Wire cross-sectional area
= Wire cross-sectional area
{title2=$A$}
The <wire cross-sectional area> is the <area> of a slice perpendicular to the wire's axis. For a round wire of radius $a$, it is $A=\pi a^2$. It enters both the uniform-flow formula $I=jA$ for <current density> and the formula $R=L/(\sigma A)$ for the <electrical resistance> of a uniform <Ohmic conductor>.