Question
Find out magnetic field at point $O$ ?



$\mathrm{B}=\frac{4 \pi \times 10^{-7} \times 10}{2 \times 5 \times 10^{-2}} \times \frac{1}{4} \,\mathrm{T}=\pi \times 10^{-5} \,\mathrm{T}$
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Use Avogadro constant as $6.023 \times 10^{23} mol ^{-1}$.
Reason : The magnetic meridian of the earth is along the axis of rotation of the earth.
