An electron travels along the positive x -axis. A magnetic field is to be applied for a brief duration to reverse the electron's direction, making it move parallel to the negative x -axis. This can be achieved by directing the magnetic field along the

Step-by-step solution

The initial velocity of the electron is v = v i . To reverse the direction of the electron such that it moves parallel to the negative x -axis, it must complete a semi-circular path. This requires a uniform magnetic field perpendicular to the initial velocity. Since the initial velocity is along the x -axis, the magnetic field must lie in the y - z plane. If the magnetic field is directed along the y -axis ( B = B j ), the magnetic force is F = -e(v i B j ) = -evB k . The electron moves in a semi-circle in the x -