Question: Q. 2. Biot-Savart’s law indicates that the moving electrons (velocity $v$ ) produce a magnetic field $B$ such that
(a) $B \perp v$.
(b) $B | v$.
(c) it obeys inverse cube law.
(d) it is along the line joining the electron and point of observation. [NCERT Exemplar]
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Solution:
Ans. Correct option : (a)
Explanation : In Biot-Savart’s law, magnetic field $B | i d l \times r$ and $i d l$ due to flow of electron is in opposite direction of
or
$$ \begin{aligned} d B & =\frac{I \cdot d l \sin \theta}{r^{2}} \ d B & =\frac{I \times d l}{r} \end{aligned} $$
According to Biot-Savart law, if magnetic field is not perpendicular to the motion of charge then it will not move in helical path, which is not possible for motion of a charge in magnetic field. So, the magnetic field is perpendicular to the direction of flow of charge.
