L03_resistance-networks4

L03_resistance-networks4 - Resistance Networks Introduction...

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Introduction When constructing electrical circuits for home use or in building equipment, we often have to use large numbers of devices like lamps, resistors, and capacitors. When using these devices, we commonly have to combine them in special ways. The purpose of this lab is to explore how we combine resistors and how do those combinations affect currents and voltages in simple circuits. Resistors Combinations Resistors are usually combined in one of two ways, parallel or series. Both are depicted in Figures 1 and 2. In the simplest of cases, when connected in parallel, both ends of the resistors are connected to each other, when in series only one end is connected. For more complicated cases, we can use the following rule: Series and Parallel Rule Resistors (or any other device) R 1 and R 2 are in series if and only if every loop that contains R 1 also contains R 2 Resistors (or any other device) R 1 and R 2 are in parallel if and only if you can find a loop that has ONLY R 1 and R 2 Series and Parallel Example Consider the circuit in Figure 3 for example. The only resistors that are in series are R 1 and R 6 . Every loop that you can trace around the circuit includes both of them. As an example loop, you can think of a loop starting at E 1 and going clockwise through R 1 , E 3 , R 4 , R 5 , R 6 and ending back at E 1 . A second loop can start at E 1 and going clockwise through R 1 , R 2 , E 2 , R 6 and ending back at E 1 . It is worth noting that R 5 is in series with the combination of R 3 and R 4 but not in series with either one of them. The only resistors that are in parallel are R 3 and R 4 . Use of Meters Reminder We usually use an Ammeter to measure current and a Voltmeter to measure voltage. When drawing a schematic of a circuit, we usually label the Ammeter as “A” and the voltmeter with “V”. An example schematic is shown in Figure 4. Along with the ammeter and the voltmeter, the schematic includes 4 other devices: a power supply designated by E, and three resistors designated by R. When measuring current with an ammeter, the ammeter should be placed in series with the device we want to measure the current through. For example, as shown in the Figure, if we want to measure the current through R 1 , we place the ammeter in series with R 1 . The same current should flow through both R 1 and the ammeter. On the other hand, if we want to measure voltage, we have to connect the voltmeter in parallel with the device you want to measure the voltage across. For example, as shown in the Figure, if we want to measure the voltage across R 3 , we place the voltmeter in parallel with R 3 . Both R 3 and the voltmeter are subject to the same voltage. R
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This note was uploaded on 02/09/2011 for the course PHYS 1112L taught by Professor Adler during the Summer '10 term at Kennesaw.

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L03_resistance-networks4 - Resistance Networks Introduction...

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