Simple Nature - Light and Matter

9.1.5 Current-conducting properties of materialsOhm’s law has a remarkable property, which is that current willflow even in response to a voltage difference that is as small as wecare to make it. In the analogy of pushing a crate across a floor, itis as though even a flea could slide the crate across the floor, albeitat some very low speed. The flea cannot do this because of staticfriction, which we can think of as an effect arising from the tendencyof the microscopic bumps and valleys in the crate and floor to locktogether. The fact that Ohm’s law holds for nearly all solids hasan interesting interpretation: at least some of the electrons are not“locked down” at all to any specific atom.More generally we can ask how charge actually flows in varioussolids, liquids, and gases. This will lead us to the explanations ofmany interesting phenomena, including lightning, the bluish crustthat builds up on the terminals of car batteries, and the need forelectrolytes in sports drinks.SolidsIn atomic terms, the defining characteristic of a solid is that itsatoms are packed together, and the nuclei cannot move very far fromtheir equilibrium positions. It makes sense, then, that electrons, notions, would be the charge carriers when currents flow in solids. Thisfact was established experimentally by Tolman and Stewart, in anexperiment in which they spun a large coil of wire and then abruptlystopped it. They observed a current in the wire immediately afterthe coil was stopped, which indicated that charged particles thatwere not permanently locked to a specific atom had continued tomove because of their own inertia, even after the material of thewire in general stopped. The direction of the current showed thatit was negatively charged particles that kept moving. The currentonly lasted for an instant, however; as the negatively charged particlescollected at the downstream end of the wire, farther particleswere prevented joining them due to their electrical repulsion, as wellas the attraction from the upstream end, which was left with a netpositive charge. Tolman and Stewart were even able to determinethe mass-to-charge ratio of the particles. We need not go into thedetails of the analysis here, but particles with high mass would bedifficult to decelerate, leading to a stronger and longer pulse of current,while particles with high charge would feel stronger electricalforces decelerating them, which would cause a weaker and shorterpulse. The mass-to-charge ratio thus determined was consistent withthe m/q of the electron to within the accuracy of the experiment,which essentially established that the particles were electrons.The fact that only electrons carry current in solids, not ions, hasmany important implications. For one thing, it explains why wiresdon’t fray or turn to dust after carrying current for a long time.Electrons are very small (perhaps even pointlike), and it is easy to528 Chapter 9 Circuits