However, as noted before, the total energy of a particle in a "purely magnetic" field remains constant. What therefore happens is that energy is converted, from the part associated with the parallel motion v // to the perpendicular part. As v // decreases, the angle between v and B then increases, until it reaches 90°. At that point W ⊥ contains all the available energy, it can grow no more and no further advance into the stronger field can occur.