Theoretical information. In 1744, Lomonosov stated the principles of molecular theory and explained many phenomena from molecular-kinetic point of view

In 1744, Lomonosov stated the principles of molecular theory and explained many phenomena from molecular-kinetic point of view. According to the theory, molecules of gas or liquid are carried from one place to another and change their velocities under some impacts. Thus, molecules take part in thermal motion.

Molecules impart their energy and momentum one to the other making more than five milliard collisions with other molecules every second. It promotes smoothing out inhomogeneities and leads to the processes generally called transmission processes.

Alignment of temperatures in space is called heat conduction. Energy is transferred during heat conduction.

Immixture of different substances and mass transfer are called diffusion.

Transforming direct motion of the given layer of molecules into chaotic thermal motion is called internal friction or dynamic viscosity. The substance transfers momentum during internal friction because the fast layer molecules penetrate into the slower layer, bring larger momentum to the slow layer molecules, and speed the slow layer up. On the contrary, the fast layer slows down until speeds of all layers become the same.

The Newton’s law is applied to a steady laminar stream of a viscous liquid (see Fig. 6.2)

,

where F is a force of internal friction in the liquid or gas on area S;

is a coefficient of internal friction (dynamic viscosity);

is a velocity gradient of liquid stream in the direction perpendicular to area ;

dv is a difference of velocities of two layers separated by dz distance.

Sign minus shows that the direction of force F and the direction of velocity gradient are opposite.

The coefficient of internal friction h depends on a liquid type. The temperature of a liquid being raised, the coefficient h decreases according to the exponential law:

,

where h_o is a coefficient independent of temperature;

k =1.38 – Boltzmann’s constant;

U is the energy of a molecule necessary to change its place.

There exists the general formula to determine the coefficient

,

where v is a mean velocity of the molecules; l is amean free path of the molecules; is a density of the liquid.

There are many ways to determine the coefficient experimentally. Among them, the Stokes method is the most popular. It is based on the sped of a small ball dropped into a liquid.