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General physics. Electric current in metals. Physics presentation on the topic "electric current in metals" Presentations on physics current in metals

Electric current in metals Savvateeva Svetlana Nikolaevna, teacher of physics, MBOU "Kemetskaya secondary school" of the Bologovsky district of the Tver region. TODAY IN THE LESSON The secret becomes clear. What is hidden behind the concept "Current carriers in metals"? What are the difficulties of the classical theory of electrical conductivity of metals? Why do incandescent bulbs burn out? Why do they burn out when turned on? How to lose resistance? REPEAT

What is electric current?
What are the conditions for the existence of a current?
What actions of the current do you know?
What is the direction of current?
What is the value of current in an electrical circuit?
What is the unit of current?
On what quantities does the current strength depend?
What is the speed of current propagation in the conductor?
What is the speed of the 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?

ELECTRICAL CONDUCTIVITY OF VARIOUS SUBSTANCES

Mandelstam and Papaleksi (1913)

Stewart and Tolman (1916)

In the direction of current -< 0

By І 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 - "electronic gas";
electrons move according to Newton's laws;
free electrons collide with crystal ions. gratings;
upon collision, electrons transfer their kinetic energy to ions;
the average speed is proportional to the intensity and, therefore, the potential difference;

R= f (ρ, l, s, t)

resistance thermometers

Benefits: Helps to measure very low and very high temperatures.

superconductivity Mercury in liquid helium

The explanation is based on quantum theory.

D. Bardeen, L. Cooper, D. Schrieffer (Amer.) and

N. Bogolyubov (co-student in 1957)

Application of superconductivity!

obtaining high currents, magnetic fields;
transmission of electricity without loss.

control test

How do free electrons move in metals?
How do free electrons move in metals under the action of an electric field?
.What particles are located at the nodes of the crystal lattice of metals and what charge do they have?
What effect of electric current is used in electric lamps?
The movement of which particles is taken as the direction of the current in the conductor?
Why do metals get hot when current is passed through them?
How does the resistance of metals change when they are cooled?

1. B. 2. D. 3.B. 4.G. 5.B. 6.B. 7.B.

SOLVE THE PROBLEM

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 filament at 0°C.

(Answer: 3.6 ohm)

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 the wire at 20°C is 25 ohms, at 60°C it is 20 ohms. Find

Temperature coefficient of electrical resistance.

Lecturer: Ph.D. PhD, Associate Professor
Veretelnik Vladimir Ivanovich

Electric current in metals

1.
2.
3.
4.
5.
Experience of Tolman-Stuart.
Classical conduction theory
metals - Theory of Drude-Lorentz.
Ohm's law and Joule-Lenz's law
classical theory of electrical conductivity.
Superconductivity.
Electron-hole transition.
Transistors.

Electric current in metals

Electric current in metals is
orderly movement of electrons
by the action of an electric field.
The most compelling evidence
the electronic nature of the current in metals was
obtained in experiments with electron inertia
(Experience of Tolman and Stewart).
Coil with a large number of turns thin
the wires were brought into rapid rotation
around its axis.
Coil ends with flexible wires
were attached to sensitive
ballistic galvanometer.

Electric current in metals

The untwisted coil sharply
braked, and arose in the chain
short-term current due
charge carrier inertia.
The total charge flowing through the circuit
was measured by the rejection of the arrow
galvanometer.

Electric current in metals

When braking a rotating coil, for each
charge carrier e acts a braking force, which
plays the role of an outside force, that is, a force
non-electric origin.
External force, related to the unit of charge, according to
definition is the field strength Est
outside forces:
Therefore, in the circuit when braking the coil
an electromotive force occurs:

Electric current in metals

where l is the length of the coil wire. During the deceleration
coil, a charge q will flow through the circuit, equal to:
Here I is the instantaneous value of the current in the coil, R is
circuit impedance, 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 module
(elementary charge) is

Electric current in metals

Specific charge
Good electrical conductivity of metals
explained by the high concentration
free electrons, equal in order
the number of atoms per unit volume.
The assumption about what kind of electric current
electrons are responsible in metals,
much earlier than the experiments of Tolman and Stuart.
Back in 1900, the German scientist P. Drude
the basis of the hypothesis of the existence of free
electrons in metals created an electronic
the theory of conductivity of metals.

Electric current in metals

This theory was developed in the works of the Dutch
physics of H. Lorentz and is called classical
electronic theory.
According to this theory, electrons in metals behave
like an electron gas, in many ways similar to an ideal
gas.
The electron gas fills the space between the ions,
forming a metal crystal lattice
Due to interaction with ions, electrons can
leave the metal, only 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 the 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
thermal motion speed
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 an external
electric field in
metal conductor except
thermal motion of electrons
their ordered
movement (drift), that is
electricity.

Electric current in metals

Estimation of drift velocity
shows 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
orderly movement of electrons in
many metal conductors
orders of magnitude less than their average speed.
thermal movement.

Electric current in metals

Slow drift speed on contradict
experimental fact that the current in the entire circuit
direct current 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)
along the chain a stationary
distribution of the electric field and in it
orderly movement begins
electrons.

Electric current in metals

In the classical electronic theory of metals
it is assumed that the movement of electrons
obeys the laws of Newtonian mechanics.
This theory neglects the interaction
electrons with each other, and their interaction
with positive ions reduce only to
collisions.
It is also assumed that for each
collision, the electron transfers the entire
energy stored in the electric field and
so after the collision it starts
movement with zero drift velocity.

Electric current in metals

Although all these assumptions are
very approximate, classical electronic
theory qualitatively explains the laws of electrical
current in metallic conductors.
Ohm's law. Between impacts on
electron is acted upon by a force equal in modulus eE, in
causing it to accelerate
Therefore, by the end of the free run, the drift
the speed of the electron is

Electric current in metals

where τ is the free run time,
which, to simplify calculations
assumed to be the same for all
electrons.
Drift average value
equal to half the maximum
values:

Electric current in metals

Consider a conductor of length l and cross section S with
electron 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.
Conductor electrical resistance
equals:

Electric current in metals

Resistivity ρ and specific
conductivity σ are expressed
ratios:
Joule-Lenz law. By the end
free path electrons
acquired under the action of the field
kinetic energy

Electric current in metals

According to the assumptions made,
all this energy is transferred to the lattice at
collision and turns into heat.
During the time Δt, each electron
experiences Δt / τ collisions.
In a conductor with a cross section S and length l
there are nSl electrons.
It follows from this that the
conductor during the time Δt heat is equal to:

Electric current in metals

This ratio expresses
Joule-Lenz law.
Thus, classical electronic
theory explains the existence
electrical resistance of metals,
Ohm and Joule-Lenz laws.
However, in a number of issues the classical
electronic theory leads to the conclusions,
in conflict with experience.

Electric current in metals

This theory cannot, for example, explain why
molar heat capacity of metals, as well as molar
heat capacity of dielectric crystals, equal to 3R,
where R is the universal gas constant (law
Dulong and Petit.)
Classical electron theory cannot
explain the temperature dependence of the specific
metal resistance.
Theory gives
while from experiment
the dependence ρ ~ T is obtained.
However, the most striking example of the discrepancy between theory and
experiments is superconductivity.

Electric current in metals

For some specific
temperature Tcr, different for different
substances, resistivity
jumps down to zero.
The critical temperature for mercury is
4.1 K, aluminum 1.2 K, tin 3.7 K.
Superconductivity is 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 passing through
low temperatures into the superconducting
condition, are not conductors
at normal temperatures.
At the same time, such "good"
conductors like copper and silver
become superconductors when
low temperatures.

Electric current in metals

Substances in the superconducting
able to possess
exceptional properties.
Almost the most important
them is the ability
long time (many years)
maintain without fading
electric current excited in
superconducting circuit.

Electric current in metals

Classical electron theory is not
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
materials with higher
critical temperatures.

Electric current in metals

A significant step in this direction has been
1986, when it was discovered that one complex
ceramic compound Tcr = 35 K.
Already in the next 1987, physicists managed to create
new ceramics with a critical temperature of 98 K,
exceeding the temperature of liquid nitrogen (77 K).
The phenomenon of the transition of substances into the superconducting
state at temperatures above temperature
boiling liquid nitrogen, was called
high temperature superconductivity.
In 1988, a ceramic compound was created on
based on Tl–Ca–Ba–Cu–O elements with a critical
temperature 125 K.
It should be noted that so far the mechanism
high temperature superconductivity ceramic
material has not yet been fully elucidated.

1.
2.
3.
4.
The qualitative difference between semiconductors and
metals.
Electron-hole mechanism
conductivity of pure pure
semiconductors.
Electronic and hole conductivity
impurity semiconductors. Donor and
acceptor impurities.
Electron-hole transition.
semiconductor diode. Transistor.

Electric current in semiconductors

The semiconductors are
many chemical elements (germanium,
silicon, selenium, tellurium, arsenic, etc.),
a wide range of alloys and
chemical compounds.
Almost all inorganic substances
the world around us -
semiconductors.
Most common in nature
silicon is a semiconductor
making up about 30% of the earth's crust.

Electric current in semiconductors

Qualitative difference
semiconductors from metals
manifests itself primarily in
specific
temperature resistance.

Electric current in semiconductors

Such behavior of the dependence ρ(T) shows that
that semiconductors have a concentration
no free charge carriers
remains constant but increases with
rise in temperature.
Let us consider this mechanism qualitatively.
on the example of germanium (Ge).
In a silicon (Si) crystal, the mechanism
similar.

Electric current in semiconductors

germanium atoms have four weakly
bound electron on the outer shell.
They are called valence electrons.
In a crystal lattice, each atom
surrounded by four nearest neighbors.
Bond between atoms in germanium crystal
is covalent, i.e. carried out
pairs of valence electrons.
Each valence electron belongs to two
atoms.

Electric current in semiconductors

Valence electrons in a germanium crystal
are much more strongly bonded 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
germanium crystal all electrons are occupied in
formation of connections.
Such a crystal of electric current is not
conducts.

Electric current in semiconductors

Pair-electronic bonds in a crystal
germanium and the formation of an electron-hole pair.

Electric current in semiconductors

As the temperature rises, some
some of the valence electrons
get enough energy to
breaking covalent bonds.
Then free crystals will appear in the crystal.
electrons (conduction electrons).
At the same time, in the places where the ties are broken
there are vacancies that are not filled
electrons.
These vacancies are called
"holes".

Electric current in semiconductors

The vacancy may be filled
valence electron from the neighboring
pairs, then the hole will move to
a new place in the crystal.
If a semiconductor is placed in
electric field, then into an ordered
movement involves not only
free electrons, but also holes,
who behave positively
charged particles.

Electric current in semiconductors

Therefore, the current I in the semiconductor
is made up 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
semiconductor conductivity.

Electric current in semiconductors

In the presence of impurities
electrical conductivity of semiconductors
changes greatly.
For example, the addition of phosphorus impurities to
silicon crystal in the amount of 0.001
atomic percentage reduces the specific
resistance more than five
orders.
Such a strong influence of impurities can
be explained on the basis of the
above the concept of the structure
semiconductors.

Electric current in semiconductors

A necessary condition for a sharp
decrease in resistivity
semiconductor with the introduction of impurities
is the difference in the valency of atoms
impurities from the valency of the main
crystal atoms.
Conductivity of semiconductors at
the presence of impurities is called
impurity conductivity.

Electric current in semiconductors

There are two types of impurities
conductivity - electronic and
hole conductivity.
Electronic conductivity
occurs when the crystal
germanium with tetravalent
atoms introduced pentavalent
atoms (for example, arsenic atoms,
As).

Electric current in semiconductors

Four valence electrons of an arsenic atom
included in the formation of covalent bonds with
four adjacent germanium atoms.
The fifth valence electron turned out to be redundant.
It easily detaches from the arsenic atom and
becomes free.
An atom that has lost an electron becomes
a positive ion located at a node
crystal lattice.

Electric current in semiconductors

An admixture of atoms with valency,
exceeding the valency of the main atoms
semiconductor crystal is called
donor impurity.
As a result of its introduction in the crystal
there is a significant number of free
electrons.
This leads to a sharp decrease in the specific
semiconductor resistance - in thousands and
even millions of times.
Conductor resistivity with
high content of impurities
approach resistivity
metal conductor.

Electric current in semiconductors

Such a conductivity
conditioned by free
electrons is called
electronic, and a semiconductor,
possessing an electronic
conductivity is called
n-type semiconductor.

Electric current in semiconductors

Hole conduction occurs when
germanium crystal introduced trivalent
atoms (for example, atoms of indium, In).

Electric current in semiconductors

On fig. shows the indium atom that was created with
with their valence electrons
covalent bonds with only three neighbors
germanium atoms.
To form a bond with the fourth atom
germanium has no electron in the indium atom.
This missing electron could be
captured by an indium atom from a covalent bond
neighboring germanium atoms.
In this case, the indium atom becomes
a negative ion located at a node
crystal lattice, and in the covalent
bonds of neighboring atoms, a vacancy is formed.

Electric current in semiconductors

An admixture of atoms capable of capturing
electrons is called an acceptor
impurity.

As a result of the introduction of an acceptor impurity into
the crystal is torn apart by many covalent
bonds and vacancies (holes) are formed.
Electrons can jump to these places from
adjacent covalent bonds, resulting in
chaotic wandering of holes in the crystal.

Electric current in semiconductors

The concentration of holes in a semiconductor with
acceptor impurity
exceeds the concentration of electrons
arose due to the mechanism of its own
semiconductor electrical conductivity: np >> nn.
This type of conduction is called
hole conduction.
Impurity semiconductor with a 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 hole
conductivity in reality
due to relay race
by vacancies from one atom of germanium to
other electrons, which
carry out a covalent bond.
For n- and p-type semiconductors, the law
Ohm is performed in certain
intervals of current and voltage at
condition of constancy of concentrations
free carriers.

In modern electronic technology
semiconductor devices play
exceptional role.
Over the past three decades, they have
completely replaced electrovacuum
appliances.
Every semiconductor device has
one or more electron-hole
transitions.
An electron-hole junction (or n-p junction) is the area of contact between two
semiconductors with different types
conductivity.

Electron-hole transition. Transistor

When two semiconductors n- and
p-types, the diffusion process begins:
holes from the p-region go 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 arises negatively
charged layer.

Electron-hole transition. Transistor

Thus, at the boundary of semiconductors
an electrical double layer is formed
whose electric field prevents
the process of diffusion of electrons and holes
towards each other

Electron-hole transition. Transistor

n–p junction has an amazing
property of one-sided
conductivity.
If a semiconductor with an n–p junction
connected to a power source so that
source positive pole
connected to the n-region, and
negative - with a p-region, then
field strength in the barrier layer
increases.

Electron-hole transition. Transistor

Holes in the p-region and electrons in the n-region will be displaced from the n-p junction, thereby increasing
concentration of minor carriers in
locking layer.
The current through the n–p junction is practically not
goes.
The voltage applied to the n–p junction in
in this case is called the reverse.

Electron-hole transition. Transistor

A very slight reverse
current is due only to its own
conductivity
semiconductor materials,
i.e. the presence of a small
free
electrons in the p-region and holes in
n-region.

Electron-hole transition. Transistor

If the n–p junction is connected to
source so that positive
the source pole was connected to the p-region, and the negative one to the n-region, then the tension
electric field in the barrier layer
will decrease, making it easier
transition of the main carriers through
contact layer.

Electron-hole transition. Transistor

Holes from the p-region and electrons from
n-region, moving towards each other
friend, will cross the n-p junction, creating a current in the forward
direction.
The current through the n–p junction in this
case will increase with
increasing the source voltage.

Electron-hole transition. Transistor

The ability of the n–p junction to pass
current in almost only one
direction is used in instruments,
which are called
semiconductor diodes.
Semiconductor diodes
made from silicon crystals
or Germany.
In their manufacture, a crystal with some type of conductivity is melted
impurity providing another type
conductivity.

Electron-hole transition. Transistor

Typical volt-ampere
silicon diode characteristic

Electron-hole transition. Transistor

Semiconductors not with
one, 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 transistor
p-n-p-type is
a small plate of germanium
with a donor impurity, i.e. from
n-type semiconductor.
This disc contains two
areas with an acceptor impurity,
i.e., regions with a hole
conductivity.

Electron-hole transition. Transistor

In an n-p-n-type transistor, the main
germanium plate has
p-type conductivity, and created on
it has two regions - with n-type conductivity.
The plate of the transistor is called the base.
(B), one of the areas with
opposite type of conduction
- 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 called
conductive media in which
flow of electric current
accompanied by a transfer
substances.
Carriers of free charges in
electrolytes are
positive and negative
charged ions.

Electric current in electrolytes

Key Representatives
electrolytes widely used in
technique, are aqueous solutions
inorganic acids, salts and
grounds.
The passage of electric current through
electrolyte is accompanied by the release
substances on the electrodes.
This phenomenon has been named
electrolysis.

Electric current in electrolytes

Electric current in electrolytes
represents 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 CuCl2
dissociates in aqueous solution
copper and chloride ions:
When connecting electrodes to
current source ions under the action
electric field start
orderly movement:
positive copper ions move towards
cathode, and negatively charged
chloride ions - to the anode.

Electric current in electrolytes

Upon reaching the cathode, copper ions are neutralized
excess electrons of the cathode and
become neutral atoms
deposited on the cathode.
Chlorine ions, having reached the anode, give but
one electron.
After that, neutral chlorine atoms
join together 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 experimentally
established by the English physicist M. Faraday in
1833.
Faraday's law determines the quantities
primary products that stand out on
electrodes during electrolysis:
Mass m of the 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

The mass of the substance released on the electrode
is equal to the mass of all ions that came to
electrode:
Here m0 and q0 are the mass and charge of one ion,
is the number of ions that came to the electrode at
charge Q passing through the electrolyte.
So the electrochemical equivalent
k is equal to the ratio of the mass m0 of the ion of the given
substance to its charge q0.

Electric current in electrolytes

Since the charge of an ion is equal to the product
valency of substance n on
elementary charge e (q0 = ne), then
expression for electrochemical
equivalent k can be written as:
F = eNA is Faraday's constant.
F = eNA = 96485 C / mol.

Electric current in electrolytes

Faraday's constant numerically
equal to the charge required
pass through an electrolyte
discharge on the electrode of one
mole of a monovalent substance.
Faraday's law for electrolysis
takes the form:

test questions

1.
2.
3.
4.
5.
6.
Charge carriers in metals.
Brief information about the classical theory
conductivity of metals (Drude-Lorentz theory).
Ohm's law from classical theory (brief
output).
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.

test questions

1.
2.
3.
4.
5.
6.
7.
8.
Electrons and holes. How are they formed in pure
semiconductors?
Mechanism of conduction in pure semiconductors.
Donor and acceptor semiconductors.
Mechanism of conduction in impurity semiconductors.
How to carry out electron and hole
conductivity in semiconductors.
What is an electron-hole transition?
Explain why the electron-hole transition
can rectify alternating current.
Transistor.

test questions

What charge carriers are in
electrolytes?
2. What are electrolytes? What's happened
electrolytic dissociation?
3. Faraday's law for electrolysis.
4. The combined law of electrolysis
Faraday.

As noted in the last chapter, metals are the most common medium that conducts electric current. And charge carriers are free electrons. In this regard, there is a special terminology, according to which the conductivity of metals is called electronic conductivity, and the electrons of the metal themselves are called conduction electrons.

This fact has not been postulated in any way, but has been verified and proven independently by many scientists using different methods. For example, the German physicist Carl Rikke conducted an experiment on passing a current of 0.1 A for a year through three polished cylinders: one aluminum and two copper. At the end of the experiment (during this time, a huge charge β passed through the circuit), no changes occurred in the structure of the cylinders, except for a small diffusion (Fig. 1). And if the charge carriers were not electrons, but ions, then there would be a transfer of the substance of one cylinder to the substance of another, and, of course, as a result of such a long experiment, the chemical structure of the cylinders would change.

Rice. 1. Scheme of the Rikke experiment

Another experiment to confirm the electronic conductivity of metals was the 1912 experiment of the Russian scientists Mangelshtam and Papaleksi, after a short time also carried out by the British Stuart and Tolman. During this experiment, a coil with a large number of turns rotated rapidly, and then abruptly braked. As a result, a galvanometer closed with it in a circuit showed the presence of a small current (Fig. 2).

Rice. 2. Scheme of the Mangelshtam-Papaleksi experiment

The fact is that along with the untwisted coil, of course, the electrons in the metal also rotate. When the coil is decelerated, the electrons continue to move inside the coil by inertia for some time, thus producing a current.

Superconductivity

Definition. Superconductivity is a phenomenon when the resistance of a conductor becomes close to zero.

The discovery of the phenomenon of superconductivity was preceded by the receipt in 1908 by the Dutchman Kamerling Onnes (Fig. 4) of liquid helium. By placing a conductor sample in liquid helium, it became possible to observe the behavior of conductors at ultralow temperatures (close to 0 ). And in 1911, Onnes found that mercury at a temperature of about 4 K sharply acquires a resistance equal to zero.

Rice. 4. Camerling Onnes ()

His experiments with mercury were preceded by experiments with platinum, as a result of which he found that the purer the substance (the less impurities it contains), the faster its resistance decreases with decreasing temperature. Due to the liquid state of mercury under normal conditions, this metal was very easy to clean from impurities. And the following dependence of the specific resistance of mercury on low temperatures was established: the linear decrease is interrupted by a jump to zero (Fig. 5):

Rice. five.

The phenomenon of superconductivity is explained from the point of view of quantum physics.

To estimate how many of those same conduction electrons are in a metal, you need to understand that each metal atom provides at least one free electron. On average, the concentration of conduction electrons is:

And as a model of the behavior of free electrons, one can take the model of a gas. Each electron of the electron gas behaves like a single gas molecule. When an external electric field appears, an ordered motion is superimposed on the chaotic motion of electrons. It is this movement that causes the electric current.

The most common effect of current is the thermal effect. As already noted in the last chapter, the mechanism of this action is the collision of electrons with the nodes of the crystal lattice, as a result of which the kinetic energy of the electrons is converted into the internal energy of the conductor.

In turn, having increased internal energy, the lattice nodes begin to oscillate faster, colliding with electrons more often. That is, the electrons are decelerated more efficiently. In other words, as the temperature of the conductor increases, its electrical resistance increases.

A simple experiment confirming this theoretical conclusion can be the heating of a conductor in a circuit with a switched on lamp and measuring instruments (see Fig. 3).

Rice. 3.

As the conductor warms up, both the lamp will begin to shine less brightly, and the devices will begin to show a drop in current strength.

After qualitative confirmation of the dependence of resistance on temperature, a quantitative dependence was obtained. After a series of experiments, it was found that the relative increase in resistance is directly proportional to the absolute increase in temperature:

Here: - resistance at a given temperature, - resistance at temperature ; - temperature change relative to ; - temperature coefficient of resistance. The temperature coefficient is a tabular value known for most metals. Coefficient dimension:

Since the linear dimensions of the conductors change slightly when the temperature changes, it means that the resistivity changes, and according to the same law:

Applications of superconductivity

The use of superconductivity greatly facilitates many technical aspects of the use of electric current. First, the absence of resistance means that there is no heat loss, which is typically 15% of the total energy. As confirmation, we can cite an experiment on a two-year passage of current through a conductor immersed in liquid helium, which was interrupted only due to a lack of helium. The absence of heating and energy losses on it is extremely important for electric motors and electronic computers.

In addition, due to the lack of resistance, extremely high currents flow in superconductors, creating strong magnetic fields, which can be used in thermonuclear fusion.

A household example of the use of superconductors is the currently existing railway network with magnetic cushion trains (Fig. 6):

Rice. 6. Magnetic levitation train

High temperature superconductors

After the discovery of superconductivity, Onnes, trying to create a superconducting electromagnet, discovered that changing the current, or magnetic fields, destroy the effect of superconductivity. It was not until the middle of the twentieth century that superconducting electromagnets were created.

Also, an extremely important discovery was made in 1986. Materials have been discovered that have superconductivity at temperatures around . Such temperatures can be obtained using liquid nitrogen, which is much cheaper than liquid helium. However, when trying to create such superconducting wires and cables, they encountered the problem of the extreme fragility of such materials, which crumble during rolling. At the moment, work is underway to solve this problem.

In the next lesson, we will look at electric current in semiconductors.

Bibliography

Tikhomirova S.A., Yavorsky B.M. Physics (basic level) - M.: Mnemozina, 2012.
Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.
Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics. - M.: 2010.

Storage.mstuca.ru ().
Physics.ru ().
Elements().

Homework

WHAT IS ELECTRIC CURRENT IN METALS?

Electric current in metals - it is the ordered movement of electrons under the action of an electric field. Experiments show that when current flows through a metal conductor, there is no transfer of matter, therefore, metal ions do not take part in the transfer of electric charge.

NATURE OF ELECTRIC CURRENT IN METALS

Electric current in metal conductors does not cause any changes in these conductors, except for their heating.

The concentration of conduction electrons in a metal is very high: in order of magnitude it is equal to the number of atoms per unit volume of the metal. Electrons in metals are in constant motion. Their random motion resembles the motion of ideal gas molecules. This gave reason to believe that electrons in metals form a kind of electron gas. But the speed of the random movement of electrons in a metal is much greater than the speed of molecules in a gas.

E.RIKKE EXPERIENCE

The German physicist Carl Rikke conducted an experiment in which an electric current passed for a year through three polished cylinders pressed against each other - copper, aluminum and again copper. After completion, it was found that there are only minor traces of mutual penetration of metals, which do not exceed the results of ordinary diffusion of atoms in solids. Measurements carried out with a high degree of accuracy showed that the mass of each of the cylinders 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. It is natural to assume that it is free electrons that carry out the current in metals.

Carl Victor Eduard Rikke

EXPERIENCE L.I. MANDELSHTAMA and N.D. PAPALEKSI

Russian scientists L. I. Mandelstam and N. D. Papaleksi in 1913 staged an original experiment. The coil with the wire began to twist in different directions. Unwind, clockwise, then abruptly stop and - 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. And so it happened. We connected a telephone to the ends of the wire and heard a sound, which meant that current was flowing through it.

Mandelstam Leonid Isaakovich

Nikolai Dmitrievich Papalexy (1880-1947)

THE EXPERIENCE OF T. STUART AND R. TOLMAN

The experience of Mandelstam and Papaleksi was repeated in 1916 by the American scientists Tolman and Stuart.

A coil with a large number of turns of thin wire was brought into rapid rotation around its axis. The ends of the coil were connected with flexible wires to a sensitive ballistic galvanometer. The untwisted coil was sharply decelerated, 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.

Butler Stuart Thomas

Richard Chase Tolman

CLASSICAL ELECTRONIC THEORY

The assumption that electrons are responsible for the electric current in metals existed even before the experiment of Stewart and Tolman. In 1900, the German scientist P. Drude, based on the hypothesis of the existence of free electrons in metals, created his electronic theory of the conductivity of metals, named after classical electronic theory . According to this theory, electrons in metals behave like an electron gas, much like an ideal gas. It fills the space between the ions that form the crystal lattice of the metal

The figure shows the trajectory of one of the free electrons in the crystal lattice of a metal

MAIN PROVISIONS OF THE THEORY:

The presence of a large number of electrons in metals contributes to their good conductivity.
Under the action of an external electric field, an ordered motion is superimposed on the random motion of electrons, i.e. current occurs.
The strength of the electric current flowing through a metal conductor is:
Since the internal structure of different substances is different, the resistance will also be different.
With an increase in the chaotic motion of particles of a substance, the body is heated, i.e. heat release. Here the Joule-Lenz law is observed:

l \u003d e * n * S * Ū d

SUPERCONDUCTIVITY OF METALS AND ALLOYS

Some metals and alloys possess superconductivity, the property of having strictly zero electrical resistance when they reach a temperature below a certain value (critical temperature).

The phenomenon of superconductivity was discovered by the Dutch physicist H. Kamerling - Ohness in 1911 in mercury (T cr = 4.2 o K).

ELECTRIC CURRENT APPLICATION:

receiving strong magnetic fields
transmission of electricity from source to consumer
powerful electromagnets with superconducting winding in generators, electric motors and accelerators, in heating devices

Currently, there is a big problem in the energy sector associated with large losses during the transmission of electricity through wires.

Possible solution to the problem:

Construction of additional transmission lines - replacement of wires with large cross-sections - voltage increase - phase splitting

Class: 11

Presentation for the lesson

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Attention! The slide preview is for informational purposes only and may not represent the full extent of the presentation. If you are interested in this work, please download the full version.

Lesson Objectives:

To reveal the concept of the physical nature of the electric current in metals, experimental confirmation of the electronic theory;

Continue the formation of natural scientific ideas on the topic under study

Create conditions for the formation of cognitive interest, student activity

Formation of skills;

Formation of communicative communication.

Equipment: interactive complex SMART Board Notebook, local area network of computers, Internet.

Lesson teaching method: combined.

Epigraph of the lesson:

Strive to comprehend science ever deeper,
Longing for the knowledge of the eternal.
Only the first knowledge will shine on you light,
You will know: there is no limit to knowledge.

Ferdowsi
(Persian and Tajik poet, 940-1030)

Lesson plan.

I. Organizing moment

II. Group work

III. Discussion of the results, installation of the presentation

IV. Reflection

V. Homework

During the classes

Hello guys! Sit down. Today we will work in groups.

Tasks for groups:

I. Physical nature of charges in metals.

II. K. Rikke's experience.

III. Experience of Stuart, Tolman. Experience of Mandelstam, Papaleksi.

IV. Drude theory.

V. Volt-ampere characteristic of metals. Ohm's law.

VI. The dependence of the resistance of conductors on temperature.

VII. Superconductivity.

1. Electrical conductivity is the ability of substances to conduct an 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. For each substance under given conditions, a certain dependence of the current strength on the potential difference is characteristic.

According to the resistivity of a substance, it is customary to divide it into:

A) conductors (p< 10 -2 Ом*м)

B) dielectrics (p\u003e 10 -8 Ohm * m)

C) semiconductors (10 -2 Ohm * m> p> 10 -8 Ohm * m)

However, such a division is conditional, because under the influence of a number of factors (heating, irradiation, impurities), the resistivity of substances and their volt-ampere characteristics change, and sometimes very significantly.

3. Carriers of free charges in metals are electrons. Proven by classical experiments K. Rikke (1901) - German physicist; L.I. Mandelstam and N. D. Papaleksi (1913) - our compatriots; T. Stewart and R. Tolman (1916) - American physicists.

K. Rikke's experience

Rikke folded three pre-weighted cylinders (two copper and one aluminum) with polished ends so that the aluminum one was between the copper ones. Then the cylinders were connected to a DC circuit: a large current passed through them during the 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 are only minor traces of penetration of metals, which do not exceed the results of ordinary diffusion of atoms in solids. The results of the experiment indicated that ions do not participate in charge transfer in metals.

L.I. Mandelstam

N. . Papalexy

Experience of L. I. Mandelstam and N. D. Papaleksi

Russian scientists L. I. Mandelstam (1879-1949; founder of the school of radio physicists) and N. D. Papaleksi (1880-1947; the largest Soviet physicist, academician, chairman of the All-Union Scientific Council for Radio Physics and Radio Engineering under the Academy of Sciences of the USSR) in 1913 delivered the original an experience. They took a coil of wire and began to twist it in different directions.

Unwind, for example, clockwise, then abruptly stop and - 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 through a wire is an electric current. As planned, so it happened. We connected a telephone to the ends of the wire and heard a sound. Once a sound is heard in the phone, therefore, current flows through it.

T. Stewart

The 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 the galvanometer by means of sliding contacts. If the coil, which is in rapid rotation, is braked sharply, then the free electrons in the wire will continue to move by inertia, as a result of which the galvanometer must 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 kinetic theory, are identical solid spheres that move in straight lines until they collide with each other. It is assumed that the duration of a single collision is negligible, and that no other forces act between the molecules, except those arising at the moment of the collision. Since an electron is a negatively charged particle, then in order to comply with the condition of electrical neutrality in a solid, there must also be particles of a different kind - positively charged. Drude suggested that the compensating positive charge belongs to much heavier particles (ions), which he considered immobile. At the time of Drude, it was not clear why there are free electrons and positively charged ions in the metal, and what these ions are. Only the quantum theory of solids could give answers to these questions. For many substances, however, one can simply assume that the electron gas consists of external valence electrons weakly bound to the nucleus, which are "liberated" in the metal and are able to move freely through the metal, while atomic nuclei with electrons of inner shells (atomic cores) remain unchanged. and play the role of fixed 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 randomly around the ions.

Fundamentals of the electronic theory of conductivity of metals.

A metal can be described by the following model: the 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 almost does not depend on temperature.
Free electrons in metals are in continuous chaotic motion.
An electric current in a metal is formed only due to the ordered movement of free electrons.
Colliding with ions vibrating at the nodes of the crystal lattice, electrons give them excess energy. This is why conductors heat up when current flows.

Electric current in metals.

Superconductivity

The phenomenon of reducing resistivity to zero at a temperature other than absolute zero is called superconductivity. Materials that exhibit the ability to pass at certain temperatures other than absolute zero into a superconducting state are called superconductors.

The passage of current in a superconductor occurs without energy loss, therefore, once excited in a superconducting ring, an electric current can exist indefinitely without change.

Superconducting materials are already being used in electromagnets. Research is underway to create superconducting power lines.

The application of the phenomenon of superconductivity in wide practice may become a reality in the coming years due to the discovery in 1986 of the superconductivity of ceramics - compounds of lanthanum, barium, copper and oxygen. The superconductivity of such ceramics is retained up to temperatures of about 100 K.

Well done boys! They did an excellent job. It turned out to be a good presentation. Thank you for the lesson!

Literature.

Gorbushin Sh.A. Reference notes for the study of physics for the course of the secondary school. - Izhevsk "Udmurtia", 1992.
Lanina I.Ya. Formation of cognitive interests of students in physics lessons: A book for teachers. – M.: Enlightenment, 1985.
Physics lesson in modern school. Creative search for teachers: A book for teachers / Comp. E.M. Braverman / Edited by V.G. Razumovsky.- M.: Enlightenment, 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 .: Knowledge, 1986. (Life of wonderful ideas).