Formula Operation of Electron Output
In metals there are electrons of conductivity forming electronic gas and participating in thermal motion. Since the conductivity electrons are held inside the metal, then, therefore, near the surface there are forces acting on electrons and directed inside the metal. So that the electron can get out of the metal beyond its limits, a certain work must be made and against these forces, which was named electron Output Metal. This work is naturally different for different metals.
The potential electron energy inside the metal is constant and is equal to:
W p \u003d -eφ , where j is the potential of the electric field inside the metal.
When the electron is transitioned through the surface electronic layer, the potential energy is rapidly reduced by the value of the output and becomes outside the metal equal to zero. The distribution of electron energy inside the metal can be represented as a potential well.
In the interpretation examined above, the operation of an electron output is equal to the depth of the potential pit, i.e.
A out \u003d Eφ
This result corresponds to the classic electronic theory of metals, in which the electron velocity in the metal is assumed to the law of the Maxwell distribution and at an absolute zero temperature is zero. However, in reality, the conduction electrons are subject to quantum statistics of Fermi Dirak, according to which, with absolute zero, the electron speed and their energy is different from zero, accordingly.
The maximum value of the energy that electrons is posses with absolute zero is called the EF Fermi energy. The quantum theory of metals, based on this statistics, gives a different interpretation of the output. Electron Output Metal is equal to the difference in the height of the potential barrier Eφ and the Energy of Fermi.
A out \u003d Eφ "- E F
where φ "is the average value of the electric field potential inside the metal.
Table Operation of electrons from simple substances
The table shows the values \u200b\u200bof the output of electrons relating to polycrystalline samples, the surface of which is purified in vacuum by calcining or mechanical processing. Insufficiently reliable data enclosed in brackets.
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Substance |
Formula of substances |
Electron Output Work (W, EV) |
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aluminum |
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beryllium |
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carbon (Graphite) |
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germanium |
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manganese |
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molybdenum |
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palladium |
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praseodymium |
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tin (γ-form) |
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tin (β-form) |
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strontium |
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tungsten |
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zirconium |
In metals there are electrons of conductivity forming electronic gas and participating in thermal motion. Since the conductivity electrons are held inside the metal, then, therefore, near the surface there are forces acting on electrons and directed inside the metal. In order for the electron to come out of the metal beyond its limits, a certain work must be made and against these forces, which was called the Operation of Electron Output from Metal. This work is naturally different for different metals.
The potential electron energy inside the metal is constant and equal:
WP \u003d -Eφ, where j is the potential of the electric field inside the metal.
21. Contact difference potentials - This is the difference in potentials between the conductors, which occurs when there are two different conductors with the same temperature.
When contacting two conductors with different exit work on the conductors, electrical charges appear. And between their free ends there is a difference in potentials. The potential difference between points outside the conductors, near their surface is called the contact difference of potentials. Since the conductors are at the same temperature, then in the absence of the applied voltage, the field can exist only in the boundary layers (Volta rule). There are internal potential difference (when metals) and external (in the gap). The value of the external contact difference in potentials is equal to the difference in the work of the exit referred to the charge of the electron. If the conductors are connected to the ring then the EDC in the ring will be equal to 0. For different pairs of metals, the value of the contact difference potential ranges from the tenths of Volta to the volt units.
The effect of the thermoelectrogenerator is based on the use of the thermoelectric effect, the essence of which is that when the connection site is heated (swaying), two different metals between them is free to ends having a lower temperature, the difference of potentials occurs, or the so-called thermoelectribute force (thermo-emf). If you close such a thermoelement (thermocouple) to external resistance, then the circuit will flow the electric current (Fig. 1). Thus, with thermoelectric phenomena, there is a direct transformation of thermal energy into electric.
The magnitude of the thermoelectribution force is determined approximately according to the formula E \u003d A (T1 - T2)
22. A magnetic field - a power field operating on moving electric charges and on the bodies with a magnetic moment, regardless of the state of their movement; Magnetic component of the electromagnetic field
Moving charge q., creates a magnetic field around itself, whose induction
where - the electron speed is the distance from the electron to the field of the field, μ - relative magnetic permeability of the medium, μ 0 = 4π · 10 -7 Gn / M. - Magnetic constant.
Magnetic induction - Vector magnitude that is the power characteristic of the magnetic field (its action on charged particles) at this point of space. Determines how power the magnetic field acts on the charge moving at speed.
More specifically, this is such a vector that the Lorentz power acting on the side of the magnetic field to the charge moving at speed is equal to
23. According to the law of Bio-Savara Laplace Element contour dLfor which current flows I., creates around itself a magnetic field, the induction of which at some point K.
where is the distance from the point K. to the current element dL, α - The angle between the radius-vector and the current element dL.
Vector direction can be found by rule Maxwell (Braschik): If screwed up a tap with right thread in the direction of current in the conductor element, then the direction of movement of the bouwn handle will indicate the direction of the magnetic induction vector.
Applying the Bio-Savara-Laplace law to the contours of various types, we get:
· In the center of the circular cooler radius R. with current strength I. magnetic induction
· Magnetic induction on the axis of the circular current 
Where a. - distance from the point in which it is looking for B. to a circular plane,
· Field created by an infinitely long conductor with a current at a distance r. From the conductor
· Field created by the conductor of the final length at a distance r.from the conductor (Fig. 15) 
· Field inside toroid or infinitely long solenoid n. - number of turns per unit length of the solenoid (toroid)
Magnetic induction vector is associated with magnetic field strength by the ratio
Volumetric density of energy Magnetic field:
25 . On a charged particle moving in a magnetic field with induction B. with speed υ , on the part of the magnetic field there is a power called force of Lorentz
and the module of this force is equal 
.
The direction of Lorentz power can be determined by relief of the left hand: If you put the left hand so that perpendicular to the speed component of the induction vector was in the palm, and four fingers would be located in the direction of the speed of movement of the positive charge (or against the direction of the negative charge rate), then the bent thumb point will indicate the direction of force Lorentz
26 .Principle of operation of cyclic accelerators of charged particles.
The independence of the rotation period T of the charged particle in the magnetic field was used by American scientist Lawrence in the idea of \u200b\u200bcyclotron - accelerator of charged particles.
Cyclotron It consists of two duangs D 1 and D 2 - hollow metal semi-cylinders placed in a high vacuum. The accelerating electric field is created in the gap between dangs. A charged particle getting into this gap increases the speed of movement and flies into the half-cylinder space (duant). Duangs are placed in a constant magnetic field, and the trajectory of the particle inside the duang will be curved around the circle. When the particle will enter the gap between dangs, the polarity of the electric field changes and it becomes accelerating again. An increase in speed is accompanied by an increase in the radius of the trajectory. Variable field with a frequency ν \u003d 1 / T \u003d (b / 2π) (q / m) is applied to almost duants. The velocity of the particle is increased between duances under the action of the electric field.
27.Ampere power This is the force that acts on the conductor through which the current flows I.magnetic field
Δ l.- Explorer length, and direction coincides with the current direction in the conductor.
Ampere Power Module: 
.
Two parallel infinitely long rectilinear conductor with currents I 1. and I 2. interact with the power
where l. - the length of the conductor site, r. - Distance between the conductors.
28. Interaction of parallel currents - Ampere Law
Now it is easily possible to obtain a formula for calculating the interaction force of two parallel currents.
So, on two long direct parallel conductors (Fig. 440), which are at a distance of R from each other (which is many, once 15 less conductors' lengths), constant currents I 1, I 2 occur.
In accordance with the field theory, the interaction of the conductors is explained as follows: the electric current in the first conductor creates a magnetic field that interacts with the electric current in the second conductor. To explain the emergence of the force acting on the first conductor, the conductors "change roles" are necessary: \u200b\u200bthe second creates a field that acts on the first. Remove a mentally right screw, twist with your left hand (or use the vector product) and make sure that at currents of current in one direction, the conductors are attracted, and at currents current in opposite directions, the conductors are repelled1.
Thus, the force acting on the site with a length of the Δl of the second conductor is the power of the amper, it is equal to
where B1 is the induction of the magnetic field created by the first conductor. When recording this formula, it is taken into account that the B1 induction vector is perpendicular to the second conductor. Induction of the field created by direct current in the first conductor, at the location of the second, is equal
From formulas (1), (2) it follows that the force acting on the highlighted section of the second conductor is equal to
29. Tock with current in a magnetic field.
If it is not a conductor in the magnetic field, and the turn (or coil) with a current and position it vertically, applying the rule of the left hand to the upper and lower sides of the turn, we get that the electromagnetic forces f acting on them will be directed in different directions. As a result of these two forces, an electromagnetic torque M is arises, which will cause a turn of the turn, in this case clockwise. This moment
where D is the distance between the sides of the turn.
The turn will turn in a magnetic field until it occurs, perpendicular to the magnetic power lines of the field (Fig. 50, b). With this position through the round, the largest magnetic flux will be held. Consequently, the turn or coil with a current made into the outer magnetic field always strive to take such a position so that a larger magnetic flux is possible through the coil.
Magnetic moment, magnetic dipole moment - The main value characterizing the magnetic properties of the substance (the source of magnetism, according to the classical theory of electromagnetic phenomena, are electrical macro and microcurbs; a closed current is considered an elementary source of magnetism). Elementary particles, atomic nuclei, electronic shells of atoms and molecules have the magnetic moment. The magnetic moment of elementary particles (electrons, protons, neutrons and others), as the quantum mechanic showed, is due to the existence of their own mechanical moment - the back.
30. Magineic flow - Physical value equal to the density of streaming of power lines passing through an infinitely small DS platform. Flow F B. as an integral of magnetic induction vector AT Through the final surface S is determined through the integral over the surface.
31. Work on moving conductor with current in a magnetic field
Consider the circuit with a current formed by stationary wires and moving along it a movable jumper L (Fig. 2.17). This outline is in an external homogeneous magnetic field perpendicular to the contour plane.
On the current I (movable wire) Length L, the ampere force is directed to the right:
Let the conductor L move parallel to itself at the distance dx. At the same time work work:
da \u003d FDX \u003d IBLDX \u003d IBDS \u003d Idf
The work performed by the conductor with the current when moving is numerically equal to the product of the current on the magnetic flux crossed by this conductor.
The formula remains fair if the conductor of any shape moves at any angle to the magnetic induction vector lines.
32. Magnetization of matter . Permanent magnets can be made only from relatively few substances, but all substances placed in a magnetic field are made that they themselves become sources of the magnetic field. As a result, the magnetic induction vector in the presence of a substance differs from the magnetic induction vector in vacuo.
The magnetic moment of the atom is composed of orbital and eigenmaths of the electrons included in its composition, as well as from the magnetic moment of the nucleus (which is due to the magnetic moments of the kernel of elementary particles - protons and neutrons). The magnetic moment of the kernel is significantly less than the moments of electrons; Therefore, when considering many questions, they can be neglected and assured that the magnetic moment of the atom is equal to the vector sum of the magnetic moments of electrons. The magnetic moment of the molecule can also be considered an equal amount of magnetic moments of its electron components.
Thus, an atom is a complex magnetic system, and the magnetic moment of the atom as a whole is equal to the vector sum of the magnetic moments of all electrons
Magneticam And they call substances capable of magnifying in an external magnetic field, i.e. capable of creating their own magnetic field. The nearest field of substances depends on the magnetic properties of their atoms. In this sense, magnetics are magnetic analogues of dielectrics.
According to classical representations, an atom consists of electrons moving in orbits around a positively charged kernel, in turn, from protons and neutrons.
Magnetics are all substances, i.e. All substances are magnetized in an external magnetic field, but they have different magnetization degrees. Depending on this, all magnetics are divided into three types: 1) diamagnetics; 2) paramagnetics; 3) ferromagnetics.
Diamagnetics. - It includes many metals (for example, copper, zinc, silver, mercury, bismuth), most gases, phosphorus, sulfur, quartz, water, the vast majority of organic compounds, etc.
For diamagnetics, the following properties are characterized:
2) Own magnetic field is directed against external and slightly weakens it (M<1);
3) There are no residual magnetism (the diamagnet's own magnetic field disappears after removing the external field).
The first two properties suggest that the relative magnetic permeability of Madiamagnets is only slightly less than 1. For example, the strongest of diamagnets - bismuth - has \u003d 0.99824.
Paramagnetics - It includes alkaline and alkaline earth metals, aluminum, tungsten, platinum, oxygen, etc.
For paramagnets are characterized by the following properties:
1) very weak magnetization in an external magnetic field;
2) the own magnetic field is directed along the external and slightly enhance it (M\u003e 1);
3) There are no residual magnetism.
Of the first two properties it follows that the value is a little more than 1. For example, for one of the strongest paramagnets - platinum - relative magnetic permeabilityM \u003d 1,00036.
33.Ferromagnetics - It includes iron, nickel, cobalt, gadolinium, their alloys and compounds, as well as some alloys and manganese and chromium compounds with nonferromagnetic elements. All these substances have ferromagnetic properties only in the crystalline state.
For ferromagnets are characterized by the following properties:
1) very strong magnetization;
2) the own magnetic field is directed along the external and significantly enhances it (the values \u200b\u200bwill migrate from several hundred to several hundred thousand);
3) relative magnetic permeability is dependent on the magnitude of the magnetizing field;
4) There is residual magnetism.
Domain - macroscopic region in a magnetic crystal, in which the orientation of the vector of spontaneous homogeneous magnetization or vector of antiferromagnetism (at temperatures below the Curie or Neel point, respectively) defined - strictly orderedwise rotated or shifted, that is, polarized, relative to the directions of the corresponding vector in adjacent domains.
Domains are formations consisting of a huge number of [ordered] atoms and sometimes visible with a naked eye (the size of about 10-2 cm3).
Domains exist in ferro and antiferromagnetic, ferroelectric crystals and other substances with spontaneous distant order.
Curie point, or Curie's temperature, - The phase transition temperature of the genus of the genus associated with a jump-like change in the properties of symmetry of the substance (for example, magnetic - in ferromagnets, electrical - in ferroelectrics, crystalochemical - in ordered alloys). Named named P. Curie. At temperatures t below the point of Curie Q ferromagnets have spontaneous (spontaneous) magnetization and a certain magnetic crystalline symmetry. At the point of Curi (T \u003d Q), the intensity of the thermal motion of the ferromagnet atoms is sufficient to destroy its spontaneous magnetization ("magnetic order") and changes in symmetry, as a result, the ferromagnet becomes a paramagnet. Similarly, antiferromagnets at t \u003d q (in the so-called antiferromagnetic point of Curie or Point of Neel), it is destroyed by the characteristic magnetic structure (magnetic centers), and antiferromagnets become paramagnets. In ferroelectrics and anti-seepoelectrics at T \u003d q, the thermal motion of atoms reduces the spontaneous ordered orientation of electric dipoles of the elementary cells of the crystal lattice. In ordered alloys at the point of Curi (it is called in the case of alloys also a point.
Magnetic hysteresis It is observed in magnetically ordered substances (at a certain temperature range), for example, in ferromagnets, usually broken into domains of spontaneous (spontaneous) magnetization, in which the magnetization value (the magnetic moment of the volume unit) is the same, but the directions of various.
Under the action of an external magnetic field, the number and size of domains magnetized by field increases at the expense of other domains. The magnetization vectors of individual domains can be rotated through the field. In a sufficiently strong magnetic field, the ferromagnetic is magnetized to saturation, and it consists of one domain with the magnetization of the saturation JS, directed along the external field H.
A typical dependence of magnetization from a magnetic field in the case of hysteresis
34. Magnetic field of land
As is known, the magnetic field is a special type of power field that affects the bodies with magnetic properties, as well as on moving electrical charges. To a certain extent, the magnetic field can be considered a special kind of matter, which transmits information between electrical charges and bodies with a magnetic moment. Accordingly, the Earth's magnetic field is such a magnetic field, which is created at the expense of factors associated with the functional features of our planet. That is, the geomagnetic field is created by the earthly itself, and not by external sources, although the latter have a certain impact on the magnetic field.
Thus, the properties of the magnetic field of the Earth inevitably depend on the characteristics of its origin. The main theory explaining the occurrence of this power field is due to the current in the liquid metal core of the planet (the temperature of the nucleus is so high that metals are in a liquid state). The energy of the magnetic field of the Earth is generated by the so-called hydromagnetic dynamo mechanism, which is due to multidirectionality and an asymmetry of electrical currents. They generate an increase in electrical discharges, which leads to the release of thermal energy and the emergence of new magnetic fields. It is curious that the mechanism of hydromagnetic dynamo has the ability to "self-excitation", that is, active electrical activity inside the globe constantly generates a geomagnetic field without external influence.
35. Magnetization - vector physical quantity characterizing the magnetic state of the macroscopic physical body. Menu is usually defined as the magnetic moment of a unit of volume of the substance:
Here, M is the magnetization vector; - vector magnetic moment; V - volume.
In general, the case of inhomogeneous, for one or another reasons, environment) magnetization is expressed as
and is the function of coordinates. Where there is a total magnetic moment of molecules in the volume of DV Communication between M and the tension of the magnetic field H in diamagnetic and paramagnetic materials, usually linear (at least, with not too large magnetizing fields):
where χm is called magnetic susceptibility. In ferromagnetic materials there is no unambiguous communication between M and H due to the magnetic hysteresis and to describe the dependence use magnetic susceptibility tensor.
Magnetic field tension (Standard designation H) - vector physical value equal to the difference in the magnetic induction vector B and magnetization vector M.
In the international system of units (C): H \u003d (1 / μ 0) b - m where μ 0 is a magnetic constant.
Magnetic permeability - physical quantity, coefficient (depending on the properties of the medium), which characterizes the relationship between magnetic induction B and the voltage of the magnetic field H in the substance. For different media, this coefficient is different, therefore, they speak about the magnetic permeability of the specific medium (implying its composition, condition, temperature, etc.).
It is usually indicated by the Greek letter μ. It can be both a scalar (in isotropic substances) and the tensor (in anisotropic).
In general, the relationship between the magnetic induction and the magnetic field strength through magnetic permeability is entered as
and in general, here should be understood as a tensor that in the component record
Consider the situation: the charge Q 0 enters the electrostatic field. This electrostatic field is also created by some charged body or body bodies, but it does not interest us. For the charge Q 0, the field is the power that can work and move this charge in the field.
The operation of the electrostatic field does not depend on the trajectory. The field operation when moving the charge along a closed trajectory is zero. For this reason, the power of the electrostatic field is called conservative, and the field itself is called potential.
Potential
The system "charge is an electrostatic field" or "charge - charge" has potential energy, just as the system "gravitational field - the body" has potential energy.
The physical scalar value characterizing the energy state of the field is called potential This point of the field. The box is placed in the field Q, it has the potential energy W. The potential is the characteristic of the electrostatic field.
Recall the potential energy in the mechanics. Potential energy is zero when the body is on Earth. And when the body raises some height, they say that the body has potential energy.
Regarding potential energy in electricity, there is no zero level of potential energy. It is chosen arbitrarily. Therefore, the potential is a relative physical value.
In body mechanics, they seek to take a position with the smallest potential energy. In the electricity, under the action of the field forces, a positively charged body tends to move from a point with a higher potential to a point with a lower potential, and a negatively charged body - on the contrary.
The potential energy of the field is the work that electrostatic force performs when the charge is moved from this point of the field to a point with zero potential.
Consider a particular case when the electrostatic field is created by an electrical charge Q. To study the potential of such a field there is no need to make a charge Q. You can calculate the potential of any point of such a field located at a distance of R from charge Q.
The dielectric permeability of the medium has a known value (tabular), characterizes the medium in which there is a field. For air it is equal to one.
Potential difference
The operation of the field to move the charge from one point to another, is called the difference in potentials
This formula can be represented in another form.
Equipotential surface (line) - The surface of equal potential. The work on the movement of the charge along the equipotential surface is zero.
Voltage
The difference of potentials is called yet electric voltage Provided that third-party forces do not act or their action can be neglected.
The voltage between two points in a homogeneous electric field located on one line of tension is equal to the product of the field strength vector module for the distance between these points.
The current depends on the voltage in the circuit and the energy of the charged particle.
Superposition principle
The potential of the field created by several charges is equal to algebraic (taking into account the potential sign) the sum of the potentials of the fields of each field separately
When solving problems, many confusion arises when determining the potential sign, potential difference, work.
The figure shows the line of tension. At what point the field the potential is greater?
The right answer is point 1. Recall that the tension lines begin on a positive charge, which means a positive charge is on the left, therefore the maximum potential has an extreme left point.
If a field study is occurring, which is created by a negative charge, the field potential near charge has a negative value, it is easy to make sure that in the formula to substitute the charge with the "minus" sign. The farther from the negative charge, the fact the potential of the field is more.
If a positive charge is moving along tension lines, the potential difference and work are positive. If the negative charge is moving along lines of tension, the potential difference has a "+" sign, the work has a sign "-".
Conducting electrons do not leave spontaneously metal in a noticeable amount. This is explained by the fact that the metal is a potential pit for them. Leave the metal is possible only to those electrons whose energy is sufficient to overcome the potential barrier existing on the surface. Forces caused by this barrier have the following origin. Random removal of the electron from the outer layer of positive lattice ions leads to the occurrence in the place that the electron leaves, excessive positive charge.
Coulomb interaction with this charge causes an electron, whose speed is not very high, return back. Thus, individual electrons all the time leave the metal surface, removed from it into several interatomic distances and then rotate back. As a result, the metal is surrounded by a subtle cloud of electrons. This cloud forms together with the outer layer of ions by a double electric layer (Fig. 60.1; mugs - ions, black points - electrons). The forces acting on the electron in such a layer are directed inside the metal.
The work performed against these forces when translating an electron from a metal outward, will come to an increase in the potential energy of the electron
Thus, the potential energy of valence electrons inside the metal is less than outside the metal, by an amount equal to the depth of the potential pit (Fig. 60.2). The change in energy occurs at a length of the order of several interatomic distances, so the walls of the pits can be considered vertical.
The potential energy of the electron and the potential of the point in which the electron is located, have opposite signs. It follows that the potential inside the metal is greater than the potential in the immediate vicinity of its surface (we will simply talk for brevity "on the surface"), by magnitude
Message Metal of an excessive charge increases the potential both on the surface and inside the metal. The potential electron energy is respectively reduced (Fig. 60.3, a).
Recall that the principal and potential energy at infinity is taken for the beginning of the reference. The message of the negative charge lowers the potential inside and outside the metal. Accordingly, the potential energy of the electron increases (Fig. 60.3, b).
The total energy of the electron in the metal is composed of potential and kinetic energies. In § 51, it was found that with an absolute zero, the values \u200b\u200bof the kinetic energy of the conductivity electrons were in the range from zero to the coinciding with the energy level of ETHE. In fig. 60.4 Energy levels of conduction zone are inscribed in a potential pit (dotted line depicted with levels unoccupied). To remove from metal, different electrons need to be indicated not the same energy.
So, the electron located at the lowest level of the conduction zone, it is necessary to inform the energy for the Fermi's electron, the energy is sufficient.
The smallest energy that the electron needs to be informed in order to remove it from a solid or liquid body into a vacuum is called the operation of the output. The work output is made to designate through where F is the value called the potential of the exit.
In accordance with the above, the operation of the electron output from the metal is determined by the expression
We received this expression under the assumption that the metal temperature is 0 K. At other temperatures, the exit work is also defined as the difference in the depth of the potential well and the Fermi level, that is, the definition of (60.1) is distributed to any temperature. The same definition applies for semiconductors.
Fermi level depends on temperature (see formula (52.10)). In addition, due to the thermal expansion of the change in average distances between atoms, the depth of the potential pits is slightly changed, this leads to the fact that the operation of the output is slightly dependent on temperature.
The operation of the exit is very sensitive to the state of the metal surface, in particular to its purity. Featuring properly surface coating, you can strongly reduce the output. For example, applying an alkaline earth metal oxide layer on the surface of the tungsten (CA, SR, BA) reduces the operation of the output from 4.5 eV (for pure W) to 1.5-2.
§ 104. Operation of electron output from metal
As experience shows, the free electrons at normal temperatures are practically not leaving the metal. Therefore, in the surface layer of metal should be a delayed electric field that prevents the output of electrons from the metal into the surrounding vacuum. The work that needs to be spent to remove an electron from a metal into a vacuum is called operating work. We point out two probable reasons for the appearance of the output:
1. If the electron is deleted from the metal for some reason, then in the place that the electron left, an excessive positive charge occurs and the electron is attracted to the positive charge induced by them.
2. Separate electrons, leaving the metal, are removed from it at the distance of the order of atomic and thereby create a "electronic cloud" metal surface, the density of which is rapidly decreasing with distance. This cloud together with the outer layer of positive lattice ions forms double electric layerthe field of which is like a field of a flat condenser. The thickness of this layer is equal to several interatomic distances (10 -10 -10 -9 m). It does not create an electric field in the outer space, but prevents the release of free electrons from the metal.
Thus, the electron when departing from the metal must overcome the electric field of the double layer delaying its electrical field. The difference of potentials in this layer called superficial jump potentialdetermined by the operation of the output ( BUT) metal electron:
where e -electron charge. Since the electric field is absent outside the double layer, the medium potential is zero, and the potential inside the metal is positive and equal to . The potential energy of the free electron inside the metal is equal to - e. and it is a negative vacuum. Based on this, we can assume that the entire volume of metal for electrons of conductivity represents a potential pit with a flat bottom, the depth of which is equal to the operation of the output BUT.
Exit work is expressed in electron-volta(eV): 1 eV is equal to the work performed by the field with the movement of an elementary electric charge (charge equal to the charge of an electron) when the potential difference is passed in 1 V. Since the electron charge is 1,6 ° 10 -19 KL, then 1 eV \u003d 1,610 -19 J.
The work of the exit depends on the chemical nature of metals and from the purity of their surface and fluctuates within a few electron volts (for example, in potassium A.\u003d 2.2 eV, at platinum A.\u003d 6.3 eV). Featuring a certain way to surface coating, you can significantly reduce the operation of the output. For example, if you apply to the surface of tungsten (BUT= 4,5 eV)the layer of alkaline-land metal oxide (SA, SR, BA), the operation of the exit is reduced to 2 eV.
§ 105. Em session phenomena and their use
If you inform electrons in metals, the energy necessary to overcome the operation of the exit, the part of the electrons may leave the metal, as a result of which the phenomenon of electron emitting is observed, or electronic emission. Depending on the method of communication, the energy electron is distinguished by thermoelectronic, photoelectron, secondary electronic and auto-electron emissions.
1. Thermoelectronic emission- This is the emission of electrons with heated metals. The concentration of free electrons in metals is sufficiently high, therefore, even at mean temperatures, due to the distribution of electrons in speeds (by energies), some electrons have energy sufficient to overcome the potential barrier at the metal boundary. With increasing temperature, the number of electrons, the kinetic energy of the thermal motion of which is more than the operation of the output, the phenomenon of thermoelectronic emission becomes noticeable.
The study of the patterns of thermoelectronic emission can be carried out with the help of the simplest two-electrode lamp - vacuum diode.representing a dumping cylinder containing two electrodes: cathode K.and anode BUT.In the simplest case, the cathode serves a thread from the refractory metal (for example, tungsten), incandescent by electric shock. Anode most often has the shape of a metal cylinder surrounding the cathode. If the diode is turned on in the chain, as shown in Fig. 152, then when influencing the cathode and feed on the anode of positive voltage (relative to the cathode) in the anode chain of the diode there is a current. If you change the polarity of the battery B. A, the current stops, no matter how much the cathode is gated. Consequently, the cathode eats negative particles - electrons.
If you maintain the temperature of the rolled cathode to constant and remove the dependence of the anode current I. and from anodic voltage U. but, - Volt-ampere characteristic(Fig. 153), it turns out that it is not linear, that is, for a vacuum diode, the Ohm law is not performed. The dependence of thermoelectronic current I.from anodic voltage in the field of small positive values U.describes The law of three second(Installed by the Russian physicist S. A. Boguslavsky (1883-1923) and American physicist I. Lengmur (1881-1957)):
where AT-the coefficient depending on the shape and size of the electrodes, as well as their mutual location.
With an increase in the anode voltage, the current increases to a maximum value. I. of us called top saturation. This means that almost all electrons leaving the cathode reaches the anode, so a further increase in field strength cannot lead to an increase in thermoelectronic current. Consequently, the density of the saturation current characterizes the emission ability of the cathode material.
Saturation current density is determined richardson's formula - Dead,derived theoretically based on quantum statistics:
where BUT -electron output from the cathode, T. - thermodynamic temperature, WITH- constant, theoretically equal milking of all metals (this is not confirmed by the experiment, which appears to be explained by surface effects). Reducing the output operation leads to a sharp increase in the density of the saturation current. Therefore, oxide cathodes are used (for example, nickel, coated with alkaline earth metal oxide), the operation of which is equal to 1-1.5 eV.
In fig. 153 presented volt-ampere characteristics for two temperatures of the cathode: T. 1 I. T. 2, and T. 2 \u003e T. 1 . WITHincreasing the temperature of the cathode emitting electrons from the cathode intensively, while the saturation current increases. For U. A \u003d 0 there is an anode current, i.e., some electrons emitted by the cathode have energy sufficient to overcome the operation of the output and reach the anode without an electric field application.
The effect of thermoelectronic emission is used in the devices in which the electron flow is needed in vacuo, for example, in electronic lamps, X-rays, electronic microscopes, etc. Electronic lamps are widely used in electrical and radio engineering, automation and telemechanics for rectifying variable currents, strengthening Electrical signals and variable currents, generating electromagnetic oscillations in t. d. Depending on the purpose of the lamps, additional control electrodes are used.
2. Photo electronic emission- This is the emission of electrons from metal under the action of light, as well as short-wave electromagnetic radiation (for example, X-ray). The main patterns of this phenomenon will be disassembled when considering the photovoltaic effect.
3. Secondary electronic emission- This is the emitting of electrons with the surface of metals, semiconductors or dielectrics in the bombardment of their electron beam. The secondary electron flow consists of electrons reflected by the surface (elastic and non-reflected electrons reflected), and "truly" secondary electrons - electrons, knocked out of metal, semiconductor or dielectric by primary electrons.
The ratio of the number of secondary electrons n. 2 to the primary n. 1 , caused emissions called secondary electronic emission ratio:
Coefficient depends on the nature of the surface material, the energy of bombarding particles and their drop in the surface. In semiconductors and dielectrics more than metals. This is explained by the fact that in metals, where the concentration of electrons of conductivity is large, secondary electrons, often facing them, lose their energy and cannot leave the metal. In semiconductors and dielectrics, due to the low concentration of electrons of the conductivity of the collision of secondary electrons, they occur much less often and the probability of the output of secondary electrons from the emitter increases several times.
For example in fig. 154 provides a qualitative dependence of the secondary emission coefficient from energy E.falling electrons for CCL. With increasing electrons increases, since the primary electrons are deeper into the crystal lattice and, therefore, they knock out more secondary electrons. However, at some energy of primary electrons begins to decrease. This is due to the fact that with an increase in the depth of penetration of primary electrons, the secondary is increasingly harder to break into the surface. Value Max for CLDs12 (for pure metals it does not exceed 2).
The phenomenon of secondary emission is used in photoelectronic multipliers(FEU) applicable to enhance weak electrical currents. FEU is a vacuum tube with a photocathode to and anode A, between which several electrodes are located - emitters(Fig. 155). Electrons, eliminated from the photocathoda under the action of light, fall on the Emitter E 1, pass the accelerating potential difference between K and E 1. From Emitter E 1 is knocked out electrons. Thus, an electronic stream is sent to Emitter E 2, and the multiplication process is repeated on all subsequent emitters. If the FEU contains n.emitters, then on the anode A, called collector,it turns out reinforced B. n. times photoelectronic current.
4. Auto-electronic emission- This is the emission of electrons from the surface of the metals under the action of a strong external electric field. These phenomena can be observed in a dumped tube, the configuration of the electrodes of which (cathode - edge, the anode is the inner surface of the tube) allows approximately 10 3 to voltages to receive electrical fields with a strength of about 10 7 V / m. With a gradual increase in the voltage already at the intensity of the field in the surface of the cathode, approximately 10,5 -10 6 V / m occurs a weak current due to electrons emitted by the cathode. The strength of this current increases with an increase in the voltage on the tube. Currents occur at a cold cathode, so the described phenomenon is also called Cold emissions.An explanation of the mechanism of this phenomenon is possible only on the basis of quantum theory.
