(a)
Electrostatic force on the first sphere, F=0.2N
Charge on this sphere, q1=0.4μC=0.4×10−6C
Charge on the second sphere, q2=−0.8μC=−0.8×10−6C
Electrostatic force between the spheres is given by the relation,
F=q1q24πε0r2
And, 14πε0=9×109Nm2C−2
Where, ε0= Permittivity of free space
r2=q1q24πε0F
=04×10−6×8×10−6×9×1090.2
=144×10−4
r=√144×10−4
=0.12m
The distance between the two spheres is 0.12 m.
(b) Equal and Opposite Force acts on the other sphere (By newton's Third Law). Hence 0.2 N attractive.