Conductivity and Resistivity
Electrical conductivity (σ) measures how easily a material conducts electric current, while resistivity (ρ) measures how strongly it opposes current flow. They are simply reciprocals of each other: σ = 1/ρ. Conductivity is measured in siemens per meter (S/m) and resistivity in ohm-meters (Ω·m).
These properties are intrinsic to the material, independent of the sample's geometry. They are determined by the number of charge carriers (free electrons or ions) and their mobility within the material. Metals have high conductivity due to their sea of free electrons, while insulators have very few mobile charge carriers and extremely high resistivity.
Conversion Formula
Material Properties
| Material | Conductivity (S/m) | Resistivity (Ω·m) | % IACS |
|---|---|---|---|
| Silver | 6.30 × 107 | 1.59 × 10-8 | 108.6% |
| Copper | 5.80 × 107 | 1.72 × 10-8 | 100% |
| Aluminum | 3.77 × 107 | 2.65 × 10-8 | 65% |
| Iron | 1.00 × 107 | 1.00 × 10-7 | 17.2% |
| Sea Water | 5.0 | 0.2 | ~0% |
Applications
- Wire sizing calculations use resistivity to determine voltage drop and heating for a given current.
- Material selection for electrical contacts requires high conductivity to minimize contact resistance.
- Non-destructive testing (eddy current testing) uses conductivity variations to detect material defects.
- IACS (International Annealed Copper Standard) rates conductivity as a percentage of pure copper, widely used in the wire and cable industry.
Frequently Asked Questions
Why is copper used for wiring instead of silver?
While silver has about 5% higher conductivity than copper, copper is much cheaper (about 1/100th the cost per kg) and has excellent mechanical properties for drawing into wire. Aluminum is even cheaper and lighter, which is why it is used for overhead power lines, despite having only 65% of copper's conductivity.
What is % IACS?
IACS stands for International Annealed Copper Standard. It defines 100% IACS as the conductivity of pure annealed copper at 20 degrees C (5.80 x 10^7 S/m). Materials are rated as a percentage of this standard. Silver exceeds 100% IACS (about 108%), while most alloys and other metals are well below 100%.
Does temperature affect conductivity?
Yes, significantly. For metals, conductivity decreases with increasing temperature because thermal vibrations of atoms scatter conducting electrons. For semiconductors and ionic conductors, conductivity typically increases with temperature as more charge carriers are thermally activated. This is why metal resistors have positive temperature coefficients while thermistors (semiconductors) have negative ones.