Quick questions on Coulomb's law and point charges: WACE Year 12 Physics Unit 3

$$F=\frac{kq_1 q_2}{r^2},\qquad k=\frac{1}{4\pi\varepsilon_0}=8.99\times10^9\ \text{N m}^2\text{C}^{-2}.$$

Coulomb's law and Newton's law of gravitation share the same inverse-square form, but with two key differences. Gravity is always attractive, whereas the electrostatic force can be attractive or repulsive depending on sign. The electrostatic force is also vastly stronger: between a proton and an electron it exceeds their gravitational attraction by roughly $10^{39}$ times, which is why gravity is irrelevant at the atomic scale.

When more than two charges are present, the net force on one charge is the vector sum of the individual Coulomb forces from each other charge (the principle of superposition). Calculate each pairwise force separately, then add them as vectors, resolving into components if they are not along the same line.