The concept necessary to develop this exercise is that of a magnetic field on a surface. The magnetic field is a tool to describe how a magnetic force is distributed in the space around and within something magnetic.
It can be defined as,
[tex]B = \frac{\mu_0 I}{2\pi R}[/tex]
Where,
R= The distance from the point
I = Current
[tex]\mu_0 =[/tex] Permeability constant in free space
Our values are given as,
[tex]\mu_0 = 4\pi * 10^{-7}H/m[/tex]
[tex]I = 200 A[/tex]
[tex]R = 20m[/tex]
Replacing ,
[tex]B = \frac{(4\pi*10^{-7})(200)}{2\pi 20}[/tex]
[tex]B = 2*10^{-6}T[/tex]
Therefore the magnetic field strength on the ground directly under such a transmission line is [tex]2*10^{-6}T[/tex]
