# 11: Chapter 11


• 11.1: Prelude to Direct-Current Circuits
An amplifier circuit takes a small-amplitude signal and amplifies it to power the speakers in earbuds. Although the circuit looks complex, it actually consists of a set of series, parallel, and series-parallel circuits.
• 11.2: Electromotive Force
All voltage sources have two fundamental parts: a source of electrical energy that has a electromotive force (emf) and an internal resistance r. The emf is the work done per charge to keep the potential difference of a source constant. The emf is equal to the potential difference across the terminals when no current is flowing. The internal resistance r of a voltage source affects the output voltage when a current flows. The voltage output of a device is called its terminal voltage.
• 11.3: Resistors in Series and Parallel
Basically, a resistor limits the flow of charge in a circuit and is an ohmic device where V=IR. Most circuits have more than one resistor. If several resistors are connected together and connected to a battery, the current supplied by the battery depends on the equivalent resistance of the circuit.

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