Coulomb's Law & Electric Fields

📋 Teacher notes

• •Coulomb's law: F = kQ₁Q₂/r², where k ≈ 8.99×10⁹ N·m²/C² (often written 1/4πε₀).
• •Like charges REPEL (positive force by the sign convention here), unlike charges ATTRACT — the direction follows automatically from the signs of Q₁ and Q₂ in the formula.
• •The force obeys an INVERSE-SQUARE law: doubling the separation cuts the force to a quarter, not a half — a very commonly tested numeric trap.
• •Coulomb's law has the same mathematical FORM as Newton's law of gravitation (F=Gm₁m₂/r²) — both are inverse-square laws — but electric forces can be attractive OR repulsive, while gravity is always attractive.
• •The released charges shown here accelerate because force causes acceleration (F=ma) — as they separate, the force (and so the acceleration) drops rapidly, which is why the motion visibly slows down even though the charges keep moving apart.