A particle of mass $m$ and charge $q$ moves with a constant velocity $v$ along the positive $x$ direction. It enters a region containing a uniform magnetic field $B$ directed along the negative $z$ direction, extending from $x = a$ to $x = b$. The minimum value of $v$ required so that the particle can just enter the region $x > b$ is
A$qbB/m$
B$q\left( {b - a} \right)\,B/\,m$
C$qaB/\,m$
D$q\left( {b + a} \right)\,B/\,m$
Medium
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B$q\left( {b - a} \right)\,B/\,m$
b The radius $r$ of the circular path is given by (see Fig.)
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