ตอบ: # "0.5 m" # คำอธิบาย: #F = (kq ^ 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 "#