Physics---electrostatics4

Let Q and q be the charges on the 0.1m sphere and small charge respectively, then:
Potential energy at 0.5 m apart is:
W = Qq/4πε0(0.5) = Qq/2πε0
which is also the work done against the electric field when bringing from infinity to this position.
And the force is:
F = Qq/4πε0(0.5)2 = Qq/πε0
Then, when the small charge is moved to a distance of 1.0 m,
New potential energy at is:
W = Qq/4πε0
So work done in bringing the small charge from 0.5 m to 1.0 m is:
Qq/4πε0 - Qq/2πε0 = -Qq/2πε0 = -W
And the new force is:
F = Qq/4πε0(1)2 = Qq/4πε0 = F/4