A plausible explanation for the magnetic properties of materials is the orbital motion of the atomic electrons. FIGURE 29.50 shows a simple, classical model of an atom in which a negative electron orbits a positive nucleus. In this picture of the atom, the electron’s motion is that of a current loop! It is a microscopic current loop, to be sure, but a current loop nonetheless. Consequently, an orbiting electron acts as a tiny magnetic dipole, with a north pole and a south pole. You can think of the orbiting electron as an atomic-size magnet.

FIGURE 29.50 A classical orbiting electron is a tiny magnetic dipole.

However, the atoms of most elements contain many electrons. Unlike the solar system, where all of the planets orbit in the same direction, electron orbits are arranged to oppose each other: one electron moves counterclockwise for every electron that moves clockwise. Thus the magnetic dipole moments of individual orbits tend to cancel each other and the net magnetic dipole moment is either zero or very small.

The cancellation continues as the atoms are joined into molecules and the molecules into solids. When all is said and done, the net magnetic dipole moment of any bulk matter due to the orbiting electrons is so small as to be negligible. There are various subtle magnetic effects that can be observed under laboratory conditions, but orbiting electrons cannot explain the very strong magnetic effects of a piece of iron.