Biot-Sawart law indicates that the moving electrons (velocity $\vec{v}$ ) produce a magnetic field $\vec{B}$ such that

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$\vec{B} ∥ \vec{v}$

$\vec{B} ⊥ \vec{v}$

It obeys inverse cube law.

It is along the line joining the electron and point of observation

answer is A.

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Detailed Solution

According to Biot-Savart law, $d \vec{B} = \frac{μ_{0}}{4 π} \frac{i (\vec{d l} \times \vec{r})}{r^{3}}$

$\Rightarrow d \vec{B} = \frac{μ_{0}}{4 π} \frac{(\frac{d q}{d t}) (\vec{d l} \times \vec{r})}{r^{3}}$

$\Rightarrow d \vec{B} = \frac{μ_{0}}{4 π} \frac{e (\frac{\vec{d l}}{d t} \times \vec{r})}{r^{3}}$

$\Rightarrow d \vec{B} = \frac{μ_{0}}{4 π} \frac{(d q) (\vec{v} \times \vec{r})}{r^{3}}$

It indicates that the moving electrons (velocity $\vec{v}$ ) produce a magnetic field $\vec{B}$ such that $\vec{B} ⊥ \vec{v}$ .

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