Module 3 : MAGNETIC FIELD
Lecture 15 : Biot- Savarts' Law

we have   \begin{eqnarray*} r &=& a/\sin\theta\\ l &=& a\cot\theta\\ dl &=& -(a/\sin^2\theta) d\theta \end{eqnarray*}

Thus  \begin{displaymath}\frac{\vec{dl}\times\hat r}{r^2} = \frac{\sin\theta}{a}d\theta\hat k\end{displaymath}

The direction of the magnetic field at a distance $a$from the wire is tangential to a circle of radius $a$,as shown.

\includegraphics{fig3.18.eps}

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