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Measurement of the speed of movement of gas molecules. The Stern

The study of diffusion and Brownian motion gives some idea of the speed of random motion of gas molecules. One of the simplest and most obvious experiments to determine the experience Of. Stern, made 1920 g. The essence of this experience lies in the following.

On a horizontal table, which can rotate around the axis Of (rice. 3.2), perpendicular to the table are strengthened cylindrical surface And ö. The surface In solid, and in the surface And there is a narrow slit, parallel to the axis Of. Along the axis is vertical platinum On silver plated wire, which is included in the electric circuit. When a current wire is heated and its surface the evaporation of silver. Molecules of silver flying in all directions and is mostly deposited on the inside cylindrical surface And. Only a narrow beam of molecules of silver flies through the crack in the surface and settles in the area M on the surface In. Width of plaque in M is determined by the slit width in the surface And. To molecules of silver is not dissipated by collisions with air molecules, the whole installation is covered with a cap, from beneath which the air is pumped out. The narrower the slit in the surface And, the narrower the plaque in Meters and the more accurate can be determined the speed of motion of molecules.

 

IMG 20131121 0002 crThe very definition of speed V based on the following idea. If the installation to bring into rotation around the axis O with a constant angular velocity w, during the time t, during which the molecule will fly from the gap to the surface In, last time to turn and the RAID will move from region M To region. Therefore, the flight time of the molecules along the radius r and the time offset of the point M on the surface In the distance L=KM equally. As the molecule flies evenly, it

 

where v is the unknown velocity, rAnd — the radius of the cylindrical surface And. Since the linear velocity of points on the surface In equal wr,the time t can be expressed by another formula.

As w, r and rA when performing the experience remain constant and defined in advance, it, measuring L, you can find the speed of the molecule with. In the stern she was close to 500 m/s.

As the plaque in It is blurred, it is possible to conclude, that the molecules of silver are flying to the surface with the difference In speed. Average speeds of the molecules can be mathematically expressed by the formula

As an example, note, that at 0°C the average speed of hydrogen molecules is equal to 1840 m/s, and nitrogen 493 m/s. The change in the thickness of plaque in the area To give an idea about the distribution of molecules over velocities of their motion. It turns out, what a small number Molekthe street has speed, several times higher than the average speed.

(Think, where on the rice. 3.2 left a trail of molecules, speed greater than average velocity v, and how to change the position of the plaque, increasing the current in the wire O.)

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