Question

1. Two point charges q1 = 5.25 x 10-9 C and q2 = -8.75 x 10-9 C are 1.00 m apart in air. (a) What

force exist between them? Is this an attractive or a repulsive force? (b) The charges are allowed

to touch and separate again by 1.00 m. What force exists between them now? Is this an

attractive or a repulsive force?

Answer 1

The Coulomb's law says

$$F=k\frac{|q_1||q_2|}{r^2}$$

(a)

$$F=9*10^9*\frac{5.25*10^{-9}*8.75*10^{-9}}{1.00^2}=4.13*10^{-7}\:\rm N$$

The force is attraction.

(b)

$$q_1'=q_2'=\frac{q_1+q_2}{2}=-3.5*10^{-9}\:\rm C$$ $$F=9*10^9*\frac{3.5*10^{-9}*3.5*10^{-9}}{1.00^2}=1.10*10^{-7}\:\rm N$$

The force is repulsion.