To solve this problem it is necessary to apply the concepts related to Magnetic Force.
The magnetic force as magnitude of a vector is described by
[tex]F= IlBsin\theta[/tex]
Where,
[tex]\theta =[/tex] Angle between the wire and the magnetic field
L = Length
B = Magnetic Field
I = Current
Since the lightning bolt is perpendicular to the earth then the angle is 90°, moreover we need find the force per meter, then
[tex]\frac{F}{l} = IBsin\theta[/tex]
Replacing with our values
[tex]\frac{F}{l} = (20000)(3*10^{-5})sin(90)[/tex]
[tex]\frac{F}{l} = 0.6N/m[/tex]
Therefore the force per meter on lightning volt is 0.6N/m
