Question

Four charges are rigidly fixed along the Y-axis as shown. A positive charge approches the system along the `X`axis with initial speed just enough to cross the origin. Then its total energy at the origin is

A. zero
B. positive
C. negative
D. data insufficient

Answer 1

Correct Answer - B

There exists a point `P` on the x-axis (other than the origin), where not electric field is zero. Once the charge `Q` reaches point `P`, attractive forces of the two `-ve` charge will dominate and automatically cause the charge `Q` to cross the origin.
Now if `Q` is projected with just enough velocity to reach `P`, its `K.E.` at `P` is zero,
But while being attracted towards origin it acquires `K.E.` and hance its net energy at the origin is positive.
(`P.E.` at origin = zero).