What Current Flows Through a Resistor?

Our digital technology depends on mathematics to compute current flow and design its devices. Mathematics describes current flow by an idealization, Kirchhoff's current law. All the electrons that flow into a node flow out. This idealization describes real circuits only when stray capacitances are included in the circuit design. Motivated by Maxwell's equations, we propose that current in Kirchhoff's law be defined as \[{\varepsilon }_{\mathrm{0\ \ }}\frac{\mathrm{\partial }\boldsymbol{\mathrm{E}}}{\mathrm{\partial }t}\ \mathrm{+}\mathrm{\ }\mathrm{\ }\widetilde{\boldsymbol{\mathrm{J}}},\] the sum of (1) displacement current (2) the flux of charge associated with mass. The flux of charge associated with mass $\widetilde{\boldsymbol{\mathrm{J}}}$ includes, for example, the polarization of dielectrics as well as the movement of electrons. Kirchhoff's law becomes exact and universal when current is defined this way. This current is the source of the magnetic field; it is the source of $\boldsymbol{\mathrm{curl}}\boldsymbol{\mathrm{\ }}\boldsymbol{\mathrm{B}}$ in Maxwell's equations. Kirchoff's laws and Maxwell's equations can use the same definition of current.