A student is studying how the dimensions of a uniform metallic wire affect its electrical properties.
(i) Define resistivity and state its SI unit.
(ii) If the length of the wire is doubled without changing its cross-sectional area, explain qualitatively how its resistance will change and give a reason.
(iii) If, instead, the diameter of the original wire is doubled while keeping its length unchanged, describe qualitatively how its resistance will change and give a reason.
(iv) Explain why the resistivity of the material remains the same in both the above modifications.