Class: 11
Presentation for the lesson
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Lesson Objectives:
Expand the concept of the physical nature of electric current in metals, experimental confirmation of the electronic theory;
Continue the formation of natural scientific ideas on the topic being studied
Create conditions for the formation of cognitive interest and activity of students
Formation of skills;
Formation of communicative communication.
Equipment: SMART Board Notebook interactive complex, local computer network, Internet.
Lesson teaching method: combined.
Lesson epigraph:
Strive to comprehend science more and more deeply,
Thirst for the knowledge of the eternal.
Only the first knowledge will shine upon you,
You will find out: there is no limit to knowledge.
Ferdowsi
(Persian and Tajik poet, 940-1030)
Lesson plan.
I. Organizational moment
II. Group work
III. Discussion of results, installation of presentation
IV. Reflection
V. Homework
During the classes
Hello guys! Sit down. Today our work will take place in groups.
Group assignments:
I. Physical nature of charges in metals.
II. Experience of K.Rikke.
III. Experience of Stewart, Tolman. Mandelstam's experience, Papaleksi.
IV. Drude's theory.
V. Current-voltage characteristics of metals. Ohm's law.
VI. Dependence of conductor resistance on temperature.
VII. Superconductivity.
1. Electrical conductivity is the ability of substances to conduct electric current under the influence of an external electric field.
According to the physical nature of charges - carriers of electric current, electrical conductivity is divided into:
A) electronic
B) ionic,
B) mixed.
2. Each substance under given conditions is characterized by a certain dependence of the current strength on the potential difference.
Based on specific resistance, substances are usually divided into:
A) conductors (p< 10 -2 Ом*м)
B) dielectrics (p > 10 -8 Ohm*m)
B) semiconductors (10 -2 Ohm*m> p>10 -8 Ohm*m)
However, this division is conditional, since under the influence of a number of factors (heating, irradiation, impurities), the resistivity of substances and their current-voltage characteristics change, and sometimes very significantly.
3. The carriers of free charges in metals are electrons. Proven by classical experiments K. Riecke (1901) – German physicist; L.I. Mandelstam and N.D. Papaleksi (1913) - our compatriots; T. Stewart and R. Tolman (1916) - American physicists.
Experience of K. Rikke
Rikke stacked three pre-weighed cylinders (two copper and one aluminum) with polished ends so that the aluminum one was between the copper ones. The cylinders were then connected to a direct current circuit: a large current passed through them for a year. During that time, an electric charge equal to approximately 3.5 million C passed through the electric cylinders. The secondary interaction of the cylinders, carried out with up to 0.03 mg, showed that the mass of the cylinders did not change as a result of the experiment. When examining the contacting ends under a microscope, it was found that there were only minor traces of metal penetration, which did not exceed the results of the usual diffusion of atoms in solids. The experimental results indicated that ions do not participate in charge transfer in metals.
L.I. Mandelstam
N. . Papalexi
The experience of L. I. Mandelstam and N. D. Papaleksi
Russian scientists L. I. Mandelstam (1879-1949; founder of the school of radiophysicists) and N. D. Papaleksi (1880-1947; the largest Soviet physicist, academician, chairman of the All-Union Scientific Council on Radiophysics and Radio Engineering at the USSR Academy of Sciences) in 1913 staged the original experience. They took a coil of wire and began to twist it in different directions.
They will spin, for example, clockwise, then abruptly stop and then back.
They reasoned something like this: if electrons really have mass, then when the coil suddenly stops, the electrons should continue to move by inertia for some time. The movement of electrons along a wire is an electric current. It happened as we planned. We connected a telephone to the ends of the wire and heard a sound. Since sound is heard in the phone, therefore, current flows through it.
T. Stewart
Experience of T. Stewart and R. Tolman
Let's take a coil that can rotate around its axis. The ends of the coil are connected to a galvanometer using sliding contacts. If the coil, which is in rapid rotation, is sharply braked, then the free electrons in the wire will continue to move by inertia, as a result of which the galvanometer should register a current pulse.
Drude theory
Electrons in a metal are considered as an electron gas, to which the kinetic theory of gases can be applied. It is believed that electrons, like gas atoms in the kinetic theory, are identical solid spheres that move in straight lines until they collide with each other. It is assumed that the duration of an individual collision is negligible, and that no forces other than those arising at the moment of collision act between the molecules. Since an electron is a negatively charged particle, in order to comply with the condition of electrical neutrality, a solid must also contain particles of a different type - positively charged. Drude suggested that the compensating positive charge belonged to much heavier particles (ions), which he considered immobile. In Drude's time, it was not clear why there were free electrons and positively charged ions in a metal, and what these ions were. Only the quantum theory of solids could provide answers to these questions. For many substances, however, we can simply assume that the electron gas consists of outer valence electrons weakly bound to the nucleus, which are “freed” in the metal and are able to move freely throughout the metal, while the atomic nuclei with the electrons of the inner shells (atomic cores) remain unchanged and play the role of immobile positive ions of the Drude theory.
Electric current in metals
All metals are conductors of electric current and consist of a spatial crystal lattice, the nodes of which coincide with the centers of positive ions, and free electrons move chaotically around the ions.
Basic principles of the electronic theory of conductivity of metals.
- A metal can be described by the following model: a crystal lattice of ions is immersed in an ideal electron gas consisting of free electrons. In most metals, each atom is ionized, so the concentration of free electrons is approximately equal to the concentration of atoms 10 23 - 10 29 m -3 and is almost independent of temperature.
- Free electrons in metals are in continuous chaotic motion.
- Electric current in a metal is formed only due to the ordered movement of free electrons.
- Colliding with ions oscillating at the nodes of the crystal lattice, electrons give them excess energy. This is why conductors heat up when current passes.
Electric current in metals.
Superconductivity
The phenomenon of resistivity decreasing to zero at a temperature other than absolute zero is called superconductivity. Materials that exhibit the ability to transition to a superconducting state at certain temperatures other than absolute zero are called superconductors.
The passage of current in a superconductor occurs without loss of energy, therefore, once excited in a superconducting ring, the electric current can exist indefinitely without change.
Superconducting materials are already used in electromagnets. Research is underway aimed at creating superconducting power lines.
The application of the phenomenon of superconductivity in widespread practice may become a reality in the coming years thanks to the discovery in 1986 of the superconductivity of ceramics - compounds of lanthanum, barium, copper and oxygen. The superconductivity of such ceramics persists up to temperatures of about 100 K.
Well done boys! They did an excellent job. It was a good presentation. Thank you for the lesson!
Literature.
- Gorbushin Sh.A. Basic notes for studying physics for a secondary school course. – Izhevsk “Udmurtia”, 1992.
- Lanina I.Ya. Formation of cognitive interests of students in physics lessons: A book for teachers. – M.: Education, 1985.
- Physics lesson in a modern school. Creative search for teachers: A book for teachers / Comp. E.M. Braverman / Edited by V.G. Razumovsky.- M.: Education, 1993
- Digelev F.M. From the history of physics and the life of its creators: A book for students. - M.: Education, 1986.
- Kartsev V.L. Adventures of great equations. - 3rd edition - M.: Znanie, 1986. (Life of wonderful ideas).
Description of the presentation by individual slides:
1 slide
Slide description:
ELECTRIC CURRENT IN METALS The presentation was developed by the teacher of CS and PT Karakasheva I.V. St. Petersburg 2016
2 slide
Slide description:
Lesson objectives: Educational: to introduce students to the conductivity of metals and its technical uses; reveal the concept of the physical nature of electric current in metals; continue the formation of natural scientific ideas on the topic under study; create conditions for the formation of cognitive interest; expand the scientific and technical horizons of students Developmental: create conditions for the development of communication skills; create conditions for the development of students’ analytical abilities, the ability to analyze, compare, compare, generalize, and draw conclusions; create conditions for the development of memory, attention, imagination Educational: promote the development of the ability to defend one’s point of view; promote the development of a culture of relationships when working in a team
3 slide
Slide description:
What is called metal? The most famous of the early definitions of metal was given in the middle of the 18th century by M.V. Lomonosov: “Metal is a light body that can be forged. There are only six such bodies: gold, silver, copper, tin, iron and lead.” Two and a half centuries later, much has become known about metals. More than 75% of all elements of D.I. Mendeleev’s table are metals, and finding an absolutely accurate definition for metals is an almost hopeless task.
4 slide
Slide description:
In 1900, the German scientist P. Drude, based on the hypothesis of the existence of free electrons in metals, created the electronic theory of metal conductivity. This theory was developed in the works of the Dutch physicist H. Lorentz (1904) and is called classical electronic theory. She gave a simple and visual explanation of most of the electrical and thermal properties of metals. Paul Drude Karl Ludwig - German physicist Hendrik Anton Lorenz - Dutch physicist Classical electron theory
5 slide
Slide description:
The movement of electrons obeys the laws of classical mechanics. Electrons do not interact with each other. Electrons interact only with ions of the crystal lattice; this interaction is reduced to collision. In the intervals between collisions, electrons move freely. Conduction electrons form an “electron gas”, similar to an ideal gas. “Electronic gas” obeys the laws of ideal gas. During any collision, the electron transfers all the accumulated energy. Basic principles of the theory
6 slide
Slide description:
The metal has a crystal lattice, at the nodes of which there are positive ions that oscillate around the equilibrium position, and free electrons that can move throughout the entire volume of the conductor (electron gas, subject to the laws of an ideal gas) Structure of the metal
7 slide
Slide description:
The average speed of thermal motion of electrons at room temperature is approximately 105 m/s. Structure of a metal In a metal, in the absence of an electric field, electrons move chaotically and collide, most often with ions of the crystal lattice.
8 slide
Slide description:
Electric current in metals Under the influence of an electric field, free electrons begin to move in an orderly manner between the ions of the crystal lattice. Electric current flows through a conductor due to the presence of free electrons in it that have escaped from atomic orbits
Slide 9
Slide description:
Electric current in metals Electric current in metals is the ordered movement of electrons under the influence of an electric field. When current flows through a metal conductor, no substance transfer occurs; therefore, metal ions do not take part in the transfer of electric charge. This was confirmed in the experiments of the German physicist E. Ricke in 1901.
10 slide
Slide description:
Experiments by E.Rikke In these experiments, an electric current of 0.1 A was passed for a year through three well-polished cylinders pressed against each other. The total charge passed through the cylinders during this time exceeded 3.5 MK. After completion, it was found that there were only minor traces of mutual penetration of metals, not exceeding the results of ordinary diffusion of atoms in solids. Measurements showed that the mass of each cylinder remained unchanged. Since the masses of copper and aluminum atoms differ significantly from each other, the mass of the cylinders would have to change noticeably if the charge carriers were ions. Therefore, free charge carriers in metals are not ions. The huge charge that passed through the cylinders was apparently carried by particles that are the same in both copper and aluminum.
11 slide
Slide description:
Experimental proof of the existence of free electrons in metals Experimental proof that the current in metals is created by free electrons was given in experiments by L.I. Mandelstam and N.D. Papaleksi (1913, the results were not published), as well as the experiments of T. Stewart and R. Tolman (1916). L.I. Mandelstam 1879-1949 N. D. Papaleksi 1880-1947 T. Stewart
12 slide
Slide description:
The coil connected to the telephone spun around its axis in different directions and slowed down sharply. If electrons really have mass, then when the coil suddenly stops, the electrons should continue to move by inertia for some time. The movement of electrons through a wire is an electric current, and the phone should make a sound. Since sound is heard in the phone, therefore, current flows through it. But no measurements or quantitative calculations were made in these experiments. Experience of L.I. Mandelstam and N.D. Papaleksi (1912)
Slide 13
Slide description:
Experience of T. Stewart and R. Tolman A coil with a large number of turns of thin wire was driven into rapid rotation around its axis. The ends of the coil were connected using flexible wires to a sensitive ballistic galvanometer. The untwisted coil was sharply slowed down, and a short-term current arose in the circuit due to the inertia of the charge carriers. The total charge flowing through the circuit was measured by the deflection of the galvanometer needle.
Slide 14
Slide description:
The experiment of T. Stewart and R. Tolman The direction of the current indicated that it was caused by the movement of negatively charged particles. By measuring the charge passing through the galvanometer during the entire existence of the current in the circuit, T. Stewart and R. Tolman experimentally determined the specific charge of the particles. He turned out to be equal
15 slide
Slide description:
Volt - ampere characteristic of metals Electric current in metals Charge carriers - electrons Conductivity - electronic The conductor through which the current flows heats up. A conductor through which current flows has a magnetic effect on surrounding bodies.
16 slide
Slide description:
Dependence of conductor resistance on temperature Resistance is a physical quantity that characterizes the ability of a conductor to resist the establishment of electric current in it. Specific resistance is the resistance of a cylindrical conductor of unit length and unit cross-sectional area. When heated, the dimensions of the conductor change little, but mainly the resistivity changes.
Slide 17
Slide description:
Dependence of conductor resistance on temperature Specific resistance of a conductor depends on temperature: where ro is the resistivity at 0 degrees, t is the temperature, α is the temperature coefficient of resistance
18 slide
Slide description:
Dependence of conductor resistance on temperature For metal conductors, as the temperature increases, the resistivity increases, the resistance of the conductor increases, and the electric current in the circuit decreases. The resistance of a conductor with a change in temperature can be calculated using the formula: R = Ro (1 + α t), where Ro is the resistance of the conductor at 0 degrees Celsius t is the temperature of the conductor α is the temperature coefficient of resistance
Slide 19
Slide description:
Application of current in metals Transfer of electricity from source to consumers In electric motors and generators In heating devices
20 slide
Slide description:
Contradictions of classical electronic theory Classical electronic theory explains the existence of electrical resistance of metals, Ohm's and Joule-Lenz's laws. However, in a number of issues, the classical electronic theory leads to conclusions that are in conflict with experiment. This theory cannot explain why the molar heat capacity of metals, as well as the molar heat capacity of dielectric crystals, is equal to 3R, where R is the universal gas constant (Dulong and Petit's law). The presence of free electrons does not affect the heat capacity of metals. Classical electronic theory also cannot explain the temperature dependence of the resistivity of metals. The theory gives the relation, while from experiment the dependence ρ ~ T is obtained. However, the most striking example of the discrepancy between theory and experiment is superconductivity.
21 slides
Slide description:
Superconductivity According to classical electronic theory, the resistivity of metals should monotonically decrease with cooling, remaining finite at all temperatures. This dependence is actually observed experimentally at relatively high temperatures. At temperatures of the order of several kelvins, the resistivity of many metals ceases to depend on temperature and reaches a certain limiting value. In 1911, the Dutch scientist Geike Kamerling-0nnes discovered that when the temperature of mercury drops to 4.1 K, its resistivity abruptly decreases to zero. (1853-1926) Geike Kamerling -0nnes, Dutch scientist
22 slide
Slide description:
Superconductivity At a certain temperature Tcr, different for different substances, the resistivity abruptly decreases to zero. This phenomenon is called superconductivity. Materials that exhibit the ability to transition to a superconducting state at certain temperatures other than absolute zero are called superconductors. Dependence of resistivity ρ on absolute temperature T at low temperatures: a – normal metal; b – superconductor
Slide 23
Slide description:
Superconductivity G. Kamerlingh Onnes was awarded the Nobel Prize in Physics in 1913 “for his studies of the properties of matter at low temperatures.” Later it was found that more than 25 chemical elements - metals - become superconductors at very low temperatures. The lowest temperature is for tungsten - 0.012 K, the highest for niobium - 9 K. Superconductivity is observed not only in pure metals, but also in many chemical compounds and alloys and some semiconductors. Moreover, the elements themselves that make up the superconducting compound may not be superconductors. For example, NiBi, Au2Bi, PdTe, PtS and others. At the same time, such “good” conductors as copper and silver do not become superconductors at low temperatures.
24 slide
Slide description:
Superconductivity The first theoretical explanation of superconductivity was given in 1935 by brothers Fritz and Heinz London. A more general theory was constructed in 1950 by L. D. Landau and V. L. Ginzburg. However, these theories did not reveal the detailed mechanisms of superconductivity. Superconductivity was first explained at the microscopic level in 1957 in the work of American physicists John Bardeen, Leon Cooper and John Schrieffer. The central element of their theory, called the BCS theory, is the so-called Cooper pairs of electrons. It was later discovered that superconductors are divided into two large families: type I superconductors (which, in particular, include mercury) and type II (which are usually alloys of different metals). The work of A. A. Abrikosov in the 1950s played a significant role in the discovery of type II superconductivity.
25 slide
Slide description:
Superconductivity In 1962, the English physicist Brian Josephson discovered the effect that received his name. In 1986, Karl Müller and Georg Bednorz discovered a new type of superconductors, called high-temperature superconductors. In early 1987, it was shown that compounds of lanthanum, strontium, copper and oxygen (La-Sr-Cu-O) experience a jump in resistance to almost zero at a temperature of 36 K. In early March 1987, a superconductor was obtained for the first time at temperatures above boiling of liquid nitrogen (77.4 K): it was discovered that the compound of yttrium, barium, copper and oxygen (Y-Ba-Cu-O) has this property.
26 slide
Slide description:
Superconductivity In 1988, a ceramic compound (a mixture of thallium, calcium, barium and copper oxides) with a critical temperature of 125 K was created. In 2003, a ceramic compound Hg-Ba-Ca-Cu-O(F) was discovered, the critical temperature for which is 138 K. Moreover, at a pressure of 400 kbar, the same compound is a superconductor at temperatures up to 166 K. In 2015, a new record was set for the temperature at which superconductivity is achieved. For H2S (hydrogen sulfide) at a pressure of 100 GPa, a superconducting transition was recorded at a temperature of 203 K (-70°C).
Slide 27
Slide description:
Properties of superconductors Since there is no resistance in superconductivity, no heat is generated when an electric current passes through a conductor. This property of superconductors is widely used. For each superconductor, there is a critical current value that can be achieved in the conductor without violating its superconductivity. This happens because when current passes, a magnetic field is created around the conductor. And the magnetic field destroys the superconducting state. Therefore, superconductors cannot be used to produce an arbitrarily strong magnetic field. When energy passes through a superconductor, there is no loss of energy. One of the areas of research by modern physicists is the creation of superconducting materials at room temperatures.
28 slide
Slide description:
Superconductivity Currently, over 500 pure elements and alloys are known that exhibit the property of superconductivity. Based on their behavior in sufficiently strong magnetic fields, they are divided into type 1 and type 2 superconductors. Type I superconductors completely displace the magnetic field. Type 1 superconductors include all superconducting elements except Nb and V, and some alloys.
Slide 29
Lecturer: Ph.D. Sc., Associate ProfessorVeretelnik Vladimir Ivanovich
Electric current in metals
1.2.
3.
4.
5.
Tolman-Stewart experiment.
Classical conduction theory
metals - Drude-Lorentz theory.
Ohm's law and Joule-Lenz's law from
classical theory of electrical conductivity.
Superconductivity.
Electron-hole transition.
Transistors.
Electric current in metals
Electric current in metals isordered movement of electrons under
action of an electric field.
The most convincing evidence
the electronic nature of current in metals was
obtained in experiments with electron inertia
(The experience of Tolman and Stewart).
Coil with a large number of turns of thin
the wire was driven into rapid rotation
around its axis.
Coil ends with flexible wires
were attached to the sensitive
ballistic galvanometer.
Electric current in metals
Untwisted reel sharplyslowed down, and a problem appeared in the chain
short-term current due to
inertia of charge carriers.
The total charge flowing through the circuit is
measured by needle drop
galvanometer.
Electric current in metals
When braking the rotating coil for eachcharge carrier e acts as a braking force, which
plays the role of an external force, that is, a force
non-electrical origin.
External force per unit charge, according to
definition is the field strength Est
third party forces:
Consequently, in the circuit when the coil is braking
electromotive force arises:
Electric current in metals
where l is the length of the coil wire. During brakingcoil, a charge q will flow through the circuit equal to:
Here I is the instantaneous value of the current in the coil, R is
total resistance of the circuit, υ0 – initial linear
wire speed.
Hence the specific charge e/m of free current carriers
in metals is equal to:
According to modern data, the electron charge modulus
(elementary charge) is equal to
Electric current in metals
Specific chargeGood electrical conductivity of metals
due to high concentration
free electrons, equal in order
quantities for the number of atoms per unit volume.
Assumption about what kind of electric current
in metals electrons are responsible, arose
much earlier than the experiments of Tolman and Stewart.
Back in 1900, the German scientist P. Drude
basis of the hypothesis about the existence of free
electrons in metals created electron
theory of conductivity of metals.
Electric current in metals
This theory was developed in the works of the Dutchphysics by H. Lorentz and is called classical
electronic theory.
According to this theory, electrons in metals behave
like an electron gas, much like an ideal gas
gas.
Electron gas fills the space between the ions,
forming a metal crystal lattice
Due to interaction with ions, electrons can
leave the metal only by overcoming the so-called
potential barrier.
The height of this barrier is called the work function.
At ordinary (room) temperatures, electrons do not
enough energy to overcome potential
barrier.
Electric current in metals
According to the Drude–Lorentz theory,electrons have the same average
energy of thermal motion, as well as
monatomic ideal molecules
gas
This allows us to estimate the average
speed of thermal movement
electrons according to the formulas of molecular kinetic theory.
At room temperature it
turns out to be approximately equal to 105 m/s.
Electric current in metals
When applying externalelectric field in
metal conductor except
thermal motion of electrons
their orderly appears
movement (drift), that is
electricity.
Electric current in metals
Estimation of drift speedshows that for metal
conductor with a cross section of 1 mm2, along which
a current of 10 A flows, this value lies in
within 0.6–6 mm/s.
So the average speed
ordered movement of electrons in
metal conductors for many
orders of magnitude less than their average speed
thermal movement.
Electric current in metals
Low drift speed does not contradictthe experimental fact that the current in the entire circuit
DC is installed practically
instantly.
Closing the circuit causes propagation
electric field with a speed c = 3·108 m/s.
After a time of the order of l / s (l is the length of the chain)
a stationary one is installed along the chain
electric field distribution and in it
orderly movement begins
electrons.
Electric current in metals
In classical electronic theory of metalsit is assumed that the movement of electrons
obeys Newton's laws of mechanics.
This theory neglects the interaction
electrons among themselves, and their interaction
with positive ions are reduced only to
collisions.
It is also assumed that for each
collision, the electron transfers to the lattice all
energy accumulated in the electric field and
so after the collision he starts
movement with zero drift speed.
Electric current in metals
Although all these assumptions arevery close, classical electronic
the theory qualitatively explains the laws of electrical
current in metal conductors.
Ohm's law. In the interval between collisions on
electron acts on a force equal in magnitude eE to
As a result, it gains acceleration
Therefore, by the end of the free run, the drift
electron speed is
Electric current in metals
where τ is the free travel time,which, to simplify calculations
assumed to be the same for everyone
electrons.
Average drift speed
equal to half the maximum
values:
Electric current in metals
Consider a conductor of length l and cross-section S withelectron concentration n.
The current in a conductor can be written as:
where U = El is the voltage at the ends of the conductor.
The resulting formula expresses Ohm's law for
metal conductor.
Electrical resistance of the conductor
equals:
Electric current in metals
Resistivity ρ and specificconductivity σ are expressed
ratios:
Joule-Lenz law. By the end
free path of electrons
acquired under the influence of the field
kinetic energy
Electric current in metals
According to the assumptions made,all this energy is transferred to the lattice when
collision and turns into heat.
During the time Δt, each electron
experiences Δt/τ collisions.
In a conductor with cross section S and length l
there are nSl electrons.
It follows that what is allocated in
conductor during time Δt heat is equal to:
Electric current in metals
This ratio expressesJoule-Lenz law.
Thus, classical electronic
theory explains existence
electrical resistance of metals,
Ohm's and Joule-Lenz's laws.
However, in a number of issues the classical
electron theory leads to conclusions
in conflict with experience.
Electric current in metals
This theory cannot, for example, explain whymolar heat capacity of metals, as well as molar
the heat capacity of dielectric crystals is 3R,
where R is the universal gas constant (law
Dulong and Petit.)
Classical electron theory cannot either
explain the temperature dependence of specific
metal resistance.
The theory gives
while from the experiment
the dependence ρ ~ T is obtained.
However, the most striking example of the divergence between theory and
experiments is superconductivity.
Electric current in metals
At some certaintemperature Tcr, different for different
substances, resistivity
decreases abruptly to zero.
The critical temperature of mercury is
4.1 K, aluminum 1.2 K, tin 3.7 K.
Superconductivity is not observed
only for elements, but also for many
chemical compounds and alloys.
Electric current in metals
For example, a compound of niobium with tin(Ni3Sn) has a critical temperature
18 K.
Some substances that pass through
low temperatures into superconducting
condition, are not conductors
at normal temperatures.
At the same time so “good”
conductors like copper and silver are not
become superconductors when
low temperatures.
Electric current in metals
Substances in superconductingcondition have
exceptional properties.
Almost the most important of them
them is the ability
long time (many years)
maintain without attenuation
electric current excited in
superconducting circuit.
Electric current in metals
Classical electron theory is notis able to explain the phenomenon
superconductivity. Explanation
the mechanism of this phenomenon was given
only 60 years after its discovery
based on quantum mechanical
representations.
Scientific interest in superconductivity
increased as new ones were discovered
materials with higher
critical temperatures.
Electric current in metals
A significant step in this direction occurred in1986, when it was discovered that one complex
ceramic connection Tcr = 35 K.
Already in the next 1987, physicists were able to create
new ceramics with a critical temperature of 98 K,
exceeding the temperature of liquid nitrogen (77 K).
The phenomenon of transition of substances into superconducting
state at temperatures exceeding the temperature
boiling point of liquid nitrogen was called
high temperature superconductivity.
In 1988, a ceramic connection was created on
based on elements Tl–Ca–Ba–Cu–O with critical
temperature 125 K.
It should be noted that until now the mechanism
high temperature superconductivity ceramic
materials is not fully understood.
1.
2.
3.
4.
The qualitative difference between semiconductors and
metals
Electron-hole mechanism
conductivity of pure unadulterated
semiconductors.
Electronic and hole conductivity
impurity semiconductors. Donor and
acceptor impurities.
Electron-hole transition.
Semiconductor diode. Transistor.
Electric current in semiconductors
Semiconductors includemany chemical elements (germanium,
silicon, selenium, tellurium, arsenic, etc.),
a huge number of alloys and
chemical compounds.
Almost all inorganic substances
the world around us -
semiconductors.
The most common in nature
silicon is a semiconductor
making up about 30% of the earth's crust.
Electric current in semiconductors
Qualitative differencesemiconductors from metals
manifests itself primarily in
specific
temperature resistance.
Electric current in semiconductors
This course of the dependence ρ(T) shows thatthat semiconductors have a concentration
no free charge carriers
remains constant but increases with
rising temperature.
Let us consider this mechanism qualitatively
using the example of germanium (Ge).
In a silicon (Si) crystal, the mechanism
similar.
Electric current in semiconductors
Germanium atoms have four weakbound electrons in the outer shell.
They are called valence electrons.
In a crystal lattice, each atom
surrounded by four closest neighbors.
Bonding between atoms in a germanium crystal
is covalent, i.e. it is carried out
pairs of valence electrons.
Each valence electron belongs to two
atoms.
Electric current in semiconductors
Valence electrons in a germanium crystalmuch more strongly bound to atoms than in
metals
Therefore, the electron concentration
conductivity at room temperature in
semiconductors are many orders of magnitude smaller,
than metals.
Near absolute zero temperature in
In a germanium crystal, all electrons are occupied in
formation of connections.
Such an electric current crystal does not
conducts.
Electric current in semiconductors
Pair-electron bonds in a crystalgermanium and the formation of an electron-hole pair.
Electric current in semiconductors
As the temperature rises, somesome valence electrons can
get enough energy to
breaking covalent bonds.
Then free ones will appear in the crystal
electrons (conduction electrons).
At the same time, in places where connections are broken
vacancies are created that are not filled
electrons.
These vacancies are called
"holes".
Electric current in semiconductors
Vacant position may be filledvalence electron from the neighboring
pairs, then the hole moves to
a new place in the crystal.
If a semiconductor is placed in
electric field, then into an ordered
the movement involves not only
free electrons, but also holes,
who behave positively
charged particles.
Electric current in semiconductors
Therefore, the current I in the semiconductorconsists of electronic In and
hole IP currents:
I = In + Ip.
Electron-hole mechanism
conductivity appears only
in pure (i.e. without impurities)
semiconductors. It is called
own electric
conductivity of semiconductors.
Electric current in semiconductors
If there are impuritieselectrical conductivity of semiconductors
changes a lot.
For example, adding phosphorus impurities to
silicon crystal in the amount of 0.001
atomic percent reduces specific
resistance by more than five
orders of magnitude.
Such a strong influence of impurities can
be explained based on the above
above ideas about the structure
semiconductors.
Electric current in semiconductors
A necessary condition for sharpReducing resistivity
semiconductor upon introduction of impurities
is the difference in valence of atoms
impurities from the valency of the main
atoms of the crystal.
Conductivity of semiconductors at
the presence of impurities is called
impurity conductivity.
Electric current in semiconductors
There are two types of impurityconductivity – electronic and
hole conductivity.
Electronic conductivity
occurs when a crystal
germanium with tetravalent
atoms introduced pentavalent
atoms (for example, arsenic atoms,
As).
Electric current in semiconductors
Electric current in semiconductorsElectric current in semiconductors
The four valence electrons of the arsenic atomincluded in the formation of covalent bonds with
four neighboring germanium atoms.
The fifth valence electron turned out to be redundant.
It is easily detached from the arsenic atom and
becomes free.
An atom that has lost an electron becomes
positive ion located at the site
crystal lattice.
Electric current in semiconductors
An impurity of atoms with valency,exceeding the valency of the main atoms
semiconductor crystal is called
donor admixture.
As a result of its introduction into the crystal
there is a significant number of free
electrons.
This leads to a sharp decrease in specific
semiconductor resistance - in thousands and
even millions of times.
Conductor resistivity with
high content of impurities may
approach resistivity
metal conductor.
Electric current in semiconductors
Such conductivityconditioned by free
electrons is called
electronic, but a semiconductor,
possessing electronic
conductivity is called
n-type semiconductor.
Electric current in semiconductors
Hole conduction occurs whengermanium crystal introduced trivalent
atoms (for example, indium atoms, In).
Electric current in semiconductors
In Fig. shows the indium atom that created withusing their valence electrons
covalent bonds with only three neighboring
germanium atoms.
To form a bond with the fourth atom
germanium the indium atom does not have an electron.
This missing electron could be
captured by an indium atom from a covalent bond
neighboring germanium atoms.
In this case, the indium atom turns into
negative ion located at the site
crystal lattice, and in a covalent
bonds between neighboring atoms, a vacancy is formed.
Electric current in semiconductors
An admixture of atoms capable of capturingelectrons, called acceptor
impurity.
As a result of the introduction of an acceptor impurity into
crystal, many covalent bonds are broken
connections and vacancies (holes) are formed.
Electrons can jump to these places from
neighboring covalent bonds, which leads to
chaotic wandering of holes throughout the crystal.
Electric current in semiconductors
Hole concentration in a semiconductor withacceptor impurity significantly
exceeds the concentration of electrons, which
arose due to the mechanism of its own
electrical conductivity of a semiconductor: np >> nn.
This type of conductivity is called
hole conductivity.
Impurity semiconductor with hole
conductivity is called a semiconductor
p-type.
The main free charge carriers in
p-type semiconductors are holes.
Electric current in semiconductors
It should be emphasized that the holeconductivity in reality
due to relay movement
by vacancies from one germanium atom to
other electrons that
make a covalent bond.
For n- and p-type semiconductors the law
Ohm is performed in certain
ranges of current and voltage at
condition of constant concentrations
free media.
In modern electronic technology
semiconductor devices play
exceptional role.
Over the past three decades they have almost
completely replaced electric vacuum
devices.
Any semiconductor device has
one or more electron-hole
transitions.
An electron-hole junction (or n–p junction) is the region of contact between two
semiconductors with different types
conductivity.
Electron-hole transition. Transistor
When two semiconductors n- andp-types the diffusion process begins:
holes from the p-region move to the n-region, and electrons, on the contrary, from the n-region to the p-region.
As a result, in the n-region near the zone
contact concentration decreases
electrons and arises positively
charged layer.
In the p-region the concentration decreases
holes and occurs negatively
charged layer.
Electron-hole transition. Transistor
Thus, at the semiconductor boundaryan electrical double layer is formed,
whose electric field prevents
process of diffusion of electrons and holes
towards each other
Electron-hole transition. Transistor
The n–p junction has an amazingproperty of one-sided
conductivity.
If a semiconductor with an n–p junction
connected to a current source so that
source positive pole
connected to the n-region, and
negative – with p-region, then
field strength in the blocking layer
increases.
Electron-hole transition. Transistor
Holes in the p-region and electrons in the n-region will shift away from the n–p junction, thereby increasingconcentrations of minority carriers in
barrier layer.
The current through the n–p junction is practically not
coming.
The voltage applied to the n–p junction in
This case is called the reverse.
Electron-hole transition. Transistor
Very minor inversethe current is due only to its own
conductivity
semiconductor materials,
i.e. the presence of a small
concentrations of free
electrons in the p-region and holes in
n-regions.
Electron-hole transition. Transistor
If the n–p junction is connected tosource so that it is positive
the pole of the source was connected to the p-region, and the negative pole to the n-region, then the voltage
electric field in the blocking layer
will decrease, which makes it easier
transition of main carriers through
contact layer.
Electron-hole transition. Transistor
Holes from the p-region and electrons fromn-regions, moving towards each other
friend, will cross the n–p junction, creating a current in the direct
direction.
The current strength through the n–p junction in this
case will increase with
increasing source voltage.
Electron-hole transition. Transistor
The ability of an n–p junction to passcurrent is practically only in one
direction is used in devices,
which are called
semiconductor diodes.
Semiconductor diodes
made from silicon crystals
or Germany.
During their manufacture, a crystal with any type of conductivity is melted into
admixture providing another type
conductivity.
Electron-hole transition. Transistor
Typical current-voltagecharacteristics of silicon diode
Electron-hole transition. Transistor
Semiconductor devices are notone, but with two n–p junctions
are called transistors.
Transistors are of two types:
p–n–p transistors and n–p–n transistors.
Electron-hole transition. Transistor
For example, a germanium transistorp–n–p type is
small plate of germanium
with a donor impurity, i.e. from
n-type semiconductor.
This record creates two
areas with an acceptor impurity,
i.e. areas with hole
conductivity.
Electron-hole transition. Transistor
In an n–p–n-type transistor, the maingermanium plate has
p-type conductivity, and those created on
There are two regions with n-type conductivity.
The plate of the transistor is called the base
(B), one of the areas with
opposite type of conductivity
– collector (K), and the second –
emitter (E).
Electron-hole transition. Transistor
1.
2.
3.
4.
Electrolytes. Charge carriers in
electrolytes.
Electrolysis. Electrolytic
dissociation.
Faraday's law for electrolysis.
Faraday's combined law for
electrolysis.
Electric current in electrolytes
Electrolytes are commonly calledconducting media in which
flow of electric current
accompanied by transfer
substances.
Carriers of free charges in
electrolytes are
positive and negative
charged ions.
Electric current in electrolytes
The main representativeselectrolytes widely used in
technology are aqueous solutions
inorganic acids, salts and
grounds.
Passage of electric current through
electrolyte is accompanied by the release
substances on the electrodes.
This phenomenon is called
electrolysis.
Electric current in electrolytes
Electric current in electrolytesrepresents the movement of ions of both
signs in opposite directions.
Positive ions move towards
negative electrode (cathode),
negative ions to positive
electrode (anode).
Ions of both signs appear in water
solutions of salts, acids and alkalis in
as a result of the splitting of part of the neutral
molecules.
This phenomenon is called electrolytic
dissociation.
Electric current in electrolytes
For example, copper chloride CuCl2dissociates in aqueous solution into
copper and chlorine ions:
When connecting electrodes to
current source ions under the influence
electric field begin
orderly movement:
positive copper ions move towards
cathode, and negatively charged
chlorine ions - to the anode.
Electric current in electrolytes
Upon reaching the cathode, copper ions are neutralizedexcess cathode electrons and
transform into neutral atoms
deposited on the cathode.
Chlorine ions, reaching the anode, give off
one electron.
After this, neutral chlorine atoms
combine in pairs to form molecules
chlorine Cl2.
Chlorine is released at the anode in the form of bubbles.
Electric current in electrolytes
The law of electrolysis was experimentallyestablished by the English physicist M. Faraday in
1833.
Faraday's law determines quantities
primary products released into
electrodes during electrolysis:
Mass m of substance released on
electrode, is directly proportional to the charge Q,
passed through the electrolyte:
m = kQ = kIt.
The value k is called electrochemical
equivalent.
Electric current in electrolytes
Mass of substance released on the electrodeequal to the mass of all ions arriving at
electrode:
Here m0 and q0 are the mass and charge of one ion,
– number of ions arriving at the electrode at
passing charge Q through the electrolyte.
Thus, the electrochemical equivalent
k is equal to the ratio of the mass m0 of a given ion
substance to its charge q0.
Electric current in electrolytes
Since the charge of an ion is equal to the productvalence of substance n on
elementary charge e (q0 = ne), then
expression for electrochemical
the equivalent of k can be written as:
F = eNA – Faraday's constant.
F = eNA = 96485 C/mol.
Electric current in electrolytes
Faraday's constant numericallyequal to the charge required
pass through electrolyte for
discharge on the electrode of one
mole of monovalent substance.
Faraday's law for electrolysis
takes the form:
Control questions
1.2.
3.
4.
5.
6.
Charge carriers in metals.
Brief information about classical theory
conductivity of metals (Drude-Lorentz theory).
Ohm's law from classical theory (brief
conclusion).
Joule-Lenz law from classical theory
conductivity (brief conclusion).
What physical problems cannot be explained
classical theory of conductivity of metals.
Brief information about superconductivity.
Control questions
1.2.
3.
4.
5.
6.
7.
8.
Electrons and holes. How are they formed in pure
semiconductors?
Conduction mechanism of pure semiconductors.
Donor and acceptor semiconductors.
Conduction mechanism of impurity semiconductors.
How to implement electron and hole
conductivity in semiconductors.
What is an electron-hole transition?
Explain why electron-hole transition
can rectify alternating current.
Transistor.
Control questions
What charge carriers are there inelectrolytes?
2. What are electrolytes? What's happened
electrolytic dissociation?
3. Faraday's law for electrolysis.
4. United law of electrolysis
Faraday.
Electric current in metals Svetlana Nikolaevna Savvateeva, physics teacher at the Kemetskaya Secondary School, Bologovsky district, Tver region. TODAY IN CLASS The secret becomes clear. What is hidden behind the concept of “Current carriers in metals”? What are the difficulties of the classical theory of electrical conductivity of metals? Why do incandescent lamps burn out? Why do they burn out when turned on? How to lose resistance? LET'S REPEAT
- What is electric current?
- What are the conditions for the existence of current?
- What effects of current do you know?
- What is the direction of the current?
- What value determines the current strength in an electrical circuit?
- What is the unit of current?
- What quantities does the current depend on?
- What is the speed of current propagation in a conductor?
- What is the speed of ordered movement of electrons?
- Does resistance depend on current and voltage?
- How is Ohm's law formulated for a section of a chain and for a complete chain?
Mandelstam and Papaleksi (1913)
Stewart and Tolman (1916)
In the direction of the current -< 0
According to I J I - q ⁄ m = e ⁄ m ) these are electrons!
Rikke's experience (German) – 1901 Year! M = const, these are not ions!
NATURE OF CHARGE CARRIERS IN METALS
Electric current in metals is the directed movement of electrons.
Theory of electrical conductivity of metals
P. Druse, 1900:
- free electrons – “electron gas”;
- electrons move in accordance with Newton's laws;
- free electrons collide with crystal ions. gratings;
- upon collision, electrons transfer their kinetic energy to the ions;
- the average speed is proportional to the tension and therefore the potential difference;
R= f (ρ, l, s, t)
resistance thermometers
Advantages: Helps measure very low and very high temperatures.
superconductivity Mercury in liquid heliumThe explanation is based on quantum theory.
D. Bardeen, L. Cooper, D. Schrieffer (American) and
N. Bogolyubov (Soviet student in 1957)
Application of superconductivity!
- obtaining high currents and magnetic fields;
- transmission of electricity without losses.
- How do free electrons move in metals? A. In a strictly defined order. B. Disorderly. B. Orderly.
- How do free electrons move in metals under the influence of an electric field? A. Disorderly. B. Orderly. B. Ordered in the direction of the electric field. D. Ordered in the direction opposite to the electric field.
- .What particles are located at the sites of the crystal lattice of metals and what charge do they have? A. Negative ions. B. Electrons. B. Positive ions.
- What effect of electric current is used in electric lamps? A. Magnetic. B. Thermal. B. Chemical. G. Light and thermal.
- The movement of which particles is taken to be the direction of current in a conductor? A. Elektronov. B. Negative ions. B. Positive charges.
- Why do metals heat up when current passes through them? A. Free electrons collide with each other. B. Free electrons collide with ions. B. Ions collide with ions.
- How does the resistance of metals change when they are cooled? A. Increases. B. Decreases. B. Does not change. 1. B. 2.G. 3.B. 4.G. 5.B. 6.B. 7.B.
1. The electrical resistance of the tungsten filament of an electric lamp at a temperature of 23°C is 4 Ohms.
Find the electrical resistance of the thread at 0°C.
(Answer: 3.6 ohms)
2. The electrical resistance of a tungsten filament at 0°C is 3.6 ohms. Find electrical resistance
At a temperature of 2700 K.
(Answer: 45.5 ohms)
3. The electrical resistance of a wire at 20 °C is 25 Ohms, at a temperature of 60°C it is 20 Ohms. Find
Temperature coefficient of electrical resistance.
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