What is the Resistivity:
Resistance is the property of the conductor to oppose the electric current to flow through it. when the electromotive force applied to the conductor electric current flows due to the movement of free electrons. These electrons collide with the atoms while moving. So the flow of the electrons is opposed. Due to this, some kinetic energy is converted into the heat energy. Crystals of the different materials are different. So all the matters do not oppose the flow of the current equally. So the resistance of different matter is different. Symbol of the resistance is R and its unit is ohm.
If an emf of 1V is applied across the two ends of a wire and as a result is a current of 1 A flow through it, then the resistance of the conductor is said to be 1 Ω.
The resistance of a conductor depends on the following conditions:
1)It is directly proportional to its length l.
R α l
2)It is inversely proportional to its cross-section area a.
R α 1/a
3)It is dependent on the material of the conductor.
4)It depends on the temperature.
Resistivity or specific resistance:
R α L and R α 1/a
∴ δ = Ra/l
so the unit of the specific resistance is ohm meter or Ω m.
Factors affecting resistivity:
The resistivity of material depends upon the following factors.
Effect of temperature:
The kinetic energy of the atom increase when the temperature is increased. So when there is a flow of electron the possibility of collision with atom increases. So resistivity increases. In the case of semiconductors like silicon, the covalent bonds are broken when a temperature is increased. So resistivity decrease. In the case of the insulators and electrolyte also resistivity decreases with the increase in temperature.
Effect of alloying:
The resistivity of a material increase when some impurity is added in the pure matter. The resistivity of the alloy is higher than that of the constituent metals. Alloying to some extent is sometimes necessary as alloying improve some mechanical properties.
Effect of mechanical stressing:
The crystalline structure of a material gets distorted when it undergoes mechanical treatment. Due to this, the localized strain is produced in the material. This interferes with the movement of electrons. As a result of the resistivity increases. That is why the resistivity of hand drown copper is more than the that of annealed copper conductor.