and physical properties material. The indicator is measured in Ohms (Ohm), the symbol is R.
Ohm's law - a mathematical approach to determining resistance
The ratio established by Georg Ohm defines the relationship between voltage, current, resistance, based on the mathematical relationship of concepts. The validity of the linear relationship - R \u003d U / I (ratio of voltage to current strength) - is not observed in all cases.
Unit [R] = B/A = Ohm. 1 ohm is the resistance of a material carrying a current of 1 ampere at a voltage of 1 volt.
Empirical formula for calculating resistance
Objective data on the conductivity of a material follows from its physical characteristics, which determine both its proper properties and reactions to external influences. Based on this, the conductivity depends on:
- size.
- Geometry.
- Temperatures.
Atoms of a conducting material collide with directed electrons, preventing their further advancement. At a high concentration of the latter, the atoms are not able to resist them and the conductivity is high. Large resistance values are typical for dielectrics, which are characterized by almost zero conductivity.
One of the defining characteristics of each conductor is its resistivity - ρ. It determines the dependence of resistance on the conductor material and external influences. This is a fixed (within the same material) value that represents conductor data. following sizes- length 1 m (ℓ), cross-sectional area 1 sq.m. Therefore, the relationship between these quantities is expressed by the relation: R = ρ* ℓ/S:
- The conductivity of a material decreases as its length increases.
- An increase in the cross-sectional area of the conductor entails a decrease in its resistance. This pattern is due to a decrease in the density of electrons, and, consequently, the contact of material particles with them becomes more rare.
- An increase in the temperature of the material stimulates an increase in resistance, while a decrease in temperature causes it to decrease.
It is advisable to calculate the cross-sectional area according to the formula S \u003d πd 2 / 4. A tape measure will help in determining the length.
Relationship with power (P)
Based on the formula of Ohm's law, U = I*R and P = I*U. Therefore, P = I 2 *R and P = U 2 /R.
Knowing the magnitude of the current strength and power, the resistance can be determined as: R \u003d P / I 2.
Knowing the magnitude of voltage and power, the resistance is easy to calculate by the formula: R \u003d U 2 /P. 
The resistance of the material and the values of other associated characteristics can be obtained using special measuring instruments or based on established mathematical patterns.
To date, one of the most important characteristics any material is its electrical resistance. This fact is explained by the unprecedented spread in the history of mankind. electrical machines, which made us take a different look at the properties of the surrounding materials, both artificial and natural. The concept of "electrical resistance" has become as important as heat capacity, etc. It is applicable to absolutely everything that surrounds us: water, air, metal, even vacuum.
Each modern man must have an understanding of this characteristic of materials. The question "what is electrical resistance" can only be answered if the meaning of the term "electric current" is known. Let's start with this...
The material manifestation of energy is the atom. Everything consists of them connected in groups. The current physical model states that the atom is like a smaller model star system. In the center is the nucleus, which includes particles of two types: neutrons and protons. The proton carries an electrical positive charge. At different distances from the nucleus, other particles rotate in circular orbits - electrons that carry a negative charge. The number of protons always corresponds to the number of electrons, so the total charge zero. The farther from the nucleus is the orbit of the electron (valence), the weaker the force of attraction that holds it in the structure of the atom.
In a current-generating machine, the magnetic field releases from the orbits. Since an “extra” proton remains in a lost electron, the force of attraction “tear off” another valence electron from the outer orbit of a neighboring atom. The entire structure of the material is involved in the process. As a result, the movement of charged particles (atoms with positive charge and free electrons with negative), which is called electric current.
The material in the structure of which the electrons of the outer orbits can easily leave the atom is called a conductor. Its electrical resistance is small. This is the metal group. For example, for the production of wires, aluminum and copper are mainly used. According to Ohm's law, electrical is the ratio of the voltage created by the generator to the strength of the passing current. By the way, in Omaha.
It is easy to guess that there are materials in which there are very few valence electrons or atoms are very far from each other (gas), so their internal structure cannot provide current flow. They are called dielectrics and are used to insulate conductive lines in electrical engineering. Their electrical resistance is very high.
Everyone knows that a wet dielectric begins to conduct electricity. In the light of this fact, the question "does the electrical resistance of water exist" is of particular interest. The answer is contradictory: yes and no. As mentioned earlier, if there are practically no valence electrons in the material, and the structure itself consists more of emptiness than particles (remember the periodic table and hydrogen with a single electron in orbit), then under normal conditions, conductivity cannot exist. This description fits perfectly with water: a combination of two gases, which we call a liquid. Indeed, being completely free of dissolved impurities, it is a very good dielectric. But since in nature solutions of salts are always present in water, it is provided by them. Its level is affected by the saturation of the solution and temperature. That is why there can be no unambiguous answer to the question, because water can be different.