A metal wire \( \mathrm{PQ} \) of mass \( 10 \mathrm{gm} \) lies at rest on two horizontal metal...

A metal wire \( \mathrm{PQ} \) of mass \( 10 \mathrm{gm} \) lies at rest on two horizontal metal rails separated by \( 4.90 \mathrm{~cm} \) (figure). A vertically downward magnetic field of magnitude \( 0.800 \mathrm{~T} \) 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 \( P Q \) starts sliding on the rails. Find the coefficient of friction. Neglect magnetic force acting on wire \( P Q \) due to metal rails \( \left(\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^{2}\right) \)