A particle travelling perpendicular to a magnetic field would experience a force given by \\\\(F=q(\\vec{v}\ imes \\vec{B})\\\\).This force, thus, acts perpendicular to the direction of motion of electron. Hence it only changes the direction of electron but not the speed of electron perpendicular to the direction of motion. This force would cause a constant centripetal acceleration. Thus the speed of electron does not change at all.