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Magnetic dipoles consist of two poles with opposite charges generating a magnetic field. Monopoles are hypothetical particles, and current loops can model behavior within magnetic materials. Magnetic dipole moment is measured as the force of the poles multiplied by the distance between them or the current multiplied by the area within the current loop. The direction of momentum can be determined by the right hand rule.
A magnetic dipole can be considered the fundamental observable unit of magnetism. More intuitively, a dipole consists of two points, or poles, that are equal but have opposite charges. The interaction between these two charged monopoles generates a force vector field in the surrounding area known as the magnetic field. Familiar examples of magnetic dipoles include bar magnets, electrons, and planet Earth itself.
While it is often simpler to consider magnetic materials such as bar magnets as having two monopoles of electric charge, this model fails to describe behavior within a magnet. Furthermore, monopolies have never actually been observed. Rather, monopolies are hypothetical particles. Interestingly, the existence of monopolies has been postulated by theoretical physics and the existence and nature of monopolies has been an active open question in science.
A second model with which magnetic dipoles can be considered is that of current loops. Hans Christian Oersted discovered in 1820 that a closed electric circuit, or closed current loop, generates a magnetic field. He did this by placing an electrically charged wire near a compass and noting that the compass needle moved. The current loop was creating a magnetic field which affected the magnetic needle, or dipole, inside the compass. The dipole of a magnetic material such as a bar magnet can be modeled by imagining the structure filled with small current loops. Models using these current loops predict behavior within magnetic materials with great success.
The force of a dipole is measured as the magnetic dipole moment. Momentum is a vector, which means it has a magnitude, or dimension, as well as a direction. When considering magnetic poles, such as those of a bar magnet, the magnetic moment (m) is defined as the force of the poles (p) multiplied by the distance between the poles (L), which can be represented by the equation m = pL. The direction of momentum points from the south pole of the magnet to its north pole.
The magnetic moment can also be defined for a magnetic dipole created by an electric current. In this case, the magnetic moment is equal to the current (I) multiplied by the area within the current loop (s), which can be represented by the equation m = Is. The direction of this moment can be determined by the right hand rule. To use this rule, a person holds their right hand forward and lets their fingers curl or fist in the same direction as the current. The person’s right thumb, when held straight, will point in the direction of the magnetic dipole moment.
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