It is impossible to imagine the life of a modern person without electricity. Volts, Amps, Watts - these words are heard when talking about devices that operate on electricity. But what is electric current and what are the conditions for its existence? We will talk about this further, providing a brief explanation for novice electricians.

Definition

Electric current is the directed movement of charge carriers - this is a standard formulation from a physics textbook. In turn, charge carriers are called certain particles of matter. They may be:

• Electrons are negative charge carriers.
• Ions are positive charge carriers.

But where do charge carriers come from? To answer this question, you need to remember basic knowledge about the structure of matter. Everything that surrounds us is matter; it consists of molecules, its smallest particles. Molecules are made up of atoms. An atom consists of a nucleus around which electrons move in given orbits. Molecules also move randomly. The movement and structure of each of these particles depends on the substance itself and the influence of the environment on it, such as temperature, stress, and others.

An ion is an atom whose ratio of electrons and protons has changed. If the atom is initially neutral, then the ions, in turn, are divided into:

• Anion is a positive ion of an atom that has lost electrons.
• Cations are an atom with “extra” electrons attached to the atom.

The unit of current measurement is Ampere, according to which it is calculated using the formula:

where U is voltage, [V], and R is resistance, [Ohm].

Or directly proportional to the amount of charge transferred per unit time:

where Q – charge, [C], t – time, [s].

Conditions for the existence of electric current

We figured out what electric current is, now let's talk about how to ensure its flow. For electric current to flow, two conditions must be met:

1. Presence of free charge carriers.
2. Electric field.

The first condition for the existence and flow of electricity depends on the substance in which the current flows (or does not flow), as well as its state. The second condition is also feasible: for the existence of an electric field, the presence of different potentials is required, between which there is a medium in which charge carriers will flow.

Let us remind you: Voltage, EMF is the potential difference. It follows that in order to fulfill the conditions for the existence of current - the presence of an electric field and electric current, voltage is needed. These can be the plates of a charged capacitor, a galvanic element, or an EMF generated under the influence of a magnetic field (generator).

We have figured out how it arises, let’s talk about where it is directed. Current, mainly in our usual use, moves in conductors (electrical wiring in an apartment, incandescent light bulbs) or in semiconductors (LEDs, the processor of your smartphone and other electronics), less often in gases (fluorescent lamps).

So, the main charge carriers in most cases are electrons; they move from minus (a point with a negative potential) to a plus (a point with a positive potential, you will learn more about this below).

But an interesting fact is that the direction of current movement was taken to be the movement of positive charges - from plus to minus. Although in fact everything happens the other way around. The fact is that the decision on the direction of the current was made before studying its nature, and also before it was determined how the current flows and exists.

Electric current in different environments

We have already mentioned that in different environments, electric current can differ in the type of charge carriers. Media can be divided according to the nature of their conductivity (in descending order of conductivity):

1. Conductor (metals).
2. Semiconductor (silicon, germanium, gallium arsenide, etc.).
3. Dielectric (vacuum, air, distilled water).

In metals

Metals contain free charge carriers, they are sometimes called "electric gas". Where do free charge carriers come from? The fact is that metal, like any substance, consists of atoms. Atoms move or vibrate one way or another. The higher the temperature of the metal, the stronger this movement. At the same time, the atoms themselves generally remain in their places, actually forming the structure of the metal.

In the electron shells of an atom there are usually several electrons whose connection with the nucleus is rather weak. Under the influence of temperatures, chemical reactions and the interaction of impurities, which are in any case in the metal, electrons are torn away from their atoms, and positively charged ions are formed. The detached electrons are called free and move chaotically.

If they are affected by an electric field, for example, if you connect a battery to a piece of metal, the chaotic movement of electrons will become orderly. Electrons from a point at which a negative potential is connected (the cathode of a galvanic cell, for example) will begin to move towards a point with a positive potential.

In semiconductors

Semiconductors are materials in which in the normal state there are no free charge carriers. They are in the so-called forbidden zone. But if external forces are applied, such as an electric field, heat, various radiations (light, radiation, etc.), they overcome the band gap and move into the free zone or conduction band. Electrons break away from their atoms and become free, forming ions - positive charge carriers.

Positive carriers in semiconductors are called holes.

If you simply transfer energy to a semiconductor, for example, heat it, a chaotic movement of charge carriers will begin. But if we are talking about semiconductor elements, such as a diode or transistor, then an EMF will arise at the opposite ends of the crystal (a metallized layer is applied to them and the leads are soldered), but this does not relate to the topic of today’s article.

If you apply an EMF source to a semiconductor, then the charge carriers will also move to the conduction band, and their directional movement will also begin - holes will go in the direction with a lower electric potential, and electrons - in the direction with a higher one.

In vacuum and gas

A vacuum is a medium with a complete (ideal case) absence of gases or a minimized (in reality) amount of gas. Since there is no matter in a vacuum, there is no place for charge carriers to come from. However, the flow of current in a vacuum marked the beginning of electronics and a whole era of electronic elements - vacuum tubes. They were used in the first half of the last century, and in the 50s they began to gradually give way to transistors (depending on the specific field of electronics).

Let us assume that we have a vessel from which all the gas has been pumped out, i.e. there is a complete vacuum in it. Two electrodes are placed in the vessel, let's call them anode and cathode. If we connect the negative potential of the EMF source to the cathode and the positive potential to the anode, nothing will happen and no current will flow. But if we start heating the cathode, current will begin to flow. This process is called thermionic emission - the emission of electrons from a heated electron surface.

The figure shows the process of current flow in a vacuum tube. In vacuum tubes, the cathode is heated by a nearby filament on the figure (H), such as in a lighting lamp.

At the same time, if you change the polarity of the power supply - apply minus to the anode, and apply plus to the cathode - no current will flow. This will prove that current in a vacuum flows due to the movement of electrons from the CATHODE to the ANODE.

Gas, like any substance, consists of molecules and atoms, which means that if the gas is under the influence of an electric field, then at a certain strength (ionization voltage) electrons will break away from the atom, then both conditions for the flow of electric current will be satisfied - field and free media.

As already mentioned, this process is called ionization. It can occur not only from applied voltage, but also from heating the gas, X-ray radiation, under the influence of ultraviolet radiation, and other things.

Current will flow through the air, even if a burner is installed between the electrodes.

The flow of current in inert gases is accompanied by luminescence of the gas; this phenomenon is actively used in fluorescent lamps. The flow of electric current in a gaseous medium is called a gas discharge.

In liquid

Let's say that we have a vessel with water in which two electrodes are placed, to which a power source is connected. If the water is distilled, that is, pure and does not contain impurities, then it is a dielectric. But if we add a little salt, sulfuric acid or any other substance to water, an electrolyte is formed and current begins to flow through it.

An electrolyte is a substance that conducts electric current due to dissociation into ions.

If you add copper sulfate to water, a layer of copper will deposit on one of the electrodes (cathode) - this is called electrolysis, which proves that the electric current in the liquid is carried out due to the movement of ions - positive and negative charge carriers.

Electrolysis is a physical and chemical process that involves the separation of the components that make up the electrolyte on the electrodes.

This is how copper plating, gilding and coating with other metals occurs.

Conclusion

To summarize, for electric current to flow, free charge carriers are needed:

• electrons in conductors (metals) and vacuum;
• electrons and holes in semiconductors;
• ions (anions and cations) in liquids and gases.

In order for the movement of these carriers to become ordered, an electric field is needed. In simple words, apply a voltage to the ends of a body or install two electrodes in an environment where electric current is expected to flow.

It is also worth noting that current influences a substance in a certain way; there are three types of influence:

• thermal;
• chemical;
• physical.

Useful

Ohm's law for a circuit section states: current is directly proportional to voltage and inversely proportional to resistance.

If you increase the voltage acting in an electrical circuit several times, then the current in this circuit will increase by the same amount. And if you increase the circuit resistance several times, the current will decrease by the same amount. Similarly, the greater the pressure and the less resistance the pipe provides to the movement of water, the greater the water flow in the pipe.

Electrical resistance- a physical quantity that characterizes the properties of a 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.

Any body through which electric current flows exhibits a certain resistance to it.

The electronic theory 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. Different metal conductors, having different atomic structures, offer different resistance to electric current.

The resistance of the conductor does not depend on the current in the circuit and voltage, but is determined only by the shape, size and material of the conductor.

The greater the resistance of a conductor, the worse it conducts electric current, and, conversely, the lower the resistance of the conductor, the easier it is for electric current to pass through this conductor.

Question 2. Apparent movements of celestial bodies. Laws of planetary motion.

A) On a dark night, we can see about 2,500 stars in the sky (including 5,000 in the invisible hemisphere), which differ in brightness and color. They seem to be attached to the celestial sphere and revolve around the Earth with it. To navigate among them, the sky was divided into 88 constellations. A special place among the constellations was occupied by the 12 zodiacal constellations through which the annual path of the Sun passes - the ecliptic. To navigate among the stars, astronomers use various celestial coordinate systems. One of them is the equatorial coordinate system (Fig. 15.1). It is based on the celestial equator - the projection of the earth's equator onto the celestial sphere. The ecliptic and equator intersect at two points: the spring and autumn equinox. Any star has two coordinates: α – right ascension (measured in hourly units), b – deviation (measured in degree units). The star Altair has the following coordinates: α = 19 h 48 m 18 s; b = +8° 44 '. The measured coordinates of stars are stored in catalogs, and star maps are built from them, which astronomers use when searching for the necessary luminaries. The relative position of the stars in the sky does not change; they rotate daily along with the celestial sphere. The planets, along with their daily rotation, make a slow movement among the stars, and are called a wandering star.

The apparent movement of the planets and the Sun was described by Nicolaus Copernicus using the geocentric system of the world.

B) The movement of planets and other celestial bodies around the Sun occurs according to Kepler’s three laws:

Kepler's first law– under the influence of gravity, one celestial body moves in the gravitational field of another celestial body along one of the conic sections - a circle, ellipse, parabola or hyperbola.

Kepler's second law- each planet moves in such a way that the radius vector of the planet describes equal areas in equal periods of time.

Kepler's third law- the cube of the semimajor axis of the orbit of a body, divided by the square of its period of revolution and the sum of the masses of the bodies, is a constant value.

a 3 /[Т 2 *(M 1+ M 2) ] = G/4П 2 G – gravitational constant.

Moon moves around Earth in an elliptical orbit. The change in lunar phases is determined by a change in the type of illumination on the side of the Moon. The movement of the Moon around the Earth is explained by lunar and solar eclipses. The phenomenon of ebb and flow is caused by the attraction of the Moon and the large size of the Earth.

Electricity. Ohm's law

If an insulated conductor is placed in an electric field, then the free charges q a force will act in the conductor. As a result, a short-term movement of free charges occurs in the conductor. This process will end when the own electric field of the charges arising on the surface of the conductor completely compensates for the external field. The resulting electrostatic field inside the conductor will be zero (see § 1.5).

However, in conductors, under certain conditions, continuous ordered movement of free electric charge carriers can occur. This movement is called electric shock . The direction of the electric current is taken to be the direction of movement of positive free charges. For an electric current to exist in a conductor, an electric field must be created in it.

A quantitative measure of electric current is current strength I – scalar physical quantity equal to the charge ratio Δ q, transferred through the cross section of the conductor (Fig. 1.8.1) during the time interval Δ t, to this time interval:

In the International System of Units (SI) current is measured in amperes (A). The current unit of 1 A is established by the magnetic interaction of two parallel conductors with current (see § 1.16).

Direct electric current can only be created in closed circuit , in which free charge carriers circulate along closed trajectories. The electric field at different points of such a circuit is constant over time. Consequently, the electric field in a direct current circuit has the character of a frozen electrostatic field. But when an electric charge moves in an electrostatic field along a closed path, the work done by electric forces is zero (see § 1.4). Therefore, for the existence of direct current, it is necessary to have a device in the electrical circuit that is capable of creating and maintaining potential differences in sections of the circuit due to the work of forces non-electrostatic origin. Such devices are called DC sources . Forces of non-electrostatic origin acting on free charge carriers from current sources are called outside forces .

The nature of external forces may vary. In galvanic cells or batteries they arise as a result of electrochemical processes; in direct current generators, external forces arise when conductors move in a magnetic field. The current source in the electrical circuit plays the same role as the pump, which is necessary to pump fluid in a closed hydraulic system. Under the influence of external forces, electric charges move inside the current source against electrostatic field forces, due to which a constant electric current can be maintained in a closed circuit.

When electric charges move along a direct current circuit, external forces acting inside the current sources perform work.

Physical quantity equal to the work ratio A st external forces when moving a charge q from the negative pole of the current source to the positive pole to the magnitude of this charge is called electromotive force of the source(EMF):

Thus, the EMF is determined by the work done by external forces when moving a single positive charge. Electromotive force, like potential difference, is measured in volts (V).

When a single positive charge moves along a closed direct current circuit, the work done by external forces is equal to the sum of the emf acting in this circuit, and the work done by the electrostatic field is zero.

A DC circuit can be divided into separate sections. Those areas where no external forces act (i.e. areas that do not contain current sources) are called homogeneous . Areas containing current sources are called heterogeneous .

When a single positive charge moves along a certain section of the circuit, work is performed by both electrostatic (Coulomb) and external forces. The work of electrostatic forces is equal to the potential difference Δφ 12 = φ 1 – φ 2 between the initial (1) and final (2) points of the inhomogeneous section. The work of external forces is equal, by definition, to the electromotive force 12 acting in a given area. Therefore the total work is equal to

The German physicist G. Ohm in 1826 experimentally established that the current strength I, flowing along a homogeneous metal conductor (i.e., a conductor in which no external forces act), is proportional to the voltage U at the ends of the conductor:

Where R= const.

Size R usually called electrical resistance . A conductor with electrical resistance is called resistor . This ratio expresses Ohm's law for a homogeneous section of a chain: The current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.

The SI unit of electrical resistance of conductors is ohm (Ohm). A resistance of 1 ohm has a section of the circuit in which a current of 1 A occurs at a voltage of 1 V.

Conductors that obey Ohm's law are called linear . Graphical dependence of current strength I from voltage U(such graphs are called volt-ampere characteristics , abbreviated as CVC) is depicted by a straight line passing through the origin of coordinates. It should be noted that there are many materials and devices that do not obey Ohm's law, for example, a semiconductor diode or a gas-discharge lamp. Even with 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.

For a section of a circuit containing an emf, Ohm's law is written in the following form:

According to Ohm's law

Adding both equalities, we get:

I (R + r) = Δφ CD + Δφ ab + .

But Δφ CD = Δφ ba = – Δφ ab. That's why

This formula will express Ohm's law for a complete circuit : the current strength in a complete circuit is equal to the electromotive force of the source divided by the sum of the resistances of the homogeneous and inhomogeneous sections of the circuit.

Resistance r heterogeneous area in Fig. 1.8.2 can be thought of as internal resistance of the current source . In this case, the area ( ab) in Fig. 1.8.2 is the internal portion of the source. If points a And b short with a conductor whose resistance is small compared to the internal resistance of the source ( R << r), then the chain will flow short circuit current

Short circuit current - the maximum current that can be obtained from a given source with electromotive force and internal resistance r. For sources with low internal resistance, the short circuit current can be very high and cause destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have short-circuit currents of several hundred amperes. Short circuits in lighting networks powered from substations (thousands of amperes) are especially dangerous. To avoid the destructive effects of such large currents, fuses or special circuit breakers are included in the circuit.

In some cases, to prevent dangerous values ​​of short circuit current, some external resistance is connected in series to the source. Then resistance r is equal to the sum of the internal resistance of the source and the external resistance, and during a short circuit the current strength will not be excessively large.

If the external circuit is open, then Δφ ba = – Δφ ab= , i.e. the potential difference at the poles of an open battery is equal to its emf.

If the external load resistance R turned on and current is flowing through the battery I, the potential difference at its poles becomes equal

Δφ ba = – Ir.

In Fig. 1.8.3 shows a schematic representation of a direct current source with an equal emf and internal resistance r in three modes: “idling”, load operation and short circuit mode (short circuit). The electric field strength inside the battery and the forces acting on the positive charges are indicated: – electric force and – external force. In short circuit mode, the electric field inside the battery disappears.

To measure voltages and currents in DC electrical circuits, special instruments are used - voltmeters And ammeters.

Voltmeter designed to measure the potential difference applied to its terminals. He connects parallel the section of the circuit where the potential difference is measured. Any voltmeter has some internal resistance R B. In order for the voltmeter not to introduce a noticeable redistribution of currents when connected to the circuit being measured, its internal resistance must be large compared to the resistance of the section of the circuit to which it is connected. For the circuit shown in Fig. 1.8.4, this condition is written as:

R B >> R 1 .

This condition means that the current I B = Δφ CD / R B flowing through the voltmeter is much less than the current I = Δφ CD / R 1, which flows through the tested section of the circuit.

Since there are no external forces acting inside the voltmeter, the potential difference at its terminals coincides, by definition, with the voltage. Therefore, we can say that a voltmeter measures voltage.

Ammeter designed to measure current in a circuit. The ammeter is connected in series to an open circuit so that the entire measured current passes through it. The ammeter also has some internal resistance R A. Unlike a voltmeter, the internal resistance of an ammeter must be quite small compared to the total resistance of the entire circuit. For the circuit in Fig. 1.8.4 The resistance of the ammeter must satisfy the condition

Conditions for the existence of direct electric current.

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

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 occurrence of external forces in different current sources are different. For example, in batteries and galvanic cells, external forces arise due to the occurrence of chemical reactions, in power plant generators they arise when a conductor moves in a magnetic field, in photocells - when light acts on electrons in metals and semiconductors.

Electromotive force of the current sourceis the ratio of the work of external forces to the amount of positive charge transferred from the negative pole of the current source to the positive one.

Basic concepts.

Current strength- a scalar physical quantity equal to the ratio of the charge passing through the conductor to the time during which this charge 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 - a scalar physical quantity equal to the ratio of the total work of Coulomb and external forces when moving a positive charge in an area to the value of this charge.

WhereA - complete work of external and Coulomb forces,q - electric charge.

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

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

Conductivitycalled the reciprocal of resistance

WhereG - conductivity.

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

Conditions for the existence of current:

1. The presence of free charges.

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

When current passes through a conductor, it has the following effects:

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

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

· Chemical (chemical reactions when current passes through certain substances). For example: electrolysis.

We can also talk about

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

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

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

Basic quantities describing the process of current passing through a conductor.

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

2. Voltage U. The voltage is numerically equal to the work of the electric field in moving a unit positive charge along the field lines 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 section of a circuit.

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

I = U/R;

Ohm established that 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. 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 a 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 alternating current circuits and for alternating electromagnetic fields is described by the concepts of impedance and characteristic impedance.

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

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

The resistance of a conductor is the same characteristic of a conductor as its mass. The resistance of a conductor does not depend either on the current in the conductor or 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 resistivity of the conductor, showing what resistance a conductor with a length of 1 m and a cross-sectional area of ​​1 m2, made of a given material, will have.

Conductors that obey Ohm's law are called linear. There are many materials and devices that do not obey Ohm's law, for example, 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 is the conductor resistance at temperature T, R 0 is the conductor resistance at 0ºC, α is the temperature coefficient of resistance.