Measurement of the distribution function of silver atoms by the Stern-Lammert method

Measurement of the distribution function of silver atoms by the Stern-Lammert method

MOLECULAR KINETIC THEORY is a branch of molecular physics that studies the properties of matter based on ideas about their molecular structure and certain laws of interaction between atoms (molecules) that make up matter. It is believed that the particles of matter are in continuous, disorderly motion and this movement is perceived as heat.

Until the 19th century. a very popular basis for the theory of heat was the theory of caloric or some liquid substance flowing from one body to another. The heating of the bodies was explained by an increase, and cooling - by a decrease in the caloric contained within them. For a long time, the concept of atoms seemed unnecessary for the theory of heat, but many scientists already then intuitively associated heat with the movement of molecules. So, in particular, the Russian scientist M.V. Lomonosov thought. It took a long time before the molecular-kinetic theory finally triumphed in the minds of scientists and became an integral part of physics.

Velocity distribution of molecules. For a gas in a closed vessel, the result of numerous collisions of molecules with each other and with the walls of the vessel is a fairly rapid establishment of a universal velocity distribution of molecules, which was theoretically obtained by Maxwell in 1860. At the level of the macroscopic description of gas, the Maxwellian velocity distribution corresponds to the state of thermal equilibrium in gas: pressure and temperature in all places inside the vessel are the same.

Gas molecules, even in equilibrium, move randomly, colliding with each other and with the vessel wall, continuously changing their speed. This means that at every moment of time there are molecules in the gas that have very different velocities. At the same time, since the pressure and temperature in the gas remain constant, no matter how the velocity of the molecules changes, the mean value of its square remains constant. This turns out to be possible only in the presence of a constant in time and the same distribution of molecules by velocities in all parts of the vessel.

In the 1920s, a real opportunity appeared to experimentally test the Maxwellian law of molecular velocity distribution. The first device for this purpose, consisting of two coaxial cylinders, was designed by the German physicist Stern. A platinum filament heated by an electric current was stretched along the axis of the device, from the surface of which silver atoms evaporated. Under the conditions of a vacuum created inside the device, a narrow beam of these atoms, moving in the radial direction, passed through a longitudinal slit on the surface of the inner cylinder and settled in the form of a narrow vertical strip on the surface of the outer cylinder. If the entire device is brought into rotation, then during the time while the silver atoms fly through the gap between the cylinders, the device manages to turn through a certain angle and the position of the beam trace on the outer cylinder will shift relative to the initial one. It is not difficult to establish a connection between this displacement and the magnitude of the velocity in the beam of molecules and the angular velocity of rotation of the device. Studies of the wake profile, which is blurred due to the presence of a velocity distribution in the beam, made it possible to establish a qualitative picture of this distribution, which approximately corresponded to the Maxwell one.

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