Which factor increases the electrical resistivity of metals?

• A. Decreasing temperature
• B. Presence of grain boundaries, dislocations, impurity atoms, and vacancies
• C. Perfect crystal lattice
• D. Formation of a protective oxide layer

Answer: (B)

Electrical resistivity in metals is set by how often conduction electrons are scattered as they move. A near-perfect crystal lets electrons flow with little scattering, so resistivity is low. Real metals, however, have imperfections—grain boundaries, dislocations, impurity atoms, and vacancies—that disrupt electron motion and act as scattering centers. Each scattering event impedes current, increasing resistivity.

Temperature changes mainly affect lattice vibrations (phonons): lowering temperature reduces these vibrations and tends to decrease resistivity, while raising temperature increases scattering and raises resistivity. A protective oxide layer mainly affects surfaces or contacts and doesn’t change the bulk resistivity of the metal itself. Therefore, the presence of defects and impurities is the factor that increases resistivity.