Resistivity
1 option
graphic representation of elements.
electrets
source
resistors
rheostats
capacitor)
The work done by the sources is equal to the product of the EMF of the source and the charge carried in the circuit.
determines the relationship between the EMF of the power supply, with internal resistance.
proportional to the resistance of the conductor in the circuit of the algebraic sum.
the amount of heat released in the conductor when an electric current passes through it is equal to the product of the square of the current strength and the resistance of the conductor and the time for the current to pass through the conductor.)
is directly proportional to the voltage in this section and inversely proportional to its resistance.
potentiometer
ammeter
resistor
capacitor
A physical quantity that characterizes the speed of doing work.
voltage
power)
resistance
there is no correct answer.
Ohm's law for a complete circuit:
Dielectrics that retain polarization for a long time after the removal of an external electric field.
ferroelectrics
electrets
potential
piezoelectric effect
electric capacitance
Dielectrics)
electrets
ferroelectrics
piezoelectric effect
Electron)
antielectron
neutral
part of the chain between two nodes;
closed part of the circuit;
graphic representation of elements;
part of a chain between two points;
element of an electrical circuit designed to use electrical resistance.
Nuclear power plants.
Thermal power plants
mechanical power plants
hydroelectric power plants
Wind power plants.
voltage
current strength
voltage and current
resistance
power
transformer
battery
electromagnet
two opposite electric charges located at a short distance from each other.
absolute permittivity of vacuum.
a value equal to the ratio of the charge of one of the capacitor plates to the voltage between them.
alignment of dipoles along lines of force electric field.
a device consisting of two conductors of any shape separated by a dielectric.
1 ohm = 1 V / 1 A
1 V = 1 J / 1 C
1 C = 1 A * 1 s
1 A = 1 ohm / 1 V
voltage
resistance
current strength
transformer
switch
The current transformer is...
scalar
vector
mechanical
answers A, B
perpendicular
magnetic system
flat magnetic system
insulation
no correct answer
What electricity?
A device consisting of two conductors of any shape separated by a dielectric
Joule–Lenz law
Determine the resistance of the filament of an electric lamp with a power of 100 W, if the lamp is designed for a voltage of 220 V.
Current in electrical circuit 2 A at a voltage of 5 V at its ends. Find the resistance of the conductor.
Substances that almost do not conduct electricity.
Which of the following particles has the least negative charge?
The section of the circuit is...?
In the device for burning wood, the voltage drops from 220 V to 11 V. The transformer passport states: “Power consumption - 55 W, efficiency - 0.8”. Determine current strength flowing through the primary and secondary windings of the transformer.
Convert fuel energy into electrical energy.
A rheostat is used to regulate the circuit ...
A device consisting of a coil and an iron core inside it.
The dipole is
Find the wrong ratio:
When connected in parallel, the capacitor……=const
The rotating part of the generator.
In a circuit with a voltage of 250 V, two lamps designed for the same voltage were connected in series. One lamp is 500W and the other is 25W. Determine the resistance of the circuit.
What size is magnetic flux F?
A set of turns forming an electrical circuit in which the EMF induced in the turns is summed.
The earth and the conductive layers of the atmosphere form a kind of capacitor. Observations have established that the strength of the Earth's electric field near its surface is on average 100 V/m. Find the electric charge, assuming that it is evenly distributed over the entire earth's surface.
Option 2
This is a device for measuring EMF.
a graphic representation of an electrical circuit showing the order and nature of the connection of elements.
ordered movement of charged particles in a conductor.
a set of devices designed to pass an electric current.
a set of devices designed to use electrical resistance.
Michael Faraday
James Maxwell
Georg Ohm
Mikhail Lomonosov
Charles Pendant
resistor
potentiometer
ammeter
potentiometers
resistors
rheostats
chain section
electrical circuit
directly proportional to the voltage at the ends of the conductor
directly proportional to the voltage at the ends of the conductor and its resistance
inversely proportional to the voltage at the ends of the conductor
inversely proportional to the voltage at the ends of the conductor and its resistance
electric charge and conductor cross section
What is the potential of a point?
is the potential difference between two points of the electric field.
is the absolute permittivity of vacuum.
called a value equal to the ratio of the charge of one of the capacitor plates to the voltage between them.
called a device consisting of two conductors of any shape, separated by a dielectric.
called the work to move a unit charge from a point in the field to infinity.
resistor
fuse
cable, wire, electrical circuit bus
receiver electrical energy
What charge carriers exist?
electrons
positive ions
negative ions
neutral
all of the above
nodes 4, branches 4;
nodes 2, branches 4;
nodes 3, branches 5;
nodes 3, branches 4;
nodes 3, branches 2.
conductivity
resistivity
voltage
potential
will not
will be, but not for long
all answers are correct
The electric current density is determined by the formula:
A magnetic system in which all rods have the same shape, design and dimensions, and mutual arrangement of any rod in relation to all yokes is the same for all stubble.
symmetrical magnetic system
asymmetrical magnetic system
flat magnetic system
spatial magnetic system
direct magnetic system
magnetic system
autotransformer
cooling system
current transformer
voltage transformer
autotransformer
pulse transformer
mechanical transformer.
What is an electrical circuit?
The source emf is expressed by the formula:
For the first time, phenomena in electrical circuits were deeply and carefully studied by:
The capacitance of the capacitor is C \u003d 10 μF, the voltage on the plates is U \u003d 220V. Determine the charge on the capacitor.
These are, in the simplest case, rheostats that are switched on to regulate the voltage.
The part of a circuit between two points is called:
Series circuit resistance:
The strength of the current in the conductor ...
What energy does an electric lamp consume from the network in 2 hours if its resistance is 440 ohms and the mains voltage is 220 V?
Symbol
An incandescent lamp with a resistance of R = 440 ohms is connected to a network with a voltage of U = 110 V. Determine the current in the lamp.
How many nodes and branches are in the diagram?
The reciprocal of the resistance
Will direct current flow in the circuit if instead of EMF source– turn on the charged capacitor?
In the power supply circuit of the heating device, connected to a voltage of 220 V, the current strength is 5 A. Determine the power of the device.
Provides physical protection for the active ingredient and also acts as a reservoir for the oil.
A transformer designed to convert pulse signals with a pulse duration of up to tens of microseconds with minimal distortion of the pulse shape.
3 option
orderly movement of electric charges.
a special kind of matter that exists around any electric charge.
ordered movement of charged particles in a conductor.
random movement of particles of matter.
interaction of electric charges.
receiver connection wire
only power supply
receiver
all elements of the chain
ballast equipment
resistor
potentiometer
current strength
voltage
resistance
current work
Determine the receiver power if the resistance is 100 ohms and the receiver current is 5 mA.
A partially or fully ionized gas in which the densities of positive and negative charges practically match.
magnetic flux
no clear answer
The globe is a big magnet.
It is impossible to get a magnet with one pole.
The magnet has two poles: north and south, they are different in their properties.
A magnet is a directed movement of charged particles.
A magnet suspended on a thread is located in a certain way in space, indicating north and south.
Michael Faraday
Amp Andre
Maxwell James
Oersted Hans
Pendant Charles
aluminum
all answers are correct
magnetic cores
windings of inductors
housings for household appliances
plug housings
aluminum
The magnitude of the induced emf depends on...
current strength
voltage
coil rotation speed in a magnetic field
conductor length and magnetic field strength
answers 1, 2
the current in a closed circuit is directly proportional to the electromotive force and inversely proportional to the resistance of the entire circuit.
the current in a closed circuit is directly proportional to the resistance of the entire circuit and inversely proportional to the electromotive force.
the resistance in a closed circuit is directly proportional to the current of the entire circuit and inversely proportional to the electromotive force.
The electromotive force in a closed circuit is directly proportional to the resistance of the entire circuit and inversely proportional to the current.
The electromotive force in a closed circuit is directly proportional.
Receiver power formula:
At parallel connection capacitor ……=const
voltage
inductance
What is Peak - transformer
transformer designed to convert pulse signals with a pulse duration of up to tens of microseconds with minimal distortion of the pulse shape
transformer powered by a voltage source.
a version of a transformer designed to convert electrical energy in electrical networks and in installations designed to receive and use electrical energy.
transformer powered by a current source.
a transformer that converts a sinusoidal voltage into a pulsed voltage with a polarity changing every half cycle.
An isolating transformer is...
a transformer designed to convert pulse signals with a pulse duration of up to tens of microseconds with minimal distortion of the pulse shape.
a transformer designed to convert pulse signals with a pulse duration of up to tens of microseconds with minimal distortion of the pulse shape.
transformer powered by a current source.
transformer, the primary winding of which is not electrically connected to the secondary windings.
transformer powered by a voltage source.
What is an electric field?
Kirchhoff's First Law
The capacitor has an electric capacitance C=5 pF. What charge is on each of its plates if the potential difference between them is U=1000 V?
What value is equal to the ratio of the electric charge that has passed through the cross section of the conductor to the time of its passage?
The unit of measurement of the potential of an electric field point ...
Which statement do you think is not correct?
In 1820, who experimentally discovered that the electric current is associated with a magnetic field?
Capacitor capacitance C \u003d 10 mF; capacitor charge Q = 4 ∙ Determine the voltage on the plates.
The magnetic materials are
Dielectrics are used to make
Semiconductor materials include:
The units of magnetic induction are
Choose the correct statement:
The capacitor has two plates. The area of each plate is 15 . A dielectric is placed between the plates - waxed paper 0.02 cm thick. Calculate the capacitance of this capacitor. (e=2.2)
Determine the receiver power if the resistance is 110 ohms and the receiver current is 5 mA.
4-option
random movement of charged particles
movement of atoms and molecules.
the movement of electrons.
directed movement of free electrons.
ion movement.
a graphical representation of an electrical circuit showing the order and nature of the connections of the elements;
a set of devices intended for the passage of electric current by mandatory elements;
decent movement of charged particles, a closed circuit, under the influence of an electric field;
an electrical circuit element designed to use its electrical resistance;
work done per unit of time or value, numerically equal to the rate of energy conversion.
thermal
radioactive
magnetic
physical
all answers are correct
human growth
the masses of man
current strength
physical condition of a person
not envy
galvanometer
wattmeter
source
resistor
Determine the amount of heat released in the heating device for 0.5 h if it is connected to a 110 V network and has a resistance of 24 ohms.
When condensates are connected in series …..=const
voltage
inductance
decrease
will increase
Will not change
not enough data
decrease and increase
An element of an electrical circuit designed to use its electrical resistance is called
chain section
resistor
receiver
connecting wires
power supply only
ballast equipment
all elements of the chain
on the rate of change of the magnetic field
from the speed of rotation of the coil
from the electromagnetic field
from the number of its turns
Newton's first law
Kirchhoff's first law
Kirchhoff's second law
Ohm's law
Dielectrics with very high permittivity
electrets
piezoelectric effect
electron
potential
ferroelectrics
Magnetic materials are used to make
radio elements
wire shielding
windings of electrical machines
anchors of electrical machines
Who coined the term "electron" and calculated its charge?
A. Becquerel
E. Rutherford
D. Stoney
Symbol
Ammeter
Voltmeter
Galvanometer
Generator
a transformer designed to convert pulse signals with a pulse duration of up to tens of microseconds with minimal distortion of the pulse shape.
a version of a transformer designed to convert electrical energy in electrical networks and in installations designed to receive and use electrical energy.
transformer powered by a voltage source.
transformer powered by a current source.
a version of a transformer designed to convert electrical energy in electrical networks and in installations designed to receive and use electrical energy.
A current of 1 A flows in a closed circuit. The external resistance of the circuit is 2 ohms. Determine the internal resistance of the source, the EMF of which is 2.1 V.
Electric current in metals is...
What is a resistor?
Electric current acts on a conductor...
The resistance of the human body to electric current depends on ...
Ohm's law is expressed by the formula
The distance between the plates of a flat capacitor was doubled. Its electric capacitance...
Capacitor capacitance C \u003d 10 mF; capacitor charge q \u003d 4 * C. Determine the stress on the plates.
For 2 hours at direct current, a charge of 180 C was transferred. Determine the strength of the current.
The outer part of the chain covers...
Strength induction current depends on what?
The algebraic sum of the EMF in the circuit is equal to the algebraic sum of the voltage drops on all elements of this circuit:
The smallest current that is deadly to a person is ...
A battery with an EMF of 4.8 V and an internal resistance of 3.5 ohms is connected to a light bulb with a resistance of 12.5 ohms. Determine the battery current.
Determine the power factor of the motor, the winding impedance of which is 20 ohms, and active resistance 19 ohm.
If a neon lamp with a power of 4.8 W is designed for a voltage of 120 V, then the current consumption is:
The power transformer is...
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1-option |
Option 2 |
3 option |
4-option |
Modern life is unthinkable without radio and television, telephone and telegraph, all kinds of lighting and heating devices, machines and devices based on the use of electric current.
An electric current is a directed movement of electrically charged particles. Depending on the interaction of electric current with certain substances, these substances are divided into conductors, dielectrics and semiconductors. Conductors are materials that conduct electric current well, dielectrics - substances that do not conduct current.
Semiconductors occupy an intermediate position between conductors and dielectrics in terms of their resistance to the passage of electric current.
For the emergence and existence of an electric current, the presence of free charged particles and a force that causes their ordered movement is necessary. Usually the source of such power is electrical voltage at the ends of an electrical circuit. If the voltage does not change with time, then a direct current flows in the circuit; if it changes, an alternating current flows.
Source direct current- an electrical element (see fig.), in which, during a chemical reaction, a potential difference is formed at the terminals. As a result of the conductor connecting the terminals of an electrical element, an electric current arises.
Depending on the resistance of the conductor to electric current, the current strength changes. Current is measured in amperes (A).
For getting alternating current use special machines - alternators, which convert mechanical energy into electric current.
Various properties of electric current are widely used. So, the property of the current to heat the conductor when passing through it is used in heating devices.
A current-carrying conductor creates a magnetic field around it. This property of electric current is used in electric motors of electromagnetic relays.
Electric current causes the deposition of pure metals from the electrolytic solution on the electrodes. This phenomenon of electrolysis is widely used in industry (see. Electrochemical methods processing).
Electricity is of great importance for various environments connection. Direct electric current is used to transmit telegraph messages in the form of pulses of various durations (Morse code, see Telegraph communication), in computer technology - to transmit commands and words from one device of an electronic computer to another.
To transmit information in radio electronics, an alternating electric current is used (see Radio transmitter).
Alternating electric current can be transformed: increase or decrease its voltage using a special device - a transformer.
1-5. Electrical conductivity of matter, molecules, atoms (chemical and physical concepts of the structure of atomic orbits, the electron field in outer orbits atomic nucleus, ionization in molecules).
The atoms of the chemical elements that make up any substance consist of a positively charged nucleus and negatively charged electrons moving around it. Atoms are usually electrically neutral, since the charge of the nucleus is equal to the sum charge of the surrounding electrons.
If an electron is separated from a neutral atom (molecule), then the atom turns into a positive ion. An electron separated from an atom joins another neutral atom, forming a negative ion, or remains free.
Such free electrons are called conduction electrons, and the process of ion formation is called ionization. The number of free electrons or ions per unit volume of a substance is called the concentration of electric charge carriers.
In a substance placed in an electric field, under the action of field forces, a directed movement of conduction electrons or ions occurs, called an electric current. The property of a substance to create an electric current under the influence of an electric field is called the electrical conductivity of a substance. The degree of electrical conductivity is estimated by specific electrical conductivity. The electrical conductivity of a substance (body) depends on the concentration of charge carriers. At a high concentration, the conductivity of a substance is greater than at a low one. All substances, depending on the electrical conductivity, are divided into conductors, dielectrics (electrical insulating materials) and semiconductors.
Conductors have high conductivity, these include metals and their alloys, coal, electrolytes (aqueous solutions of salts, alkali acids) and melts.
Dielectrics, on the other hand, have negligible conductivity. These include gases, mineral oils, varnishes and big number solid non-metallic bodies.
Semiconductors have intermediate conductivity between conductors and dielectrics. These include metals such as silicon, germanium, selenium, metal oxides, etc.
Each electron in an atom can only have certain values of energy, i.e., be only in allowed energy states or levels, since a change in the energy of an electron can occur only in certain portions - quanta. The transition of an electron to a higher energy level, i.e., to a more distant orbit, requires the expenditure of energy to overcome the attraction of the electron to the nucleus. Thus, electrons farther from the nucleus have higher energies. The transition of an electron to a lower level is accompanied by the emission of energy by the atom.
In solids formed by a combination of atoms, due to the mutual influence of neighboring atoms energy levels change somewhat, forming energy zones.
These zones are separated by regions in which electrons cannot reside, called band gaps.
Energy zones corresponding to permitted levels are divided into filled and free. For the occurrence of electrical conductivity, it is necessary for a part of the electrons of the filled zone to go to the free zone. The possibility of such a transition is determined by the band gap, which is proportional to the energy that must be expended for the indicated transition of electrons.
The difference in the electrical conductivity of conductors, semiconductors and dielectrics is caused by the peculiarities of their structure. According to the zone theory solid body in metal conductors, high electrical conductivity is due to the fact that the filled zone is closely adjacent to the free zone (Fig. 1-3, a).
Rice. 1-3. Energy levels. a - conductor; b - dielectric; c - semiconductor; 1 - free zone; 2 - band gap; 3 - filled zone.
As a result, the electrons in the metal can pass from the levels of the filled zone to the levels of the free zone. In other words, electrons can move from orbits less distant from the nucleus to more distant orbits or leave the boundaries of the conductor atom, becoming free. Easily occurring significant concentration of electrons and provides a large electrical conductivity of the conductors.
When an electric voltage is applied to the ends of a metal conductor, an electric field arises in it. Under the influence of the forces of this field, the movement of free electrons is ordered, and they drift in the direction opposite to the direction of the field (since they have a negative charge), i.e., an electric current arises in the conductor.
If the free zone of a given substance is separated from the filled one (Fig. 1-3, b) by a sufficiently wide band gap, then the latter makes it practically impossible for electrons to pass into the free zone.
Thus, both the concentration of free electrons and the conductivity of the substance will be negligible and, therefore, it will be a dielectric.
For semiconductors, the band gap is much narrower than for dielectrics (Fig. 1-3, b). Consequently, for the transition of electrons to the free zone, a small excitation is required, for example, due to an increase in the thermal motion of atoms with increasing temperature, and therefore semiconductors have a conductivity that is intermediate between the conductivity of conductors and dielectrics.
Conductors in which an electric current is created by the movement of some electrons are called conductors with electronic conductivity or conductors of the first kind. Their main representatives are metals and their alloys.
Conductors in which an electric current is created by the movement of positive and negative ions are called conductors with ionic conductivity or conductors of the second kind - these are electrolytes, which include aqueous solutions of acids, salts and alkalis.
The movement of freely charged particles in the direction of the field lines of force. Electricity.
Free electrons in a metallic conductor (a conductor of the first kind) in the absence of an external electric field are in a state of random movement, and the amount of electricity transferred through any cross section of the conductor is on average equal to zero.
If there is an electric field in the conductor directed along the wire, the forces of this field act on free electrons and they acquire acceleration in the direction opposite to the direction of the field. Thus, a uniformly accelerated motion in the indicated direction is superimposed on the random motion of electrons. Accelerated motion occurs until the electron collides with an ion of the crystal lattice of the wire metal, after which the process begins to repeat.
If there is a longitudinal electric field in the wire, a certain amount of electricity will pass through any cross section of the wire. The phenomenon of movement of charged particles under the influence of an electric field in a conductor is called an electric current.
Resistivity is a property of a material that characterizes its ability to resist the passage of electric current.
Characteristics of electrical materials
One of the main characteristics in electrical engineering is electrical conductivity, measured in S/m. It serves as a proportionality factor between the field strength vector and the current density. Often denoted by the Greek letter gamma γ. Resistivity is the reciprocal of electrical conductivity. As a result, the formula mentioned above takes the following form: the current density is directly proportional to the field strength and inversely proportional to the resistivity of the medium. The unit of measurement is ohm m.
It should be noted that the concept under consideration retains its relevance not only for solid media. For example, current is conducted by electrolyte liquids and ionized gases. Therefore, in each case, the concept of resistivity can be introduced, since an electric charge passes through the medium. But it will be difficult to find values in reference books, for example, for a welding arc, to put it mildly, for the simple reason that these problems are not dealt with sufficiently. Why? It's not relevant. It can be seen that from the moment Davy discovered the incandescence of a platinum plate with an electric current until the introduction of incandescent bulbs into everyday life, almost a century passed - for the same reason, the importance and significance of the discovery was not immediately realized.
Depending on the value of the resistivity value, materials are divided as follows:
- For conductors - less than 1/10000 ohm m.
- Dielectrics have over 100 million ohm m.
- Semiconductors in terms of resistivity are between dielectrics and conductors.
These values characterize only the ability of the body to resist the passage of electric current and do not affect other aspects (elasticity, heat resistance). For example, magnetic materials can be both conductors, dielectrics, and semiconductors.
How is conductivity formed in a material?
In modern physics, resistance and conductivity are usually explained by the band theory. It applies to solid crystalline bodies, whose lattice atoms are assumed to be immobile. According to this concept, the energy of electrons and other types of charge carriers is determined by several rules. There are three main zones inherent in each material:
- The valence band contains electrons associated with atoms. In this region, the electron energy is graded in steps, and the number of levels is limited. It is the outermost of all the layers belonging to the atom.
- Forbidden zone. Charge carriers cannot be in this region. Serves as a dividing line between two other zones. Metals are often absent.
- The free zone is located above the two previous ones. Here, electrons participate freely in the creation of an electric current, and the energy can be absolutely any. There are no levels.
Dielectrics are characterized by a very high location of the free zone. Therefore, under any natural conditions conceivable on Earth, these materials do not conduct electric current. The width of the forbidden zone is also large. Metals have many free electrons. And the valence band is also the conduction region at the same time - there are no forbidden states at all. As a result, such materials have low resistivity.
Intermediate energy levels are formed at the contact boundary of atoms, due to which unusual effects arise, which are used, among other things, by semiconductor physics. When inhomogeneities are created quite intentionally by the introduction of impurities (acceptors and donors). As a result, new energy states are formed, which, during the flow of electric current, exhibit new properties that the original material did not possess.
Semiconductors have a small band gap. Therefore, under the action external forces electrons can leave the valence region. The reason for this may be not only electrical voltage, but also heating, and irradiation, and some other types of influences. In dielectrics and semiconductors, as the temperature decreases, electrons move to lower levels, and eventually the entire valence band is filled, and the conduction band is completely free. As a result, no electric current will flow. In accordance with quantum theory It is possible to characterize the class of semiconductors as materials with a band gap less than 3 eV.
Fermi energy
An important place in the theory of conductivity, as well as in the explanation of phenomena occurring in semiconductors, is occupied by the Fermi energy. Stealthiness adds to the vague definition of the term in the literature. AT foreign literature it is said that the Fermi level is a certain value in eV, while the Fermi energy is the difference between it and the lowest in the crystal. Here are some of the most common and understandable suggestions:
- The Fermi level is the maximum of all at which an electron can be in metals at a temperature of 0 K. Therefore, the Fermi energy will be the difference between this figure and the minimum level at absolute zero.
- Fermi energy level - the probability of finding electrons at which is 50% at all temperatures except absolute zero.
The Fermi energy is determined only for a temperature of 0 K, while the level exists under any conditions. In thermodynamics, the same concept characterizes the total chemical potential of all electrons. In addition, the Fermi level is defined as the work that must be expended in order to supplement the object with one electron. This parameter not only determines the conductivity of the material, but also helps to understand the physics of semiconductors.
The Fermi level does not have to exist physically. There are cases when the place of passage was in the middle of the forbidden zone. Physically, such a level does not exist, and there are no electrons there. However, this parameter can be observed with a voltmeter: the potential difference between two points in the circuit (reading on the display) is proportional to the difference in the Fermi levels of these points and inversely proportional to the charge of the electron. Pretty simple dependency. In turn, these parameters can be related to conductivity and resistivity using Ohm's law for the circuit section.
Materials with low resistivity
Conductors include not only most metals, but also graphite, as well as electrolytes. Such materials have low resistivity. In metals, positively charged ions form crystal lattice sites surrounded by a cloud of electrons. They are usually called common because they are part of the conduction band.
Although it is not entirely clear what exactly an electron is, it is customary to describe it as a particle moving inside a crystal at a thermal speed of hundreds of kilometers per second. This is much more than it takes to bring out spaceship into orbit. At the same time, the drift speed, which forms an electric current under the action of the intensity vector, barely reaches one centimeter per minute. In turn, the field propagates in the medium at the speed of light (100,000 km/s).
As a result of such relationships, it becomes possible to express the conductivity in terms of the following physical quantities(see picture):
- Electron charge, e.
- Free carrier concentration, n.
- Mass of an electron, me.
- Thermal speed of carriers,
- Electron mean free path, l.
The Fermi level for metals ranges from 3 to 15 eV, and the concentration of free carriers is almost independent of temperature. Therefore, the specific conductivity, and hence the resistance, is almost completely determined by the structure of the molecular lattice and its proximity to the ideal, freedom from defects. These parameters determine the mean free path of electrons, which can be found in reference books if calculations are required (for example, in order to determine the resistivity).
Metals with a cubic lattice have the best conductivity. This includes, among other things, copper. Transition metals have much higher resistivity. Conductivity decreases with increasing temperature and at high AC frequencies. In the latter case, the skin effect is also observed. The dependence on temperature is linear above a certain limit named after the Dutch physicist Peter Debye.
There are also less linear relationships. For example, heat treatment of steel increases the number of defects, which naturally reduces the specific conductivity of the material. An exception to this rule is annealing. This process reduces the density of defects, due to which the resistivity also decreases. Deformation also has a significant effect. For some alloys, machining leads to a noticeable increase in resistivity.
Materials with high resistivity
In some cases, it is required to specifically increase the resistivity. First of all, this situation occurs in cases with heating devices and resistors. electronic circuits. That's when the turn of alloys with high resistivity (more than 0.3 μOhm m) comes. When used as part of measuring instruments a minimum potential requirement is imposed at the interface with the copper contact.
The most famous was nichrome. Despite the fact that many heating devices are constructed from cheaper fechral (more fragile, but cheaper). Depending on the purpose, copper, manganese and some other metals may be included in the alloys. This is quite an expensive pleasure. For example, one manganin resistor can cost 30 cents. And this is on Aliexpress, where prices are traditionally lower than store prices. There is even an alloy of palladium with iridium. The price of this material should not be spoken aloud.
PCB resistors are often made from pure metals in the form of films by sputtering. For this, chromium, tantalum, tungsten, as well as alloys, among others, nichrome, are widely used.
Substances that do not conduct electricity
Dielectrics are characterized by very high resistivity. But this is not their key feature. Dielectrics are materials that can redistribute their charge under the influence of an electric field. As a result, accumulation can occur. What is used in capacitors. The degree of charge redistribution is characterized by the permittivity. This parameter shows how many times the capacitance of the condenser increases, where this or that material is used instead of air. In addition, some dielectrics can conduct and radiate vibrations when subjected to alternating current. There is also ferroelectricity due to temperature changes.
In the process of changing the direction of the field, losses occur. Just as the magnetic intensity is partially converted into heat when exposed to mild steel. Dielectric losses depend mainly on frequency. If necessary, non-polar insulators are used as materials, the molecules of which are symmetrical and do not have a pronounced electric moment. Polarization occurs due to the fact that the charges are strongly associated with the crystal lattice, and is of the following types:
- Electronic polarization arises as a result of deformation of the outer energy shells of atoms. Completely reversible. It is typical for non-polar dielectrics in any phase of a substance. Due to the small weight of electrons, it appears almost instantly (units fs).
- Ionic polarization propagates two orders of magnitude slower and is characteristic of substances with an ionic crystal lattice. Accordingly, materials are used at frequencies up to 10 GHz and have a large value permittivity(for titanium dioxide - up to 90).
- The dipole-relaxation polarization is much slower. The execution time is hundredths of a second. Dipole-relaxation polarization is more typical for gases and liquids and depends, respectively, on viscosity (density). The influence of temperature is also traced: the effect has a peak at a certain value.
- Spontaneous polarization is observed in ferroelectrics.
- the ability of a substance to conduct an electric current.
Conductivity occurs in an electric field.
Electrical conductivity is inherent in all substances, but in order for it to be significant, it is necessary that there are free charges in the substance.
Electrical conductivity is also called specific electrical conductivity - a quantitative measure of this ability.
The electrical conductivity is inversely proportional to the resistivity.
Electrical conductivity is usually denoted by the Greek letter? and is measured in the SI system in Siemens per meter, in the CGS the dimension of electrical conductivity is the reciprocal second (s -1). It establishes a relationship between current density and electric field strength.
In the general case, electrical conductivity is a tensor of the second rank, but in many substances this tensor reduces to a scalar.
The concept of electrical conductivity can be applied when Ohm's law is satisfied. In many heterogeneous systems, Ohm's law is not valid, and even with very small applied fields, the dependence of current on voltage is non-linear.
Electrical conductivity is due to the predominant movement of charged particles, charge carriers in the direction of the electric field. Charge carriers can be electrons, holes or ions. To ensure conductivity, charge carriers must be free.
In an electric field, a force acts on a charge carrier, Where q is the charge, and is the electric field strength. Under the action of this force, the charge carrier accelerates and gains energy. However, this acceleration is not unlimited. To prevent him are collisions with other charge carriers, ions or neutral atoms. During such collisions, the energy of the electron is dissipated and converted into heat. The passage of current through a substance is always accompanied by the release of heat. The value of electrical conductivity thus depends not only on the concentration of free charge carriers and the field strength, but also on the frequency of collisions of charge carriers, which is described by the so-called mean free path.
From a quantum mechanical point of view, scattering events, i.e., collisions of charge carriers with various structural defects, are also determining factors for conductivity. One of the conclusions of the band theory states that free quasi-particles - electrons and holes, moving through an ideal crystal, as if through a vacuum, not feeling the presence of ions at the nodes of the crystal lattice. Scattering of charge carriers occurs on crystal lattice defects: impurity atoms, crystal atoms displaced from their position due to thermal vibrations, m. An important role in determining conductivity is played by the Pauli exclusion principle, which prohibits charge carriers from passing into states occupied by other charge carriers of the same kind.
The conductivity of various media lies in a very wide range - from infinitely small to infinitely large. Infinitely small conductivity has vacuum, in which there are no charged particles, infinitely high - superconductors. Depending on the value of conductivity, materials are divided into conductors and insulators. An intermediate position between these two groups is occupied by semiconductors.
Conductivity of various media
There are no vacuum electric charges, so its conductivity is infinitesimal. However, if electrons are injected into a vacuum, it becomes a good conductor. This phenomenon is used in vacuum lamps. Electrons in them are injected into vacuum from a heated cathode due to the phenomenon of thermionic emission. Vacuum conductivity is limited by the formation of a space charge region - a negatively charged electron cloud between the cathode and anode, which prevents the escape of electrons from the cathode.
As in vacuum, there are usually no free charge carriers in gases. They can be injected from the cathode. However, when moving towards the anode, electrons injected into the gas experience collisions with gas atoms and scatter. On the one hand, this reduces the conductivity, but on the other hand, the electrons accelerated electric field to high speeds, can ionize gas atoms, knocking out electrons from them and creating positive ions. New electrons and ions move towards the anode or cathode, respectively, increasing the electric current. Depending on the applied voltage and chemical composition gas, these phenomena lead to the emergence of a number of different types of gas discharges, stratification of the gap between the anode and cathode into zones with different properties, i.e.
Most liquids do not have free charge carriers and are insulators. The exception is electrolytes, such as water or solutions of salts in water. In electrolytes, some of the neutral molecules dissociate, forming negatively and positively charged ions. The electrical conductivity of electrolytes is due to the movement of these ions towards the anode and cathode, respectively. At the anode and cathode, ions are reduced or oxidized, enter into chemical reactions. All this leads to various galvanic effects.
In metals there are free charge carriers - electrons. The band structure of metals is characterized by a half-filled valence band. However, only electrons with energies close to the level chemical potential can be accelerated by an electric field. The Pauli exclusion principle gets in the way of accelerating electrons with lower energy. Thus, only electrons with energies lying in the gap (K
