Question

A metal wire PQ of mass 10 g lies at rest on two horizontal metal rails separated by 5 cm as shown in fig. A vertically downward magnetic field of magnitude 0.80 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.

Answer 1

For equilibrium, the magnetic force is equal to the friction force.
$BIl=\mu mg$ where $\mu$ is the coefficient of friction and current $I=V/R=6/20A$
$\Rightarrow \mu =\frac{BIl}{mg}=\frac{0.80\times \left(6/20\right)\times \left(5/100\right)}{\left(10/1000\right)\left(10\right)}=0.12=3/25$