Charged ball in a vertical circle

A small ball of mass $$2 \times 10^{-3}$$ kg having a charge of $$1 \times 10^{-6}$$ C is suspended by a string of length 0.8 m.Another identical ball having the same charge is kept at the point of suspension. Determine the minimum velocity which should be imparted to the lower ball so that it can make a complete revolution.(Mass of the other ball doesn't matter)

Note: This problem is taken from some other source.

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