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Unformatted text preview: Lab 3: Simple DC Circuits 1 Introduction This lab will allow you to acquire handson experience with the basic principles of simple electric circuits. These circuits consist of discrete resistors and light bulbs that are connected to a DC power supply using conducting wires. Discrete resistors are usually made from poor electrical conductors such as carbon. Con ducting wires have negligible resistance because they are usually made from copper, which is an excellent electric conductor. In the first part of this lab, you will test Ohms law, which describes the properties of resistors. In the next part, you will verify Kirchhoffs rules that apply to all circuits that are in steady state, i.e. circuits with a constant current flow. You will discover that it is pos sible to derive rules for how resistors combine in series and parallel circuits using Kirchhoffs rules. The ex periments will allow you to test your derivations. You will also learn how the resistance of a wire is related to its length and area of cross section. In, the last part of the lab, you will perform some simple exper iments with circuits consisting of light bulbs. These experiments will test your understanding, and allow you to figure out how these circuits are wired. EXERCISES 1 AND 2 PERTAIN TO THE BACKGROUND CONCEPTS AND EXER CISES 310 PERTAIN TO THE EXPERI MENTAL SECTIONS. 2 Background When a (typical) resistor is connected to a power sup ply, the current I flowing through the resistor is pro portional to the electric potential difference V across the terminals of the resistor. This relationship is called Ohms law, and is expressed by the following equation, I = V R (1) Here, R is the resistance. The SI unit of resistance is the ohm, . Consider a closed circuit loop consisting MT65 MT66 MT69 MT67 MT68 MT70 MT80MT111MT119MT101MT114MT32MT115MT117MT112MT112MT108MT121 MT82 MT49 MT82 MT50 MT82 MT51 Figure 1: A circuit containing three resistors of a network of resistors connected to a DC power sup ply or battery as shown in figure 1. When any closed loop in this circuit (such as ABCDA or BEFCB) is traversed, the algebraic sum of the changes in elec tric potential is equal to zero. This is a statement of Kirchhoffs voltage rule , and it follows from the law of conservation of energy. At any junction in the circuit (such as B), the current flowing into the junction is the same as the sum of currents flowing out of the junction. This is known as Kirchhoffs current rule , and it is a consequence of the fact that charge is conserved. Using Kirchhoffs rules and Ohms law it is possible to derive rules for how resistors can be combined in circuits. When a circuit is made up of resistors ( R 1 , R 2 , R 3 , etc.) connected in series, it can be shown that the total or effective resistance R T is just the sum of the individual resistances, R T = R 1 + R 2 + R 3 (2) When these resistors are connected in parallel, the effective resistance is given by, 1 R T =...
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