What determines how well a metal conducts electricity?
Electrical conductivity depends on the internal structure of the material. As you might know, electricity is flow of electrons. Materials that have more free electrons are better conductors. For example, metals are good conductors while insulators such as glass or vaccum have very low conductivity. Also, electrical conductivity is more or less strongly dependent on temperature. In metals, electrical conductivity decreases with increasing temperature, whereas in semiconductors, electrical conductivity increases with increasing temperature. Over a limited temperature range, the electrical conductivity can be approximated as being directly proportional to temperature.
