Question

A negative charge of −0.50 × 10⁻⁶𝐶 exerts a repulsive force of magnitude 0.20 N on an unknown charge 0.20 m away. What is the unknown charge (magnitude and sign)? 

Answer 1

$$q2 = - \frac{F * R^2} {k * q1} = - \frac{0.2 * 0.2^2} {9 * 10^9 * 0.5 * 10^{-6}}= - 1.78 * 10^{-6}$$