A circular loop of mass m and radius r in X-Y plane of a horizontal table as shown in figure. A uniform magnetic field B is applied parallel to X-axis. The current I in the loop, so that its one edge just lifts from the table is

Options

A. `(mg)/(pi r^(2) B)`
B. `(mg)/(2pi r^(2) B)`
C. `(mg)/(pi r B)`
D. `(pi r B)/(mg)`

Answer

C

Explanation

Deflecting torque `ge` Restoring torque
`I pi r^(2)B sin 90^(@) ge mgr`
`I ge (mg)/(pi rB)`
`I_(min) = (mg)/(pi rB)`