Raman wants to draw a graph to show how resistivity (rho) of a wire changes with length(l) of wire. What should his graph look like? Why is the answer (D) and not (C)?

Solution:
Resistivity ρof a wire is independent of its shape and size, i.e., it is independent of its length and area. 
Resistivity depends on the material of the wire.
So, resistivity remains constant on varying the length of the wire.
Hence, graph of resistivity vs length of the wire is a straight line parallel to length axis as shown below:



Hence, the correct answer is option (D).

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