An electron moving with a speed $u$ along the positive $x -$ axis at $y = 0$ enters a region of uniform magnetic field which exists to the right of $y -$ axis. The electron exits from the region after some time with the speed $v$ at coordinate $y,$ then :

v > u, y < 0

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v = u, y > 0

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v > u, y > 0

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v = u, y < 0

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Solution

The correct option is A $v = u, y < 0$
$\to v = v^i$
$\to B = u (-^k)$
${\to F}_{m a g} = - e (\to v \times \to B) = e u B (^i \times^k) = - e u B^j$
Thus the electron gets deflected towards $- v e y -$ axis.
Since no work is done during motion in a magnetic field, electron's speed will remain constant.

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