A clock face has charges `-q, -2q, ,.....-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 what time does the hour hand point in the direction of the electric field at the centre of the dial.

Answer

C

Explanation

Six vectors of equal magnitudes are as shown in figure. a.
Now resultant of two vectors of equal magnitudes `(=Esay)` at `120^@ ` is also E and passing through their bisector line.
So, resultant of 1 and 5 is also `E` in the direction of 3. Similarly, resultant of 2 and 6 is also in the direction of 4.
Finally resultant of `2E` in the direction of 3 and `2E` in the direction of 4 passes through the bisector line of 3 and 4 (or 9.30)`