Every conductor possesses some resistance. It may be very high (insulator) or maybe low (conductor). Resistance is effectively helpful in controlling the flow of electric current.
So before understanding resistivity and what are the factors affecting resistivity, you have to understand resistance. What is it, and how it works?
What is resistance?
Resistance is the measure of a material that shows how much current it can oppose. SI unit of resistance is the ohm (Ω).
Resistance of a material can be measured by:
R = ρ(L/A)
L = length of the conductor
A = area of the cross-section of the conductor
ρ = resistivity
From the above equation, resistance (R) is directly proportional to the (L) length of the conductor and (ρ) resistivity. And the resistance (R) is inversely proportional to (A) area of the cross-section of the conductor.
So, the resistance of a material is affected by its length, area of cross-section, material, and temperature.
What is resistivity?
Resistivity can be defined as the resistance between opposite faces of a unit cube of that material. Resistivity is expressed by rho (ρ) and its SI unit is ohms per meter cube.
Factors affecting resistivity
Few factors affecting resistivity of material are:
- Age hardening
- Mechanical stressing
These effects are discussed below in detail.
Alloys are substances that are the combination of two or more metals or a combination of metal and non-metals. An alloy has a higher resistivity than the pure metal.
Alloying increases the resistivity of the metals. Alloying elements greatly increase the lattice imperfections which increases the scattering of electrons that leads to increased resistivity of the metal.
With the change in temperature, the resistance of material changes. So it is quite obvious that the resistivity of the material also depends on the temperature.
As the temperature increases, the resistivity of the pure metallic conductor increases i.e. the temperature coefficient of the resistance of pure metals is positive.
The resistivity of the semiconductor or insulator decreases with an increase in the temperature i.e. temperature coefficient of the resistance of insulators is negative.
And there is a material known as “superconductor” which shows zero resistivity at absolute zero temperature.
Read More: What is a superconductor?
Due to age hardening the resistivity of the metal increases.
Age hardening also known as precipitation hardening is a technique that imparts strength to the metals and their alloys.
In age hardening, the metal goes through a heat treatment which increases the tensile and yield strength of the metal. And the alloys develop the ability to wear external force without any deformation.
During the age-hardening process, the crystal lattice of the metal undergoes distortion due to which the mobility of electrons is decreased which decreases the mean free path. This results in electrical resistivity of the metal/alloys/
Mechanical stressing can affect the resistivity of the material. Mechanical stressing distorts the crystal structure of the metal which tends to harden the material. This increases the tensile strength and resistivity of the material.
If the material is conductor, mechanical stressing will decrease its conductivity (as it increases its resistivity). To restore the conductivity of the material a heat treatment process known as annealing is used.
Annealing restores the regularity in the crystal structure which restores the conductivity of the material.
- In simple terms, resistance is the opposing force to the flow of electric current. Which is measured in ohms.
- Whereas resistivity is the resistance between opposite faces of the conductor. And it is measured in ohms per meter cube.
- The resistivity of the materials depends on a few factors, which are temperature, mechanical stressing, age hardening, and alloying.