Question

The magnetic field $B$ due to a current-carrying circular loop of the radius $12 \, cm$ at its centre is $0.5\times 10^{- 4} \, T$ . The magnetic field due to this loop at a point on the axis at a distance of $5 \, cm$ from the centre -

• A. $3.9 \, \times \, 10^{- 5} \, T$
• B. $5.2 \, \times \, 10^{- 5} \, T$
• C. $2.1 \, \times \, 10^{- 5} \, T$
• D. $9 \, \times \, 10^{- 5} \, T$

Answer 1

Answer: (A)

$B_{0} \, =\frac{\mu _{0} I}{2 a}$
At axial point
$B=\frac{\mu_{0} I a^{2}}{2\left(a^{2}+x^{2}\right)^{3 / 2}}$
$\frac{B}{B_{0}}=\frac{a^{3}}{\left(a^{2}+x^{2}\right)^{3 / 2}}$
$\Rightarrow B=B_{0} \frac{a^{3}}{\left(a^{2}+x^{2}\right)^{3 / 2}}$
$=0.5 \times 10^{-4} \times \frac{(12 cm )^{3}}{\left(144 cm ^{2}+25 cm ^{2}\right)^{3 / 2}}$
$= \, 3.9 \, \times \, 10^{- 5} \, T.$