Question

A dog's fur is combed, and the comb gains a charge of 8.0 nC. Find the electric force between the fur and comb when they are 2.0 cm apart. 

Answer 1

There is no exact answer to this question. Since both dog and comb can not be considered as a point charges if they only 2cm apart.

But, we can attempt to apply Coulomb's law anyway, keeping in mind that the result will be definitely wrong.

$$F = k\dfrac{q_1q_2}{r^2}$$

where $q_1 = q_2 = 8\times 10^{-9}C$ are the modulus of the charges on comb and fur (they are equal according to the charge conservation law), $r = 2cm = 0.02m$ is the distance, and $k = 9\times 10^9Nm^2/C^2$ is the constant. Thus, obtain:

$$F = 9\times 10^9 \cdot \dfrac{(8\times10^{-9})^2}{(0.02)^2} \approx 1.4\times 10^{-3}N$$

Answer. $1.4\times 10^{-3}N$ (taking into account the initial note).