Electricity and Magnetism
All atoms contain negatively charged electrons and positively charged protons along with (with the exception of hydrogen) neutral neutrons. If there were no way to separate these charges there would be no such phenomena as electricity or magnetism. Static electricity consists of an unbalance of positive and negative charges. An abundunce of electrons produces a net negative charge, and an abundance of protons (or a deficiency of electrons) a positive. If charges flow through space or along a conductor, an electric current results. Electric currents produce magnetism.
The unit of charge would logically be the charge on one electron, first measured by Nobel laureate Robert Millikan in his famous oil drop experiment. This is such a tiny amount of charge, however, that it would not make a good standard unit.
Luckily for students and scientists alike, electrical units were not defined until after the metric system began. There never have been any archaic units in this field. All of the electrical units are metric and they fit together in a single, logical system. The beginning of the system is in the definition of the unit of electric current, the ampere.
The ampere is the amount of electric current flowing in two electrical conductors (wires) that will exert a magnetic force of 200 nanonewtons per meter of length on each other when the wires are spaced one meter apart. This is the fundamental beginning of all of the electrical units.
One ampere flowing for one second of time passes a coulomb of charge along the wire. A coulomb of negative charge is that of 6 280 000 000 000 000 000 electrons. (R. A. Millikan) If 96 500 coulombs (one faraday) of charge are used to electroplate a metal such as silver, a mole of the metal will be deposited. This is as many grams of the metal as its atomic weight (for silver this is 108 grams, or about four ounces). Metals with a valence of two (two loose electrons per atom), such as copper, require two faradays to plate a mole, etc. This kind of relationship (electrochemistry) leads to the easy determination of atomic weights of all of the elements. (H G J Moseley)
The volt is the difference in EMF (electromotive force: the push behind the electrons) which will give a coulomb of electrons a joule of energy.
The ohm is the unit of resistance to flow of electrons (perfect conductors do not exist except at very low temperatures) which will require a volt of EMF to drive one ampere of current.
The watt is the amount of power required to keep pushing a coulomb of charge each second (an ampere) with (against) an EMF of one volt. A one volt source connected to a one ohm resistance will cause one ampere of current to flow and produce one joule of heat energy each second. A kilowatt-hour of electric energy is exactly 3 600 000 joules. (One watt is the power of one joule of energy (or work) each second of time.) It's interesting to note that the conversion of electricity to heat is 100% efficient. There is no loss whatsoever. It is also a fact that the electrons flowing in ordinary copper wires carrying normal direct currents move at about the speed of a snail, and when carrying 60Hz alternating current oscillate within only about 1/500mm or 2 microns of distance.
Electric charge can be stored in chemical form in electric cells (which can be strung together to form batteries of two or more cells) and used at a later time. Some of these cells are rechargeable by driving the electrons in the opposite direction. Charges can also be stored in capacitors which are essentially pairs of conducting planes separated by an insulating material such as plastic, glass, mica, or even air. The unit of capacitance is the farad which is that capacitance that will allow one volt of EMF to store one coulomb of charge. In actual practice capacitors are generally in the microfarad and picofarad ranges. The earliest example of a capacitor is the Leyden jar. (Anton von Muschenbroek)