An electron with a speed u along the positive x-axis at y = 0 enters a region of uniform magnetic field B= -Bo k which exists to the right of y-axis. [Fig.] The electron exists from the region after sometime with the speed v at ordinate y, then

The energy of a charged particle moving in magnetic field remains constant because the magnetic field does not do any work. Therefore, kinetic energy is constant, i.e.U = V Lorentz force, F= q(v x B) The force on electron will act along - ve y-axis initially .The electron will undergo circular motion in clockwise direction and emerge out the field. So, y < 0.