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## 12.6 Resistance

Resistance is the property of a substance by which it opposes the flow of current through it.

For a given body, the resistance is defined as the ratio of applied potential difference to the resulting current,

R = This property of the substance is due to frequent collisions of electrons with the atoms of the conductor.

SI unit of resistance is named ohm ) which is equivalent to V/A. Its dimensions areW(

[R] = .

### 12.6.1 Dependence of Resistance on Physical Dimensions

For a given body, its resistance is found to be

1. directly proportional to its length (L), and
2. inversely proportional to its area of cross-section (S).

That is, R µ L and R µ or R µ = r where r is a constant called resistivity.

### 12.6.2 Resistivity or Specific Resistance (r)

The constant r is the characteristic of the material depends on its temperature of which the conductor is made. If L = unity and S = unity, then

R = r

It can also be defined as the resistance of a cube of material of side 1 m when current flows perpendicular to opposite faces.

The SI unit of resistivity is ohm metre (W m). ### Email Based Assignment Help in Resistance

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