Physics M5

Radius of action is:

• the the smallest distance between two molecules at which the van der Waals forces become

  small enough to be neglected;

• the greatest distance between two molecules at which the van der Waals forces become small enough to be neglected;

• the greatest distance between two molecules at which the van der Waals forces become maximum

Sphere of molecular action is:

• a sphere centered at the molecule and with radius, equal to the radius of action of that molecule;

• the smallest distance between two molecules at which the van der Waals forces become small enough to be neglected;

• the greatest distance between two molecules at which the van der Waals forces become maximum.

At very small distances:

• the molecules of liquids interact with the repulsive van der Waals forces;

• the molecules of liquids interact with the attractive van der Waals forces;

• the molecules of liquids do not interact with each other.

At larger distances:

• the molecules of liquids interact with the attractive van der Waals forces with the forces decreasing away from the molecule;

• the molecules of liquids interact with the repulsive van der Waals forces;

• the molecules of liquids interact with the van der Waals forces of maximum repulsion.

When the molecule is submerged deeper than radius of action r:

• the molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a zero net force;

• the net force is maximum;

• the net force is normal to the free surface.

When the molecule is in the surface layer with thickness r (radius of action):

• the net force is not zero, it is normal to the free surface, and is directed inwards-cohesion force;

• the molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a zero net force;

• the net force is minimum.

The cohesion pressure is:

• the sum of the cohesion forces acting on all molecules in unit area of the surface layer of the liquid;

• the sum of the surface tension forces per unit length;

• the sum of the surface tension forces per unit time.

The molecules of the surface layer:

• have have higher potential energy than the molecules in the bulk of the liquid;

• have lower potential energy than the molecules in the bulk of the liquid;

• have the same potential and kinetic energies as in the bulk of the liquid.

The free liquid:

• takes a shape with minimum potential energy of the surface tension forces;

• takes a shape with maximum potential energy of the surface tension forces;

• takes a shape with minimum kinetic energy of the surface tension forces.

When a molecule emerges from the bulk to the surface of the liquid:

• its kinetic energy decreases, and its potential energy increases;

• its kinetic energy increases, and its potential energy decreases;

• Its potential and kinetic energies decrease.