4. State microscopic form of Ohm’s law.

The microscopic form of Ohm's law relates the electric field E in a material to the current density J through the material. It is given by the equation:

J = σE

where:

• J is the current density (the amount of current per unit area)
• E is the electric field
• σ is the conductivity of the material (the ability of the material to conduct electric current)

This equation states that the current density in a material is directly proportional to the electric field applied to the material. The proportionality constant is the conductivity of the material. This form of Ohm's law is useful for understanding the behavior of electric fields and currents on very small scales.