Body
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.