CHARGE consists in product of difference of POTENTIALS at points A and B and value of test charge. Potential at A \[{{V}_{A}}=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{q}{a}\] Potential at B \[{{V}_{B}}=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{q}{a}\] Thus, work done in carrying a test chargeQ from A to B. \[W=({{V}_{A}}-{{V}_{B}})\,(-Q)=0\]