Charge in motion. It can take the form of a sudden discharge of static electricity, such as lightning. Or it could be a controlled process in generators, batteries, solar or fuel cells. Today we will consider the very concept of "electric current" and the conditions for the existence of an electric current.

Electric Energy

Most of the electricity we use comes in the form of alternating current from the electrical grid. It is created by generators that work according to Faraday's law of induction, due to which a changing magnetic field can induce an electric current in a conductor.

Generators have spinning coils of wire that pass through magnetic fields as they spin. As the coils rotate, they open and close with respect to the magnetic field and create an electrical current that changes direction with each turn. The current goes through a full cycle back and forth 60 times per second.

Generators can be powered by steam turbines heated by coal, natural gas, oil, or a nuclear reactor. From the generator, the current passes through a series of transformers, where its voltage increases. The diameter of the wires determines the amount and strength of current they can carry without overheating and wasting energy, and voltage is only limited by how well the lines are insulated from ground.

It is interesting to note that the current is carried by only one wire, not two. Its two sides are designated as positive and negative. However, since the polarity of alternating current changes 60 times per second, they have other names - hot (main power lines) and grounded (passing underground to complete the circuit).

Why is electricity needed?

There are many uses for electricity: it can light up your house, wash and dry your clothes, lift your garage door, boil water in a kettle, and power other household items that make our lives so much easier. However, the ability of the current to transmit information is becoming increasingly important.

When connected to the Internet, a computer uses only a small part of the electric current, but this is something without which a modern person cannot imagine his life.

The concept of electric current

Like a river current, a stream of water molecules, an electric current is a stream of charged particles. What is it that causes it, and why doesn't it always go in the same direction? When you hear the word flow, what do you think of? Perhaps it will be a river. It's a good association, because that's the reason the electric current got its name. It is very similar to the flow of water, only instead of water molecules moving along the channel, charged particles move along the conductor.

Among the conditions necessary for the existence of an electric current, there is an item that provides for the presence of electrons. Atoms in a conductive material have many of these free charged particles that float around and between the atoms. Their movement is random, so there is no flow in any given direction. What does it take for an electric current to exist?

The conditions for the existence of electric current include the presence of voltage. When it is applied to a conductor, all free electrons will move in the same direction, creating a current.

Curious about electric current

Interestingly, when electrical energy is transmitted through a conductor at the speed of light, the electrons themselves move much more slowly. In fact, if you walked leisurely next to a conductive wire, your speed would be 100 times faster than the electrons are moving. This is due to the fact that they do not need to travel huge distances to transfer energy to each other.

Direct and alternating current

Today, two different types of current are widely used - direct and alternating. In the first, the electrons move in one direction, from the "negative" side to the "positive" side. The alternating current pushes the electrons back and forth, changing the direction of the flow several times per second.

Generators used in power plants to produce electricity are designed to produce alternating current. You probably never noticed that the light in your house is actually flickering as the current direction changes, but it happens too fast for the eyes to recognize.

What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. As far back as the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a house was possible only at very high voltages. But the problem was that sending really high voltage was extremely dangerous for people.

The solution to this problem was to reduce the stress outside the home before sending it inside. To this day, direct electric current is used for transmission over long distances, mainly because of its ability to easily convert to other voltages.

How electric current works

The conditions for the existence of an electric current include the presence of charged particles, a conductor, and voltage. Many scientists have studied electricity and found that there are two types of it: static and current.

It is the second that plays a huge role in the daily life of any person, as it is an electric current that passes through the circuit. We use it daily to power our homes and more.

What is electric current?

When electric charges circulate in a circuit from one place to another, an electric current is produced. The conditions for the existence of an electric current include, in addition to charged particles, the presence of a conductor. Most often it is a wire. Its circuit is a closed circuit in which current flows from a power source. When the circuit is open, he cannot complete the journey. For example, when the light in your room is off, the circuit is open, but when the circuit is closed, the light is on.

Current power

The conditions for the existence of an electric current in a conductor are greatly influenced by such a voltage characteristic as power. This is a measure of how much energy is being used over a given period of time.

There are many different units that can be used to express this characteristic. However, electrical power is almost measured in watts. One watt is equal to one joule per second.

Electric charge in motion

What are the conditions for the existence of an electric current? It can take the form of a sudden discharge of static electricity, such as lightning or a spark from friction with a woolen cloth. More often, however, when we talk about electric current, we mean a more controlled form of electricity that makes lights and appliances work. Most of the electrical charge is carried by the negative electrons and positive protons within the atom. However, the latter are mostly immobilized inside atomic nuclei, so the work of transferring charge from one place to another is done by electrons.

Electrons in a conductive material such as a metal are largely free to move from one atom to another along their conduction bands, which are the higher electron orbits. A sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to move through a conductor in the form of an electric current.

If we draw an analogy with water, then take, for example, a pipe. When we open a valve at one end to let water enter the pipe, we don't have to wait for that water to work its way all the way to the end of the pipe. We get water at the other end almost instantly because the incoming water pushes the water that is already in the pipe. This is what happens in the case of an electric current in a wire.

Electric current: conditions for the existence of an electric current

Electric current is usually viewed as a flow of electrons. When the two ends of the battery are connected to each other with a metal wire, this charged mass passes through the wire from one end (electrode or pole) of the battery to the opposite. So, let's call the conditions for the existence of an electric current:

1. charged particles.
2. Conductor.
3. Voltage source.

However, not all so simple. What conditions are necessary for the existence of an electric current? This question can be answered in more detail by considering the following characteristics:

• Potential difference (voltage). This is one of the prerequisites. Between the 2 points there must be a potential difference, meaning that the repulsive force that is created by charged particles in one place must be greater than their force at another point. Voltage sources, as a rule, do not occur in nature, and electrons are distributed fairly evenly in the environment. Nevertheless, scientists managed to invent certain types of devices where these charged particles can accumulate, thereby creating the very necessary voltage (for example, in batteries).
• Electrical resistance (conductor). This is the second important condition that is necessary for the existence of an electric current. This is the path along which charged particles travel. Only those materials that allow electrons to move freely act as conductors. Those who do not have this ability are called insulators. For example, a metal wire will be an excellent conductor, while its rubber sheath will be an excellent insulator.

Having carefully studied the conditions for the emergence and existence of electric current, people were able to tame this powerful and dangerous element and direct it for the benefit of mankind.

The actions of an electric current are the phenomena that an electric current causes. From them, you can judge the presence of current.

Coating some metals with a thin layer of others (nickel plating, chromium plating, copper plating, silver plating, gilding, etc.) - electroplating

Current strength Effect of current on the human body 0 - 0.5 m. A Absent 0.5 - 2 m. A Loss of sensitivity 2 -10 m. A Pain, muscle contractions 10 -20 m. A Growing effect on muscles, some damage 16 m. A Current, above which a person can no longer get rid of the electrodes 20 -100 m. A Respiratory paralysis 100 m. A - 3 A Fatal ventricular fibrillation (immediate resuscitation is necessary) More than 3 A Cardiac arrest. (If the shock was brief, the heart can be resuscitated.) Severe burns.

Electric current is the ordered movement of charged particles. The following conditions are necessary for the existence of an electric current: 1. The presence of free electric charges in a conductor; 2. The presence of an external electric field for the conductor.

Do liquids conduct electricity? Electrolytes - solutions of salts, alkalis or acids capable of conducting electric current. An electric current in an electrolyte (liquid) is the directed movement of ions in an electric field. (m=kit)

Compare the experiments carried out in the figures. What do the experiences have in common and how do they differ? To create an email fields use a current source - a device in which any type of energy is converted into electrical energy. Devices that separate charges, i.e., create an electric field, are called current sources.

The first electric battery appeared in 1799. It was invented by the Italian physicist Alessandro Volta (1745 - 1827) - an Italian physicist, chemist and physiologist, inventor of a constant electric current source. His first current source - "voltaic column" was built in strict accordance with his theory of "metallic" electricity. Volta put several dozen small zinc and silver circles on top of each other alternately, laying paper moistened with salted water between them.

Battery (battery) - the common name for a source of electricity for autonomous power supply of a portable device. It can be a single galvanic cell, a battery, or their connection into a battery to increase the voltage.

The battery is a reusable chemical current source. If two carbon electrodes are placed in a salt solution, the galvanometer does not show the presence of current. If the battery is pre-charged, then it can be used as an independent current source. There are different types of batteries: acid and alkaline. In them, charges are also separated as a result of chemical reactions. Electric batteries are used for energy storage and autonomous power supply of various consumers.

Sealed small batteries (GMA). GMA are used for small-sized consumers of electrical energy (telephone radio handsets, portable radio receivers, electronic watches, measuring instruments, cell phones, etc.).

Battery (from Latin accumulator - collector) - a device for storing energy for the purpose of its subsequent use.

Electrophore Machine Until the end of the 18th century, all technical current sources were based on electrification by friction. The most effective of these sources was the electrophore machine (the machine's disks are rotated in opposite directions. As a result of the friction of the brushes on the disks, charges of the opposite sign accumulate on the conductors of the machine) Mechanical current source - mechanical energy is converted into electrical energy.

Electromechanical generator. The charges are separated by doing mechanical work. It is used for the production of industrial electricity. Generator (from lat. generator - manufacturer) - a device, apparatus or machine that produces a product.

Thermoelement Thermocouple Thermoelement (thermocouple) - two wires from different metals must be soldered from one edge, then the soldering point is heated, then a current appears in them. The charges are separated when the junction is heated. Thermoelements are used in temperature sensors and in geothermal power plants as a temperature sensor. Thermal current source - internal energy is converted into electrical energy

Photocell Solar battery Photocell. When some substances are illuminated with light, a current appears in them, the light energy is converted into electrical energy. In this device, the charges are separated by the action of light. Solar panels are made up of photovoltaic cells. They are used in solar batteries, light sensors, calculators, video cameras. Light energy is converted into electrical energy with the help of solar panels.

Classification of current sources Current source Photocell Method of charge separation Application Action of light Solar batteries Heating Thermocouple Measuring the temperature of junctions Performing Electromechanics. Manufacture of a mechanical generator for industrial electricity. energy Work Electroplating Chemical Flashlights, Reaction Element Radios Accumulator Chemical Cars Reaction

Current strength is a physical quantity that characterizes the action of the current I n Designated - n Measured in amperes - A n The measuring device is an ammeter, connects in series. n Device for regulation - rheostat.

Why does resistance decrease? n The distance in the diagram from the tip of the arrow to the pole of the rheostat is the distance that the charge travels along the wire with high resistance. By moving the rheostat slider to the left, we reduce this distance, and, consequently, the resistance of the circuit.

Definition of current strength: Current strength is a physical quantity showing how much charge has passed through the cross section of the conductor per unit time.

Unit of current ANDRE-MARI AMPERE (1775 - 1836) - French physicist and mathematician. The current in a metal conductor is

Voltage is a physical quantity that characterizes the work of an electric field in moving a charge. n Denoted - U Measured in volts - V n Instrument for measuring voltmeter, connected in parallel. n

For the existence of a direct electric current, the presence of free charged particles and the presence of a current source are necessary. in which the conversion of any type of energy into the energy of an electric field is carried out.

Current source - a device in which any type of energy is converted into the energy of an electric field. In a current source, external forces act on charged particles in a closed circuit. The reasons for the appearance of external forces in various current sources are different. For example, in batteries and galvanic cells, external forces arise due to the flow of chemical reactions, in generators of power plants they arise when a conductor moves in a magnetic field, in photocells - when light acts on electrons in metals and semiconductors.

The electromotive force of the current source called the ratio of the work of external forces to the value of the positive charge transferred from the negative pole of the current source to the positive.

Basic concepts.

Current strength - a scalar physical quantity equal to the ratio of the charge that has passed through the conductor to the time for which this charge has passed.

where I - current strength, q - amount of charge (amount of electricity), t - charge transit time.

current density - vector physical quantity equal to the ratio of the current strength to the cross-sectional area of ​​​​the conductor.

where j -current density, S - cross-sectional area of ​​the conductor.

The direction of the current density vector coincides with the direction of motion of positively charged particles.

Voltage - scalar physical quantity equal to the ratio of the total work of the Coulomb and external forces when moving a positive charge in the area to the value of this charge.

where A - full work of third-party and Coulomb forces, q - electric charge.

Electrical resistance - a physical quantity characterizing the electrical properties of a circuit section.

where ρ - specific resistance of the conductor, l - the length of the conductor section, S - cross-sectional area of ​​the conductor.

Conductivity is the reciprocal of the resistance

where G - conductivity.

Ohm's laws.

Ohm's law for a homogeneous section of a chain.

The current strength in a homogeneous section of the circuit is directly proportional to the voltage at a constant section resistance and inversely proportional to the section resistance at a constant voltage.

where U - tension in the area R - section resistance.

Ohm's law for an arbitrary section of the circuit containing a direct current source.

where φ 1 - φ 2 + ε = U voltage in a given section of the circuit,R - electrical resistance of a given section of the circuit.

Ohm's law for a complete circuit.

The current strength in a complete circuit is equal to the ratio of the electromotive force of the source to the sum of the resistances of the external and internal sections of the circuit.

where R - electrical resistance of the outer section of the circuit, r - electrical resistance of the internal section of the circuit.

Short circuit.

It follows from Ohm's law for a complete circuit that the current strength in a circuit with a given current source depends only on the resistance of the external circuit R.

If a conductor with resistance is connected to the poles of the current source R<< r, then only the EMF of the current source and its resistance will determine the value of the current in the circuit. This value of the current strength will be the limit for this current source and is called the short circuit current.

Electromotive force. Any current source is characterized by electromotive force, or, for short, EMF. So, on a round battery for a flashlight it is written: 1.5 V. What does this mean? Connect two metal balls carrying charges of opposite signs with a conductor. Under the influence of the electric field of these charges, an electric current arises in the conductor ( fig.15.7). But this current will be very short-lived. The charges quickly neutralize each other, the potentials of the balls become the same, and the electric field disappears.

Third party forces. In order for the current to be constant, it is necessary to maintain a constant voltage between the balls. This requires a device current source), which would move charges from one ball to another in the direction opposite to the direction of the forces acting on these charges from the electric field of the balls. In such a device, in addition to electric forces, the charges must be affected by forces of non-electrostatic origin ( fig.15.8). Only one electric field of charged particles ( Coulomb field) is not capable of maintaining a constant current in the circuit.

Any forces acting on electrically charged particles, with the exception of forces of electrostatic origin (i.e., Coulomb), are called outside forces. The conclusion about the need for external forces to maintain a constant current in the circuit will become even more obvious if we turn to the law of conservation of energy. The electrostatic field is potential. The work of this field when moving charged particles in it along a closed electric circuit is zero. The passage of current through the conductors is accompanied by the release of energy - the conductor heats up. Therefore, there must be some source of energy in the circuit that supplies it to the circuit. In it, in addition to the Coulomb forces, third-party, non-potential forces must necessarily act. The work of these forces along a closed contour must be different from zero. It is in the process of doing work by these forces that charged particles acquire energy inside the current source and then give it to the conductors of the electric circuit. Third-party forces set in motion charged particles inside all current sources: in generators at power plants, in galvanic cells, batteries, etc. When a circuit is closed, an electric field is created in all conductors of the circuit. Inside the current source, the charges move under the influence of external forces vs. Coulomb forces(electrons from a positively charged electrode to a negative one), and in the external circuit they are set in motion by an electric field (see Fig. fig.15.8). The nature of extraneous forces. The nature of outside forces can be varied. In power plant generators, external forces are forces acting from the magnetic field on electrons in a moving conductor. In a galvanic cell, for example, the Volta cell, chemical forces act. The Volta element consists of zinc and copper electrodes placed in a solution of sulfuric acid. Chemical forces cause the zinc to dissolve in the acid. Positively charged zinc ions pass into the solution, and the zinc electrode itself becomes negatively charged. (Copper dissolves very little in sulfuric acid.) A potential difference appears between the zinc and copper electrodes, which determines the current in a closed electrical circuit. Electromotive force. The action of external forces is characterized by an important physical quantity called electromotive force(abbreviated EMF). The electromotive force of the current source is equal to the ratio of the work of external forces when moving the charge along a closed circuit to the value of this charge:

Electromotive force, like voltage, is expressed in volts. We can also talk about the electromotive force in any part of the circuit. This is the specific work of external forces (the work of moving a unit charge) not in the entire circuit, but only in this area. Electromotive force of a galvanic cell is a value numerically equal to the work of external forces when moving a unit positive charge inside the element from one pole to another. The work of external forces cannot be expressed in terms of the potential difference, since external forces are non-potential and their work depends on the shape of the charge trajectory. So, for example, the work of external forces when moving a charge between the terminals of a current source outside the source itself is equal to zero. Now you know what EMF is. If 1.5 V is written on the battery, then this means that third-party forces (chemical in this case) do 1.5 J of work when moving a charge of 1 C from one pole of the battery to another. Direct current cannot exist in a closed circuit if external forces do not act in it, that is, there is no EMF.

PARALLEL AND SERIES CONNECTION OF CONDUCTORS

Let us include in the electrical circuit as a load (current consumers) two incandescent lamps, each of which has some specific resistance, and each of which can be replaced by a conductor with the same resistance.

SERIAL CONNECTION

Calculation of the parameters of the electrical circuit with a series connection of resistances:

1. the current strength in all series-connected sections of the circuit is the same 2. the voltage in a circuit consisting of several sections connected in series is equal to the sum of the voltages in each section 3. the resistance of a circuit consisting of several series-connected sections is equal to the sum of the resistances of each section

4. the work of an electric current in a circuit consisting of series-connected sections is equal to the sum of the work in individual sections

A \u003d A1 + A2 5. the power of the electric current in a circuit consisting of series-connected sections is equal to the sum of the powers in the individual sections

PARALLEL CONNECTION

Calculation of the parameters of the electrical circuit with a parallel connection of resistances:

1. the current strength in an unbranched section of the circuit is equal to the sum of the current strengths in all parallel connected sections

3. when the resistances are connected in parallel, the values ​​\u200b\u200bthat are inverse to the resistance are added:

(R - conductor resistance, 1/R - electrical conductivity of the conductor)

If only two resistors are connected in parallel in a circuit, then about:

(when connected in parallel, the total resistance of the circuit is less than the smaller of the included resistances)

4. The work of an electric current in a circuit consisting of parallel-connected sections is equal to the sum of the work in individual sections: A=A1+A2 5. The power of the electric current in a circuit consisting of sections connected in parallel is equal to the sum of the powers in the individual sections: P=P1+P2

For two resistances: i.e. the greater the resistance, the less current it has.

The Joule-Lenz law is a physical law that allows you to determine the thermal effect of the current in the circuit, according to this law: , where I is the current in the circuit, R is the resistance, t is the time. This formula was calculated by creating a circuit: a galvanic cell (battery), a resistor and an ammeter. The resistor was dipped into a liquid, into which a thermometer was inserted and the temperature was measured. This is how they deduced their law and imprinted themselves forever in history, but even without their experiments it was possible to deduce the same law:

U=A/q ​​A=U*q=U*I*t=I^2*R*t but despite this honor and praise to these people.

Joule Lenz's law determines the amount of heat released in a section of an electrical circuit with finite resistance when current passes through it. A prerequisite is the fact that there should be no chemical transformations in this section of the chain.

WORK OF ELECTRIC CURRENT

The work of an electric current shows how much work was done by an electric field when moving charges through a conductor.

Knowing two formulas: I \u003d q / t ..... and ..... U \u003d A / q, you can derive a formula for calculating the work of an electric current: The work of an electric current is equal to the product of the current strength and the voltage and the time the current flows in the circuit.

The unit of measure for the work of electric current in the SI system: [ A ] \u003d 1 J \u003d 1A. b. c

LEARN, GO! When calculating the work of an electric current, an off-system multiple unit of electric current work is often used: 1 kWh (kilowatt-hour).

1 kWh = ...........W.s = 3,600,000 J

In each apartment, to account for the electricity consumed, special electricity meters are installed, which show the work of the electric current, completed over a certain period of time when various household electrical appliances are turned on. These meters show the work of electric current (electricity consumption) in "kWh".

You need to learn how to calculate the cost of electricity consumed! We carefully understand the solution of the problem on page 122 of the textbook (paragraph 52)!

ELECTRIC CURRENT POWER

The power of the electric current shows the work of the current done per unit of time and is equal to the ratio of the work done to the time during which this work was done.

(power in mechanics is usually denoted by the letter N, in electrical engineering - by letter R) as A = IUt, then the power of the electric current is equal to:

or

The unit of electric current power in the SI system:

[P] = 1 W (watt) = 1 A. B

Kirchhoff's laws – rules that show how currents and voltages are related in electrical circuits. These rules were formulated by Gustav Kirchhoff in 1845. In the literature, they are often called Kirchhoff's laws, but this is not true, since they are not laws of nature, but were derived from Maxwell's third equation with a constant magnetic field. But still, the first name is more familiar to them, therefore we will call them, as is customary in the literature - Kirchhoff's laws.

Kirchhoff's first law – the sum of the currents converging in the node is equal to zero.

Let's figure it out. A node is a point that connects branches. A branch is a section of a chain between nodes. The figure shows that current i enters the node, and currents i 1 and i 2 leave the node. We compose an expression according to the first Kirchhoff law, given that the currents entering the node have a plus sign, and the currents emanating from the node have a minus sign i-i 1 -i 2 =0. Current i, as it were, spreads into two smaller currents and is equal to the sum of currents i 1 and i 2 i=i 1 +i 2. But if, for example, the current i 2 entered the node, then the current I would be defined as i=i 1 -i 2 . It is important to take into account the signs when compiling an equation.

Kirchhoff's first law is a consequence of the law of conservation of electricity: the charge coming to the node in a certain period of time is equal to the charge leaving the node in the same time interval, i.e. the electrical charge in the node does not accumulate and does not disappear.

Kirchhoff's second law – the algebraic sum of the EMF acting in a closed circuit is equal to the algebraic sum of the voltage drops in this circuit.

Voltage is expressed as the product of current and resistance (according to Ohm's law).

This law also has its own rules for application. First you need to set the direction of the contour bypass with an arrow. Then sum the EMF and voltage, respectively, taking with a plus sign if the value coincides with the bypass direction and minus if it does not. Let's make an equation according to the second Kirchhoff's law, for our scheme. We look at our arrow, E 2 and E 3 coincide with it in direction, which means a plus sign, and E 1 is directed in the opposite direction, which means a minus sign. Now we look at the voltages, the current I 1 coincides in the direction with the arrow, and the currents I 2 and I 3 are directed oppositely. Hence:

-E 1 +E 2 +E 3 =I 1 R 1 -I 2 R 2 -I 3 R 3

On the basis of Kirchhoff's laws, methods for analyzing sinusoidal alternating current circuits have been compiled. The method of loop currents is a method based on the application of the second Kirchhoff law and the method of nodal potentials based on the application of the first Kirchhoff law.

The directed (ordered) movement of free charged particles under the action of an electric field is called electric current.

Conditions for the existence of a current:

1. The presence of free charges.

2. The presence of an electric field, i.e. potential differences. Free charges are present in conductors. The electric field is created by current sources.

When current passes through a conductor, it does the following:

Thermal (heating of the conductor by current). For example: the operation of an electric kettle, iron, etc.).

· Magnetic (appearance of a magnetic field around a current-carrying conductor). For example: the operation of an electric motor, electrical measuring instruments).

Chemical (chemical reactions during the passage of current through certain substances). For example: electrolysis.

You can also talk about

Light (accompanies thermal action). For example: the glow of the filament of an electric light bulb.

Mechanical (accompanies magnetic or thermal). For example: deformation of the conductor when heated, rotation of the frame with current in a magnetic field).

Biological (physiological). For example: electric shock to a person, use of the action of current in medicine.

The main quantities that describe the process of passing current through a conductor.

1. Current I- a scalar value equal to the ratio of the charge that has passed through the cross section of the conductor, the time interval during which the current flowed. The current strength shows how much charge passes through the cross section of the conductor per unit of time. The current is called permanent if the current does not change with time. In order for the current through the conductor to be constant, it is necessary that the potential difference at the ends of the conductor is constant.

2. Voltage U. The voltage is numerically equal to the work of the electric field in moving a single positive charge along the field lines of force inside the conductor.

3. Electrical resistance R- a physical quantity numerically equal to the ratio of the voltage (potential difference) at the ends of the conductor to the strength of the current passing through the conductor.

60. Ohm's law for a chain section.

The current strength in a circuit section is directly proportional to the voltage at the ends of this conductor and inversely proportional to its resistance:

I=U/R;

Ohm found that the resistance is directly proportional to the length of the conductor and inversely proportional to its cross-sectional area and depends on the substance of the conductor.

where ρ is the resistivity, l is the length of the conductor, S is the cross-sectional area of ​​the conductor.

61. Resistance as an electrical characteristic of a resistor. The dependence of the resistance of metal conductors on the type of material and geometric dimensions.

Electrical resistance- a physical quantity that characterizes the properties of the conductor to prevent the passage of electric current and is equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it. Resistance for AC circuits and for alternating electromagnetic fields is described in terms of impedance and wave resistance.

Resistance (often denoted by the letter R or r) is considered, within certain limits, a constant value for a given conductor; it can be calculated as

Where R is the resistance; U is the difference in electrical potentials at the ends of the conductor; I is the strength of the current flowing between the ends of the conductor under the action of a potential difference.

The resistance of a conductor is the same characteristic of a conductor as its mass. The resistance of the conductor does not depend on the current strength in the conductor, nor on the voltage at its ends, but depends only on the type of substance from which the conductor is made and its geometric dimensions: , where: l is the length of the conductor, S is the cross-sectional area of ​​the conductor, ρ is the specific resistance of the conductor, showing what resistance a conductor 1 m long and a cross-sectional area of ​​1 m 2 made of this material will have.

Conductors obeying Ohm's law are called linear. There are many materials and devices that do not obey Ohm's law, such as a semiconductor diode or a gas discharge lamp. Even for metal conductors at sufficiently high currents, a deviation from Ohm's linear law is observed, since the electrical resistance of metal conductors increases with increasing temperature.

The dependence of the conductor resistance on temperature is expressed by the formula: , where: R - conductor resistance at temperature T, R 0 - conductor resistance at a temperature of 0ºС, α - temperature coefficient of resistance.

Electric current - ordered in the direction of the movement of electric charges. The direction of the current is taken to be the direction of movement of positive charges.

The passage of current through the conductor is accompanied by the following actions:

* magnetic (observed in all conductors)
* thermal (observed in all conductors except superconductors)
* chemical (observed in electrolytes).

For the occurrence and maintenance of current in any medium, two conditions must be met:

* the presence of free electric charges in the environment
* creating an electric field in the environment.

The electric field in the medium is necessary to create a directed movement of free charges. As you know, a charge q in an electric field of strength E is affected by a force F = q * E, which forces the free charges to move in the direction of the electric field. A sign of the existence of an electric field in the conductor is the presence of a non-zero potential difference between any two points of the conductor,
However, electric forces cannot sustain an electric current for a long time. The directed movement of electric charges after some time leads to equalization of the potentials at the ends of the conductor and, consequently, to the disappearance of the electric field in it.

To maintain the current in the electric circuit, the charges, in addition to the Coulomb forces, must be affected by non-electrical forces (external forces).
A device that creates external forces, maintains a potential difference in a circuit and converts various types of energy into electrical energy, is called a current source.
For the existence of electric current in a closed circuit, it is necessary to include a current source in it.
Main characteristics

1. Current strength - I, unit of measure - 1 A (Ampere).
The current strength is a value equal to the charge flowing through the cross section of the conductor per unit time.
I = Dq/Dt.

The formula is valid for direct current, at which the current strength and its direction do not change with time. If the strength of the current and its direction change with time, then such a current is called variable.
For AC:
I = limDq/Dt ,
Dt - 0

those. I = q", where q" is the derivative of the charge with respect to time.
2. Current density - j, unit of measurement - 1 A/m2.
The current density is a value equal to the strength of the current flowing through a single cross section of the conductor:
j = I/S .

3. The electromotive force of the current source - emf. (e), the unit is 1 V (Volt). E.m.f. is a physical quantity equal to the work done by external forces when moving along an electric circuit of a single positive charge:
e \u003d Ast. / q.

4. Conductor resistance - R, unit - 1 ohm.
Under the action of an electric field in a vacuum, free charges would move at an accelerated rate. In matter, they move uniformly on average, because part of the energy is given to particles of matter in collisions.

The theory states that the energy of the ordered movement of charges is dissipated by the distortions of the crystal lattice. Based on the nature of electrical resistance, it follows that
R \u003d r * l / S,

where
l - conductor length,
S - cross-sectional area,
r is a proportionality factor, called the resistivity of the material.
This formula is well confirmed by experience.
The interaction of conductor particles with charges moving in the current depends on the chaotic motion of particles, i.e. on the temperature of the conductor. It is known that
r = r0(1 + a t) ,
R = R0(1 + a t) .

The coefficient a is called the temperature coefficient of resistance:
a = (R - R0)/R0*t .

For chemically pure metals a > 0 and equal to 1/273 K-1. For alloys, temperature coefficients are less important. The dependence r(t) for metals is linear:

In 1911, the phenomenon of superconductivity was discovered, which consists in the fact that at a temperature close to absolute zero, the resistance of some metals drops abruptly to zero.

For some substances (for example, electrolytes and semiconductors), the resistivity decreases with increasing temperature, which is explained by an increase in the concentration of free charges.
The reciprocal of resistivity is called electrical conductivity s
s = 1/r

5. Voltage - U, unit of measurement - 1 V.
Voltage is a physical quantity equal to the work done by external and electric forces when moving a single positive charge.

U \u003d (Ast. + Ael.) / q.

Since Ast./q = e, and Ael./q = f1-f2, then
U = e + (f1 - f2) .