Question

A circular coil of radius R carries an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a distance r from the centre of the coil, such that r >> R, varies as

• A. $\frac { 1 } { r }$
• B. $\frac { 1 } { r ^ { 3 / 2 } }$
• C. $\frac { 1 } { r ^ { 2 } }$
• D. $\frac { 1 } { r ^ { 3 } }$

Answer 1

Answer: (D)

$\frac { 1 } { r ^ { 3 } }$

Answer 2

$B = \frac { \mu _ { o } } { 4 \pi } \cdot \frac { 2 \pi N i R ^ { 2 } } { r ^ { 3 } } \Rightarrow B \propto \frac { 1 } { r ^ { 3 } }$