Let's solve the problem based on Magnetic Field at Focus of Parabola and learn how to solve it. First, try it yourself, then check your solution.
The Problem:
An infinite wire carrying current (I) is bent in the form of a parabola. Find the magnetic field at the focus of the parabola. Take the distance of the focus from the apex as (a).
Solution:
From Biot-Savart law, the magnetic field ar (S) is given by $$ \vec{B}= \frac{\mu_0}{4 \pi} \int\frac{I\vec{dl}\times\vec{r}}{r^3}$$
From the figure, we note that
$$ |\vec{dl}\times \vec{r}|$$=area of the parallelogram by (\vec{dl}) and (\vec{r}) $$ = 2\times1/2\times r.rd\theta$$$$=r^2d\theta$$
Hence, $$ \vec{B}=\frac{\mu_0I}{4 \pi}\int_{0}^{2\pi}\frac{d\theta}{r}$$ Using (r(1-cos\theta)=2a) as the equation to the parabola, we get $$ \vec{B}=\mu_0I/4a
$$
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Where is the dl taken??