Simple Nature - Light and Matter

imagine them passing between the cracks among the atoms withoutcreating holes or fractures in the atomic framework. For those whoknow a little chemistry, it also explains why all the best conductorsare on the left side of the periodic table. The elements in that areaare the ones that have only a very loose hold on their outermostelectrons.GasesThe molecules in a gas spend most of their time separated fromeach other by significant distances, so it is not possible for them toconduct electricity the way solids do, by handing off electrons fromatom to atom. It is therefore not surprising that gases are goodinsulators.Gases are also usually nonohmic. As opposite charges build upon a stormcloud and the ground below, the voltage difference becomesgreater and greater. Zero current flows, however, until finallythe voltage reaches a certain threshold and we have an impressiveexample of what is known as a spark or electrical discharge. If airwas ohmic, the current between the cloud and the ground wouldsimply increase steadily as the voltage difference increased, ratherthan being zero until a threshold was reached. This behavior can beexplained as follows. At some point, the electrical forces on the airelectrons and nuclei of the air molecules become so strong that electronsare ripped right off of some of the molecules. The electronsthen accelerate toward either the cloud or the ground, whicheveris positively charged, and the positive ions accelerate the oppositeway. As these charge carriers accelerate, they strike and ionize othermolecules, which produces a rapidly growing cascade.LiquidsMolecules in a liquid are able to slide past each other, so ionsas well as electrons can carry currents. Pure water is a poor conductorbecause the water molecules tend to hold onto their electronsstrongly, and there are therefore not many electrons or ions availableto move. Water can become quite a good conductor, however, withthe addition of even a small amount of certain substances calledelectrolytes, which are typically salts. For example, if we add tablesalt, NaCl, to water, the NaCl molecules dissolve into Na + and Cl −ions, which can then move and create currents. This is why electriccurrents can flow among the cells in our bodies: cellular fluid isquite salty. When we sweat, we lose not just water but electrolytes,so dehydration plays havoc with our cells’ electrical systems. It isfor this reason that electrolytes are included in sports drinks andformulas for rehydrating infants who have diarrhea.Since current flow in liquids involves entire ions, it is not surprisingthat we can see physical evidence when it has occurred. Forexample, after a car battery has been in use for a while, the H 2 SO 4Section 9.1 Current and Voltage 529