Scientists are well aware that this particle is a fundamental component of the entire material world. Accordingly, the question arose about the study and measurement of its properties. The first precise measurement of the electric charge of an electron is the merit of Robert Millikan. His experimental setup was a large and capacious flat capacitor made of two metal plates with a chamber between them. Millikan applied a constant voltage from a powerful battery to the plates of the capacitor, creating a high potential difference on them, and between the plates he placed finely atomized drops - first water, and then oil, which, as it turned out, behaves in an electrostatic field much more stable, and most importantly, evaporates much slower. First, Millikan measured the droplets' ultimate speed - that is, the speed at which the force of gravity acting on the droplets is balanced by the force of air resistance. According to this speed, the scientist determined the volume and mass of aerosol suspension droplets. After that, he sprayed an identical aerosol in the presence of an electrostatic field, that is, with the battery connected. In this case, the oil droplets remained suspended for a long time, since the forces of gravitational attraction of the Earth were balanced by the forces of electrostatic repulsion between the aerosol droplets.
The reason the oil aerosol droplets become electrified is trivial: it is a simple electrostatic charge, similar to that which accumulates, say, on laundry that we take out of a drying centrifuge, as a result of the fact that the fabric rubs against the fabric - it arises from the friction of the droplets against air filling the chamber. However, due to the microscopic size of the oil droplets in the chamber, they cannot receive a large charge, and the magnitude of the droplet charge will be a multiple of the unit electron charge. This means that, gradually lowering the external voltage, we will observe how the oil drops periodically "precipitate", and by the gradations of the voltage scale at which the next portion of the aerosol is deposited, we can judge the absolute value of the unit charge, since the electrified drops carry the fractional charge on themselves can not.
In addition, Millikan irradiated the oil suspension with X-rays and additionally ionized its organic molecules in order to increase their electrification and extend the experimental observation time, while simultaneously increasing the voltage in the chamber, and did so many times to refine the data obtained. Finally, having accumulated enough experimental data for statistical processing, Millikan calculated the value of the unit charge and published the results, which contained the calculated electron charge as accurately as possible for those years.
Millikan's experience was extremely laborious. The scientist had, in particular, to constantly measure and take into account the air humidity and atmospheric pressure - and so throughout all five years of continuous monitoring of his installation. The award for the titanic work was the 1923 Nobel Prize in Physics, awarded to Millikan for his 1913 publication. Interestingly, for all the seeming simplicity of Millikan's camera, it did not become a museum piece. Already in the 1960s, when the quark hypothesis appeared ( cm. Standard Model), modern, improved installations were built, operating according to the above-described principle, on which scientists unsuccessfully searched for free quarks. Since it was not possible to find such (quarks of various types must have electric charges equal to 1/3 and 2/3 of the electron charge), this served as an additional confirmation of the theory according to which quarks in a free form in modern nature do not occur and are always in a bound state inside other elementary particles.
Robert Andrews Millikan, 1868-1953
American physicist. Born in Morrison, Illinois, the son of a Congregational priest and female parish school teacher. After graduating from Oberlin College in Ohio, he taught Greek for some time and, concurrently, physics in elementary school. Carried away by the latter, he entered the physics department of Columbia University, after which he completed a one-year practice in the leading laboratories of Europe, and then was enrolled in the teaching staff of the University of Chicago. There he received universal recognition as an authoritative teacher (in particular, for many years physics was taught in American schools from his textbooks). In the same place, in Chicago, he spent over a number of years his famous experiment, which made it possible for the first time to determine with sufficient accuracy the charge of an electron and put Millikan in the forefront of representatives of American science. At the same time, the scientist was engaged in active social activities and, to some extent, contributed to the formation of a new image of a socially active intellectual in the minds of the mass reader.
During the First World War, as Colonel, Millikan led the US Signal Corps. The scientist devoted a lot of time to the organization of research institutions and in 1921 he actually headed the newly created California Institute of Technology in Pasadena. At the same time, Millikan did not abandon his research activities, being one of the pioneers of cosmic ray physics. As a result, he became the personified symbol of his generation of scientists, continuing the traditions of the Englishmen John Tyndall and Michael Faraday, and anticipated the emergence of such outstanding popularizing scientists as Carl Sagan.
The existence of particles with the smallest electric charge has been proven by many experiments. Consider the experiences conducted by the Soviet scientist A.F. Ioffe and, independently of him, by the American scientist R. Millikan.
Let us first get acquainted with the physical phenomenon used in these experiments. This phenomenon consists in the fact that under the influence of light ( especially ultraviolet 1) negative: body charge decreases. For example, a negatively charged zinc plate is discharged under the influence of ultraviolet light (Fig. 220).
Figure 221 shows the setup used in the experiment of A.F. Ioffe. In a closed vessel there were two metal plates P, arranged horizontally. From camera A through hole 0 into the space between the plates small charged dust particles of zinc. These dust particles were observed under a microscope.
1. Ultraviolet radiation is the same radiation that causes tanning of human skin; it is present not only in sunlight, but also in the light of special electric lamps.
Let us assume that a speck of dust k is negatively charged. Under the influence of gravity F T, it will begin to fall down. But its fall can be delayed if the lower plate is charged with a negative charge, and the upper one with a positive charge. In the electric field between the plates, an electric force F el will act on a speck of dust.This force is proportional to the charge of the dust grain: the greater the charge of the dust grain, the greater will be the force F el; acting on her. You can charge the plates in such a way that this force will balance the force of gravity: F el \u003d F T. Under these conditions, a grain of dust will be in equilibrium for an arbitrarily long time. Then the negative charge of the dust particle was reduced by acting on it with ultraviolet light. The grain of dust began to fall, since the force Fel acting on it decreased due to a decrease in the charge of the dust grain. By imparting an additional charge to the plates, and thereby increasing the electric field between the plates, the dust grain was again stopped. This was done several times.
Ioffe Abram Fedorovich (1880-1960) - Soviet physicist, academician. He owns a number of discoveries in the field of the study of solids, dielectrics and semiconductors. A. F, Ioffe is one of the major organizers of physics research in the USSR.
Experiments showed that in this case all changes in the charge of the dust grain were an integer number of times (i.e., 2.3, 4, 5, etc.) more than the initial charge of the dust grain. Consequently, the charge of the dust grain changed in certain portions. From this experience A.F. Ioffe made the following conclusion: “When illuminated with ultraviolet light, a speck of dust loses its negative charge not continuously, but in separate portions. The charge of a dust grain is always expressed in integer multiples of the elementary charge e 0. But the charge leaves a speck of dust together with a particle of matter.Consequently, in nature there is such a particle of matter that has the smallest charge, which is no longer divisible. This particle was called an electron. "
The value of the electron charge was first determined by R. Millikan. In his experiments, he used small droplets of oil, observing their movement in an electric field.
The electron mass turned out to be 9.1 10 -28 g, it is 3700 times less than the mass of a hydrogen molecule, the smallest of all molecules. Electric charge is one of the basic properties of an electron. It is impossible to imagine that this charge can be "removed" from electron, it is inseparable property of the electron. An electron is a particle with the least negative charge.
An exercise. In the described experiment, the bottom plate was negatively charged. The drop, which was previously in equilibrium, began to move upward. How has its charge changed? Has the number of electrons on it increased or decreased?
Figure 1 shows a diagram of the setup used in the experiment of A.F. Ioffe. In a closed vessel, the air from which was evacuated to a high vacuum, there were two metal plates Parranged horizontally. From the camera AND through the hole ABOUT whether small charged zinc dust has fallen into the space between the plates. These dust particles were observed under a microscope.
Let's assume that a speck of dust is negatively charged. Under the influence of gravity, it begins to fall down. But its fall can be delayed if the lower plate is charged with a negative charge and the upper one with a positive charge. In the electrostatic field between the plates, a force \\ (~ \\ vec F_ (el) \\) will act on the dust, which is proportional to the charge of the dust. If a mg = F el, then the speck of dust will remain in equilibrium for an arbitrarily long time. Then the negative charge of the dust particle was reduced by acting on it with ultraviolet light. The speck of dust began to fall as the force \\ (~ \\ vec F_ (el) \\), acting on it, decreased. By imparting an additional charge to the plates and thereby increasing the electric field between the plates, the dust particle was again stopped. This was done several times.
Experiments have shown that the charge of a grain of dust always changed abruptly, a multiple of the electron charge. From this experiment, A.F. Ioffe made the following conclusion: the charge of a dust grain is always expressed as integer multiples of the elementary charge e... There are no smaller "portions" of electric charge capable of transferring from one body to another in nature. But the charge of the dust particle leaves along with the particle of matter. Consequently, in nature there is such a particle of matter that has the smallest charge, then it is already indivisible. This particle was named electron.
The value of the electron charge was first determined by the American physicist R. Millikan. In his experiments, he used small oil droplets, observing their movement in an electrostatic field (Fig. 2). In these experiments, the speed of movement of oil droplets was measured in a uniform electrostatic field between two metal plates. A drop of oil, which does not have an electric charge due to air resistance and buoyancy force, falls at a certain constant velocity, since \\ (~ m \\ vec g + \\ vec F_A + \\ vec F_c \u003d 0 \\).
If on its way a drop meets an ion and acquires an electric charge q, then, in addition to gravity \\ (~ m \\ vec g \\), \\ (~ \\ vec F_c \\) and \\ (~ \\ vec F_A \\), the force \\ (~ \\ vec F_ (el ) \\). Then, with a steady motion \\ (~ m \\ vec g + \\ vec F_A + \\ vec F_c + \\ vec F_ (el) \u003d 0 \\). By measuring the speed of the drop, Millikan was able to determine its charge.
Literature
Aksenovich L.A. Physics in high school: Theory. Tasks. Tests: Textbook. allowance for institutions providing the receipt of general. environments, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Minsk: Adukatsya i vyhavanne, 2004 .-- P. 210-211.
Experiments by Milliken and Ioffe to measure the electron charge. Discreteness of the electric charge.
Date: 1910–1913.
Methods:quantitative comparison of direct observation with theory.
Directness of the experiment: direct observation + theoretical analysis.
The artificiality of the studied conditions: artificial conditions under which the used model is applicable.
The fundamental principles under study:discreteness of the electric charge.
In the experiment of Robert Andrews Millikan (1858-1953) microdroplets of oil were investigated TO (see fig. on the right), electrified by friction against air, as well as the capture of air ions, ionized by ultraviolet radiation. If you place such a drop in a vertical vessel with air, then it will begin to fall, and its constant falling speed will soon be established, corresponding to the balance of the Archimedes force, the force of viscous friction and the force of gravity:
where is the density, volume and radius of the drop, respectively, is the coefficient of air resistance, expressed through its viscosity according to Stokes' law, is the density of air. If now a vertically directed field with an intensity is created in the vessel, then a term will appear on the left side of the equation above, where is the charge of the drop. In the experiment, the oil passed through a special spray chamber R, directed into the space between two metal plates, the potential difference between which was up to several kilovolts (see Fig.). At first, with the voltage off, the drop began to fall, while it was observed through a microscope M, fixing the steady-state falling speed. However, before the drop fell on the bottom plate, the voltage was turned on so that the electric field lifted the drop, and the steady-state rate of the drop rising up was calculated. Turning on and off the field in time, the drop was forced to rise and fall many times, while it was easy to calculate its charge. It turned out that it was different in different dimensions, but all the time a multiple of the same value elementary charge
This value of the charge was subsequently associated with the charge of the electron. In fact, it is believed that the drop simply captured positively or negatively charged ions in the course of its movement.
If we talk about the features of the Millikan experiment, then we can say that specially purified air was used in it, and the chamber through which the drop rose and fell was illuminated with the light of an electric arc. On the one hand, this made the drop visible, and on the other hand, it ionized the air, which made it possible for the drop to capture its ions. In addition, as shown in the figure, the spray was above the top plate, in which, however, there was a small hole ABOUTthrough which only individual drops fell into the space between the plates, in which there was an electric field. In Millikan's experiment, droplets on the order of a micrometer were used.
A similar experiment was carried out by Abram Fedorovich Ioffe (1890–1960) with a difference of only a couple of years (Ioffe published his work in 1913, after Millikan, therefore, the latter is usually referred to in the literature). In his experiment, it was not oil droplets that were balanced by an electric field, but metal dust particles, which were electrified with the help of ionizing radiation (here, however, the charge had to be always positive, since the dust particle had to lose electrons as a result of absorption of quanta of this radiation). Since the density of the metal significantly exceeds the density of air, the Archimedes force is insignificant; moreover, in Ioffe's experiment, the equilibrium of the particles was observed, and not their uniform movement, which was provided by adjusting the voltage between the plates.
The peculiarity of Ioffe's experiment was that the dust particles thrown into the condenser chamber were not initially neutral, but it was possible to notice that under the action of ultraviolet radiation they lost a negative charge, which indicated precisely this sign of the electron charge. This is nothing more than a photo effect discovered and studied by Stoletov.
As a result of the experiments of Milliken and Ioffe, a fundamental fact for physics was established - the discreteness of an electric charge - and a quantitative characteristic of discreteness was found. Nevertheless, in modern theoretical physics, there are objects with fractional charges. These are quarks, the charges of which are elementary in absolute value. However, these particles do not exist in a free form, and their bound states - hadrons - already have a whole charge (in elementary units). Nevertheless, in experiments on the scattering of high-energy particles by hadrons, the values \u200b\u200bof the quark charges inside them, multiples of a third of the elementary charge, were actually obtained.
The value of the elementary electric charge is closely related to the dimensionless constant fine structure, which determines the strength of electromagnetic interaction and is known today with amazing accuracy:
One of the theoretical explanations for the discreteness of the charge was proposed at the beginning of the 20th century by Kaluza and Klein based on the concept of higher dimensions of space-time. Nevertheless, the discreteness of the electric charge remains accepted today, but not explained.
Description of the presentation by individual slides:
1 slide
Slide Description:
The experience of Joffe and Milliken. Completed by the teacher of physics MKOU "Secondary school with. legostaevo "Pronkina VS Divisibility of electric charge.
2 slide
Slide Description:
The experience of Ioffe and Milliken. By the beginning of the XX century. the existence of electrons has been established in a number of independent experiments. But, despite the enormous experimental material accumulated by various scientific schools, the electron remained, strictly speaking, a hypothetical particle. The reason is that there was not a single experiment in which single electrons would participate.
3 slide
Slide Description:
The experience of Ioffe and Milliken To answer this question, in 1910-1911, the American scientist Robert Andrews Milliken and the Soviet physicist Abram Fedorovich Ioffe independently carried out precise experiments in which it was possible to observe single electrons.
4 slide
Slide Description:
5 slide
Slide Description:
6 slide
Slide Description:
Experiment of Ioffe and Millikene In their experiments, in a closed vessel 1, the air from which was pumped out to a high vacuum, there were two horizontally arranged metal plates 2. Between them, through a tube 3, a cloud of charged metal dust particles or oil droplets was placed. They were observed through a microscope 4 with a special scale, which made it possible to observe their settling (falling) downward. Suppose that dust particles or droplets were negatively charged before being placed between the plates. Therefore, their settling (falling) can be stopped if the lower plate is charged negatively, and the upper plate is positively charged. So they did it, achieving equilibrium of the dust grain (droplets), which was observed through a microscope. Then the charge of the dust particles (droplets) was reduced by acting on them with ultraviolet or X-ray radiation. Dust particles (droplets) began to fall as the supporting electric force decreased.
7 slide
Slide Description:
the experiment of Ioffe and Milliken By imparting an additional charge to the metal plates and thereby increasing the electric field, the speck of dust was again stopped. This was done several times, each time calculating the charge of the dust grains using a special formula. The experiments of Milliken and Ioffe showed that the charges of droplets and dust particles always change abruptly. The minimum "portion" of the electric charge is an elementary electric charge equal to e \u003d 1.6 · 10-19 C. However, the charge of a grain of dust does not go away by itself, but together with a particle of matter. Consequently, in nature there is such a particle of matter that has the smallest charge, then already indivisible - the electron charge. Thanks to the experiments of Ioffe-Milliken, the existence of the electron has turned from a hypothesis into a scientifically proven fact.