Physicists represent sources of magnetic moments in materials as poles. The North and South poles are an analogy to the positive and negative charges in electrostatics. Consider a bar magnet which has magnetic poles of equal magnitude but opposite polarity. Each pole is the source of magnetic force which weakens with distance. Since s always come in pairs, their forces partially cancel each other because while one pole pulls, the other repels. This cancellation is greatest when the poles are close to each other i.e. when the bar magnet is short. The magnetic force produced by a bar magnet, at a given point in space, therefore depends on two factors: on both the strength formula_1 of its poles, and on the vector formula_2 separating them. The moment is defined as