Principle of superposition (electrostatic force) (F_net = Σ F_i (vector))

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F_net = Σ F_i (vector)

What each symbol means

SymbolWhat it stands for
\vec{F}_iForce on test charge due to i-th source charge
\vec{F}_{\mathrm{net}}Vector sum of pairwise Coulomb forces

When to use this

Point charges; add as vectors. Works because Maxwell equations linear in vacuum.