- Resistivity Definition: Resistivity is the measure of how much a material opposes the flow of electric current.
- Temperature Impact: The resistivity of most metals increases with temperature, showing a positive temperature coefficient. Semiconductors and insulators, however, see a decrease in resistivity with temperature.
- Alloying Effect: Alloying metals increases their resistivity by adding impurities that disrupt the crystal structure.
- Mechanical Stressing: Stressing a metal increases its resistivity by creating localized strains that block electron movement. Annealing can reverse this effect.
- Factors Affecting Resistivity: Temperature, alloying, mechanical stressing, age hardening, and cold working all significantly impact the resistivity of electrical materials.
Factors effecting the resistivity of electrical materials are listed below –
- Temperature.
- Alloying.
- Mechanical stressing.
- Age Hardening.
- Cold Working.
Temperature
The resistivity of materials changes with temperature. Most metals’ resistivity increases as temperature rises. The change in resistivity with temperature can be calculated using a specific formula.
Where,
ρt1 is the resistivity of material at temperature of t1o C
and
ρt2 is the resistivity of material at temperature of t2oC
α1 is temperature coefficient of resistance of material at temperature of t1o C.
If the value of α1 is positive, the resistivity of material is increase.
The resistivity of metals increase with increase of temperature. Means the metals are having positive temperature coefficient of resistance. Several metals exhibit the zero resistivity at temperature near to absolute zero. This phenomenon is “called the superconductivity”. The resistivity of semiconductors and insulators decrease with increase in temperature. Means the semiconductors and insulators are having negative temperature coefficient of resistance.
Alloying
Alloying is a solid solution of two or more metals. Alloying of metals is used to achieve some mechanical and electrical properties. The atomic structure of a solid solution is irregular as compared to pure metals. Due to which the electrical resistivity of the solid solution increases more rapidly with increase of alloy content. A small content of impurity may increase the resistivity metal considerably. Even the impurity of low resistivity increases the resistivity of base metal considerably. For example the impurity of silver (having lowest resistivity among all metals) in copper increase the resistivity of copper.
Mechanical Stressing
Mechanical stressing creates localized strains in a material’s crystal structure, increasing its resistivity by hindering electron movement. Annealing, which reduces mechanical stress, lowers resistivity by removing these localized strains. For example, hard-drawn copper has higher resistivity than annealed copper.
Age Hardening
Age hardening, or precipitation hardening, is a heat treatment process that increases an alloy’s yield strength and resistance to permanent deformation. It works by creating solid impurities that disturb the metal’s crystal structure, thus increasing resistivity.
Cold Working
Cold working, also known as work hardening or strain hardening, is a manufacturing process that increases a metal’s strength. This process disturbs the crystal structure, which interferes with electron movement and increases resistivity.
