## What is Electrical Resistance?

According to Ohm’s law, there is a relation between the current flowing through a conductor and the potential difference across it. It is given by,

V ∝ I

V = IR

V is the potential difference measured across the conductor (in volts)

I is the current through the conductor (in amperes)

R is the constant of proportionality called resistance (in ohms)

The electrical resistance of a circuit is defined as the ratio between the voltage applied to the current flowing through it. Rearranging the above relation,

R = \( \frac VI \)

The unit of electrical resistance is ohms.

1 ohm = \( \frac {1~ volt}{ 1 ~ampere}\)

### Electrical Resistance and Resistivity

So what is resistivity? How is resistivity connected to electrical resistance?

The electrical resistance of a conductor is dependent on the following factors:

- Cross-sectional area of the conductor
- Length of the conductor
- Material of the conductor
- Temperature of the conducting material

Electrical resistance is directly proportional to length (L) of the conductor and inversely proportional to the cross sectional area (A). It is given by the following relation.

R = \( \frac{ρL}{A}\)

ρ is the resistivity of the material (measured in Ωm, ohm meter)

Resistivity is a qualitative measurement of a material’s ability to resist flowing electric current. Obviously, insulators will have a higher value of resistivity than that of conductors. The resistivities of a few materials are given below for a comparison. Materials with a low value of resistivity conduct electricity very well.

^{−8}

Copper – 1.68×10

^{−8}

Aluminum – 2.82×10

^{−8}

Wood – 1.00×10

^{14}

Air – 2.30×10

^{16}

Teflon – 1.00×10

^{23}

Based on the information and values given above, can you infer why copper is used as a fuse element in tripping circuits?

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