Simple Nature - Light and Matter

your mouth is analogous to a voltage difference. When you make the“h” sound, you form your mouth and throat in a way that allows airto flow easily. The large flow of air is like a large current. Dividingby a large current in the definition of resistance means that we geta small resistance. We say that the small resistance of your mouthand throat allows a large current to flow. When you make the “f”sound, you increase the resistance and cause a smaller current toflow.Note that although the resistance of an object depends on thesubstance it is made of, we cannot speak simply of the “resistanceof gold” or the “resistance of wood.” Figure f shows four examplesof objects that have had wires attached at the ends as electricalconnections. If they were made of the same substance, they wouldall nevertheless have different resistances because of their differentsizes and shapes. A more detailed discussion will be more naturalin the context of the following chapter, but it should not be toosurprising that the resistance of f/2 will be greater than that off/1 — the image of water flowing through a pipe, however incorrect,gives us the right intuition. Object f/3 will have a smaller resistancethan f/1 because the charged particles have less of it to get through.SuperconductorsAll materials display some variation in resistance according totemperature (a fact that is used in thermostats to make a thermometerthat can be easily interfaced to an electric circuit). Morespectacularly, most metals have been found to exhibit a suddenchange to zero resistance when cooled to a certain critical temperature.They are then said to be superconductors. Theoretically,superconductors should make a great many exciting devices possible,for example coiled-wire magnets that could be used to levitatetrains. In practice, the critical temperatures of all metals are verylow, and the resulting need for extreme refrigeration has made theiruse uneconomical except for such specialized applications as particleaccelerators for physics research.But scientists have recently made the surprising discovery thatcertain ceramics are superconductors at less extreme temperatures.The technological barrier is now in finding practical methods formaking wire out of these brittle materials. Wall Street is currentlyinvesting billions of dollars in developing superconducting devicesfor cellular phone relay stations based on these materials. In 2001,the city of Copenhagen replaced a short section of its electrical powertrunks with superconducing cables, and they are now in operationand supplying power to customers.There is currently no satisfactory theory of superconductivity ingeneral, although superconductivity in metals is understood fairlywell. Unfortunately I have yet to find a fundamental explanation ofg / A superconducting segmentof the ATLAS acceleratorat Argonne National Laboratorynear Chicago. It is used toaccelerate beams of ions to a fewpercent of the speed of light fornuclear physics research. Theshiny silver-colored surfaces aremade of the element niobium,which is a superconductor atrelatively high temperatures comparedto other metals — relativelyhigh meaning the temperature ofliquid helium! The beam of ionspasses through the holes in thetwo small cylinders on the endsof the curved rods. Charge isshuffled back and forth betweenthem at a frequency of 12 millioncycles per second, so that theytake turns being positive andnegative. The positively chargedbeam consists of short spurts,each timed so that when it isin one of the segments it willbe pulled forward by negativecharge on the cylinder in frontof it and pushed forward by thepositively charged one behind.The huge currents involved wouldquickly melt any metal that wasnot superconducting, but in asuperconductor they produce noheat at all.Section 9.1 Current and Voltage 521