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Some materials conduct electricity and some don't. This page describes which are which, and why it matters.

Summary

Most materials can be classed either as conductors or electricity or as insulators. Conductors take the electricity where we want it to go, and insulators are used to prevent it going where we don't want it to go.

Safety

If you touch a point in an electrical circuit with a conductor, the electricity may pass into your body. This could be lethal.

Conductors

Every atom has a cloud of electrons, each carrying a negative electric charge. In the centre is the nucleus, incredibly dense and carrying an equal positive charge. In some types of atom, some of the electrons are not bound to particular atoms but are free to move through the material. If you connect a battery to opposite sides, electrons will be attracted to the positive pole of the battery and drawn out of the material, while an equal number will be fed in the opposite side from the battery's negative pole. This flow of electrons constitutes an electric current.

This applies to all metals, which are hence designated conductors. In some metals, the electons flow very easily and others less so. Silver is the best conductor, but being expensive it's rately used for making wires. Copper is nearly as good and hence the vast majority of wires are made of this. Aluminium is pretty good too, and can be used for overhead power cables in order to reduce the weight.

Carbon also conducts electricity and is frequently used for the brushes in an electric motor to convey the current to the rotor, as it has self-lubricating properties.

Pure water hardly conducts elctricity but most water isn't pure. Hence anything that is wet must be considered a conductor, including the human body (even if your fingers are dry).

Insulators

In an insulator, the electrons are strongly bound to individual atoms and hence can't move through the bulk of the material. Such materials are termed insulators. So if you cannect a battery to opposites sides of an insulator, the electrons will still be attracted to the positive pole and the nuclei to the negative but they will only be displaced by a tiny amount and will spring back as soon as you remove the battery, and hence no current flows.

Most things apart from metals and carbon are insulators. This includes plastics, wood, glass and fresh air, but if any of these are damp they will no longer be reliable insulators.

But if you apply enough force in the form of a high voltage, eventually the electrons will be torn away from their atoms and a current will flow. This is known as "breakdown", and is likely to be destructive.

Practical Application

Wires are nearly always made of copper as this is an excellent conductor. Wires will usually have an insulating sheath of plastic so the current can't pass from one to another if the accidentally touch or if they're bundled together as in a 2-core cable.

If you poke inside a toaster with a metal knife to free a slice of toast, if the knife touches the element, the current may flow through the knife and into your hand. Consequently, this is a very dangerous thing to do, unless you first switch off and unplug the toaster. The same applies to anything electrically powered. If it's only powered by a battery or low voltage adapter you won't get a shock, but you may cause the current to flow where it shouldn't, and possibly damage some electronic components.

Most screwdrivers are made of metal, but many have a plastic handle. It's always safer to use such a screwdriver when working on mains circuits and devices in case they are still live.

Should someone in your presence be unfortunate to receive an electric shock, they may continue tohold onto a live device or appliance since the current will cause their muscles to involuntarily contract. In such a case it would be very dangerous to touch them as you would then very likely also get an electric shock, and if you need to move them or a live appliance or wire you should use something non-conducting such as a wooden stick.