## Conceptual Physics (12th Edition)

Use Coulomb's Law, $F = k \frac{q_{1} \; q_{2}}{d^{2}}$. First calculate the force in situation A. We are interested in magnitudes only, so we ignore the signs of the charges. $$F_{A} = k \frac{(4q)(2q)}{x^{2}}$$ Next calculate the force in situation B. We are interested in magnitudes only, so we ignore the signs of the charges. $$F_{B} = k \frac{(3q)(3q)}{x^{2}} = \frac{9}{8} F_{A}$$ Finally, calculate the force in situation C. We are interested in magnitudes only, so we ignore the signs of the charges. $$F_{C} = k \frac{(2q)(2q)}{(x/2)^{2}}= 2 F_{A}$$