Question

.. A vertical wire in which a current is flowing
produces a neutral point with the earth's magnetic
field at a distance of 10 cm from the wire. What is
the current if Bh = 1.8 x 10-4 T? (Ans. 90 A)​

Answer 1

Magnetic Field due to a current carrying wire is

B = (ui/2πr)

Since, it neutralizes the Earth's magnetic field. Thus, B = Bh

ui/2πr = 1.8 × 10^-4

i = (1.8×2π×0.1/u) × 10^-4

i = 0.09 × 10^7 × 10^-4 Ampere

i = 0.09 × 10^3 Ampere

i = 90 Ampere