We can assimilate the small bead as a single point charge, with charge Q equal to
[tex]Q=-9.0 nC=-9.0 \cdot 10^{-9}C[/tex]
We want to find the electric field at a distance [tex]d=2.4 cm= 0.024 m[/tex]. The electric field of a single point charge is given by
[tex]E=k_e \frac{Q}{d^2} [/tex]
where [tex]k_e = 8.99 \cdot 10^9 N m^2 C^{-2}[/tex]
So, substituting, we find the electric field at the distance d:
[tex]E=(8.99 \cdot 10^9 N m^2 C^{-2}) \frac{-9.0\cdot 10^{-9}C}{(0.024m)^2}=-1.40\cdot 10^5 N/C [/tex]
And the negative sign means the field is directed towards the charge, since the charge is negative.
