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

Options

A. `vgtu, ylt0`
B. `v=u, ygt0`
C. `vgtu, ygt0`
D. `v=u, ygt0`

Answer

D

Explanation

Given `vecu` and `vecB` are `_|_r` to each other. Force on electron due to magnetic field is, `vecF=-e(vecuxxvecB)`. Since this force acts perpendicular to `vecu`, therefore, it does not do any work. Due to it, the magnitude of velocity of electron remains constant. Due to the perpendicular magnetic field acting on the moving electron, the electron will describe a circular path in the magnetic field in clockwise direction. As a result of it, the condition (d) i.e., `v=u` and `ylt0` is satisfied.