Question

A clock face has charges -q, - 2q, - 3q,-4q,.....-12q fixed at the position of the corresponding numerals on the dial. The clock hands do not disturb the net field due to point charges, At $x$:$30hrs$, the hour hand point in the direction of the electric field at the centre of the dial. Find $x$

Answer 1

The net electric field due to the oppositely faced charges would be in the direction of -7q, -8q, -9q, -10q, -11q and -12q and the magnitude of these charges would be -6q each.
The resultant of -10q, -11q and -12q would point in the direction of -11q and the resusltant of -7q, -8q and -9q would point in the direction of -8q.
Now finally the resultant of -11q and -8q would point in the direction of 9:30.
Thus the answer is 9.