Question

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.

Answer 1

Electrical resistivity is the inherent property of a conductor due to which it opposes the flow of current in the wire. It does not depend on the length and area of cross section of wire. It depends only on the temperature and nature of the material of the wire.

Its SI unit is Ωm (Ohm metre).

Experiment to study the factors on which the resistance of conducting wire depends:

Materials needed: nichrome wires and copper wires of specified dimensions as described below, an ammeter, a cell, a key etc.,

1. Set up an electric circuit consisting of a cell, an ammeter, a nichrome wire of length l [say, marked (1)] and a plug key, as shown in the diagram. 
   
2.  Now, plug the key. Note the current in the ammeter.
3. Replace the nichrome wire with another nichrome wire of the same thickness but twice the length, that is 2l [marked (2)]. Note the ammeter reading.
4. Now replace the wire with a thicker nichrome wire of larger cross-sectional area and of the same length l [marked (3)]. Again note down the current through the circuit.
5. Now instead of taking a nichrome wire, connect a copper wire [marked (4) ] in the circuit. Let the wire be of the same length and same area of cross-section as that of the first nichrome wire [marked (1)]. Note the value of the current. Notice the difference in the current in all cases.
6. Tabulate all the readings and calculate the resistance from the ratio of potential difference V and current I. 

It is observed that the resistance of the wire increases with the increase in length of the nichrome wire and decreases with the increase in the area of cross-section.

If we replace the nichrome wire with copper wire of the same length and area of cross-section even then resistance will be different.

These observations show that the resistance of a wire depends on the length, area of cross-section and nature of the material.