DC Circuits Manual

# DC Circuits Manual - DC CIRCUITS Physics 241/261 Fall 2014...

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The first drawing of a telephone by Alexan- der Graham Bell. Image courtesy of the Library of Congress, Manuscripts Division, The Alexander Graham Bell Family Papers. DC CIRCUITS Physics 241/261 Fall 2014 1 Introduction Electrical devices have revolutionized society, transforming how we work, spend our leisure time, and communicate. In this lab you will investigate properties of some simple direct-current (DC) circuits. By the end of this lab you should: understand how to use a digital multimeter. understand the concepts of current, voltage, and resistance, and how they are related. be able to apply the concept of conservation of charge to explain how current flows through different parts of a circuit. understand the behavior of circuits containing combinations of resistors in series and in par- allel. 2 Theory Before starting this lab, you should be familiar with the following physical concepts. If you need to review them, or if you haven’t yet discussed them in your lecture course, consult the indicated sections in Young & Freedman, University Physics. Ohm’s law and resistors, and electric power, §25.1 Resistors in series and parallel circuits, §25.3 Kirchhoff’s Rules, §25.4 Ammeters and voltmeters, §25.5 2.1 Ohm’s Law Ohm’s Law states that the voltage difference, V , across a conductor is directly proportional to the current, I , flowing through the conductor. The constant of proportionality is referred to as the resistance, R . V = IR (1) 1

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V batt I batt R 1 R 2 V batt I batt R 2 I 2 R 1 I 1 Figure 1: Series and parallel circuits, respectively. When we refer to a material as being “Ohmic” we are saying that the resistance is constant, which is not true for all materials. 2.2 Kirchhoff’s Laws Kirchhoff described two “laws” regarding circuit analysis. Strictly speaking, they are not laws since they can be derived from the definition of electric potential and the conservation of charge. However, we will refer to them as Kirchhoff’s Laws since that is the common usage. Kirchhoff’s Voltage Law, or Loop Law as it is sometimes referred to, states that if you travel a full loop through a circuit, the sum of all of the voltage drops will be zero. Kirchhoff’s Current Law, or Junction Law, states that the sum of all of the currents entering a junction is equal to the sum of all of the currents exiting a junction. Using Kirchhoff’s Laws we can derive equations for the equivalent resistance of a circuit. If we connect two resistors to a power supply in series, it would be the equivalent of having one resistor with a resistance of the sum of the two individual resistances. If we connect two resistors in parallel to a power supply, it would have the inverse equivalent resistance by adding the who resistances in inverse.
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