A straight wire of mass 500 g and length 7 m carries a current of 1.4 A. It is suspended in mid air by a uniform horizontal magnetic field B perpendicular to the length of the wire. The magnitude of magnetic field is

0.5 T

The correct answer is Option (3) → 0.5 T

Using Lorentz force,

$F_L=ILB\sin θ$

for $\sin 90°=1$,

$F=ILB$

and,

$F_{gravity}=mg$

$=0.5×9.8=4.9N$

and,

$F_L=F_G$

$1.4×7×B=4.9$

$B=0.5T$