1900 is an important dividing line for the study of Brownian motion. At this point, the appropriate physical model of Brownian motion is obvious, and the remaining problem is to make a quantitative theoretical description.
Einstein's Brownian motion theory
1905, Einstein put forward Brownian motion theory according to the principle of molecular motion theory. At about the same time, Smo Ruhoff also made the same achievement. Their theory satisfactorily answers the basic questions of Brownian motion.
It should be pointed out that the historical background of Einstein's work was the debate about molecular authenticity in the scientific community at that time. This argument has a long history, which has existed since the theory of atoms and molecules came into being. At the beginning of this century, some people, represented by physicist and philosopher Mach and chemist ostwald, once again criticized the theory of atoms and molecules. They doubt the authenticity of atoms and molecules from the perspective of positivism or phenomenology, making this debate a central issue in the forefront of science. To answer this question, apart from philosophical differences, science itself needs more powerful evidence to prove the real existence of atoms and molecules. For example, the relative atomic mass and molecular mass measured in the past are only relative comparison values of mass. If they are true, then the absolute values of relative atomic mass and relative molecular mass can and must be measured. Such a question needs someone to answer.
Because of the above situation, as Einstein pointed out in his paper, his purpose is to "find the most convincing facts that can confirm the existence of atoms of a certain size." He said: "According to the theory of thermomolecular motion, because of thermomolecular motion, an object whose size can be seen by a microscope is suspended in a liquid, and its size can be easily observed by a microscope. Perhaps the movement discussed here is the so-called Brownian molecular movement. He believes that as long as this movement and expected regularity can be actually observed, "it is possible to accurately determine the actual size of the atom. "On the other hand, if the prediction about this kind of movement proves to be incorrect, it provides strong evidence against the popular views of molecular movement.
Einstein's achievements can be roughly divided into two aspects. One is based on the principle of molecular thermal motion.
Is the statistical average of particle displacement in a certain direction in t time, that is, the root mean square value, and d is the diffusion coefficient of particles. This formula is the inevitable result that seemingly irregular Brownian motion obeys the law of molecular thermal motion.
The second aspect of Einstein's achievement is that for spherical particles, it is deduced that it can be calculated
Where η is the viscosity of the medium, A is the particle radius, R is the gas constant, and NA is the Avon Gadereau constant. According to this formula, the absolute mass of atoms and molecules can be obtained as long as the accurate diffusion coefficient d or Brownian motion average orientation is actually measured. Einstein estimated the NA value of 3.3× 1023 by using the diffusion coefficient of sugar in water measured by predecessors. One year later (1906), it was revised to 6.56× 1023.
Einstein's theoretical achievements have found a way to prove the authenticity of molecules, and also satisfactorily expounded the roots and regularity of Brownian motion. The next work is to test the reliability of this theory with sufficient experiments. Einstein said: "I don't want to compare the scarce experimental data I can get with the results of this theory here, but leave it to those who have mastered this problem in the experiment." "I hope a researcher can successfully solve this problem of great significance to thermal theory immediately!" This task put forward by Einstein was successfully completed by Belan (1870- 1942) and Svard Berg respectively. It should also be mentioned that an important experimental progress in the study of Brownian motion at the beginning of this century was that 1902 Siegmund (1865-1929) invented the ultramicroscope, with which the Brownian motion of colloidal particles can be directly seen and measured, which proved the authenticity of colloidal particles. That's why Zigmond won 1925. Swedberg used ultramicroscope to measure Brownian motion.
Brownian motion represents a random fluctuation phenomenon, and its theory also has important applications in other fields. For example, the research on the accuracy limit of measuring instruments; Research on background noise in high amplification telecommunication circuits.
Brownian motion is different from molecular thermal motion, which is related to temperature and particle number. The higher the temperature, the more intense Brownian motion, the fewer particles and the more intense molecular thermal motion.