The magnetic field at the centre of a circular current carrying-conductor of radius $r$ is $B_c$. The magnetic field on its axis at a distance $r$ from the centre is $B_a$. The value of $B_c$ : $B_a$ will be
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Magnetic field at centre of current-carrying coil.
$B_{c}=\frac{\mu_{0} I}{2 r}.........(i)$
Magnetic field at axial point due to a currentcarrying coil at distance of $\mathrm{r}$
$\mathrm{d}=\mathrm{r}$
$\mathrm{B}=\frac{\mu_{0} \mathrm{Ir}^{2}}{2\left(\mathrm{r}^{2}+\mathrm{d}^{2}\right)^{3 / 2}} \Rightarrow \mathrm{B}_{\mathrm{a}}=\frac{\mu_{0} \mathrm{Ir}^{2}}{2\left(2 \mathrm{r}^{2}\right)^{3 / 2}}........(ii)$
$\mathrm{Now}$
$\frac{\mathrm{B}_{\mathrm{c}}}{\mathrm{B}_{\mathrm{a}}}=\frac{\mu_{0} \mathrm{I}}{2 \mathrm{r}} \times \frac{2\left(2 \mathrm{r}^{2}\right)^{3 / 2}}{\mu_{0} \mathrm{Ir}^{2}}=2 \sqrt{2}$
$B_{c}: B_{a}=2 \sqrt{2}: 1$
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