Question

Two positive charges each of magnitude 5C are r metres apart in vaccum. If the force between them is 10N and C1=1/square root of 4pie permittivity. Express r in terms of C1.

Answer 1

The equation for the force:

$$F=C_1\frac{q·q}{r}=C_1\frac{q^2}{r^2},\\\space\\ 10=C_1\frac{25}{r^2},\\\space\\ 1=C_1\frac{2.5}{r^2},\\\space\\ r=2.5C_1.$$