Chapter 19 - Electric Charges, Forces, and Fields - Problems and Conceptual Exercises - Page 689: 105

(C) $4.4cm$

We can find the required distance as follows: $F=K\frac{q^2}{r^2}$ $r=q\sqrt{\frac{K}{F}}$ We plug in the known values to obtain: $r=(93\times 10^{-12})\sqrt{\frac{8.99\times 10^9}{4.0\times 10^{-6}}}$ $r=0.044m$ $r=4.4cm$