CBSE Class 12 · 2007 · Magnetic Effects of CurrentLong Answer

\textbf{Biot-Savart law:} $d\vec{B}=\frac{{\mu }_0}{4\pi }\frac{Id\vec{\ell }\times \vec{r}}{r^3}$, magnitude $\frac{{\mu }_0}{4\pi }\frac{Id\ell \sin \theta }{r^2}$. At the centre every element is at distance $R$ with $d\vec{\ell }\perp \vec{r}$ ($\theta =90^{\circ }$), and all $dB$ point the same way (along the axis), so The field lines are concentric circles close to the wire, becoming straight and axial through the centre; the loop acts like a magnetic dipole with one face a north pole and the other a south pole.