Relation between the value of a function at a point and the limit at a point

What is the relation between the normal reaction acting on a surface and the limiting friction on the surface?

Give a relation between magnetic field at a point on the axis and equatorial point of short magnetic dipole.

Write a relation between electric potential and electric field intensity at a point due to a point charge.

Relation between domain and function of composite functions

The correct relation between elevation of boiling point and mass of solute is