Answer:
The expression is [tex]B=\frac{\mu mg}{IL}[/tex]
Explanation:
If the rod is move with constant velocity, the expression of the magnetic field is:
∑Fn = Fm - fk = 0
[tex]BILsin90-\mu n=0\\B=\frac{\mu n}{ILsin90} \\B=\frac{\mu mg}{IL}[/tex]
Where
B is the magnetic field
m is the mass
g is the gravity
I is the current
L is the distance
μ is the kinetic friction between the rod and rails