\section{Direct-Current Circuits}

This unit develops the circuit viewpoint for electric charge motion. In AP Physics C: Electricity and Magnetism, the central idea is that steady currents through resistive elements obey precise relationships between current, voltage, and resistance. The treatment is restricted to direct-current (DC) circuits — configurations where sources are ideal constant-voltage batteries and fields have settled so that charge flows at a constant rate.

The flow begins with the microscopic description of current, including drift velocity and current density, then introduces resistance, resistivity, and Ohm's law at both macroscopic and microscopic levels. It continues with electric power dissipation in resistors, then builds up to multi-resistor circuits through equivalent resistance for series and parallel combinations. Kirchhoff's junction and loop rules are introduced as the systematic tool for analyzing complex networks. RC transients with a time constant and internal resistance, which affects real batteries and measurement devices, round out the unit.

\input{concepts/em/u11/e11-1-current-density.tex}

\input{concepts/em/u11/e11-2-resistance-ohm.tex}

\input{concepts/em/u11/e11-3-power.tex}

\input{concepts/em/u11/e11-4-equivalent-resistance.tex}

\input{concepts/em/u11/e11-5-kirchhoff.tex}

\input{concepts/em/u11/e11-6-rc-circuits.tex}

\input{concepts/em/u11/e11-7-internal-resistance.tex}
