Resistance increases with length and resistivity, and decreases with the cross-sectional area of the conductor.
Symbols
R
resistance
[Ω]
ρ
resistivity
[Ω·m]
l
length of the conductor
[m]
S
cross-sectional area
[m²]
Where it comes from
A longer and thinner conductor has a greater resistance (like a narrow, long pipe). The resistivity ρ depends on the material and increases with temperature.
When to use it
Choosing wires, comparing materials. Copper has a low ρ — it is a good conductor.
Example
A wire 2 m long with a cross-section of 1 mm² made of a material with ρ=1.7⋅10−8 Ω·m. Calculate its resistance.