Lecture 11-12 Ch 27

# Lecture 11-12 Ch 27 - PH 222-3A Spring 2007 Circuits...

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PH 222-3A Spring 2007 ircuits Circuits Lectures 11-12 Chapter 27 (Halliday/Resnick/Walker, Fundamentals of Physics 8 th edition) 1

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Chapter 27 Circuits In this chapter we will cover the following topics: -Electromotive force (emf) - eal and real emf devices Ideal and real emf devices -Kirchhoff’s loop rule -Kirchhoff’s junction rule -Multiloop circuits -Resistors in series -Resistors in parallel -RC circuits, charging and discharging of a capacitor 2
In order to create a current through a resistor, a potential difference must be created across its terminals. One way f doing this is to connect the resistor o a battery A device of doing this is to connect the resistor to a battery. A device that can maintain a potential difference between two terminals is called a or an Here emf "seat of an emf " "emf device." stands for electromotive force. Examples of emf devices are a battery, an electric generator, a solar cell, a fuel cell, etc. hese devices act like "charge pumps" in he sense that they move positive charg These devices act like charge pumps in the sense that they move positive charges from the low-potential (negative) terminal to the high-potential (positive) terminal. A mechanical analog is given in the figure below. High (+) reservoir In this mechanical analog a water pump transfers water from the low to the high reservoir. The water turns from the high to the low reservo through a ump Low (-) servoir returns from the high to the low reservoir through a pipe, which is the analog of the resistor. The emf (symbol ) is defined as the potential E pump reservoir difference between the terminals of the emf device when no current flows through it. 3

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The polarity of an emf device is indicated by an arrow with a small circle at its tail. Notation : The arrow points from the negative to the positive terminal of the device. When the emf device is connected to a circuit its internal mechanism transports positive charges from the negative to the positive terminal and sets up a charge flow (a.k.a. current) around the rcuit In doing so the emf devic does work on W circuit. In doing so the emf device does work on a charge , which is given by the equation . The required energy comes from chemical dW q dq d dW = E reactions in the case of a battery; in the case of a generator it comes from the mechanical force that rotates the generator shaft; in the case of a solar cell it comes from the Sun. In the circuit of the figure the energy stored in emf device B changes form: does mechanical work on he motor It produces It does mechanical work on the motor. It produces thermal energy on the resistor. It gets converted into chemical energy in emf device A. 4
Ideal and Real Emf Devices An emf device is said to be if the voltage across its terminals and does depend on the current that flows V ab i ideal not through the emf device: . V = Ideal emf device V = E An emf device is said to be if the voltage across its rminals and with current according o V b real ecreases V i E terminals with current according to the equation .

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## This note was uploaded on 09/26/2008 for the course PH 222 taught by Professor Mirov during the Spring '08 term at University of Alabama at Birmingham.

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Lecture 11-12 Ch 27 - PH 222-3A Spring 2007 Circuits...

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