Conductors and Insulators

Some materials let electricity flow through them easily. These are called conductors. Other materials block the flow of electricity. These are called insulators. Almost every electrical device depends on the difference between the two. The conducting wires inside your phone charger carry current to the phone; the plastic coating around them keeps you from getting shocked. Knowing which materials conduct and which insulate is one of the most important parts of electrical engineering.

• ConductorsLet charge flow easilyMost metals
• InsulatorsBlock charge flowPlastic, rubber, glass, wood
• Best conductorSilverThen copper, gold, aluminium
• Best insulatorGlass, rubber, certain ceramicsUsed to support power lines
• SemiconductorsIn betweenSilicon: the basis of chips
• SuperconductorsZero resistanceBelow very low temperatures

Why some materials conduct

In a metal, the outer electrons of each atom are loosely held and can move freely from atom to atom. When you apply a voltage, these "free electrons" drift through the metal in response, carrying current. Metals are good conductors because they have lots of free electrons.

In an insulator (like plastic), the electrons are tightly bound to their atoms. They cannot move freely, so no current can flow. The voltage has nothing to push around.

Common conductors

Almost all metals are good conductors. The very best are:

• Silver: the best conductor of all, but expensive. Used in special electronics and in some plug contacts.
• Copper: a slightly weaker conductor than silver but much cheaper. Used in almost all household wiring and motors.
• Gold: a less efficient conductor than copper, but it does not corrode. Used in connectors and microchip wiring where reliability matters.
• Aluminium: lighter and cheaper than copper. Used for long-distance overhead power lines.
• Iron and steel: weaker conductors, but very common in motors and rails.

Some liquids also conduct electricity, especially salt water. That is why being in water near electricity can be deadly: the dissolved salts make the water a good conductor, letting current flow through your body.

Common insulators

• Plastic: cheap, easy to mould, the most common insulator on modern wiring.
• Rubber: stretchy and tough, used for some old cable coverings and electricians gloves.
• Glass: an excellent insulator, used in the bowl-shaped ceramics seen on old electricity pylons (called pin insulators).
• Wood: a poor conductor (when dry), used for handles on some old tools.
• Air: a fairly good insulator at normal voltages. High enough voltage (millions of volts in a lightning bolt) can break down the air and force it to conduct briefly.
• Ceramic: very strong, very heat-resistant, used in spark plugs and on overhead power-line insulators.
• Pure distilled water: a much better insulator than salt water (since pure water has no dissolved ions).

Fact One important fact about water: pure water is actually a poor conductor (an insulator, by most standards). What conducts is the dissolved ions in tap and sea water. Hospitals and labs use carefully purified water for things like cleaning circuit boards, because pure water does not short out the electronics.

Semiconductors

In between conductors and insulators are semiconductors: materials that conduct only a little electricity, and only under certain conditions. The most famous semiconductor is silicon.

Silicons special power is that engineers can carefully control its conducting behaviour, building it into transistors: tiny electronic switches. Modern microchips are built from billions of silicon transistors arranged into circuits.

Almost every modern electronic device (computers, phones, TVs, washing machines, cars, satellites) depends on silicon semiconductors. The town of Cambridge has a thriving semiconductor industry; the worlds biggest semiconductor maker, TSMC, is in Taiwan; and a small region of California is so famous for chip-making that it is called Silicon Valley.

Superconductors

Some materials, when cooled to extremely low temperatures, lose ALL their electrical resistance. They become superconductors: electricity can flow through them with no losses at all. Once started, a current in a superconducting loop can keep flowing forever.

Most superconductors only work at temperatures near absolute zero (around -270 degrees Celsius), kept cold with liquid helium. "High-temperature" superconductors, discovered in 1986, work at the warmer temperature of liquid nitrogen (-196 C). Even warmer superconductors are still being researched.

Superconductors are used in:

• MRI scanners: the powerful magnets in hospital MRI machines are made of superconducting coils.
• Particle accelerators: like CERNs Large Hadron Collider.
• Maglev trains: where powerful magnets levitate the train above the track.

Did you know? A few materials behave oddly. Glass, an excellent insulator at room temperature, becomes a conductor when heated red-hot. Graphite (the soft stuff in pencils) is one of the only non-metals that conducts well at room temperature. Diamond, which is just a different arrangement of the same carbon atoms as graphite, is an excellent insulator. The arrangement of atoms makes all the difference.

Why insulators matter so much

Without good insulators, electrical engineering would be impossible. Every wire would short out to anything it touched. The insulators we use today (plastics, ceramics, special rubbers) let us build complex devices where many wires can run side by side, separated by tiny insulator coatings.

The history of electrical engineering is partly the history of finding better insulators. The early telegraph lines of the 1840s used wires hanging in mid-air to keep them apart, with glass and porcelain insulators where they touched poles. The first practical underwater telegraph cable across the English Channel in 1851 was insulated with gutta-percha, a natural rubber-like substance. Modern fibre-optic cables include layers of insulation that would have seemed magical to those early engineers.

Try this Build a simple "circuit tester" with a battery, a small bulb and two short wires. Connect the bulb so it lights when the wires touch each other. Then try touching the loose wire ends to different materials: a metal coin (lights up: a conductor), a plastic ruler (does not light: an insulator), a pencil lead (lights weakly: graphite is a conductor), a rubber eraser (does not light), the metal blade of a knife (lights up), the wooden handle of the knife (does not light). You have just tested for conductors and insulators around your home.

Deeper dive: silicon and the digital age

The element silicon is the most important material in modern electronics. Silicon is the second most abundant element in Earths crust (after oxygen), making up about 28 per cent of the rocks beneath our feet. It is the main ingredient in sand, glass and many minerals. And purified into ultra-clean form, it is the basis of every microchip.

Pure silicon is a semiconductor: it conducts electricity, but only weakly. Engineers can change its conducting behaviour dramatically by adding tiny traces of other elements (a process called doping). Adding phosphorus or arsenic creates "n-type" silicon with extra free electrons. Adding boron or gallium creates "p-type" silicon with electron "holes". Where n-type meets p-type, the boundary acts as a one-way valve, the heart of a transistor or diode.

By layering different doped regions of silicon, engineers build transistors: tiny electronic switches that can amplify or control signals. The transistor was invented in 1947 at Bell Labs in the USA. The first ones were the size of small biscuits. Today, billions of them can fit on a fingernail-sized chip.

Moores Law (a 1965 prediction by Gordon Moore, co-founder of Intel) said that the number of transistors on a chip would roughly double every 2 years. Astonishingly, the prediction has held for 60 years. A modern microchip might have 100 billion transistors. Each individual transistor is now only a few atoms across.

Every smartphone, computer, AI system and modern car owes its existence to the strange middle-ground behaviour of silicon: not quite an insulator, not quite a conductor, but trainable into doing exactly what you want.

For more, see what is electricity and circuits.