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# Two observers moving with different velocities see that a point charge produces same magnetic field at a point $A$. Their relative velocity must be parallel to , $\overrightarrow r$ where $\overrightarrow r$ is the position vector of point $A$ w.r.t. the point charge. This statement is

$\begin {array} {1 1} (a)\;True \\ (b)\;False \\ (c)\;Nothing\: can\: be\: said \\ (d)\;True\: only\: if\: the\: charge\: is\: moving\: perpendicular \: to \: \overrightarrow r \end {array}$

Magnetic field is same, $\therefore \overrightarrow V \times \overrightarrow r$ must be same.
$( \overrightarrow Vc- \overrightarrow Va) \times \overrightarrow r (\overrightarrow Vc - \overrightarrow Vb) \times \overrightarrow r$
$\Rightarrow (\overrightarrow Vb - \overrightarrow Va) \times \overrightarrow r=0$
or
$( \overrightarrow Vc-\overrightarrow Va) || \overrightarrow r$
Ans : (a)