$$P= { I }^{ 2 } \times R$$
$\left\{\begin{matrix}I=\sqrt{\frac{P}{R}}\text{; }I=-\sqrt{\frac{P}{R}}\text{, }&\left(P\geq 0\text{ and }R>0\right)\text{ or }\left(P\leq 0\text{ and }R<0\right)\\I\in \mathrm{R}\text{, }&P=0\text{ and }R=0\end{matrix}\right.$
$P=RI^{2}$
