Specific Resistance- Resistivity

We work with a variety of materials. Metals, plastics, wood, and fluids. We are frequently warned not to touch a switch with wet hands or to touch a wire outside in the rain, yet no such warnings are offered when touching plastic or wooden products.

Because some materials enable current to flow freely through them while others do not.  Some materials are conductors, while others are insulators, and practically all of them are resistors. The material’s type, size, and proportions may all have a part in determining its current flow.

Resistivity

The measure of resistance that a material offers to the flow of current through it is called resistivity or specific resistance. It is a material’s intrinsic characteristic. The temperature, composition, and pressure of a material determine its specific resistance. 

The resistivity of insulators stays high whereas that of conductors remains low. A material’s resistivity is a scalar quantity, meaning that the description of specific resistance is a number (magnitude) connected with a unit. 

Specific Resistance

When a specified quantity of voltage is supplied, specific resistance or resistivity is defined as the resistance provided per unit length and cross-sectional area. The resistivity will be represented mathematically as follows:

ρ=RA/L

Here, 

: Specific resistance

R: resistance

A: Cross- Sectional area

L: Length of the material

Specific resistance is just the reciprocal of specific conductance, which is a measure of a material’s capacity to conduct electricity.

Unit of Specific Resistance

From the specific Resistance Formula:

Unit of ρ=Unit of R×Unit of A / Unit of L

When resistance R is represented in Ohm (Ω) and distances are expressed in centimetres, the unit of resistivity is obtained (cm). The Ohm cm (Ω. cm) will be the unit of resistivity in this case.

The SI unit of specific resistance is Ohm.m (Ω.m) if the distance is measured in metres (m).

The SI unit for conductivity Siemens.m-1 (S.m-1).

As metals are good conductors of electricity and insulators are bad, metals have a very low specific resistance.

Electrical Resistivity

Specific electrical resistance, also known as electric resistivity, is a material attribute that determines how well it can resist the flow of electric current. The ohm-meter is its SI unit.

Specific Resistance of copper

The specific resistance of copper is 1.68×10-8 .m (200C). This means that the resistance between two opposite surfaces of a copper cube with a side of 1 m at 200C is 1.68×10-8  and  its conductivity is 5.96×107Sm-1. 

Copper’s resistance to current flow is negligible due to its high conductivity and low resistivity. Copper wires are commonly used in electrical circuits to conduct electricity.

Temperature Dependence of Resistivity

Temperature is one of the most important characteristics that affects the resistivity of a conductor because it affects the variation of resistance and hence resistivity of a conductor.

The resistivity of a certain metallic conductor increases as the temperature rises. When a conductor is heated, the atoms that make up the conductor begin to vibrate with higher amplitude. As a result, the frequency of collisions between ions and electrons increases. As a result, the average time between two successive impacts decreases, and the drift velocity decreases. As a result, as the temperature rises, the number of collisions increases, resulting in an increase in resistance.

Alloys resistivity grows with temperature as well, but at a considerably slower rate than metals, despite the fact that non-metals’ resistivity reduces as temperature rises. Semiconductors exhibit similar behaviour; the temperature coefficient of resistivity is negative for semiconductors and nonmetals, and its value is frequently large for semiconductor materials.

Conductivity

The ease with which electric current can flow through a material is measured by its conductivity. Specific conductance is another name for it. A substance’s conductivity is the inverse of its resistivity. The higher the resistivity value, the lower the conductivity value, and vice versa.

It is represented by

σ=1/p

Its SI unit is Siemens/Meter

The attribute of conductance is closely related to a material’s conductivity, and a material’s conductance is the reciprocal of its electrical resistance.

We know that the electrical resistance R and specific resistance ρ depend on the physical nature of the material; its dimensions or physical shape are described in terms of its length L and cross-sectional area A. As a result, a material’s conductance is determined by the type and physical qualities of the substance.

Generally the conductance of a substance is equal to the Reciprocal of electrical Resistance. It is represented by G.

G=1/R

R is the resistance.

The Siemens (S) is the SI unit of conductance and is symbolised by an inverted ohm and is represented by ℧ (mho).

Resistance indicates how difficult it is for current to pass through a substance, while conductance indicates how easy it is for current to flow through a substance. As a result, good conductors, such as copper and aluminium, have high conductance values, whereas insulators, such as plastic and wood, have low conductance values.

Conclusion

In this article we have studied resistivity and specific resistance. The measure of resistance that a material offers to the flow of current through it is called resistivity or specific resistance. It is a material’s intrinsic characteristic. The temperature, composition, and pressure of a material determine its specific resistance. 

The ease with which electric current can flow through a material is measured by its conductivity. Specific conductance is another name for it.