Directed (ordered) motion of free charged particles under the action of electric field 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 - 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 electric current and 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.
Third party forces. Electromotive force and tension.
Extraneous forces are those forces that differ in nature from the forces of the electrostatic field.
These forces can be due to chemical processes, diffusion of current carriers in an inhomogeneous medium, electric (but not electrostatic) fields generated by time variables magnetic fields, etc.
EMF - physical quantity, equal to work performed by third-party forces when moving along electrical circuit unit positive charge:
ε = A st./q Unit of measurement - 1 V (Volt)
Voltage is a physical quantity equal to the work done by external and electric forces when moving a single positive charge.
U \u003d (A st. + A el.) / q Unit of measurement - 1 V.
Electrical circuit. Homogeneous and heterogeneous section of the chain.
Homogeneous and inhomogeneous sections of the chain
Homogeneous section of the circuit - a section of the circuit on which no external forces act (no current source)
An inhomogeneous section of a circuit is a section of a circuit on which there is a current source.
Electrical circuit
Electrical circuit. External and internal section of the circuit, voltage drop.
Electrical circuit- a set of devices, elements designed for the flow of electric current, electromagnetic processes.
The electrical circuit can be divided into two sections: external and internal.
An external section, or, as they say, an external circuit, consists of one or more receivers of electrical energy, connecting wires and various auxiliary devices included in this circuit.
The inner section, or inner circuit, is the source itself.
Voltage drop- a gradual decrease in voltage along the conductor through which the electric current flows, due to the fact that the conductor has active resistance.
Conductor resistance
Resistance - a value proportional to the length of the conductor l and inversely proportional to its cross-sectional area S
The greater the resistance of the conductor, the worse it conducts electric current, and, conversely, the lower the resistance of the conductor, the easier it is for the electric current to pass through this conductor.
Conductor electrical resistivity ρ [Ohm*m] ρ=RS/l R = ρ*l/S
Ohm's law for a section of a circuit and for a closed circuit
Ohm's law for a section of an electrical circuit - the current strength in a section of an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance of the section.
Ohm's law for a complete electrical circuit - the current strength in an electrical circuit is directly proportional to the EMF of the source and inversely proportional to the impedance of the circuit (the sum of external and internal resistances)
I = ε / (R + r). where R is the resistance of the external section of the circuit,
r - internal resistance.
Serial connection of energy consumers
With a serial connection, the conductors are connected in series, that is, one after another, while I \u003d const, U \u003d U 1 +U 2 +U 3 + ... + U n and R \u003d R 1 +R 2 +R 3 + ... + R n
Parallel connection of current sources.
Work of electric current
The work of the electric current A is equal to the product of the value of the transferred charge Q and the voltage U
A=Q*U [A]=J, [U]=B, [Q]=Cl, [t]=c.
Because I=Q/t, => Q=I*t, so A=I*U*t
According to Ohm's law for the chain section I=U/R, U=I*R
A=I*U*T => A=U 2 *t/R(useful for parallel connection) => A=I 2 *R*t(useful for serial connection)
The nature of the world.
The nature of the world - wave.
17th century Christian Huygens: 1) diffraction - light bending around obstacles 2) interference - addition of waves.
19th century- Maxwell's theory (speed of light - special case electromagnetic waves) - electromagnetic theory, the propagation speed of electromagnetic waves in vacuum 3 * 10 8 m / s is equal to the speed of light in vacuum. 299 thousand km/s
17th century O. Roemer astronomically obtained the speed of light about 214.3 km / s
19th century. Fizeau speed of light is about 313 thousand km / s
The nature of the world quantum.
approximately 500 BC Pythagoras: light is a stream of particles.
17th century Isak Newton followed the same theory. Karpuskula (from lat.) - a particle.
Newton's carpuscular theory: 1) rectilinear propagation of light 2) the law of reflection 3) the formation of shadows from objects
19 Heinrich Hertz discovered the photoelectric effect.
20th century. The light has dual nature - has a corpuscular-wave dualism: during propagation - as a wave, and during radiation and absorption - as a stream of particles.
relationship between long and lambda wavelength and nu frequency
lambda=s/nu s - speed of light in vacuum [m/s] lambda [m] nu [Hz]
Laws of reflection
1. The incident beam, the reflecting beam and the perpendicular to the interface between two media, restored at the point of incidence of the beam, lie in the same plane.
2The angle of reflection γ is equal to the angle of incidence α: γ = α
Specular reflection - if the roughness is less than lambda and diffuse roughness is comparable to lambda
Diffuse reflection of light. Mirror reflection of light.
Laws of refraction of light.
The law of refraction of light: the incident and refracted rays, as well as the perpendicular to the interface between two media, restored at the point of incidence of the beam, lie in the same plane. The ratio of the sine of the angle of incidence α to the sine of the angle of refraction γ is a constant value for two given media:
The constant value n is called the relative refractive index of the second medium relative to the first. The refractive index of a medium relative to vacuum is called the absolute refractive index.
The relative refractive index of two media is equal to the ratio of their absolute refractive indices:
physical meaning refractive index is the ratio of the wave propagation speed in the first medium υ 1 to the speed of their propagation in the second medium υ 2:
The nature of light from 26.
Wave interference is the phenomenon of superposition of coherent waves; characteristic of waves of any nature (mechanical, electromagnetic, etc.)
Coherent waves are waves emitted by sources that have the same frequency and constant phase difference.
When coherent waves are superimposed at any point in space, the amplitude of oscillations (displacement) of this point will depend on the difference in distances from the sources to the point under consideration. This distance difference is called the path difference.
When coherent waves are superimposed, two limiting cases are possible:
Maximum condition:
Where 
The wave path difference is equal to an integer number of wavelengths (otherwise an even number of half-wavelengths).
In this case, the waves at the point under consideration come with the same phases and reinforce each other - the amplitude of oscillations of this point is maximum and equal to twice the amplitude.
Minimum condition:
, Where
The wave path difference is equal to an odd number of half-wavelengths.
The waves arrive at the point under consideration in antiphase and cancel each other out.
The oscillation amplitude of this point is equal to zero.
As a result of the superposition of coherent waves (wave interference), an interference pattern is formed.
When waves interfere, the amplitude of oscillations of each point does not change in time and remains constant.
When incoherent waves are superimposed, there is no interference pattern, because the amplitude of oscillations of each point changes with time.
Light interference
1802 English physicist Thomas Jung set up an experiment in which light interference was observed.
Thomas Young's experience
Two beams of light were formed from one source through slit A (through slits B and C), then the beams of light fell on screen E. Since the waves from slits B and C were coherent, an interference pattern could be observed on the screen: alternation of light and dark stripes .
Light stripes - the waves strengthened each other (the maximum condition was observed).
Dark stripes - the waves were formed in antiphase and extinguished each other (minimum condition).
If Young's experiment used a source of monochromatic light (of the same wavelength, then only light and dark bands of a given color were observed on the screen.)
If the source gave white light (i.e., complex in composition), then rainbow stripes were observed on the screen in the region of light stripes. The iridescence was explained by the fact that the conditions of the maxima and minima depend on the wavelengths.
Interference in thin films
The phenomenon of interference can be observed, for example:
Iridescent stains on the surface of the liquid during oil spills, kerosene, in soap bubbles;
The film thickness must be greater than the light wavelength.
During his experiment, Jung was able to measure the wavelength of light for the first time.
As a result of the experiment, Jung proved that light has wave properties.
Application of interference:
- interferometers - devices for measuring the length of a light wave
- enlightenment of optics (in optical devices, when light passes through the lens, the loss of light is up to 50%) - all glass parts are covered with a thin film with a refractive index slightly less than that of glass; interference maxima and minima are redistributed, and light losses are reduced.
The nature of light from 26.
DIFFRACTION OF LIGHT
Diffraction is a phenomenon inherent in wave processes for any kind of waves.
Diffraction of light- this is the deviation of light rays from rectilinear propagation when passing through narrow slots, small openings or when bending around small obstacles.
The phenomenon of light diffraction proves that light has wave properties.
To observe diffraction, you can:
Pass light from a source through a very small hole, or place the screen at a great distance from the hole. Then a complex picture of light and dark concentric rings is observed on the screen.
- or direct light onto a thin wire, then light and dark stripes will be observed on the screen, and in the case of white light - a rainbow stripe.
Diffraction grating
It is an optical instrument for measuring the wavelength of light.
A diffraction grating is a collection of a large number of very narrow slits separated by opaque gaps.
If a monochromatic wave is incident on the grating. then the slots (secondary sources) create coherent waves. A converging lens is placed behind the grille, then a screen. As a result of the interference of light from different grating slits, a system of maxima and minima is observed on the screen.
The path difference between the waves from the edges of adjacent slots is equal to the length of segment AC. If an integer number of wavelengths fit on this segment, then the waves from all slots will amplify each other. When using white light, all maxima (except the central one) have a rainbow color.
So the maximum condition is:
where k is the order (or number) of the diffraction spectrum
The more lines on the grating, the farther apart the diffraction spectra are and the smaller the width of each line on the screen, so the maxima are seen as separate lines, i.e. the resolving power of the grating increases.
The accuracy of measuring the wavelength is the greater, the more grooves per unit length of the grating.
POLARIZATION OF LIGHT
Wave polarization
The property of transverse waves is polarization.
A polarized wave is a transverse wave in which all particles oscillate in the same plane.
Light polarization
Experience with tourmaline - proof of the transverseness of light waves.
Tourmaline crystal is a transparent, green mineral with an axis of symmetry.
In a beam of light from a conventional source, there are fluctuations in the vectors of the electric field strength E and magnetic induction B in all possible directions perpendicular to the direction of propagation of the light wave. Such a wave is called a natural wave.
When passing through a tourmaline crystal, light is polarized.
At polarized light oscillations of the strength vector E occur only in one plane, which coincides with the axis of symmetry of the crystal.
The polarization of light after the passage of tourmaline is detected if a second tourmaline crystal (analyzer) is placed behind the first crystal (polarizer).
With identically directed axes of two crystals, the light beam will pass through both and only slightly weaken due to the partial absorption of light by the crystals.
Scheme of the operation of the polarizer and the analyzer behind it:
If the second crystal starts to rotate, i.e. shift the position of the symmetry axis of the second crystal relative to the first, then the beam will gradually go out and go out completely when the position of the symmetry axes of both crystals becomes mutually perpendicular.
Application of polarized light:
Smooth adjustment of illumination with two polaroids
- to extinguish glare when photographing (glare is extinguished by placing a polaroid between the light source and the reflective surface)
To eliminate the blinding effect of the headlights of oncoming cars.
Polaroid, polarizing light filter, one of the main types of optical linear polarizers; is a thin polarizing film glued to protect against mechanical damage and moisture between two transparent plates (films).
DISPERSION
A beam of white light, passing through a trihedral prism, is not only deflected, but also decomposed into component colored rays.
This phenomenon was established by Isaac Newton by conducting a series of experiments.
Newton's experiments
Experience in the decomposition of white light into a spectrum:
or 
Newton sent a beam sunlight through a small hole onto a glass prism.
Getting on the prism, the beam was refracted and gave on the opposite wall an elongated image with iridescent alternation of colors - the spectrum.
Experience in the synthesis (obtaining) of white light:
First, Newton directed the sun's beam at a prism. Then, having collected the colored rays emerging from the prism with the help of a converging lens, Newton received a white image of a hole on a white wall instead of a colored strip.
Newton's conclusions:
The prism does not change the light, but only decomposes it into its components.
- light rays that differ in color differ in the degree of refraction; Violet rays are most strongly refracted, red rays are less strongly refracted.
Red light, which is refracted less, has the highest speed, and violet light has the lowest, so the prism decomposes the light.
The dependence of the refractive index of light on its color is called dispersion.
Remember the phrase, the initial letters of the words of which give a sequence of colors of the spectrum:
"Every Hunter Wants to Know Where the Pheasant Sits."
White light spectrum:
Conclusions:
The prism splits the light
- white light is complex (composite)
Violet rays are refracted more than red ones.
The color of a beam of light is determined by its frequency of oscillation.
When moving from one medium to another, the speed of light and wavelength change, but the frequency that determines the color remains constant.
The boundaries of the ranges of white light and its components are usually characterized by their wavelengths in vacuum.
White light is a collection of wavelengths from 380 to 760 nm.
Where can one observe the phenomenon of dispersion?
When light passes through a prism
- refraction of light in water droplets, for example, on grass or in the atmosphere when a rainbow is formed
- around the lanterns in the fog.
How to explain the color of any object?
White paper reflects all the rays of various colors falling on it.
- a red object reflects only red rays, and absorbs the rays of other colors
-
The eye perceives rays of a certain wavelength reflected from an object and thus perceives the color of the object.
Spectral analysis - a set of methods for qualitative and quantitative determination of the composition of an object, based on the study of the spectra of the interaction of matter with radiation, including spectra electromagnetic radiation, acoustic waves, mass and energy distributions elementary particles and etc.
Electric current and conditions of its existence.
Electric current is an ordered, directed, movement of free charges in a conductor.
Direct current is an electric current whose characteristics do not change over time.
Conditions for the existence of an electric current
For the occurrence and maintenance of current in any medium, two conditions must be met:
-presence in the environment of free electric charges
- creation of an electric field in the environment.
In different media, the carriers of electric current are different charged particles.
The current strength I is a scalar quantity that characterizes the charge Q passing through the cross section of the conductor per unit time. Q=q*N I=Q/t
Current is measured in amperes and charge in coulombs. I=[A], Q=[Cl]
Current density - j vector quantity j V q, shows the current strength per unit S sec.
j=I/S sec Sectional area S sec. measured in square meters
And again, good day to you, dear. Without further ado, let's start our conversation today. It would seem that we have long figured out the causes of the current in the conductor. We placed a conductor in a field - electrons ran, a current arose. What else does. But it turns out that for this current to exist in the conductor constantly, it is necessary to observe certain conditions. For a clearer understanding of the physics of the process of the flow of electric current in a conductor, consider an example.
Suppose we have some conductor that we will place in an electric field as shown in figure 4.1.
Figure 4.1 - Conductor in an electric field
Let's conventionally denote the magnitude of the tension at the ends of the conductor as E 1 and E 2, and E 1 >E 2. As we found out earlier, free electrons in the conductor will begin to move towards a greater field strength, that is, to point A. However, over time, the potential formed by the accumulation of electrons at point A will become such that its own electromagnetic field E 0 created by it will be equal in absolute value to the external field, and the directions of the fields will be opposite, since the potential of point B is more positive (lack of electrons caused by the action of an external field).
Since the resulting action of two identical opposite forces is equal to zero: |E|+|(E 0)|=0, the electrons stop their ordered movement, the electric current stops. In order for the electron flow to be continuous, it is necessary: firstly, to apply an additional force of a non-potential nature, which would compensate for the influence of the conductor's own electric field and, secondly, to create a closed circuit, since the movement of electrons can occur only in conductors (previously we pointed out that dielectrics, although they have some electrical conductivity, do not pass electric current) and to ensure the constancy of the compensating force, the constancy of the fields is necessary: both external and intrinsic.
Let's start with the second point. We will consider a conductor placed in a field, as shown in Figure 4.2. Let us assume that after the interaction of the external and intrinsic electromagnetic fields has been compensated, we have applied in addition to the external field one more of the same field. The total action of the external field will be 2 |E|. The current in the conductor will continue to flow in the same direction, but exactly until 2 |E|>|E 0 |, after which the electric current will stop again. That is, the external influence must increase continuously to ensure the flow of current in an open conductor, which is impossible.
If you close the conductor so that one part of it lies outside the field, then due to the work of an additional force in addition to the external field (this force in this case should not be potential, since the work of the potential force in a closed loop is zero and does not depend on the shape of the trajectory), then an electric current will appear in the conductor, due to the influence of only the external field, since the actual field of the conductor will be completely compensated. That is why any electrical circuit must always be closed.
You can try to explain the need to introduce additional force from the following consideration: if we could partially transfer charges from end B of the conductor to end A of the conductor, the electric current would also not stop. However, such "landing" also requires energy. Hence, the introduction of additional force is still necessary. Non-potential forces are also called external forces. And their sources are current sources or generators. 
Figure 4.2 - The emergence of its own electromagnetic field in the conductor
So where can we get additional force, which, moreover, should not be created by the field, because without it we will not get current? It turns out that during the course of a chemical reduction-oxidation reaction, for example, the interaction of lead oxide and dilute sulfuric acid, free electrons are released:
In order to “attract” all the electrons released during the reaction to one point in space, several lead grids, called electrodes, are placed in a solution of sulfuric acid. One part of the electrodes is made of lead and is called the cathode, the other - the anode - is made of lead dioxide. The cathode is the source of free electrodes for the external circuit, and the anode is the receiver.
The above example corresponds to a device known to all motorists (and not only) - a lead-acid battery. Of course, the above example does not coincide much with what is happening inside the battery in reality, however, the essence of the appearance of the current reflects well. Thus, between the positive anode (few electrons) and the negative cathode (many electrons), an electric field arises, which forms external forces and creates a current in the conductor. This force depends only on the flow chemical reaction, then it is practically constant until the elements of this reaction exist - acid and lead oxide. Therefore, if we remove the electric field and connect the conductor to the anode and cathode, the electric current will still flow due to the fact that the battery creates an external force. The conductor will have its own electric field around it, which the battery needs to overcome in order to transfer the electron from the cathode to the anode. This is the essence of outside power.
Now consider the situation with the battery and the conductor connected to it. The electric field does positive work to move a positive charge (we are talking about positive charges, since the direction of their movement corresponds to the direction of the current) in the direction of decreasing the field potential. The current source carries out the separation of electric charges - positive charges accumulate on one pole, negative charges on the other. The strength of the electric field in the source is directed from the positive pole to the negative, so the work of the electric field to move the positive charge will be positive when it moves from "plus" to "minus". The work of external forces, on the contrary, is positive if the positive charges move from the negative pole to the positive, that is, from “minus” to “plus”. This is the fundamental difference between the concepts of potential difference and EMF, which must always be remembered.
Figure 4.3 shows the direction of current flow I in the conductor connected to the battery - from the positive anode to the negative cathode, however, inside the battery, third-party chemical reaction forces “drop” the electrons that came from the external circuit from the anode to the cathode and positive ions from the cathode to the anode, that is, they act against the direction of current flow and the direction of the field. 
Figure 4.3 - Demonstration of external forces in the event of an electric current
From the above considerations, the following conclusion can be drawn: the forces acting on the charge inside the current source are different from the forces acting inside the conductor. Accordingly, it is necessary to distinguish these forces from each other. To characterize external forces, the magnitude of the electromotive force (EMF) was introduced - the work performed by external forces to move a single positive charge. It is denoted by the Latin letter ε (“epsilon”) and is measured in the same way as the potential difference - in volts.
Since the potential difference and EMF are forces of different types, we can say that the EMF outside the source leads is zero. Although in ordinary life these subtleties are neglected and they say: “The voltage on the battery is 1.5V”, although strictly speaking the voltage in the circuit section is the total work of electrostatic and third-party forces to move a single positive charge. In the future, we will still encounter these concepts and they will be useful to us when calculating complex electrical circuits.
This, perhaps, is all, because the lesson turned out to be too loaded ... But the concepts of voltage and EMF must be able to distinguish.
- For the existence of an electric current, two conditions are necessary:
1) a closed electrical circuit;
2) the presence of a source of third-party non-potential forces. - Electromotive force (EMF) is the work done by external forces to move a single positive charge.
- Sources of extraneous forces in an electrical circuit are also called current sources.
- The positive terminal of the battery is called the anode, the negative terminal is called the cathode.
There will be no tasks this time, it is better to repeat this lesson in order to understand the whole physics of current flow in a conductor. As always, you can leave any questions, suggestions and wishes in the comments below! See you soon!
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 is known, 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 a conductor is the presence of 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 the circuit and converts different kinds 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 distortions crystal lattice. Coming from nature electrical resistance, 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) .
Sections: Physics
Lesson goals.
Tutorial:
the formation of students' knowledge about the conditions for the occurrence and existence of electric current.
Developing:
development logical thinking attention, skills to use the acquired knowledge in practice.
Educational:
creating conditions for the manifestation of independence, attentiveness and self-esteem.
Equipment.
- Galvanic cells, battery, generator, compass.
- Cards (attached).
- Demonstration material (portraits of outstanding physicists Ampère, Volta; posters "Electricity", "Electric charges").
Demos:
- The action of an electric current in a conductor on a magnetic needle.
- Current sources: galvanic cells, battery, generator.
Lesson plan
1. Organizational moment.
2. introduction teacher.
3. Preparation for the perception of new material.
4. Learning new material.
a) current sources;
b) the action of electric current;
c) physical operetta “Queen of Electricity”;
d) filling in the table “Electric current”;
e) safety measures when working with electrical appliances.
5. Summing up the lesson.
6. Reflection.
7. Homework:
a) Based on the knowledge gained in the lessons of life safety, special technologies, prepare and write down in a notebook a memo “Safety measures when working with electrical appliances”
b) Individual task: Prepare a report on the use of a power source in everyday life and technology.
Lesson summary
1. Organizational moment
Mark the presence of students, name the topic of the lesson, the goal.
2. Introductory speech of the teacher
With the words electricity, electric current, we are familiar from early childhood. Electric current is used in our homes, in transport, in production, in the lighting network.
But what is an electric current, what is its nature, is not easy to understand.
The word electricity comes from the word electron, which is translated from Greek as amber. Amber is the fossilized resin of ancient coniferous trees. The word current means the flow or movement of something.
3. Preparation for the perception of new material
Questions of the introductory conversation.
What are the two types of charges that exist in nature? How do they interact?
Answer: There are two types of charges in nature: positive and negative.
Positive charge carriers are protons, negative charge carriers are electrons. Like-charged particles repel each other, opposite-charged particles attract.
Is there an electric field around an electron?
Answer: Yes, there is an electric field around an electron.
What are free electrons?
Answer: These are the electrons most distant from the nucleus, they can freely move between atoms.
4. Learning new material
a) Current sources.
There are special devices on the table. What are their names? What are they needed for?
Answer: These are galvanic cells, a battery, a generator - the common name for current sources. They are necessary to supply electrical energy, create an electric field in the conductor.
We know that there are charged particles, electrons and protons, we know that there are devices called current sources.
b) Actions of electric current.
Tell me, how can we understand that there is an electric current in the circuit, by what actions?
Answer: Electric current has different types of action:
- Thermal - the conductor through which the electric current flows is heated (electric stove, iron, incandescent lamp, soldering iron).
- The chemical effect of current can be observed when an electric current is passed through a solution. blue vitriol– isolation of copper from a solution of vitriol, chromium plating, nickel plating.
- Physiological - contraction of the muscles of humans and animals through which an electric current has passed.
- Magnetic - when an electric current passes through a conductor, if a magnetic needle is placed nearby, it can deviate. This action is the main one. Demonstration of experience: battery, incandescent lamp, connecting wires, compass.
c) Physical operetta “Queen Electricity”. (Appendix No. 1)
Now senior girls will present to your attention the operetta "Queen of Electricity". Do not forget the Russian folk proverb “The fairy tale is a lie, but there is a hint in it, a lesson for good fellows.” That is, you not only listen and watch, but also take certain information from it. Your task is to write down as many physical terms as possible that occur in the representation.
d) Filling in the table “Electric current”. (Appendix No. 2)
Tell me, what one concept unites all the terms that you wrote down?
Answer: electric current.
Let's start filling out the table "Electric current".
Filling in the table, let's summarize the knowledge gained in the lesson and get new information.
In the process of filling out the table, we conclude what conditions are necessary to create an electric current.
- The first condition is the presence of free charged particles.
- The second condition is the presence of an electric field inside the conductor.
e) Safety measures when working with electrical appliances.
Where, in industrial practice, do you encounter the use of electric current? Student responses.
Answer: When working with electrical appliances.
Forbidden.
- Walk on the ground, holding electrical appliances plugged into the network. It is especially dangerous to walk barefoot on wet soil.
- Enter electrical and other electrical rooms.
- Take on broken, bare, hanging and lying on the ground wires.
- Drive nails into the wall in a place where hidden wiring can be located. It is deadly dangerous at this moment to ground on central heating batteries, water supply.
- Drilling walls in places of possible electrical wiring.
- Paint, whitewash, wash walls with external or hidden live wiring.
- Work with switched on electrical appliances near batteries or water pipes.
- Work with electrical appliances, change light bulbs, standing on the bathroom.
- Work with faulty electrical appliances.
- Repair broken electrical appliances.
5. Summing up the lesson
Following the laws of physics, time moves inexorably forward, and our lesson has come to its logical conclusion.
Let's summarize our lesson.
What do you think electric current is?
Answer: Electric current is the directed movement of charged particles.
What conditions are necessary to create an electric current?
Answer: The first condition is the presence of free charged particles.
The second condition is the presence of an electric field inside the conductor.
6. Reflection
7. Homework
a) Based on the knowledge gained in the lessons of life safety, special technologies, prepare and write down in a notebook a memo “Safety measures when working with electrical appliances”.
b) Individual task: Prepare a report on the use of a power source in everyday life and technology. (
