hi

 

see let tension in string be T

electric force be F

at the bootom of the string we can say

 

T - mg - F = m(V^2)/r

 

And at the top of the circular trajectory we can say

 

T + mg - F = m(v^2)/r

 

now in orer to be able to complete a revolution tension in string at the upper most point should be zero

 

so eqn becomes

 

mg - F = m(v^2)/r  .......(1)

 

also by energy conservation

 

(m(V^2))/2  = (m(v^2))/2 + 2mgL   ......(2)

 

using (2) in(1) we get

 

root((5mg - F) L / m) = V = velocity to be given at the lowest point

 

using the values of m , g , and L and calculating F using Kqq/r^2

 

u get V = 5.86 m/s