Conductor

[edit] Conductor

A conductor is a material that conducts heat or electricity – or both.

Conduction is the process whereby heat or electricity diffuses, or move across a material as a result of a difference between its parts. Conductivity is the rate at which conduction occurs.

[edit] Thermal conductance

A conductor is a material with a high thermal conductivity, i.e. it allows heat to pass freely through it when there is a temperature difference between it and the ambient environment or between it and another substance with which it is in contact.

Materials with high thermal conductivity tend to be poor insulators (e.g metals) as opposed to materials with low thermal conductivities that tend to be good insulators (e.g polystyrene).

In building construction, a material with a low thermal conductivity and hence a good insulator can be used to improve the thermal performance building elements such as wall and roof constructions. Polystyrene, polyurethane and mineral fibre are all excellent insulators that provide good resistance to the passage of heat. They are therefore used to help constructions attain (or exceed) the thermal performance (U-values) required by building regulations. For more information see: Insulation.

All materials conduct heat to a greater or lesser extent. If this were not the case, situations such as cold bridging would not occur. Cold bridging occurs when heat inside a building finds a pathway to the outside (when it is colder) through the layers of construction (and the air spaces) that are in contact with each other. This occurs because even poor heat conductors such as concrete and masonry will allow some heat to pass through when there is a temperature difference across them. Preventing cold bridging requires breaking this path of heat transfer using a material of low thermal conductivity, i.e. an insulator such as polystyrene, but it also means ensuring there are no air spaces on critical paths. For more information see: Cold bridge.

The internal to external thermal transmittance rate of a construction assembly is known as a U-value (i.e. the number of Watts transmitted through 1m2 of construction per degree Kelvin difference in temperature between the air on each side of the construction). For more information see: U Value.

See also Thermal conduction.

[edit] Electrical conductance

In relation to electricity, a conductor is a material which offers low resistance to the flow of electricity (the flow of electrons or electrically-charged particles) and is therefore able to conduct an electric current. Electrical conductivity (also known as ‘specific conductance’) is the rate at which a material conducts electricity. It is the reciprocal to electrical resistivity, the measure of how much a material resists the flow of an electrical current.

Most metals and plasma have high electrical conductivity, but to different extents. Silver is regarded as the best electrical conductor; copper, gold, aluminium, zinc, nickel and brass follow. Copper is an excellent conductor and is used widely in wiring systems; aluminium is better than iron. Lead is a poor conductor compared to the aforementioned.

Electrical conductivity occurs in various degrees in these metals due to the ease with which electrons in the outer shell of an atom (valence or free electrons) can be separated and are free to travel around, passing on an electric charge. Temperature also plays a part: when the temperature of metallic conductors is decreased, the electrical conductivity increases.

Other materials such as dry wood, glass, paper, rubber, diamond, air and most plastics are poor electrical conductors and are therefore said to be non-conductors or insulators.

Semi-conductors such as silicon and germanium can also conduct electricity but are not particularly efficient at doing so. They must therefore be heated or ‘doped’ with other elements, after which they can become extremely efficient electrical conductors.

[edit] Related articles on Designing Buildings Wiki

• Alternating current and direct current.
• Ampere.
• Articles about electricity.
• Cold bridge.
• Conduction.
• Consumer electronics.
• Electrical energy.
• Electrical power.
• Energy consumption.
• Glossary of electrical terms.
• Heat transfer.
• Kilowatt hour.
• Latent heat.
• Lightning conductor.
• Mains electricity.
• Thermal mass.
• U value.
• Urban heat island.