Question

Positive point charges are placed at the vertices of a star shape as shown in the figure. Direction of the electrostatic force on a negative point charge at the centre $O$ of the star is:

• A. Neither along horizontal or vertical direction.
• B. Vertically up
• C. Towards left
• D. Vertically down

Answer 1

The correct option is A Neither along horizontal or vertical direction.

The charges are on the circumference of a circle and are equally spaced. Electric field due to two q charges facing each other is zero.

Electric field due to 2q and q facing each other is $k\frac{q^2}{r^2}$ where r is the radius of the circle and its direction is towards q.

Electric field due to charges q and 3q facing each other is $k\frac{2q^2}{r^2}$ and is towards charge q.

The resultant of these two unequal forces will be neither along the horizontal or vertical direction.