The resistance of a conductor is directly proportional to its length, inversely proportional to its cross-sectional area, and depends on the material and its temperature.
Double L and R doubles; double A and R halves.
R = ρ × L / A
R
resistance (ohms)
ρ
resistivity of the material (Ω·m)
L
length of the conductor (metres)
A
cross-sectional area (m²)
In plain English: A longer wire is a longer straw — harder to push through. A thicker wire is a wider straw — easier. The material sets the baseline: copper's ρ is tiny, nichrome's is deliberately large. Warming a metal raises R roughly as R = R₀(1 + αΔT).
Where you'll meet it: Choosing wire gauges, understanding why long thin PCB traces drop voltage, designing shunt resistors and heating elements, and why sensor leads matter in precision work.