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## Answers

**OHM’S LAW:-**Let us consider a conductor through which a current I is flowing and V be the potential difference between its ends ,then Ohm’s law states that

**V ∝ I or, V = R I**……..(1) where R is the resistance of the conductor. Its SI units of resistance is ohm, and is denoted by Ω.

The resistance R not only depends on the material of the conductor but also on its dimensions .

**Resistivity:**

-The resistance R of a conductor of length l and cross-sectional area A is found to be ,

R ∝ L and R ∝ 1/A Combining, we have R = ρL /A

where ρ is called resistivity or specific resistance

and ρ depends on the material of the conductor but not on its dimensions.

**Current density :-**

Using Ohm’s law

**V = IR = IρL /A**

The current density is defined as the current through unit area normal to the current,

**I /A = j**, is called current density .

The SI units of the current density is

**A/**

*m*2**Ohm’s law in classical form:-**

If E is the magnitude of uniform electric field in the conductor whose length is L, then the potential difference V across its ends is EL. Using these, the last equation reads

E L = j ρ L or E = j ρ ……………………(1)

In vector form E = j ρ ……………………(2)

The current density is defined as the current through unit area normal to the current. It is directed along E .

Equation (1)can be written as j = σ E ……………..(2)

where σ = 1/ρ is called the conductivity.

**This is Ohm’s law in classical form.**