Problems of medium difficulty. B1. In an experiment to detect the photoelectric effect, a zinc plate is mounted on the rod of an electrometer, pre-charged negatively and illuminated with light

B1. In an experiment to detect the photoelectric effect, a zinc plate is mounted on the rod of an electrometer, pre-charged negatively and illuminated with light electric arc so that the rays fall perpendicular to the plane of the plate. How will the discharge time of the electrometer change if: a) the plate is rotated so that the rays fall at a certain angle; b) bring the electrometer closer to the light source; c) cover part of the plate with an opaque screen; d) increase illumination; e) install a light filter that blocks the infrared part of the spectrum; f) install a light filter that blocks the ultraviolet part of the spectrum?

B2. Find the wavelength lof the light with which the metal surface is illuminated if photoelectrons have kinetic energy K= 4.5×10 –20 J, and the work function of an electron from the metal A out = 7.5×10 –19 J.

B3. Define highest speed electron emitted from cesium when illuminated with light of wavelength l = 331 nm. Work function A out = 2 eV, electron mass 9.1×10 –31 kg.

Q4. Determine the speed of photoelectrons if the photocurrent stops at a retarding potential difference of 1 V.

B5. The minimum frequency of light that ejects electrons from the surface of the metal cathode is n 0 = 6.0 × 10 14 Hz. At what frequency
n light, emitted electrons are completely delayed by the potential difference U= 3.0 V?

The tasks are difficult

C1. How to charge a zinc plate attached to the electrometer rod with a positive charge using an electric arc, a glass rod and a sheet of paper? You cannot touch the plate with a stick.

C2. In the installation shown in Fig. 22.6, the photocell cathode can be made of various materials. In Fig. 22.7 shows graphs of the dependence of the blocking voltage U h, on the frequency n of the irradiating light for two different materials cathode Justify the linearity of this dependence. Which material has a higher work function?

Rice. 22.6 Rice. 22.7

C3. To determine Planck's constant, the circuit shown in Fig. 22.6. When the sliding contact of the potentiometer P is in the extreme left position, sensitive galvanometer G registers a weak photocurrent when illuminating a photocell F. By moving the sliding contact to the right, the blocking voltage is gradually increased until the photocurrent stops in the circuit. When a photocell is illuminated with violet light with a frequency n 2 = 750 THz, the blocking voltage U 32 = 2.0 V, and when illuminated with red light with a frequency n 1 = 390 THz, the blocking voltage U 31 = 0.50 V. What value of Planck's constant was obtained?

C4. For potassium, the red limit of the photoelectric effect is l max = 0.62 µm. Which maximum speed And can have photoelectrons emitted when potassium is irradiated with violet light with a wavelength l = 0.42 μm?

C5. When the surface of a certain metal is illuminated with violet light with a wavelength l 1 = 0.40 μm, the electrons knocked out by the light are completely delayed by the potential difference (blocking voltage) U 1 = = 2.0 V. What is the blocking voltage? U 2 when illuminating the same metal with red light with a wavelength l 2 = 0.77 μm?

4. The photoelectric effect is practically inertialess.

The methodology for studying the photoelectric effect can be divided into several stages:

1. Introducing students to the phenomenon of the photoelectric effect. A story about the history of its discovery (G. Hertz).

2. A story about the search for patterns of this phenomenon. Research.

3. Consideration of the basic laws of the photoelectric effect. Demonstration, revelation of existing difficulties - the impossibility of explaining all the laws of the photoelectric effect from positions already known to students (vol. new theory Sveta).

4. Proposition of the hypothesis of light quanta. A story about the work of A. Einstein. Photoelectric effect equation.

5. Explanation of all the laws of the photoelectric effect from a quantum point of view:

6. Conclusions quantum theory about the nature of light.

7. Vacuum and semiconductor photocells. Application of the photoelectric effect in technology.

It is best to introduce schoolchildren to an understanding of the phenomenon of the photoelectric effect and its laws through an experiment. In the first lesson on a topic, a series of experiments is usually offered:

1) A well-cleaned zinc plate fixed to the electrometer rod is charged negatively and illuminated with a stream of ultraviolet rays. Observe the discharge of the electrometer.

2) The discharge stops if we block the flow of rays with glass.

3) If you give the plate a positive charge, then under the same lighting the discharge of the electrometer is not observed.

4) The discharge occurs faster, the greater the light intensity.

5) Replacing the zinc plate with a copper one (then lead), repeat the experiments under the same conditions (illumination, initial charge).

During the conversation, they consistently discuss the following questions: Why can a charged plate retain a charge for a long time? In what ways can you discharge the plate? How to explain the rapid discharge of a negatively charged plate when illuminated by arc light? Will a positively charged zinc plate discharge in the same way when illuminated by ultraviolet light? Why does the electrometer not detect a change in charge in this case? Do we observe a discharge of a copper plate under the same experimental conditions? Why does the discharge of a negatively charged zinc plate stop if the light from the electric arc is blocked by a glass plate?

The discussion allows us to draw the following conclusions:

1. When exposed to light, only negatively charged metals are discharged. Therefore, under certain conditions, light can strip electrons from metals. This phenomenon is called the photoelectric effect. (Here you can also talk about the history of the discovery of the photoelectric effect.)

2. The discharge begins simultaneously with the start of illumination, therefore, the photoelectric effect is practically inertialess. (Precise experiments have shown that the time between the start of irradiation and the start of the photoelectric effect does not exceed 10-9 s.)

3. The presence of a photoelectric effect depends on the type and processing of the illuminated metal and on spectral composition radiation, the discharge speed also depends on the light energy incident per unit time.

The study of the laws of the photoelectric effect continues on a setup that allows one to study the dependence of the photocurrent on the applied voltage, intensity and spectral composition of the radiation. First, the existence of a saturation current is experimentally established, and then its dependence on the intensity of light incident on the cathode (the first law of the photoelectric effect is Stoletov’s law).

Based on the results of the experiment, graphs are drawn of the dependence of current on light intensity and voltage.

After this, illuminating the photocell with light of a certain frequency, the photocell is “locked” using a potentiometer and the blocking voltage is measured, which makes it possible to determine the maximum speed of the emitted electrons.

By changing the light filters, new data is obtained by repeating the experiments and convinces students that the speed of electron emission depends on the frequency of the incident light and does not depend on the intensity of the light (the second law of the photoelectric effect).

Next we begin to explain the laws of the photoelectric effect. The phenomenon itself and the fact that the saturation photocurrent is directly proportional to the light energy incident per unit time - the first law of the photoelectric effect - can also be explained from a wave position. Explanation of why there is a photoelectric effect threshold (red limit), why the maximum initial speed(and maximum kinetic energy photoelectrons) does not depend on the light intensity, but is determined only by its frequency (increases linearly with frequency), and an explanation for the inertia-free photoelectric effect cannot be given on the basis of the wave electromagnetic theory Sveta. Indeed, according to this theory, the ejection of electrons from a metal is the result of their “swinging” in the alternating electric field of a light wave. But then both the speed and kinetic energy of photoelectrons should depend on the amplitude of the intensity vector E electric field wave and, therefore, depending on its intensity, it takes time to “swing” the electron; the effect cannot be inertia-free. The discrepancy between the experimental facts and the established wave theory light proved its inconsistency and required the creation of a new theory.

Next they say that difficulties in explaining the laws of the photoelectric effect were not the only reason for the creation of a new theory. In 1900, M. Planck, to explain thermal radiation, was forced to express, at first glance, the absurd idea that a body emits energy not continuously, but in separate portions (quanta). This idea contradicted the prevailing ideas classical physics, where the processes and quantities that characterize them change continuously. In 1905, A. Einstein used this incomprehensible and therefore little accepted idea to explain the laws of the photoelectric effect. He went further than M. Planck and argued: light is not only emitted, but also propagates and is absorbed by quanta.

In other words, the flow monochromatic light, energy-carrying E is a stream of n particles (later called photons), each of which has energy h.

The energy of a photon is proportional to the frequency of light. The higher the frequency (smaller wavelength) of radiation, the more energy each photon carries. The energy of long-wave radiation photons is less.

Einstein further suggested: each photon interacts not with the entire substance on which the light falls, and not even with the atom as a whole, but with an individual electron of the atom. The photon gives its energy to the electron, and the electron, having received energy, escapes from the metal with a certain kinetic energy. Based on the law of conservation of energy, we can write the following equation for the elementary act of interaction of a photon with an electron:

It is important to emphasize that direct proportionality, and not equality, is observed, since some of the photons incident on the metal are reflected, and not all of the absorbed photons are torn out of the metal free electrons. The energy of part of the absorbed photons is converted into internal energy metal Therefore, the ratio of the number of electrons to the number of photons incident on the metal is significantly less than unity (for pure metals it is approximately 1000 times).

They further explain why the greatest kinetic energy of photoelectrons depends on the frequency of the incident light, and not on its intensity (the second law of the photoelectric effect). From Einstein's equation it follows: since the work function for a given substance is constant, the maximum kinetic energy of photoelectrons is proportional to the frequency of the incident light. Analyze the case when the energy light quantum is numerically equal to the work function.

Consequently, all the energy of the photon goes to perform the work function and the speed of the electrons is zero. The red limit of the photoelectric effect depends only on the work function, i.e. chemical nature metal, and can lie in any part of the optical range. For each substance there is a certain long-wave limit of the photoelectric effect (the third law of the photoelectric effect).

Thus, Einstein's equation explains all the laws of the external photoelectric effect. It allows you to calculate photoelectron velocities and determine greatest length wave at which the phenomenon of the photoelectric effect is still observed for a given substance, and also calculate the work function for a specific metal.

After analyzing Einstein's equation, it can be shown how the experimental verification of this equation was carried out. It consisted of determining Planck's constant from the results of the experiment.

Since the work function for a given substance is a constant value, the kinetic energy of the photoelectron is linear function frequency of radiation incident on the photocell.

At practical implementation such measurements have been encountered great difficulties. The first thorough measurements of Planck's constant by this method were carried out in 1915 by R. Millikan. He obtained a value close to what was already known from the theory of thermal radiation.

In our country in 1928 it was confirmed by experiments linear dependence kinetic energy of photoelectrons from the frequency of incident light and the value of Planck’s constant was obtained.

To consolidate Einstein's equations, solve problems to calculate the speed and energy of electrons, the red limit of the photoelectric effect, and the work function.

COMPTON EFFECT

The formation of ideas about the photon, begun when studying: the photoelectric effect, continues when studying subsequent issues of the course - the Compton effect, light pressure, chemical action Sveta.

Tests

Preparation for the Unified State Exam. 11th grade

(Here only tests are published on two topics out of eleven submitted, for the entire course for the 11th grade. Full text problems published on the Physics website: in the section “ Additional materials». – Red.)

I propose a system of tests designed to prepare students for the Unified State Exam. Each is designed for one lesson, includes six options and is like a thematic fragment of the Unified State Exam. The level of difficulty of the five tasks is differentiated. Each contains three multiple-choice tests and two problems (one easier, the other more difficult). Three minutes after the start of the test, I collect the answers to the tests, and the students begin to solve the problems. Thus, the pace (question per minute) turns out to be as close as possible to the conditions of the Unified State Exam.

Tasks are formalized traditionally: a brief condition, a drawing, calculation formulas with brief explanations, substitution of numerical data, checking units of physical quantities. Full transparency of the summing up of the test results is ensured by detailed information to students and a grading system. The solved test is worth 1 point, the 4th problem is worth 2 points, the more difficult 5th problem is worth 3 points. The assessment for the test is given depending on the total score received by the student for correct answers to questions and tasks, on the following scale: 7–8 points – “5”, 5–6 points – “4”, 3–4 points – “3” ", less than 3 - "2".

This structure of the test work allows you to combine current control mastering the material (tasks 1–3) with checking the depth of understanding physical theory(tasks 4, 5). Having summary data on the answer to each question and the solution to each problem, you can get an idea of ​​the dynamics of each student’s learning of the material. For example, if a student regularly answers the first three questions correctly, but fails to cope with the fourth and fifth tasks, this means that he has a sufficient (at the reproductive level) understanding of the course material. On the contrary, if a student regularly solves the fifth problem, but answers the remaining questions incorrectly, then this indicates a fairly deep, but fragmented study of the course.

Literature

Kasyanov V.A. Physics-11: Thematic and lesson planning. – M.: Bustard, 2002.

Kasyanov V.A. Unified state exam in physics in Russia and SAT-II in the USA. – Physics (“PS”, No. 40/03.

Konoplich R.V., Orlov V.A., Dobrodeev N.A., Tatur A.O. Collection of test tasks for thematic and final control. Physics-11. – M.: Intellect-Center, 2002.

Konoplich R.V., Orlov V.A., Dobrodeev N.A., Tatur A.O. Collection of test tasks for thematic and final control. Physics-10. – M.: Intellect-Center, 2002.

Kirik L.A. Physics-11. Multi-level independent and control work. – M.: Ilexa, 2003.

Kirik L.A. Physics-10. Multi-level independent and control work. – M.: Ilexa, 2003.

Orlov V.A., Khannanov N.K., Fadeeva A.A. Educational and training materials for preparing for the unified state exam. Physics. – M.: Intellect-Center, 2003.

Pigalitsyn L.V. Thematic tests in physics. 11th grade. – N. Novgorod: Nizhny Novgorod Humanitarian Center, 1997.

Test No. 10. Quantum theory electromagnetic radiation substances

Option 1

1. Photon momentum r is related to its frequency by the relation ( h– Planck’s constant):

2. The photoelectric effect is a phenomenon:

A) blackening of the photographic emulsion under the influence of light;

B) the emission of electrons from the surface of the metal under the influence of light;

C) glow of some substances in the dark;

D) radiation from a heated solid.

3. The figure shows a diagram energy levels atom. Which arrow indicates the transition with the emission of a photon of the highest frequency?

A) 1; B) 2; B) 3; D) 4.

4. When an electron in a hydrogen atom moves from one orbit to another, closer to the nucleus, photons with an energy of 3.03 10 –19 J are emitted. Determine the frequency of the atom’s radiation.

5. The work function of an electron from zinc is 3.74 eV. Determine the red limit of the photoelectric effect for zinc. What speed will electrons ejected from zinc receive when irradiated with ultraviolet radiation with a wavelength of 200 nm?

Option 2

1. Photon energy is directly proportional to ( – wavelength):

A) –2; B) –1; IN) ; D) 2.

2. Which of the graphs correctly depicts the dependence of the photocurrent (during the photoelectric effect) on the voltage between the electrodes at constant illumination in a standard experiment?

3. Atoms of one element that were in states with energies E 1 and E 2, during the transition to the ground state, they emitted photons with wavelengths 1 and 2, respectively, with 1 > 2. For the energies of these states the following relation is valid:

A) E 1 > E 2 ; B) E 1 < E 2 ;

IN) E 1 = E 2 ; D) | E 1 | < | E 2 |.

4. When an electron in a hydrogen atom passes from the third stationary orbit to the second, a photon is emitted corresponding to a wavelength of 0.652 μm (red line of the hydrogen spectrum). How much energy does the hydrogen atom lose in this process?

5. For some metals, the red limit of the photoelectric effect is light with a wavelength of 690 nm. Determine the work function of an electron from this metal and the maximum speed that electrons will acquire under the influence of radiation with a wavelength of 190 nm.

Option 3

1. The wavelength kr corresponding to the red boundary of the photoelectric effect is equal to ( A– work function, h– Planck’s constant):

A) ; B) ; IN) ; G) .

2. Saturation photocurrent during the photoelectric effect when the incident light flux decreases:

A) increases; B) decreases; B) does not change;

D) increases or decreases depending on the conditions of the experiment.

3. What number on the given diagram of atomic energy levels indicates a transition with the emission of a photon of maximum frequency?

A) 1; B) 2; B) 3; D) 4.

4. The human eye perceives light with a wavelength of 500 nm if the light rays bring energy of at least 20.8 10 –18 J into the eye every second. How many quanta of light hit the retina every second?

5. What maximum speed will photoelectrons acquire when they are torn from the surface of molybdenum by radiation with a frequency of 3 10 15 Hz? The electron work function for molybdenum is 4.27 eV.

A) E. Rutherford; B) J.J. Thomson;

B) F. Joliot-Curie; D) I.V. Kurchatov.

2. Which of the following continuations of the statement is correct? When transitioning between two different stationary states, an atom can:

A) emit and absorb photons of any energy;

B) emit and absorb photons only with certain energy values;

C) emit photons of any energy, but absorb only certain energy values;

D) absorb photons of any energy, and emit only certain energy values.

3. Which of the following phenomena: I – spontaneous emission; II - stimulated emission - used in optical quantum generators?

A) I; B) II; B) I and II; D) neither I nor II.

4. At what electromagnetic wavelength is the photon energy equal to 9.93 10 –19 J?

5. The red limit of the photoelectric effect for rubidium is 0.81 microns. What voltage must be applied to the photocell in order to trap electrons emitted from rubidium ultraviolet rays wavelength 0.1 microns?

Option 5

1. What is the energy of a photon with frequency ?

A) hWith 2 ; B) hWith; IN) h; G) h/With.

2. When the cathode of a vacuum photocell is illuminated with monochromatic light, photoelectrons are released. How will the maximum energy of photoelectrons change when the frequency of light increases by a factor of 2?

A) Will not change; B) will increase by 2 times;

C) will increase by less than 2 times;

D) will increase more than 2 times.

3. For this energy level diagram, indicate the correct statement:

A) E 1 > E 4 ; B) E 4 > E 2 ;

IN) E 2 > E 3; G) E 2 > E 4 .

4. To ionize a nitrogen atom, an energy of 14.53 eV is required. Find the wavelength of radiation that will cause ionization.

5. The work function of electrons from cadmium is 4.08 eV. With what wavelength of light should cadmium be illuminated so that the maximum speed of the emitted electrons is 7.2 10 5 m/s?

Option 6

1. The frequency of red light is almost 2 times less than the frequency of violet. The momentum of the “red” photon in relation to the momentum of the “violet” photon:

A) 4 times more; B) 4 times less;

B) 2 times more; D) 2 times less.

2. What is the nature of the forces that deflect a-particles at small angles from straight trajectories in Rutherford's experiment?

A) Gravitational; B) Coulomb;

B) electromagnetic; D) nuclear.

3. When illuminating the surface of a body with work function A Monochromatic light frequency releases photoelectrons. What determines the difference ( h – A)?

A) Average kinetic energy of photoelectrons;

B) average speed photoelectrons;

B) maximum kinetic energy of photoelectrons;

D) maximum speed of photoelectrons.

4. When electrons in a hydrogen atom move from the 4th stationary orbit to the 2nd, a photon is emitted, giving green line in the spectrum of hydrogen. Determine the wavelength of this line if 2.53 eV of energy is lost when a photon is emitted.

5. A negatively charged zinc plate was illuminated with monochromatic light with a wavelength of 300 nm. The red limit for zinc is 332 nm. What is the maximum potential that a zinc plate acquires?

Answers

Test No. 11. High energy physics

Option 1

1. When a nucleus emits a particle, a daughter nucleus is formed having:

A) higher charge and the same mass number;

B) lower charge and the same mass number;

B) higher charge and lower mass number;

D) lower charge and higher mass number.

2. Number radioactive nuclei in the sample changes with time, as shown in the figure. Half-life of sample material:

A) 1 year; B) 1.5 years; B) 2 years; D) 2.5 years.

3. During the radioactive decay of uranium, nuclear reaction What isotope is formed in this case?

4. The half-life of a radioactive element is 400 years. How much of a sample of this element decays after 1200 years?

5. Determine the binding energy per nucleon in the nucleus of an atom if the mass of the latter is 22.99714 amu.

Option 2

1. As a result of natural radioactive decay, the following are formed:

A) only -particles;

B) only -particles;

B) only -quanta;

D) -particles, -particles, -quanta, neutrinos.

2. The number of radioactive nuclei in a sample changes over time, as shown in the figure. Find the half-life of the material.

A) 2 ms; B) 2.5 ms; B) 3 ms; D) 3.5 ms.

3. What particle X formed as a result of a nuclear reaction

4. What fraction of nuclei of a radioactive isotope with a half-life of 2 days will remain after 16 days?

5. By firing protons at boron nuclei, beryllium is produced. What other nuclei are produced in this reaction and how much energy is released?

Option 3

1. How many protons are in the nucleus

A) Z; B) A – Z; B) A + Z; G) Z – A.

2.

A) Flow of hydrogen nuclei; B) flow of helium nuclei;

B) neutron flux; D) electron flow.

3. The nucleus of an atom can spontaneously split into two fragments. One of the fragments is barium, the other is krypton. How many neutrons are released during fission?

A) 1; B) 2; B) 3; D) 4.

4. Determine whether the reaction occurs with the absorption or release of energy

5. When bombarded with boron particles, the emission of neutrons is observed. Write the equation for a nuclear reaction leading to the emission of one neutron. What is the energy output of this reaction?

Option 4

1. Indicate the second product of a nuclear reaction

A) Neutron; B) proton;

B) electron; D) -particle.

2. What is radiation?

A) Neutron flux;

B) flow of fast electrons;

B) flow of quanta of electromagnetic radiation;

D) proton flux.

3. In nuclear reactors, the neutron multiplication factor in a fission chain reaction must be:

A) > 1; B) = 1; IN)< 1; Г) 1.

4. Determine the energy that is released during the annihilation of an electron and a positron if the mass of the electron is 9.1 10 –31 kg.

5. What is the electrical power nuclear power plant with an efficiency of 25%, consuming 220 g of uranium-235 isotope per day?

Option 5

1. What particle is emitted by the atomic nucleus during -decay?

A) Neutron only; B) only -quantum;

B) electron and antineutrino; D) positron and neutron.

2. What forces act between neutrons in the nucleus?

A) Gravitational; B) nuclear;

B) Coulomb; D) nuclear and gravitational.

3. In the depths of the Sun, temperatures reach tens of millions of degrees. This is explained:

A) fast rotation The sun around its axis;

B) fission of heavy nuclei;

IN) thermonuclear fusion light nuclei;

D) the reaction of combustion of hydrogen in oxygen.

4. When an aluminum isotope is bombarded with -particles, it turns out radioactive isotope phosphorus, which then decays to release positrons. Write the equations for both reactions.

5. When a boron isotope is bombarded with neutrons, -particles are formed. Write an equation for this reaction and find its energy output.

Option 6

1. Mass of the nucleus of a helium atom more mass nucleus of a hydrogen atom in:

A) 2 times; B) 3 times; B) 4 times; D) 6 times.

2. The complete transformation of elements was first observed in the reaction, as a result of which two nuclei appeared:

A) hydrogen; B) helium; B) beryllium; D) boron.

3. What fraction of radioactive nuclei decays after a time interval equal to two half-lives?

A) 25%; B) 50%; C) 75% D) 100%.

4. In the process of thermonuclear fusion, 5 10 4 kg of hydrogen is converted into 49,644 kg of helium. Determine how much energy is released during this process.

5. The power of a nuclear reactor when using 0.2 kg of uranium-235 isotope per day is 32,000 kW. How much of the energy released by nuclear fission is used usefully?

Answers

Olga Pavlovna Sorokina Graduated from the Faculty of Computational Mathematics and Cybernetics of Gorky State University. N.I. Lobachevsky in 1988. Since 1993 he has been teaching mathematics, physics, computer science and ICT (the last two years). Teacher of the highest qualification category. Author of two articles with pedagogical content. Credo: “By teaching others, we learn ourselves.” She and her husband are raising two children. He devotes all his free time to self-education. Loves to cook, bake pies and cakes.

Demonstrates simple experience. If a negatively charged zinc plate connected to an electroscope (a device that shows the presence of electric charge), illuminate with light ultraviolet lamp, then very quickly the electroscope needle will move to zero state. This indicates that the charge has disappeared from the surface of the plate. If the same experiment is performed with a positively charged plate, the electroscope needle will not deflect at all. This experiment was first carried out in 1888 by Russian physicist Alexander Grigorievich Stoletov.

Alexander Grigorievich Stoletov

What happens to a substance when light falls on it?

We know that light is electromagnetic radiation, flow quantum particles- photons. When electromagnetic radiation falls on a metal, some of it is reflected from the surface, and some is absorbed by the surface layer. When absorbed, a photon gives up its energy to the electron. Having received this energy, the electron does work and leaves the surface of the metal. Both the plate and the electron have negative charge, so they repel each other and the electron escapes from the surface.

If the plate is positively charged, the negative electron knocked out from the surface will be attracted again by it and will not leave its surface.

History of discovery

The phenomenon of the photoelectric effect was discovered in early XIX century.

In 1839, the French scientist Alexandre Edmond Becquerel observed the photovoltaic effect at the interface of a metal electrode and a liquid (electrolyte).

Alexander Edmond Becquerel

In 1873, the English electrical engineer Smith Willoughby discovered that if selenium is exposed to electromagnetic radiation, its electrical conductivity changes.

Conducting research experiments electromagnetic waves In 1887, German physicist Heinrich Hertz noticed that a charged capacitor discharges much faster if its plates are illuminated with ultraviolet radiation.

Heinrich Hertz

In 1888, the German experimental physicist Wilhelm Galwachs discovered that when a metal is irradiated with short-wave ultraviolet radiation, the metal loses its negative charge, that is, the phenomenon of the photoelectric effect is observed.

A huge contribution to the study of the photoelectric effect was made by the Russian physicist Alexander Grigorievich Stoletov, who conducted detailed experiments on the study of the photoelectric effect in 1888-1890. To do this, he designed a special device consisting of two parallel disks. One of these disks cathode, made of metal, was inside a glass case. Another disk anode, was a metal mesh applied to the end of the case made of quartz glass. Quartz glass was not chosen by the scientist by chance. The fact is that it transmits all types of light waves, including ultraviolet radiation. Ordinary glass blocks ultraviolet radiation. Air was pumped out of the housing. A voltage was applied to each of the disks: negative to the cathode, positive to the anode.

Stoletov's experience

During the experiments, the scientist illuminated the cathode through glass with red, green, blue and ultraviolet light. The magnitude of the current was recorded by a galvanometer, in which the main element was a mirror. Depending on the magnitude of the photocurrent, the mirror was deflected by different angle. Greatest effect exposed to ultraviolet rays. And the more of them there were in the spectrum, the stronger the impact of light.

Stoletov discovered that only negative charges are released under the influence of light.

The cathode was made of various metals. The most sensitive to light were metals such as aluminum, copper, zinc, silver, and nickel.

In 1898, it was discovered that the negative charges released during the photoelectric effect are electrons.

And in 1905, Albert Einstein explained the phenomenon of the photoelectric effect as special case law of conservation and transformation of energy.

External photoeffect

External photoeffect

The process of electrons leaving a substance under the influence of electromagnetic radiation is called external photoeffect , or photoelectron emission. Electrons emitted from the surface are called photoelectrons. Respectively, electric current, which is formed during their ordered movement, is called photocurrent.

First law of the photoelectric effect

The strength of the photocurrent is directly proportional to the density luminous flux . The higher the radiation intensity, the more electrons will be knocked out of the cathode in 1 s.

The intensity of the light flux is proportional to the number of photons. As the number of photons increases, the number of electrons leaving the metal surface and creating a photocurrent increases. Consequently, the current increases.

Second law of the photoelectric effect

The maximum kinetic energy of electrons ejected by light increases linearly with the frequency of light and does not depend on its intensity.

The energy possessed by a photon incident on the surface is equal to:

E = h ν ,Where ν - frequency of the incident photon; h - Planck's constant.

Having received energy E , the electron performs a work function φ . The rest of the energy is the kinetic energy of the photoelectron.

The law of conservation of energy implies the following equality:

h·ν=φ + W e , Where W e - the maximum kinetic energy of an electron at the moment of departure from the metal.

h·ν=φ + m v 2 /2

Third law of the photoelectric effect

For each substance there is a red limit of the photoelectric effect, that is, a minimum frequency of light νmin(or maximum length waves λ max), at which the photoelectric effect is still possible, and if ν˂ ν min, then the photoelectric effect no longer occurs.

The photoelectric effect appears starting from a certain frequency of light νmin . At this frequency, called "red" border of the photoelectric effect, electron emission begins.

h ν min = φ .

If the photon frequency is lower νmin , its energy will not be enough to “knock out” an electron from the metal.

Internal photoelectric effect

If, under the influence of radiation, electrons lose connection with their atoms, but do not leave solid and liquid semiconductors and dielectrics, but remain inside them as free electrons, then this photoelectric effect is called internal. As a result, electrons are redistributed along energy states. The concentration of charge carriers changes and a photoconductivity(increase in conductivity when exposed to light).

The internal photoelectric effect also includes valve photoelectric effect, or photoelectric effect in the barrier layer. This photoelectric effect occurs when, under the influence of light, electrons leave the surface of a body and pass into another, contacting body - a semiconductor or electrolyte.

Application of photoelectric effect

All devices whose operating principle is based on the photoelectric effect are called photocells. The world's first photocell was Stoletov's device, created by him to conduct experiments to study the photoelectric effect.

Photocells are widely used in a wide variety of devices in automation and telemechanics. Without photocells, it is impossible to control computer numerical control (CNC) machines, which can create parts according to drawings without human intervention. With their help, sound is read from film. They are part of various control devices, help to stop and block the device in right moment. With the help of photocells, street lighting turns on at nightfall and turns off at dawn. They help control turnstiles in the metro and beacons on land, and lower the barrier when a train approaches a crossing. They are used in telescopes and solar panels.

In 1887 the German scientist Hertz discovered the influence of light on electrical discharge. Studying spark discharge, Hertz discovered that if the negative electrode is illuminated with ultraviolet rays, the discharge occurs at a lower voltage on the electrodes.

It was further discovered that when light is illuminated on a negatively charged metal plate connected to an electroscope, the needle of the electroscope moves down. This indicated that the metal plate illuminated by the electric arc was losing its negative charge. Positive charge the metal plate does not lose color when illuminated.

Loss metal bodies when illuminated by rays of negative light, it is called the photoelectric effect or simply the photoelectric effect.

The phenomena have been studied since 1888 by the famous Russian scientist A.G. Stoletov.

Stoletov studied the photoelectric effect using an installation consisting of two small disks. A solid zinc plate and a thin mesh were installed vertically opposite each other, forming a capacitor. Its plates were connected to the poles and then illuminated by the light of an electric arc.

Light penetrated freely through the mesh onto the surface of a solid zinc disk.

Stoletov established that if the zinc plate of a capacitor is connected to the negative pole of a voltage source (it is a cathode), then a galvanometer connected to the circuit shows the current. If the cathode is the grid, then there is no current. This means that the illuminated zinc plate emits negatively charged particles, which determine the existence of a current in the gap between it and the grid.

Stoletov, studying the photoelectric effect, the physics of which had not yet been revealed, took disks from a wide variety of metals for his experiments: aluminum, copper, zinc, silver, nickel. Connecting them to the negative pole of the voltage source, he observed how, under the influence of an arc in his circuit pilot plant an electric current occurred. This current is called photocurrent.

As the voltage between the plates of the capacitor increased, the photocurrent increased, reaching its value at a certain voltage maximum value, called saturation photocurrent.

Investigating the photoelectric effect, the physics of which is inextricably linked with the dependence of the saturation photocurrent on the magnitude incident on the cathode plate, Stoletov established next law: the value of the saturation photocurrent will be directly proportional to the light flux incident on the metal plate.

This law is called Stoletov.

It was later established that photocurrent is a flow of electrons torn out of a metal by light.

The theory of the photoelectric effect has found widespread practical application. Thus, devices were created based on this phenomenon. They are called photocells.

The photosensitive layer - the cathode - covers almost the entire inner surface glass container, with the exception of a small window for light access. The anode is a wire ring fixed inside the cylinder. There is a vacuum in the cylinder.

If you connect the ring to the positive pole of the battery, and the photosensitive metal layer through a galvanometer to its negative pole, then when the layer is illuminated by an appropriate light source, a current will appear in the circuit.

You can turn off the battery completely, but even then we will observe a current, only a very weak one, since only an insignificant part of the electrons ejected by the light will fall on the wire ring - the anode. To enhance the effect, a voltage of about 80-100 V is required.

The photoelectric effect, the physics of which is used in such elements, can be observed using any metal. However, most of them, such as copper, iron, platinum, tungsten, are sensitive only to alkali metals- potassium, sodium and especially cesium are sensitive to visible rays. They are used for the manufacture of photocell cathodes.