Conductors & Insulators
11 min read
Quick Answer
A conductor is a material whose outer electrons move freely, so charge passes through it easily. An insulator keeps its electrons bound, so almost no charge passes. The common conductors are metals; the common insulators are plastics, glass and ceramics, and many orders of magnitude separate the two groups.
Intuition
The metal spoon and the wooden spoon
Leave a metal spoon in a mug of hot tea and within a minute the handle is too hot to hold. Leave a wooden spoon in the same mug and the handle stays cool. It is the same tea in both cases, so the difference has to sit in the spoon: metal carries heat along its length, and wood barely does.
Electricity sorts materials along the same line, and for a closely related reason. Metals conduct heat well and electricity well, because both depend on the same loosely held outer electrons being free to move about. Wood, plastic, glass and ceramic hold their electrons firmly in place, and neither heat nor charge gets very far.
A material whose outer electrons can wander is a conductor. Copper, aluminium, silver, gold and steel are all conductors, and copper is the one you meet most, because it conducts nearly as well as silver at a small fraction of the price. A material that keeps every electron attached to its own atom is an insulator: PVC, rubber, glass, air, dry paper, most plastics and ceramics.
An ordinary mains cable puts both kinds side by side in cross-section. The copper in the middle carries current to where you want it. The coloured plastic around it keeps that current out of everywhere else — and keeps your fingers off what is inside. A cable works because it is a conductor and an insulator doing complementary jobs.
The gap between the two families is enormous. Comparing equal shapes of copper and of good insulating plastic, the plastic resists something like a million million million million times more strongly — a ratio with about twenty-four zeros in it. Few properties in nature spread that far, and that spread is what makes electrical engineering workable at all: you can route current with confidence, because every alternative path is so absurdly bad.
Practitioner
Resistivity: the number that separates them
Conduction is a property of the material, and the number that captures it is resistivity, symbol ρ. It says how strongly a standard shape of that material resists. Multiply by length and divide by cross-sectional area and you get the resistance of a particular object:
Low resistivity means a conductor; high resistivity means an insulator. Separating the geometry out is what lets you compare materials fairly — a thin copper wire and a thick one are made of equally good copper.
Worked example — Same wire, different metal
Take a 10 m length of wire with a cross-section of 1 mm², first in copper, resistivity 16.8 mΩ·mm²/m, then in a nichrome resistance alloy, resistivity 1.1 Ω·mm²/m.
Multiplying resistivity by length and dividing by area gives the copper wire's resistance, 168 mΩ, and the nichrome wire's, 11 Ω. The shapes are identical; the ratio between them is 65.5 and comes entirely from the material.
Nichrome is used for heating elements and copper is not, and that ratio is the reason. A conductor good enough to carry current without wasting energy makes a hopeless heater, and vice versa.
The ordinary conductors rank in a settled order. Silver is the best of them, copper follows closely and costs far less, gold is worse than either but does not corrode, and aluminium is worse still while being much lighter. Among insulators, dry air, PTFE, polyethylene and good ceramics are excellent, and damp anything is much worse than the same material dry.
Silicon and germanium sit deliberately between the two families, conducting far worse than metals and far better than plastics. Their conductivity can also be set by adding tiny amounts of other elements, and it is that controllability that semiconductor materials and every chip built from them rest on.
Whether something counts as an insulator depends on the job as much as on the material. Air insulates until the field across it gets high enough to break it down, at which point it conducts spectacularly. Distilled water insulates reasonably well; tap water does not. Insulation is always specified with conditions attached, because a material is only a good insulator for a given voltage, temperature and cleanliness.
Where the same shape is used to hold charge apart rather than to block current, the material is called a dielectric — a different name for the same class of material, chosen for a different property.
Engineer
Bands, carriers, and why no insulator is perfect
Underneath all of it are energy bands. In an isolated atom electrons occupy discrete levels; pack atoms into a solid and those levels smear into bands of allowed energies separated by gaps of forbidden ones. What matters is the highest occupied band and whether an electron in it has anywhere to go.
In a metal the highest occupied band is only partly filled, or overlaps the next one. An electron can pick up an arbitrarily small amount of energy from an applied field and move into a neighbouring state, so conduction happens at any field, at any temperature, and the carrier population is enormous — of order one mobile electron per atom. In an insulator the highest occupied band is completely full and the next empty band lies a large energy gap above it. An electron has nowhere to go without acquiring a big lump of energy at once, and at room temperature almost none of them can.
"Almost none" is doing real work in that sentence, and it is where the Layer 1 picture has to be corrected. An insulator has a very large resistance rather than an infinite one, and Ohm's law applies to it exactly as it applies to a resistor.
Worked example — How much leaks through an insulator
Two adjacent tracks on a board sit at 5 V apart, separated by board material whose insulation resistance measures 100 MΩ.
Dividing voltage by resistance gives the leakage current between them: 50 nA. A digital circuit will never notice it. In a picoammeter front end, an electrometer, or a circuit that must hold a charge on a capacitor for minutes, it becomes the dominant error and the reason such designs use guard rings and PTFE standoffs.
Temperature moves the two families in opposite directions. Heating a metal makes the lattice vibrate more, scattering the already-abundant electrons more often, so resistance rises. Heating an insulator or semiconductor promotes more electrons across the gap, so resistance falls — sharply, because the population depends exponentially on the gap-to-thermal-energy ratio. Temperature effects on resistance develops this.
Charge also gets carried by things other than free electrons. In an electrolyte the carriers are ions, in a gas discharge they are ions and electrons together, and in a semiconductor the absence of an electron behaves as a positive carrier in its own right. Copper turns out to be the special case rather than the general one.
Measured through a material and measured across its surface, insulation resistance gives two different numbers, and the surface one collapses with humidity, dust, flux residue and fingerprints. Most real insulation failures on a board happen across the surface rather than through the bulk.
Professional
Choosing, and the ways insulation fails
Choosing a conductor is mostly an argument about cost, weight and space, and copper wins it by default. Aluminium has a resistivity of 26.5 mΩ·mm²/m, so to reach the same resistance it needs 1.58 times the cross-sectional area — a real penalty in a connector or a winding, and irrelevant in an overhead transmission line where weight dominates and space is free. Silver turns up where contact resistance matters more than money, and gold plating where a contact must not oxidise, gold's advantage being chemical rather than electrical.
An insulator carries more ratings than a conductor does, and resistivity is rarely the limiting one. Dielectric strength sets the voltage per unit thickness the material survives before it breaks down, and temperature class sets where it softens, embrittles or degrades. Comparative tracking index describes how well the surface resists forming a carbonised conducting path under a contaminated, wet, electrically stressed surface — a slow failure that IEC 60112 exists to quantify, and one that ends in a short across a barrier that measured perfectly when new.
Insulation ages; conductors mostly do not. Heat, ultraviolet light, ozone, vibration and repeated flexing all degrade polymer insulation, and the failure is usually cracking followed by ingress rather than a gradual fall in resistance. Old equipment therefore gets more dangerous rather than merely less reliable, and cable specifications quote a temperature rating and a bend radius alongside the electrical numbers.
A joint between two different metals is a materials problem rather than an electrical one. Aluminium grows a tough oxide immediately on exposure to air, and that oxide is an insulator; an aluminium-to-copper joint made without the right preparation and jointing compound develops resistance over time, heats, and accelerates its own decline. Where two metals with different electrode potentials sit together in a damp environment, galvanic corrosion adds a second mechanism. Connector platings are chosen at least as much for these reasons as for conductivity.
The same property governs static behaviour. A surface that insulates too well accumulates charge and cannot release it, which is precisely the hazard in ESD control: ESD-safe materials are deliberately slightly conductive — high enough resistance to be safe to touch, low enough to bleed charge away. "Insulator" and "conductor" are the ends of a continuum, and a great deal of practical work is done with materials taken from somewhere along it.
For how resistivity turns into the resistance of a real cable run, see wire resistance and voltage drop and wire and cable.
Common mistakes
- Treating an insulator as an absolute barrier — it is a very large resistance, and leakage through it matters in high-impedance, low-current and long-hold-time circuits.
- Confusing resistivity with resistance — resistivity belongs to the material, resistance to a particular object. A thick copper bar and a thin copper wire have the same resistivity and wildly different resistance.
- Assuming insulation performance is constant — humidity, surface contamination, temperature and age all move it, usually in the wrong direction, and surface leakage moves furthest.
- Ignoring the oxide on aluminium — a joint that measures fine on assembly can heat and fail months later. Aluminium terminations need the right preparation and hardware.
- Reading "conductor" as "perfect wire" — copper has resistivity too. Over a long run at high current, that resistance is the design constraint.
Frequently asked questions
What makes a material a conductor?
Its outer electrons are not bound to individual atoms and can move through the material under an applied field. Metals are conductors because their highest occupied energy band is only partly filled.
Why is copper used instead of silver?
Silver conducts slightly better but costs far more and tarnishes. Copper gives nearly the same conductivity at a price that makes kilometres of it practical, which is why it dominates wiring.
Do insulators conduct any current at all?
Yes, a very small one. An insulator is a very high resistance rather than an infinite one, and its leakage becomes significant in electrometers, long-hold sample-and-hold circuits and high-voltage barriers.
Is water a conductor?
Pure water conducts poorly. Ordinary water conducts through dissolved ions, and it is the salts, not the water, that carry the charge — which is why tap water, sweat and seawater are hazardous while distilled water is much less so.
Where do semiconductors fit?
Between the two, with the special property that their conductivity can be set deliberately by doping and changed at will by an applied voltage. That controllability is what makes transistors possible.
Knowledge check
Two wires have identical length and cross-section, one copper and one nichrome. The copper one measures 168 mΩ. What does the nichrome one measure? (Show answer)
Board material between two tracks measures 100 MΩ, and the tracks sit 5 V apart. How much current flows between them? (Show answer)
Does a thick copper bar have a lower resistivity than a thin copper wire? (Show answer)
Why does the resistance of a metal rise with temperature while the resistance of an insulator falls? (Show answer)
Why are ESD-safe work surfaces made slightly conductive rather than insulating? (Show answer)
References
- CRC Press, CRC Handbook of Chemistry and Physics — electrical resistivity of the elements at 20 °C, the source of the copper and aluminium figures used above.
- IEC 60112, Method for the determination of the proof and the comparative tracking indices of solid insulating materials — the surface-tracking test referred to in Layer 4.