# Electrical Resistivity; definition, formula & units

### Electrical resistivity is a key parameter for any material used in electrical circuits, electronic components and many other items.

resistivity is a measure of the resistance of a given size of a specific material to electrical conduction.

resistivity may also be referred to as the specific electrical resistance, or volume resistivity, although these terms are less widely used.

Although materials resist the flow of electrical current, some are better at conducting it than others.the resistivity is a figure that enables comparisons of the way in which different materials allow or resist current flow.

the resistivity of different materials plays a major role in selecting the materials used for electrical wire, within many electronic components including resistors, integrated circuits and many more.

## Resistivity definition & units

the electrical resistivity of a specimen of a material may also be known as its specific electrical resistance. it is a measure of how strongly a material opposes the flow of electric current.

#### Resistivity definition:

the resistivity of a substance is the resistance of a cube of that substance having edges of unit length, with the understanding that the current flows normal to opposite faces and is distributed uniformly over them.

the electrical resistivity is the electrical resistance per unit length and per unit of cross-sectional area at a specified temperature.

the si unit of electrical resistivity is the ohm⋅metre (ω⋅m). it is commonly represented by the greek letter ρ, rho.

As an example, if a solid cube of material with dimensions of 1 M3 has sheet contacts on two opposite faces which do not introduce any resistance themselves, and the resistance between the contacts is 1 Ω, then the resistivity of the material is said to be 1 &Omega: ˙⋅m.

## Resistivity formula / equation

$\rho =\frac{E}{J}$

Where:
ρ is the resistivity of the material in ohm metres, Ω⋅m
E is the magnitude of the electric field in volts per metre, V⋅m^-1

$\rho =R\frac{A}{l}$

Where:
R is the electrical resistance of a uniform specimen of the material measured in ohms
l is the length of the piece of material measured in metres, m
A is the cross-sectional area of the specimen measured in square metres, m^2

for example, keeping the material resistivity constant, the resistance of the sample can be increased by increasing the length, or decreasing the cross sectional area. it can also be seen from the resistivity equations that increasing the resistivity of the material will increase the resistance assuming the same dimensions. similarly decreasing the resistivity will decrease the resistance.

## Material resistivity levels

materials are put into different categories according to their level or resistivity. a summary is given in the table below.

Resistivity Regions for Different Categories of Materials
Material type Resistivity region
Electrolytes
Variable*
Insulators
~10^16
Metals
~10^-8
Semiconductors
Variable*
Superconductors
0

*   The level of conductivity of semiconductors is dependent upon the level of doping. With no doping they appear almost like an insulator, but with doping charge carriers are available and the resistivity falls dramatically. Similarly for electrolytes, the level of resistivity varies widely.

Check out more details in our . . . . table of the resistivity of different materials.

## Practical implications of resistivity

materials used as conductors, for example in electrical and general connecting wire need to be able to have a low level of resistivity. this means that for a given cross sectional area, the resistance of the wire will be low. selecting the right material depends on knowing its properties, one of which is its resistivity.

As an example, copper is a good conductor as it provides a low level of resistivity, its cost is not too high, and it also provides other physical characteristics that are useful in many electrical and electronic applications. The resistivity of copper is around 1.7 x 10-8亚博绑定银行卡有危险吗 ohm metre (or 17. nΩm), although figures will vary slightly according to the grade of the copper

materials like copper and even aluminium provide low levels of resistivity and this makes them ideal for use as electrical wires and cables - copper often being the favourite. silver and gold have much low resistivity values, but as they are considerably more expensive, they are not widely used. however, silver is sometimes used to plate wires where its low resistivity is essential, and gold flashing is used for the mating surfaces of many electronic connectors to ensure the best contacts. gold is also good for electrical connectors as it does not tarnish or oxidise like other metals.

Other materials are required to act as insulators, carrying as little current as possible. The resistivity of an insulator will be many orders of magnitude higher. One example is air and this has a very resistivity figure of over 1.5 x 1014亚博绑定银行卡有危险吗, which can be seen to be very, very much higher than the resistivity of copper.

electrical resistivity is important in many other electronic components. in resistors for example the resistivity of various materials is key in making resistors have the right resistance.

resistivity is also key in other electronic components as well. for integrated circuits the resistivity of the materials in the chip is very important. some areas need to have a very low resistance and able to connect different areas of the ic internally, whereas other materials need to isolate different areas. again, the resistivity is important to make this happen.

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