Figure shows two large cylindrical shells having uniform linear charge densities $+\mathrm{\lambda }$  and $-\mathrm{\lambda }$ . Radius of inner cylinder is ‘a’ and that of outer cylinder is ‘b’. A charged particle of mass m, charge q revolves in a circle of radius r. Then, its speed ‘v’ is: (Neglect gravity and assume the radii of both the cylinders to be very small in comparison to their length.)