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Chapter 26

Direct-Current Circuits

PowerPoint ® Lectures for

University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman

Lectures by James Pazun

Goals for Chapter 26

• To study resistors in series and parallel • To consider Kirchoff’s Rules • To see the design and learn about the use of electronic measuring instruments • To mentally assemble R-C circuits • To study the applications of circuits in household wiring Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

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Introduction

In the last chapter, we gained insight about how current flows through a resistor in simple examples like a light bulb attached to a battery. • Now, imagine many thousands of circuits wired onto flat wafers with structure so tiny that microscopy would be necessary to view their patterns. Understanding the next step and mastering more complex circuit patterns is the goal for Chapter 26.

Resistors in series and parallel I

• If we took three resistors and considered the different ways they could be connected, we arrive at the four possibilities illustrated in Figure 26.1. • Some of the combinations will be sequential (like the line at a phone booth), some will be en masse (like a marching band moving in rows). The former are analogous to resistors in series, the latter to resistors in parallel.

Resistors in series and parallel II

• If you have ever wired a Christmas tree with a series of lights (resistors) in series, you know what happens if just one burns out. The lights have become an open circuit and will not function.

• Car headlights are a good example of resistors wired in parallel. If one light burns out, the circuit changes but still functions to allow the driver a safe trip to repair. See Figure 26.2 below.

Resistors in series and parallel III—combinations

• Consider Problem-Solving Strategy 26.1.

• Follow Example 26.1 guided by Figure 26.3 below.

• Follow Example 26.2.

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Kirchoff’s Rules I—junctions

The algebraic sum of the currents into any junction is zero. Figures 26.6 and 26.7 illustrate this rule and are shown below.

Kirchoff’s Rules II—loops

• The algebraic sum of the potential differences in any loop, including those associated with emfs and those of resistive elements, must equal zero.

Kirchoff’s Rules III—examples and strategy

• Read through Problem-Solving Strategy 26.2. Figure 26.9 illustrates this strategy.

• Refer to Example 26.3, illustrated by Figure 26.10.

• • •

Kirchoff’s Rules IV—examples

• Refer to Example 26.4, illustrated by Figure 26.11.

Consider Example 26.5.

Refer to Example 26.6, illustrated by Figure 26.12.

Review Example 26.7.

• •

D’Arsonval’s galvanometer

We’ll call it simply “meter” henceforth.

The meter is a coil of wire mounted next to a permanent magnet. Any current passing through the coil will induce magnetism in the coil. The interaction of the new electromagnetism and the permanent magnet will move the meter indicator mounted to the coil.

The ammeter

• The ammeter (sometimes prefixed with milli or micro because the currents to be measured are routinely thousandths or millionths of an ampere) may be used to measure current OR voltage. A shunt resistor makes this conversion as shown below in Figure 26.15.

• Consider Example 26.8 to follow a current example. Consider Example 26.9 to follow a voltage example.

• • • •

Ammeters and voltmeters in combination

An ammeter and a voltmeter may be used together to measure voltage and power.

Figure 26.16 below illustrates meters set to measure resistance.

Refer to Example 26.10.

Follow Example 26.11.

Ohmmeters and digital multimeters

• An ohmmeter is designed specifically to measure resistance. • Refer to Figure 26.17 and Figure 26.18 below to see an ohmmeter wiring diagram and a photograph of a digital multimeter. The multimeter can measure current, voltage, or resistance over a wide range.

• •

R-C circuits

Some excellent examples of variable resistance and potential are likely common to our experience.

The physiology of a heartbeat, the medical intervention of a pacemaker, and charging a capacitor to take a flash picture are ideas shown below in Figures 26.20 and 26.21.

• •

Charging, discharging and timing …

Consider Figures 26.23 and 26.24.

Follow Examples 26.12 and 26.13.

Power distribution systems—a home

• Potential, resistors, outlets, input from the power company … no wonder electricians are integral contractors in home construction!

• •

Fuses, circuit breakers, and GFI

A fuse will melt and a breaker will open the circuit if maximum current is reached. See Figure 26.26.

GFI stops further current flow when a sudden drop in resistance indicates that someone has offered a new path to ground. I don’t know if it will save this worker we see in Figure 26.27 who didn’t use a grounded drill.

The wiring diagram for a typical kitchen

• • Consider Figure 26.28 below.

Follow Example 26.14.