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Direct-Current Circuits

Direct-Current Circuits - Chapter 7 Direct-Current Circuits...

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Chapter 7 Direct-Current Circuits 7.1 Introduction .......................................................................................................... 7-2 7.2 Electromotive Force ............................................................................................. 7-3 7.3 Resistors in Series and in Parallel ........................................................................ 7-5 7.4 Kirchhoff’s Circuit Rules ..................................................................................... 7-7 7.5 Voltage-Current Measurements ........................................................................... 7-9 7.6 RC Circuit .......................................................................................................... 7-10 7.6.1 Charging a Capacitor .................................................................................. 7-10 7.6.2 Discharging a Capacitor ............................................................................. 7-14 7.7 Summary ............................................................................................................ 7-16 7.8 Problem-Solving Strategy: Applying Kirchhoff’s Rules ................................... 7-16 7.9 Solved Problems ................................................................................................ 7-19 7.9.1 Equivalent Resistance ................................................................................. 7-19 7.9.2 Variable Resistance .................................................................................... 7-20 7.9.3 RC Circuit ................................................................................................... 7-21 7.9.4 Parallel vs. Series Connections .................................................................. 7-22 7.9.5 Resistor Network ........................................................................................ 7-25 7.10 Conceptual Questions ........................................................................................ 7-26 7.11 Additional Problems .......................................................................................... 7-26 7.11.1 Resistive Circuits ........................................................................................ 7-26 7.11.2 Multiloop Circuit ........................................................................................ 7-27 7.11.3 Power Delivered to the Resistors ............................................................... 7-27 7.11.4 Resistor Network ........................................................................................ 7-27 7.11.5 RC Circuit ................................................................................................... 7-28 7.11.6 Resistors in Series and Parallel .................................................................. 7-28 7-1
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Direct-Current Circuits 7.1 Introduction Electrical circuits connect power supplies to loads such as resistors, motors, heaters, or lamps. The connection between the supply and the load is made by soldering with wires that are often called leads , or with many kinds of connectors and terminals. Energy is delivered from the source to the user on demand at the flick of a switch. Sometimes many circuit elements are connected to the same lead, which is the called a common lead for those elements. Various parts of the circuits are called circuit elements, which can be in series or in parallel, as we have already seen in the case of capacitors. Elements are said to be in parallel when they are connected across the same potential difference (see Figure 7.1.1a) . Figure 7.1.1 Elements connected (a) in parallel, and (b) in series. Generally, loads are connected in parallel across the power supply. On the other hand, when the elements are connected one after another, so that the current passes through each element without any branches, the elements are in series (see Figure 7.1.1b) . There are pictorial diagrams that show wires and components roughly as they appear, and schematic diagrams that use conventional symbols, somewhat like road maps. Some frequently used symbols are shown below: Voltage Source Resistor Switch Often there is a switch in series; when the switch is open the load is disconnected; when the switch is closed, the load is connected. 7-2
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One can have closed circuits, through which current flows, or open circuits in which there are no currents. Usually by accident, wires may touch, causing a short circuit . Most of the current flows through the short, very little will flow through the load. This may burn out a piece of electrical equipment such as a transformer. To prevent damage, a fuse or circuit breaker is put in series. When there is a short the fuse blows, or the breaker opens. In electrical circuits, a point (or some common lead) is chosen as the ground . This point is assigned an arbitrary voltage, usually zero, and the voltage V at any point in the circuit is defined as the voltage difference between that point and ground.
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