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Resistivity - Cost
General Information
Physical Insights
Example Uses
Simple Questions
Further Questions
This chart is useful for selecting cheap conductors and insulators.
General Information
This chart is important for designing components requiring good electrical insulation (e.g. plug casings) or good electrical conductivity (e.g. electric power cables) at a good price.
Good electrical conductors are usually good thermal conductors and good electrical insulators are good thermal insulators.
The similarity between electrical and thermal properties means that the chart can also be used to identify materials requiring good thermal insulation (e.g. for kiln walls) or thermal conductivity (e.g. for frying pans).
Physical Insights
Metals are much better conductors than other materials because they have metallic bonds in which valence electrons are pooled. These electrons are free to move and carry current.
Pure metals are much better conductors than alloys - this is because electrons travelling through the material are obstructed by irregularities such as alloying atoms.
Polymers and ceramics are good insulators because they are covalently bonded and so their electrons are all tightly bound.
Semiconductors are ‘doped’ with elements which provide extra electrons (n-type) or provide positive holes (p-type) which can move freely.
Example Uses
Gold (and silver) have the best conductivities but are too expensive to be widely used, except as connectors in electronic applications. In general the next best materials, copper and aluminium, are used (e.g. electric cables)
Polymers like urea formaldehyde are insulators, and are used in electrical components to protect the user (e.g. light switch casings)
Simple Questions
Why is copper used instead of brass for electric cables?
When alloying atoms are added to pure metals they interrupt the flow of electrons through the metal, lowering the conductivity. Hence nearly pure metals are used when high conductivity is required (e.g. power transmission, tracks in micro-circuits, etc. Brass has a much worse conductivity then copper, beside the increased ductility of copper also makes it easier to process.
Why do metals feel cold to touch?
Of course all materials we touch are at ambient temperature, unless they have only recently been removed from a hotter or colder location. As a result our body is significantly hotter and when we touch something we tend to warm it up. Poor conductors tend to warm up very quickly local to our finger so that they feel warm when we touch them. Metals on the other hand are good conductors so that our body heat is conducted away from the point of contact which remains largely at room temperature, thus feeling cold.
Select materials for an electric drill body.
It needs to be a good insulator to protect the user. Plastics are ideal because they are also easy to form into complex shapes by injection of vacuum moulding. Something like ABS is suitable.
Select materials for the main body of a 13A fuse.
This must be an insulator, commonly a sintered ceramic such as alumina is used being stiff and easy to form into a tube.
Select materials for a cavity wall or attic insulation.
Electrical and thermal conductivity tend to increase or decrease together. The low cost insulators tend to be the foams, the glass fibre nests and other low density systems. Paper based thermal insulators are also an option.
Further Questions
If polymers are bad insulators why do trees get struck by lightning?
Why must silicon used in semiconductors be really pure?
Why do electronic components get hot?
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