Ohm’s law states that the current through a resistor is directly proportional to the voltage across the resistor. Denoting the constant of proportionality with $R$ (the resistance), one can express Ohm's law mathematically as $$\begin{equation}V=RI\end{equation}$$ or $$\begin{equation}I=\frac{V}{R}\end{equation}$$ where voltage $V$ and current $I$ are shown in Fig. 1. It is important to note that the value of the current that we obtain using Ohm's law depends on how we define voltage $V$. As shown in the figure, the current obtained using Ohm's law is assumed to flow from the $+$ to the $-$ nodes (however, depending on the value of $V$ this current can be either positive or negative).

If we introduce the conductance as the inverse of the resistance $C=1/R$, Ohm's law becomes $V=I/C$ or $I=CV$. The unit for resistance is ohm ($Ω$); the unit for conductance is siemens ($S$), also known as mho.

• Ohm's law
  Voltage source and resistor (analytical)
  Voltage source and resistor (numerical)
  Voltage source and resistor (design)