Why Is Copper Used for Wires? Free Electrons and the Metal of Electricity
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Inside the walls of every building, under every street and inside every motor, phone and computer, the electricity is moving through copper, and the reason is a number: at room temperature copper conducts electricity better than any metal except silver, and silver costs a hundred times as much. A copper atom holds one electron so loosely that in the solid metal it belongs to no atom at all, and a wire is a river of such electrons, waiting to be pushed. The metal was the first that humans worked, ten thousand years ago, and it is the one the electrical age could not have happened without.
Electrons that belong to nobody
In a metal the atoms are packed in a crystal and their outermost electrons are shared across the whole lattice, a sea of charge through which a voltage can drive a current. How well a metal conducts depends on how many such electrons there are and how easily they move, which is to say how rarely they are scattered by the vibrating atoms and by impurities. Copper has one free electron per atom, a lattice that scatters them little, and a crystal structure that packs them tightly, and the result is a conductivity of about 60 million siemens per metre, which silver beats by about five percent and gold, aluminium and iron fall short of by a fifth, a third and five sixths. Impurities matter enormously: a few hundredths of a percent of arsenic or phosphorus halves the conductivity, which is why electrical copper is refined to 99.99 percent by electrolysis, dissolving crude copper from one electrode and plating it pure onto another.
Why not silver, and when aluminium
Silver would make slightly better wire and is used where a little goes a long way, in the contacts of switches and relays and as a plating on high-frequency conductors, since at radio frequencies the current runs only in the outer skin of a wire. For everything else the price rules it out. Aluminium is the rival: it conducts about 60 percent as well as copper by volume but is a third the weight, so an aluminium cable of the same conductance is half the weight of a copper one, and cheaper, which is why every high-voltage transmission line strung across the countryside is aluminium wrapped around a steel core. Aluminium is not used in house wiring because it creeps under the screws of terminals, loosening and overheating, and because its oxide skin makes bad joints; a wave of American house fires in the 1970s ended the experiment. The other things copper does well:
- •Conducts heat almost as well as it conducts electricity, so it is in every heat sink, kettle base and saucepan bottom
- •Bends without breaking and can be drawn into wire finer than a hair, which aluminium and iron cannot match
- •Resists corrosion, forming a green patina that seals the surface, which is why roofs and statues last centuries
- •Kills bacteria on contact, a property known to the ancients and used for door handles and hospital surfaces now
- •Alloys usefully: with tin to make bronze, with zinc to make brass, with nickel to make coins
Where it comes from
Copper occurs as the pure metal in a few places, which is why it was the first metal worked, hammered into beads and blades in Anatolia and the Balkans from about 8000 BC; the smelting of copper from its ores by 5000 BC, and the discovery around 3300 BC that adding tin made a harder metal, bronze, named an age. Modern copper comes from ores that hold less than one percent metal, mostly copper sulphides in the huge porphyry deposits of the Andes and the American southwest, mined in open pits kilometres across; Chile produces about a quarter of the world's supply, Peru, China, Congo and the United States most of the rest. The ore is crushed, concentrated by flotation, smelted to crude metal and refined electrolytically, and the world uses about 25 million tonnes a year, a third of it recycled, since copper can be melted and reused indefinitely and a scrapped building or car is worth mining.
The electric century
Copper made the telegraph, the telephone, the dynamo and the electric motor, and a wire drawn from it now carries every kind of signal and power from a hearing aid to a power station. The next century needs more of it. An electric car uses about four times the copper of a petrol one, a wind turbine several tonnes, and the grids that must be built to connect them a great deal more; estimates of demand by 2040 run to double the present supply, and the new mines that would meet it take a decade to open and are opposed in most of the places they would go. Substitution is limited, because nothing else does what copper does at the price, and the metal that began the age of metals is one of the constraints on the next one.
The takeaway
Copper is used for wires because its single loosely held outer electron per atom moves through the metal's lattice with little scattering, giving the highest conductivity of any metal but silver at a fraction of the price, along with ductility, corrosion resistance and good heat conduction. Aluminium, lighter and cheaper but a poorer conductor and a worse joint, takes the overhead lines; copper is mined from low-grade ores mainly in Chile, refined to near purity by electrolysis, endlessly recycled, and in growing demand for the electrified world.