The black spheres, or circles, represent the points of attraction, or better, the centers of the particles. In the 'Negative' example, the north faces the point of attraction and it is the south pole that faces outwards. We will use the letter 'u' to represent a negative orientation. In the 'Positive' example, the south pole faces the center (or point of attraction), and it is the north pole that faces outwards. We will represent this orientation with an 'n'.