Electric Charge and Electric Force

Charge comes in two signs, is conserved by every charging process, and is quantized in units of $e$. The force between two point charges has magnitude $F = k|q_1||q_2|/r^2$, directed along the line joining them: toward each other for unlike charges, apart for like ones. The absolute values are deliberate, because the sign of a charge product is not a direction and cannot know which way $+x$ points.

Three errors dominate. Reading a direction off the signed product instead of off the geometry, which scrambles as soon as a third charge joins. Scaling the force as $1/r$ rather than $1/r^2$, so doubling the separation halves instead of quarters. And handing the larger charge the larger force, when $F = k|q_1||q_2|/r^2$ is symmetric in the two charges and only the accelerations differ. A fourth, quieter one is writing $k$ and $\varepsilon_0$ into the same expression, which double-counts a single constant.