5) Half a ring with total charge Q and radius a exerts an electric field at point P (geometrical center of the semicircle). On the point P is placed a negative charge -Q. The negative charge is located at equilibrium held by the electric force, find the mass of the charge in terms of the variables of the problem.

5) Half a ring with total charge Q and radius a exerts an electric field at point P (geometrical center of the semicircle). On the point P is placed a negative charge -Q. The negative charge is located at equilibrium held by the electric force, find the mass of the charge in terms of the variables of the problem.