A metal wire `PQ` of mass `10 gm` lies on two horizontal metal rails

A metal wire `PQ` of mass `10 gm` lies on two horizontal metal rails separated by `4.90 cm`(figure).A vertically downward magnetic field of magnitude `0.800 T` exists in the space.The resistance of the circuit is slowly decreased and it is found that when the resistance goes below `20.0 Omega` the wire `PQ` starts sliding on the rails.Find the coefficient of friction.Neglect magnetic force acting on wire `PQ` due to metal rails `(g=9.8 m//g^(2))`