Answer:
[tex]10.9\ \text{mT}[/tex]
Explanation:
I = Current = 500 kA
L = Length of wire = 180 m
m = Mass of train = [tex]100\times 1000=100000\ \text{kg}[/tex]
g = Acceleration due to gravity = [tex]9.81\ \text{m/s}^2[/tex]
B = Magnetic field
The gravitational force and magnetic force will balance each other
[tex]mg=BIL\\\Rightarrow B=\dfrac{mg}{IL}\\\Rightarrow B=\dfrac{100000\times 9.81}{500\times 10^3\times 180}\\\Rightarrow B=0.0109\ \text{T}[/tex]
The magnitude of the magnetic field needed to levitate the train is [tex]10.9\ \text{mT}[/tex]