An electron of mass $m$ and charge $e$ is projected normally from the surface of a sphere of radius $a$ with speed $v_0$ into a uniform magnetic field $B$ perpendicular to the plane of the paper. The centre of the sphere is at a distance $b$ from a wall. If the electron strikes the wall symmetrically with respect to the $x$-axis,what should be the magnetic field $B$ such that the charged particle just touches the wall?

  • A
    $B = \frac{2bmv_0}{(b^2 - a^2)e}$
  • B
    $B = \frac{2bmv_0}{(a^2 + b^2)e}$
  • C
    $B = \frac{mv_0}{(\sqrt{b^2 - a^2})e}$
  • D
    $B = \frac{2mv_0}{(\sqrt{b^2 - a^2})e}$

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