What is electrical resistivity?
Electrical resistivity is a measure of a material’s property to oppose the flow of electric current. This is expressed in Ohm-meters (Ω⋅m). The symbol of resistivity is usually the Greek letter ρ (rho). A high resistivity means that a material does not conduct well electric charge.
Electrical resistivity is defined as the relation between the electrical field inside a material, and the electric current through it as a consequence:
In which ρ is the resistivity of the material (Ωm), E is the magnitude of the electrical field in the material (V/m), J is the magnitude of the electric current density in the material (A/m2 ).
If the electrical field (E) through a material is very large and the flow of current (J) very small, it means that the material has a high resistivity.
What is electrical conductivity?
Electrical conductivity is the inversion of resistivity, and is a measure of how well a material conducts electric current:
in which σ is the conductivity of the material expressed in Siemens per meter (S/m). In electrical engineering often κ (kappa) is used instead of σ.
Factors That Affect Electrical Conductivity
There are three main factors that affect the conductivity or resistivity of a material:
- Cross-Sectional Area: If the cross-section of a material is large, it can allow more current to pass through it. Similarly, a thin cross-section restricts current flow.
- Length of the Conductor: A short conductor allows current to flow at a higher rate than a long conductor. It’s a bit like trying to move a lot of people through a hallway.
- Temperature: Increasing temperature makes particles vibrate or move more. Increasing this movement (increasing temperature) decreases conductivity because the molecules are more likely to get in the way of current flow. At extremely low temperatures, some materials are superconductors.
Table of Resistivity and Conductivity at 20°C
|Material||ρ (Ω•m) at 20 °C|
|σ (S/m) at 20 °C|
|Grain oriented electrical steel||4.60×10−7||2.17×106|
|GaAs||5×10−7 to 10×10−3||5×10−8 to 103|
|Carbon (amorphous)||5×10−4 to 8×10−4||1.25 to 2×103|
|Carbon (graphite)||2.5×10−6 to 5.0×10−6 //basal plane|
3.0×10−3 ⊥basal plane
|2 to 3×105 //basal plane|
3.3×102 ⊥basal plane
|Drinking water||2×101 to 2×103||5×10−4 to 5×10−2|
|Wood (damp)||1×103 to 4||10−4 to 10-3|
|Glass||10×1010 to 10×1014||10−11 to 10−15|
|Wood (oven dry)||1×1014 to 16||10−16 to 10-14|
|Air||1.3×1016 to 3.3×1016||3×10−15 to 8×10−15|
|Teflon||10×1022 to 10×1024||10−25 to 10−23|