What is electrical resistivity of a material? What is its unit? Describe an experiment to study the factors on which the resistance of conducting wire depends.

Resistance of a uniform metallic conductor is directly proportional to the length (l) and inversely proportional to the area of cross-section.

i.e. R

R

Or R =

ρ is constant of proportionality and is known as electrical resistivity of the material

S.I. unit of electrical resistivity of a material is: Ω m.

An experiment to study the factors on which the resistance of conducting wire depends can be demonstrated as follows:

1. Complete an electrical circuit which consists of a cell, an ammeter, a nichrome wire of length l.

2. Note down the reading from Ammeter.

3. Replace the nichrome wire with twice the length of earlier nichrome wire having same thickness.

4. Note down the ammeter reading in the given circuit.

5. Replace the nichrome wire with thicker nichrome wire having a large cross-section area. Note down the current through circuit.

6. Now, replace the nichrome wire with copper wire having same length and same area of cross-section.

7. Note down the value of current and observe the difference in the current in all the cases.

Following observations were noted:

1. The ammeter reading decreases to one-half when the length of wire is doubled.

2. The ammeter reading is increased when thicker wire of same material and same length is used.

3. Change in ammeter reading is observed when copper wire is used.

So, Resistance of conducting wire depends on various factors:

1. Length

2. Area of cross-section

3. Nature of material