Physics · Ch 11 — Magnetic Materials
Origin of Magnetism in Materials
Origin of Magnetism in Materials
To understand why some materials are magnetic and others are not, the earlier, purely macroscopic description of a bar magnet (as a pair of poles) is not enough -- the explanation has to start from the building blocks of matter: atoms. In every atom, negatively charged electrons revolve about a central nucleus (made of protons and neutrons); just as a chemical element's outer-shell electrons determine its chemical behaviour, it is these same orbiting (and, as the next section shows, also spinning) electrons that determine an atom's magnetic behaviour. …
Worked out. To find which face of a current loop behaves as a North pole and which as a South pole, a circular loop carrying a clockwise current (as viewed from above) is drawn on a glass plate and a wire loop with clockwise current is placed on it; applying the right-hand rule, the top surface (as viewed) is found to behave as a South pole. If the same glass plate is then turned over and the identical loop viewed from its other face, the current now appears to flow anticlockwise (even though the real current direction in the wire has not changed at all -- only the viewing side has), so by the right-hand rule that same physical face now reads as a North pole. The activity demonstrates that a current loop's polarity is a property of the current's sense as seen from a given face, not an intrinsic label on the wire -- the same right-hand-rule logic used here to assign N/S to a macroscopic current loop is what is appli …