(i) The relation between resistance and resistivity is:
$$R = \rho \frac{l}{A}$$
Rearranging: $\rho = \dfrac{RA}{l}$
SI unit of $\rho = \dfrac{\Omega \times m^2}{m} = $ Ω m
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(ii) Given: $l = 3$ m, $R = 60\ \Omega$, $A = 4 \times 10^{-7}$ m²
$$\rho = \frac{RA}{l} = \frac{60 \times 4 \times 10^{-7}}{3} = \frac{240 \times 10^{-7}}{3}$$
$$\boxed{\rho = 8 \times 10^{-6}\ \Omega\ \text{m}}$$
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(iii) Electrical resistivity will not change.
Resistivity is a characteristic property of the material, not of the shape or size of the conductor. Stretching doubles the length but also halves the area of cross-section, so the resistance changes (increases 4 times), but resistivity remains the same.
Source: Chapter 11, Section 11.5
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