Where,
K=14πϵo=9×109
Total potential at point a, Va=9×109×3×10−910−2+9×109×(−3×10−9)2×10−2=13.5×102 V
Total potential at point b, Vb=9×109×3×10−92×10−2+9×109×(−3×10−9)10−2=−13.5×102 V
Potential difference between point a and b, Vab=Va−Vb=27×102 V
Work done in moving the charge q′ from a to b, Wab=q′Vab
Wab=2×10−9×2700=5.4×10−6 J
From work-energy theorem : Wab=ΔK.E
⇒ 5.4×10−6=12×0.005×v2
⇒v=0.0465 ms−1=4.65 ms−1