Find out, what is the difference between the action of molecular forces inside the liquid and on its surface.

The average value of the resultant molecular forces of attraction, attached to the molecule M₁ which is located inside the liquid (rice. 10.1), close to zero. The random fluctuations of the resultant force of this molecule M₁ to make only chaotic motion inside the liquid. Several different molecules M₂ and M₃, in the surface layer of the liquid.

Will describe around the molecules of the sphere of molecular action radius r_m (order 10^-9 m). Then the molecules M₂ in the lower hemisphere will be a lot of molecules, and at the top is much less, because underneath is liquid, and on top — steam and air. Therefore, for molecules M₂ the resultant molecular forces of attraction in the lower hemisphere R_W a lot more of the resultant molecular forces in the upper hemisphere R_p. Note, what power R_p so small, that it can be neglected. The resultant molecular forces of attraction, attached to the molecule M₃, less, than for molecules M₂, since is determined only by the action of molecules in the blackened area. Significantly, what Ravnodenstvie for molecules M₂ and M₃ face down into the liquid perpendicular to its surface.

Thus, all the molecules of the liquid, in the surface layer thickness, equal to the radius of molecular action (rice. 10.1), lured into the fluid. But the space inside the liquid occupied by other molecules, so the surface layer creates pressure on the liquid, which is called molecular pressure.

If liquid gets inside any body, between the fluid and the body is formed a layer of liquid, which molecular force is directed away from the body inside the fluid, t. e. squeeze the liquid, and the body is not acted upon. It follows, to determine the value of the molecular pressure experienced by not, since it does not affect the body, immersed in the liquid. Theoretical calculations showed, what's the molecular pressure is very high. For example, for water it is about 11*10⁸ PA, and for broadcast 1,4*10⁸ PA.

Now it becomes clear, why using an external pressure considerably difficult to compress a liquid. Indeed, for this purpose the pressure should be of the same order, and molecular pressure of the fluid itself. Since the latter is very large, it is difficult to do. Therefore, at not too high pressures practically liquid can be considered incompressible.