Question

What value must q2 have if the electric potential at point a is to be zero?

Answer 1

 let r be side of square 
V (B) = k q2/r + k Q / sqrt[r^2 + r^2] = 0 given 
q2/r + Q / r root2 = 0 
q2 = - Q / root2 
q2/Q = - 1/root2 
q2/Q = - root2 /2 = - 0.707 

q2 equals negative 0.707 times Q