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A matel wire PQ of mass 10g lies at rest...

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.

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