A matel wire PQ of mass 10g lies at rest on two horizontal metal rails separated by 4.90 cm . 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.

Explanation

Wire `PQ` begins to slide when magnetic force is just equal to the force of friction i.e.
`mumg=ilBsintheta (theta=90^@)`
here `i=E/R=6/20=0.3A`
`:. mu==(ilB)/(mg)=((0.3)(4.9xx10^I-2)(0.8))/((10xx10^-3)(9.8))`
`=0.12`