<P> Another situation this formula is not exact for is with alternating current (AC), because the skin effect inhibits current flow near the center of the conductor . Then, the geometrical cross-section is different from the effective cross-section in which current actually flows, so the resistance is higher than expected . Similarly, if two conductors are near each other carrying AC current, their resistances increase due to the proximity effect . At commercial power frequency, these effects are significant for large conductors carrying large currents, such as busbars in an electrical substation, or large power cables carrying more than a few hundred amperes . </P> <P> Aside from the geometry of the wire, temperature also has a significant effect on the efficacy of conductors . Temperature affects conductors in two main ways, the first is that materials may expand under the application of heat . The amount that the material will expand is governed by the thermal expansion coefficient specific to the material . Such an expansion (or contraction) will change the geometry of the conductor and therefore its characteristic resistance . However, this effect is generally small, on the order of 10 . An increase in temperature will also increase the number of phonons generated within the material . A phonon is essentially a lattice vibration, or rather a small, harmonic kinetic movement of the atoms of the material . Much like the shaking of a pinball machine, phonons serve to disrupt the path of electrons, causing them to scatter . This electron scattering will decrease the number of electron collisions and therefore will decrease the total amount of current transferred . </P> <Table> <Tr> <Th> Material </Th> <Th> ρ (Ω m) at 20 ° C </Th> <Th> σ (S / m) at 20 ° C </Th> </Tr> <Tr> <Td> Silver, Ag </Td> <Td> 1.59 × 10 </Td> <Td> 6.30 × 10 </Td> </Tr> <Tr> <Td> Copper, Cu </Td> <Td> 1.68 × 10 </Td> <Td> 5.96 × 10 </Td> </Tr> <Tr> <Td> Aluminum, Al </Td> <Td> 2.82 × 10 </Td> <Td> 3.50 × 10 </Td> </Tr> </Table> <Tr> <Th> Material </Th> <Th> ρ (Ω m) at 20 ° C </Th> <Th> σ (S / m) at 20 ° C </Th> </Tr>

What type of solids are the best conductors
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