Key Equations
Resistivity & Superconductivity — AQA A-Level Physics
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Resistivity equation
$$\rho = \frac{RA}{L}$$
- Where:
- $ρ$ = resistivity (Ω m)
- $R$ = resistance (Ω)
- $A$ = cross-sectional area (m²)
- $L$ = length (m)
Material property. Rearranges to R = ρL/A for calculations.
Cross-sectional area of a wire
$$A = \frac{\pi d^{2}}{4}$$
- Where:
- $A$ = cross-sectional area (m²)
- $d$ = diameter (m)
Essential for the resistivity practical. Convert d from mm to m first.
Resistance from resistivity
$$R = \frac{\rho L}{A}$$
- Where:
- $R$ = resistance (Ω)
- $ρ$ = resistivity (Ω m)
- $L$ = length (m)
- $A$ = cross-sectional area (m²)
The form used in most exam calculations. Gradient of R vs L graph = ρ/A.