Electron mean free path

Electrical conductivity

Electrical conductivity 
<http://en.wikipedia.org/wiki/Electrical_conductivity> is a measure of 
how well a material transports an electric charge 
<http://en.wikipedia.org/wiki/Electric_charge>.^[9] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-8> This is 
an essential property in electrical wiring systems. Copper has the 
highest electrical conductivity rating of all non-precious metals 
<http://en.wikipedia.org/wiki/Precious_metals> (electrical conductivity 
of copper = 101% IACS (International Annealed Copper Standard); 
electrical resistivity of copper = 16.78 nΩ•m at 20 °C). Oxygen-Free 
Electronic (OFE) copper 
<http://en.wikipedia.org/wiki/Oxygen-free_copper> achieves a minimum of 
101% IACS.

The solid state theory of metals ^[10] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-9> helps 
to explain the unusually high electrical conductivity of copper. In a 
copper atom <http://en.wikipedia.org/wiki/Atom>, the outermost 4s energy 
zone, or conduction band <http://en.wikipedia.org/wiki/Conduction_band>, 
is only half filled, so many electrons 
<http://en.wikipedia.org/wiki/Electron> are able to carry electric 
current <http://en.wikipedia.org/wiki/Electric_current>. When an 
electric field <http://en.wikipedia.org/wiki/Electric_field> is applied 
to a copper wire, the conduction of electrons accelerates towards the 
electropositive <http://en.wikipedia.org/wiki/Electropositive> end, 
thereby creating a current. These electrons encounter resistance to 
their passage by colliding with impurity atoms, vacancies, lattice ions, 
and imperfections. The average distance travelled between collisions, 
defined as the “mean free path 
<http://en.wikipedia.org/wiki/Mean_free_path>,” is inversely 
proportional to the resistivity of the metal. What is unique about 
copper is its long mean free path (approximately 100 atomic spacings at 
room temperature). Furthermore, this mean free path increases rapidly as 
copper is chilled.^[11] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-10>

Silver, an expensive precious metal 
<http://en.wikipedia.org/wiki/Precious_metal>, is the only metal with a 
higher electrical conductivity rating than copper (i.e., electrical 
conductivity of silver = 106% IACS, electrical resistivity of silver = 
15.9 nΩ•m at 20°C).^[12] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-11>[13] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-12> The 
high cost of silver combined with its low tensile strength 
<http://en.wikipedia.org/wiki/Tensile_strength> limits its use to 
special applications, such as joint plating and sliding contact 
surfaces. With the exception of silver, copper conducts electricity with 
less resistance than any other metallic material.^[14] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-13>

Because of its superior conductivity, annealed 
<http://en.wikipedia.org/wiki/Annealing_%28metallurgy%29> copper became 
the international standard to which all other electrical conductors are 
compared. In 1913, the International Electrotechnical Commission 
<http://en.wikipedia.org/wiki/International_Electrotechnical_Commission> 
set the conductivity of copper in its International Annealed Copper 
Standard (IACS) to 100%. Today, copper conductors used in building wire 
often exceed the 100% IACS standard.

The main grade of copper used for electrical applications, such as 
building wire, motor <http://en.wikipedia.org/wiki/Motor> windings, 
cables and busbars <http://en.wikipedia.org/wiki/Busbar>, is 
electrolytic-tough pitch (ETP) copper 
<http://en.wikipedia.org/wiki/Oxygen-free_copper#Specification> (CW004A 
or ASTM <http://en.wikipedia.org/wiki/ASTM> designation C100140). This 
copper is at least 99.90% pure and has an electrical conductivity of at 
least 101% IACS. ETP copper contains a small percentage of oxygen 
<http://en.wikipedia.org/wiki/Oxygen> (0.02 to 0.04%). If high 
conductivity copper needs to be welded 
<http://en.wikipedia.org/wiki/Welding> or brazed 
<http://en.wikipedia.org/wiki/Braze> or used in a reducing atmosphere, 
then oxygen-free high conductivity copper 
<http://en.wikipedia.org/wiki/Oxygen-free_copper> (CW008A or ASTM 
designation C10100) may be used.^[15] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-14>

Several electrically conductive metals are lighter than copper, but 
since they require larger cross sections to carry the same current, they 
are unacceptable when limited space is a major requirement.^[16] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-15>[17] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-16>

Aluminium has 61% of the conductivity of copper.^[18] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-17> The 
cross sectional area of an aluminium conductor must be 56% larger than 
copper for the same current carrying capability.^[19] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-18> The 
need to increase the thickness of aluminium wire 
<http://en.wikipedia.org/wiki/Aluminium_wire> restricts its use in 
several applications,^[20] 
<http://en.wikipedia.org/wiki/Copper_wire_and_cable#cite_note-19> such 
as in small motors and automobiles. In some applications such as aerial 
electric power transmission 
<http://en.wikipedia.org/wiki/Electric_power_transmission> cables, 
thickness is an advantage and copper is rare.