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978 Chapter 22 | Magnetism
Figure 22.5 Unlike poles attract, whereas like poles repel.
Figure 22.6 North and south poles always occur in pairs. Attempts to separate them result in more pairs of poles. If we continue to split the magnet, we will eventually get down to an iron atom with a north pole and a south pole—these, too, cannot be separated.
Real World Connections: Dipoles and Monopoles
Figure 22.6 shows that no matter how many times you divide a magnet the resulting objects are always magnetic dipoles. Formally, a magnetic dipole is an object (usually very small) with a north and south magnetic pole. Magnetic dipoles have a vector property called magnetic momentum. The magnitude of this vector is equal to the strength of its poles and the distance between the poles, and the direction points from the south pole to the north pole.
A magnetic dipole can also be thought of as a very small closed current loop. There is no way to isolate north and south magnetic poles like you can isolate positive and negative charges. Another way of saying this is that magnetic fields of a magnetic object always make closed loops, starting at a north pole and ending at a south pole.
With a positive charge, you might imagine drawing a spherical surface enclosing that charge, and there would be a net flux of electric field lines flowing outward through that surface. In fact, Gauss’s law states that the electric flux through a surface is proportional to the amount of charge enclosed.
With a magnetic object, every surface you can imagine that encloses all or part of the magnet ultimately has zero net flux of magnetic field lines flowing through the surface. Just as many outward-flowing lines from the north pole of the magnet pass through the surface as inward-flowing lines from the south pole of the magnet.
Some physicists have theorized that magnetic monopoles exist. These would be isolated magnetic “charges” that would only generate field lines that flow outward or inward (not loops). Despite many searches, we have yet to experimentally verify the existence of magnetic monopoles.
The fact that magnetic poles always occur in pairs of north and south is true from the very large scale—for example, sunspots always occur in pairs that are north and south magnetic poles—all the way down to the very small scale. Magnetic atoms have
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