Revise Electric force between point charges for Cambridge 9702 (18.3)

Coulomb's Law is the cornerstone for understanding how electric charges interact. It specifically describes the electrostatic force that exists between two stationary point charges. For uniformly charged spheres, the charge can be treated as if it were concentrated at the center, allowing the law to be applied to larger objects as well. This force always acts directly along the straight line that connects the centres of these two charges, dictating whether they are drawn together or pushed apart.

$F = \frac{1}{4\pi\epsilon_0} \frac{Q_1Q_2}{r^2}$

Let's break down the formula. $Q_1$ and $Q_2$ represent the magnitudes of the point charges, measured in Coulombs (C). $r$ is the distance separating their centres, in metres (m). The constant term $k_e = 1/(4\pi\epsilon_0)$ is crucial. Here, $\epsilon_0$ is the permittivity of free space, a fundamental physical constant with an approximate value of $8.85 \times 10^{-12} \text{ F m}^{-1}$. The combined constant, $k_e$, is approximately $8.99 \times 10^9 \text{ N m}^2 \text{ C}^{-2}$, which is often used to simplify calculations.

What happens when more than two charges are present? The principle of superposition applies. The net electrostatic force on any one charge is the vector sum of the individual forces exerted on it by all other charges. Each force is calculated independently using Coulomb's Law and then all forces are added together as vectors, taking their directions into account.