Q.Three equal charges of $10\times10^{-8}$ C each, located at the corners of a right triangle whose sides are 15 cm, 20 cm and 25 cm respectively. Find the force exerted on the charge located at the $90^\circ$ angle.

The principle of superposition states that when several charges act on a given charge at once, the resultant (net) force on it is the VECTOR SUM of the individual Coulomb forces exerted by every other charge, each computed one pair at a time exactly as though the rest were not present. For N surrounding charges acting on a test charge $q_0$, this gives $\vec{F}=\dfrac{q_0}{4\pi\epsilon_0}\displaystyle\sum_{n=1}^{N}\dfrac{q_n}{r_{0n}^2}\hat{r}_{0n}$ -- a genuine vector sum, never a simple addition of magnitudes, since the individual pairwise forces generally point in different directions and must be combined using vector-addition methods (components, or the parallelogram/triangle law, or the law of cosines when only two forces at a known angle are involved). …