28 - Direct Current Circuits

28 - Direct Current Circuits - 858 Chapter 28 C HAPTE R 2 8...

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858 CHAPTER 28 Direct Current Circuits Chapter 28 Direct Current Circuits CHAPTER OUTLINE 28.1 Electromotive Force 28.2 Resistors in Series and Parallel 28.3 Kirchhoff’s Rules 28.4 RC Circuits 28.5 Electrical Meters 28.6 Household Wiring and Electrical Safety ± An assortment of batteries that can be used to provide energy for various devices. Batteries provide a voltage with a ﬁxed polarity, resulting in a direct current in a circuit, that is, a current for which the drift velocity of the charges is always in the same direction. (George Semple) 858

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T his chapter is concerned with the analysis of simple electric circuits that contain batteries, resistors, and capacitors in various combinations. We will see some circuits in which resistors can be combined using simple rules. The analysis of more complicated circuits is simpliﬁed using two rules known as Kirchhoff’s rules, which follow from the laws of conservation of energy and conservation of electric charge for isolated systems. Most of the circuits analyzed are assumed to be in steady state, which means that currents in the circuit are constant in magnitude and direction. A current that is constant in direction is called a direct current (DC). We will study alternating current (AC), in which the current changes direction periodically, in Chapter 33. Finally, we describe electrical meters for measuring current and potential difference, and discuss electrical circuits in the home. 28.1 Electromotive Force In Section 27.6 we discussed a closed circuit in which a battery produces a potential difference and causes charges to move. We will generally use a battery in our discus- sion and in our circuit diagrams as a source of energy for the circuit. Because the potential difference at the battery terminals is constant in a particular circuit, the current in the circuit is constant in magnitude and direction and is called direct current. A battery is called either a source of electromotive force or, more commonly, a source of emf. (The phrase electromotive force is an unfortunate historical term, describ- ing not a force but rather a potential difference in volts.) The emf of a battery is the maximum possible voltage that the battery can provide between its terminals. You can think of a source of emf as a “charge pump.” When an electric potential difference exists between two points, the source moves charges “uphill” from the lower potential to the higher. Consider the circuit shown in Figure 28.1, consisting of a battery connected to a resistor. We shall generally assume that the connecting wires have no resistance. ± 859 + Resistor Battery Figure 28.1 A circuit consisting of a resistor connected to the terminals of a battery.
The positive terminal of the battery is at a higher potential than the negative termi- nal. Because a real battery is made of matter, there is resistance to the ﬂow of charge within the battery. This resistance is called internal resistance r . For an idealized bat-

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This note was uploaded on 02/24/2011 for the course PHYS 102 taught by Professor Wang during the Spring '11 term at Nanjing University.

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28 - Direct Current Circuits - 858 Chapter 28 C HAPTE R 2 8...

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