Thermal motion of atoms and molecules

Random motion is ubiquitous. At the molecular level, the thermal motions of atoms and molecules are random. Further, motions in macroscopic systems are often described by random processes. For example, the motion of stirred coffee is a turbulent flow that can be characterized by random velocity components. Randomness means that the movement of an individual portion of the medium (i.e., a molecule, a water parcel, etc.) cannot be described deterministically. However, if we analyze the average effect of many individual random motions, we often end up with a simple macroscopic law that depicts the mean motion of the random system (see Box 18.1). 

Numerous sources of noise in physical measurements are encountered. From the environment and instrumentation side, for example, they include building vibrations, air-flow fluctuations, electric mains fluctuations, stray radiation from nearby electrical apparatus, and interference from high-frequency (radio) transmissions. But also system-intrinsic sources Uke random thermal motion of atoms and molecules, and even the basic quantum nature of matter and energy itself, contribute to the reproducibility of a particular signal. 

The linear instability theory of the behavior of a system near the bifurcation point can be successfully applied to many self-organization problems, such as thermal convection in hydrodynamics4 and crystal growth in solution.5 In these theories, various initial fluctuations play important roles. Occasionally the fluctuations arise from the thermal motion of atoms or molecules. If a system reaches an unstable mode over... 

Thermal energy. Energy associated with the random motion of atoms and molecules. (6.1)... 

Temperature measurement, or thermometry, is the process of determining the quantitative value of the degree of heat, or the amount of sensible thermal energy contained in matter. The temperature of matter is a manifestation of the motion of atoms and molecules, either relative to each other or because of motions within themselves. Absolute zero is a temperature that creates a state in which molecules come to complete rest. 

The thermal movement of molecules often serves as a prototype of random motion. In fact, molecular diffusion is the result of the random walk of atoms and molecules through gaseous, liquid, solid, or mixed media. This section deals with molecular diffusion of organic substances in gases (particularly air) and in aqueous solutions. Diffusion in porous media (i.e., mixes of gases or liquids with solids) and in other media will be discussed in the following section. 

The reason for the smah-step random mohon of atoms and molecules in hquids is the incessant collisions that they suffer with each other. In a beautiful series of papers, Einstein showed that this Brownian mohon is a consequence of the natural motion of the molecules controlled by the temperature of the system [2]. Therefore, Brownian motion is also called thermal mohon of the molecules. Obviously, the molecules of the liquid move faster when the liquid is heated, causing more agitated Brownian movement of the big parhcles. Similarly, if you make the liquid less viscous, the molecules can move more easily, also resulting in faster particle motion. 

The primary structme, as we have already mentioned, is the sequence of units in the chain, that is, the text . It is created during the biosynthesis of each molecule and is memorized in other words, it is not distorted by the thermal motion rniless the whole molecule is destroyed. For a book, it is obvious that thermal motion of atoms in the page is far insufficient to swap letters in a word (making no from on etc.) for a molecular book, it is much less obvious — but still true, because the sequence of letters in either a DNA or a protein is fixed by strong covalent bonds which are practically never broken by thermal fluctuations under normal conditions. 

Eventually, the answer was found by Albert Einstein and the Polish physicist Marian Smoluchowski (1872-1917), then a professor at the University of Lviv. The title of one of Einstein s papers on the theory of Brownian motion is rather telling On the motion of particles suspended in resting water which is required by the molecular-kinetic theory of heat . Einstein and Smoluchowski considered chaotic thermal motion of molecules and showed that it explains it all a Brownian particle is fidgeting because it is pushed by a crowd of molecules in random directions. In other words, you can say that Brownian particles are themselves engaged in chaotic thermal motion. Nowadays, science does not make much distinction between the phrases Brownian motion and thermal motion — the only difference lies back in history. The Einstein-Smoluchowski theory was confirmed by beautiful and subtle experiments by Jean Perrin (1870-1942). This was a long awaited, clear and straightforward proof that all substances are made of atoms and molecules. ... 

---