What is Characteristic; resistor; Ohm’s law?

The characteristic of a dipole is the curve $i(u)$ (or $u(i)$) it imposes. A resistor has a linear characteristic through the origin, Ohm’s law

$R$ being its resistance (ohms, $1\,\Omega = 1\,\mathrm{V}/\mathrm{A}$) and $G = 1/R$ its conductance (siemens). It receives $p = Ri^2 = u^2/R \geq 0$, dissipated as heat (Joule effect). A wire of length $\ell$, cross-section $S$ and resistivity $\rho$ has $R = \rho\ell/S$.