Simple Nature - Light and Matter

11.7 Electromagnetic Properties of MaterialsDifferent types of matter have a variety of useful electrical and magneticproperties. Some are conductors, and some are insulators.Some, like iron and nickel, can be magnetized, while others haveuseful electrical properties, e.g., dielectrics, discussed qualitativelyin the discussion question on page 592, which allow us to makecapacitors with much higher values of capacitance than would otherwisebe possible. We need to organize our knowledge about theproperties that materials can possess, and see whether this knowledgeallows us to calculate anything useful with Maxwell’s equations.11.7.1 ConductorsA perfect conductor, such as a superconductor, has no DC electricalresistance. It is not possible to have a static electric fieldinside it, because then charges would move in response to that field,and the motion of the charges would tend to reduce the field, contraryto the assumption that the field was static. Things are a littledifferent at the surface of a perfect conductor than on the interior.We expect that any net charges that exist on the conductor willspread out under the influence of their mutual repulsion, and settleon the surface. As we saw in chapter 10, Gauss’s law requires thatthe fields on the two sides of a sheet of charge have |E ⊥,1 − E ⊥,2 |proportional to the surface charge density, and since the field insidethe conductor is zero, we infer that there can be a field on or immediatelyoutside the conductor, with a nonvanishing componentperpendicular to the surface. The component of the field parallelto the surface must vanish, however, since otherwise it would causethe charges to move along the surface.On a hot summer day, the reason the sun feels warm on yourskin is that the oscillating fields of the light waves excite currentsin your skin, and these currents dissipate energy by ohmic heating.In a perfect conductor, however, this could never happen, becausethere is no such thing as ohmic heating. Since electric fields can’tpenetrate a perfect conductor, we also know that an electromagneticwave can never pass into one. By conservation of energy, weknow that the wave can’t just vanish, and if the energy can’t bedissipated as heat, then the only remaining possibility is that all ofthe wave’s energy is reflected. This is why metals, which are goodelectrical conductors, are also highly reflective. They are not perfectelectrical conductors, however, so they are not perfectly reflective.The wave enters the conductor, but immediately excites oscillatingcurrents, and these oscillating currents dissipate the energy both byohmic heating and by reradiating the reflected wave. Since the partsof Maxwell’s equations describing radiation have time derivatives inthem, the efficiency of this reradiation process depends strongly onfrequency. When the frequency is high and the material is a goodconductor, reflection predominates, and is so efficient that the waveSection 11.7 Electromagnetic Properties of Materials 707