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Copper Cables (for Peterson).

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Posted by Alexander on September 15, 2001 00:54:19 UTC

Copper, as all metals, forms crystals in solid form. When liquid copper crystallizes, if you do not take very special measures, then as temperature drops, it starts crystallizing in many places at once, thus forming microcrystallic structure where each microcrystal is oriented in random direction (the one which moving atoms in the begining of crystal growth happen to "freeze" in. Therefore, neigboring crystals have small irregular gaps between them (because you can not perfectly match two tilted lattices on the border). Those gaps between randomly orienter microcrystals are major contributor into all properties of metals. For example, single large crystal of metal is usually very soft - atomic layers in metal crystals easily slide on top of each other. Not the case with polycrystal piece of metal - layers in microcrystals can not slide too far before they bump into differently oriented neighboring lattice and stop - thus polycrystals are much stronger. Filling gaps in soft iron metal with carbon atoms makes much stronger steel.

Gaps are usually small - 1-5 atoms across only, but this is enough for oxygen atoms from atmosphere to enter them and react (with copper, for example), forming microscopic layers of CuO (green rust) in gaps between microcrystals. Because CuO is a semiconductor, it influences passing via copper wire current dramatically differently that conductor. Semiconductors do not obey linear Ohm law I=V/R as all metals do. Instead, they obey nonlinear exponential dependance of current versus voltage applied: I(V)=Io (exp(eV/kT)-1). Thus, if you plug pure harmonic tone V=Vo sin(wt) in this equation, current will NOT be pure harmonic: I=Io exp(eVo sin(wt)/kT).If you take a Fourie-transform of the above non-harmonic function, you find that instead of only ONE frequency w it now has a SET of frequencies: w, 2w, 3w, 4w, etc. So, current via speakers have MORE harmonics than original voltage on the output of amplifier. When we hear those higher frequencies where there should not be any, our ear immediately picks them and we say: distortions (they are even actually called "harmonic distortions"). Because still microcrystals touch each other, most current passes via copper, so wire still conducts well and you will not notice essential degradation in conductivity. But even small harmonic distortions (0.1-0.5%) can be heard by human ear.

So, rusted copper wires may slightly contribute into distortions (heavily if it is heavily rusted). If wire would consist of a single copper crystal, it would not have much CuO inside, but making large monocrystals of metals is extremely expensive process.

That is why engineers try to gold-plate wires and copper connectors, use low-oxygen plastic insulation and other tricks to reduce CuO content in wires.

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