Circuits—from flashlights to computers—are the controlled motion of charges through conductors and resistors.

• This chapter focuses on DC circuits, meaning direct current, in which potentials and currents are constant.

• You’ll learn to draw circuit diagrams.

《 LOOKING BACK Section 26.4 Sources of potential

Any circuit, no matter how complex, can be analyzed with Kirchhoff’s two laws:

• The junction law (charge conservation) relates the currents at a junction.

• The loop law (energy conservation) relates the voltages around a closed loop.

• We’ll also use Ohm’s law for resistors.

Circuits use energy to do things, such as lighting a bulb or turning a motor. You’ll learn to calculate power, the rate at which the battery supplies energy to a circuit and the rate at which a resistor dissipates energy. Many circuit elements are rated by their power consumption in watts.

Resistors, like capacitors, often occur in series or in parallel. These combinations of resistors can be simplified by replacing them with a single resistor with equivalent resistance.

《 LOOKING BACK Sections 27.3, 27.4, and 27.5 Current, resistance, and Ohm’s law

Capacitors are charged and discharged by current flowing through a resistor. These important circuits are called RC circuits. Their uses range from heart defibrillators to digital electronics. You’ll learn that capacitors charge and discharge exponentially with time constant $\tau =RC$.

《 LOOKING BACK Section 26.5 Capacitors

We live in an electronic era, and electric circuits surround you: your household wiring, the ignition system in your car, your music and communication devices, and your tablets and computers. Electric circuits are one of the most important applications of physics, and in this chapter you will see how the seemingly abstract ideas of electric charge, field, and potential are the foundation for many of the things we take for granted in the 21st century.