[tex]F = K_o . \frac{Q}{d^{2}} [/tex]
[tex]K_o = 9.10^{9}
d = 1 cm = 0,01 m = 1.10^{-2} m
F = 9.10^{-1}
microcoulomb = 10^{-6} [/tex]
[tex]Q = \frac{F.d^{2}}{K_o} = \frac{9.10^{-1}.1.10^{-2}.1.10^{-2} }{9.10^{9}} = \frac{1.10^{-5} }{1.10^{9}} = 1.10^{-14} = 10.10^{-6}.1.10^{-8} [/tex]
[tex]Q = 1.10.^{-7} microcoulomb[/tex]