A flat, rigid rectangular conducting sheet of length \(L\), width \(w\), thickness \(d\), and electrical resistivity \(\rho\) moves to the right with instantaneous speed \(v\) into a region containing a uniform magnetic field \(B\) directed perpendicular to its face. At an instant when a length \(x\) (where \(0 < x < L\)) of the sheet has entered the field region, the current flowing across the width \(w\) encounters an effective electrical resistance of \(R = \dfrac{\rho w}{x d}\). Which of the following expressions represents the magnitude of the magnetic retarding force \(F_B\) acting on the sheet at this instant?