PhysicsA straight conducting rod of length I and mass m is suspended in a horizontal plane by a pair of flexible strings in a magnetic field of magnitude B. To remove the tension in the supporting strings, the magnitude of the current in the wire is

A straight conducting rod of length I and mass m is suspended in a horizontal plane by a pair of flexible strings in a magnetic field of magnitude B. To remove the tension in the supporting strings, the magnitude of the current in the wire is

A
mgB/l
B
mgl/B
C
mg/lB
D
lB/mg

Solution:

If we want strings to have zero tension force, weight of rod should be supported by maximum magnetic force on rod due to magnetic field.
$F_{B} = mg B . I . lsin 90^{\circ} = mg I = \frac{mg}{BI}$