Resistivity · Grade 11
Resistance depends on the material, the length and the thickness of a conductor. Resistivity ρ is the property of the material itself: copper has a tiny resistivity, nichrome (used in heating elements) a much larger one. This topic also covers how components behave (their I–V characteristics) and how thermistors and LDRs in potential dividers make sensors.
Key facts
- R = ρL ÷ A; ρ is in Ω m. Longer → more resistance; thicker → less.
- Cross-sectional area of a wire: A = πd² ÷ 4.
- Stretching a wire to n times its length (same volume) multiplies its resistance by n².
- Potential divider: V_out = V_in × R₂ ÷ (R₁ + R₂).
- Thermistor: resistance falls as temperature rises. LDR: resistance falls as light increases.
- Superconductors have zero resistivity below their critical temperature.