# Two equal charges of magnitude 1.1 x 10-7 C experience an electrostatic force of 4.2 x 10-4 N. How far apart are the centers of the two charges?

$\text{0.5 m}$
$F = \frac{k {q}^{2}}{d} ^ 2$
d = qsqrt(k/F) = 1.1 × 10^-7 "C" × sqrt((9 × 10^9 "Nm"^2//"C"^2)/(4.2 × 10^-4 "N")) = "0.5 m"