In the adjoining figure a charge Q is fixed. Another charge q is moved along a circular arc MN of radius r . . .

Question : In the adjoining figure a charge Q is fixed. Another charge q is moved along a circular arc MN of radius r around it, from the point M to the point N such that the length of the arc MN=l. The work done in this process is : [ISC 2016]

a) Zero
b) Qql/4πε₀
c) Qql/2ε₀r²
d) Qq/2πε₀r²

Doubt by Mohini

Solution : 

We know, 
Work done = Potential Difference × Charge
W=(V_N-V_M)×q

But V_N=V_M=kQ/r
So V_N-V_M=0
Hence
W=(0)×q
W=0

∴ a) Zero is the correct option.