Resistance
A measure of the opposition to current flow in a conductor, measured in ohms (Ω). Defined by Ohm's law: R = V/I, where V is potential difference (in volts) and I is current (in amperes). Resistance depends on the material's resistivity (ρ), its length (L), and cross-sectional area (A): R = ρL/A.
Formula
R = ρL/A
Real World
Apple chose pure copper interconnects in the M-series chip because copper's very low resistivity (1.68 × 10⁻⁸ Ω·m) minimises resistance in nanometre-scale wires, reducing heat and power loss.
Exam Focus
When using R = ρL/A, convert all lengths to metres and areas to m² before substituting — unit errors are the most penalised mistake in resistivity calculations.
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