If you are really adventurous, you could try some extra modifications to this simple setup. For example, while the particle is moving in some way, zoom way out and check the Contained radio button. The magnetic field will suddenly disappear from all space except that between the black circles. What happens to the particle? If you check Infinite, the magnetic field blinks back into existence everywhere. Can you use these two buttons to cause the particle to move back to the very center of the two circles?