• # question_answer A coil of number of turn N, radius R is carrying current I have the value of magnetic field ${{B}_{0}}$ at the centre. The value of magnetic field at the axis of that coil at the point which is very far away (X>>R) from the centre is proportional to :- A)  $\frac{1}{X}$               B)  $\frac{1}{{{X}^{2}}}$C)  $\frac{1}{{{X}^{3}}}$                 D)  $\frac{1}{{{X}^{4}}}$

${{B}_{axis}}=\frac{{{\mu }_{0}}NI{{R}^{2}}}{2{{({{X}^{2}}+{{R}^{2}})}^{3/2}}}\approx \frac{{{\mu }_{0}}NI{{R}^{2}}}{2{{X}^{3}}}$ here ${{X}^{2}}+{{R}^{2}}\approx {{X}^{2}}$