A rigid ring is made to roll along the ceiling of a room, where exists a uniform horizontal magnetic field of induction B perpendicular to the plane of the ring. The velocity of the center of the ring is constant and its modulus is v. A charge particle P of mass m is fixed on the ring. The charge q on the particle is  $\frac{kmg}{Bv}$ so that there is no force of interaction between the ring and the particle then find the value of k?