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Physics II week 3- Ch. 21

by: ShayD

Physics II week 3- Ch. 21 1012

GPA 3.74

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About this Document

This study guide combines the textbook and lecture slides. The notes cover: electric current, electromotive force, DC circuits, resistance/ resistivity, ohm's law, capacitors both parallel/ in seri...
Basic Physics II
David Hornes
Class Notes
direct current circuits, resistance, voltage, current, electric current, electromotive force, DC circuits, resistance/ resistivity ohm's law, capacitors parallel/series, ammeters, voltmeters
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This 5 page Class Notes was uploaded by ShayD on Saturday February 13, 2016. The Class Notes belongs to 1012 at University of Missouri - St. Louis taught by David Hornes in Spring 2016. Since its upload, it has received 28 views. For similar materials see Basic Physics II in Physics 2 at University of Missouri - St. Louis.


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Date Created: 02/13/16
Dudaie 1 Physics II­ Chapter 21 Chapter 21­ Electric Current & Direct­Current Circuits  1. Electric Current  a. The flow of electric charge from one place to another is known as  electric current i. I=∆Q/∆ t 1. 1 A= 1C/s} ampere or amp b. Often charge is carried by electrons moving through a metal wire i. Do example 21­1  c. When the charge flows through a closed path and returns to its starting point the closed path is referred to as electric circuit i. In this chapter we consider direct current circuits­ also known dc circuit­ is a current in which always flows in the same  direction  ii. **when circuits with currents that periodically reverse their  directions are AC circuits (chapter 24)** d. Batteries and electromotive force i. A battery performs a similar functions in an electric circuits 1. To put it simply, a battery uses chemical reactions to  produce a difference in electric potential between 2 ends  (terminals) a. The terminal that corresponding to a high electric  potential is denoted by + b. The terminal that corresponding to a low electric  potential is denoted by – 2. The switch is open creating an open circuit;  no closed path Dudaie 2 ii. The electric potential between the terminals is referred to as  electromotive force (emf ε); this happens when the battery is  disconnected form the circuit iii. The direction of the current in an electric circuit is the direction  in which a positive test charge would move 2. Resistance and Ohm’s Law a. In an ideal case nothing about the wire prevents their free motion b. Real wires under normal conditions always affect the electrons to  some extent} resistance  i. Ohm’s law: 1. V=IR a. I­ current  b. R­resistance c. V­ volts  2. 1 ohm (Ω) = 1 V/A 3. Although it’s not a law of nature it’s a good rule of  thumb c. Resistivity (ρ) i. In a wire of length L and cross sectional area A, the resistivity  of the wire depends of the material from which it’s made of 1. The resistance of a wire is proportional to L and  inversely proportional to A a. R=ρ( ) A i. Do problem 21­2  3. Energy and Power in Electric Circuits  a. When a charge moves across a potential difference V, its electric  potential energy, U, changes by the amount  Dudaie 3 ∆ U= (∆Q )V i. 1. The rate at which energy changes  P=∆U /∆t ∆Q V /(∆t) a. =  ii. Power is measured by multiplying amps and current  1. P=IV a. Watts, W iii.  Applying ohm’s law to this case we can write the power  dissipated in a r2sistor  1. P=IV =I R iv. We can also modify 2hm’s law to solve for current  1. P=IV=V /R   a. Conceptual check point 21­2 b. Example 21­4  4. Circuits Contains Capacitors  a. Voltage is the difference in charge between two points b. Current is the rate at which charge is flowing i. Series of capacitors, ae when resistors connected one after  another  1. A capacitor is a device used to store an electric charge,  consisting of one or more pairs of conductors separated  by an insulator c. Capacitor in parallel i. In a series circuit, the current through each of the components  is the same, and the voltage across the circuit is the sum of the  voltages across each component ii. The simplest way to combine capacitors is connecting them in  parallel  Dudaie 4 iii. We equivalent charge of the individual capacitors  1. (C 1C +2 )= 3 eq a. Q=C ε}eqarad, F 2. Connecting capacitors in parallel produces an equivalent  capacitance greater than the greatest individual  capacitance  3. Example 21­8 d. Capacitor in series  i. Capacitors connected in a series follow the same rule as  capacitors in parallel  ii. In a parallel circuit, the voltage across each of the components  is the same, and the total current is the sum of the currents  through each component iii. Active Example 21­3  5. Ammeters and Voltmeters Dudaie 5 a. Voltmeter measures voltage i. A voltmeter measures the potential drop between any two point in a circuit  ii. A real voltmeter always allow some current to flow through it  1. An ideal voltmeter would be one in which resistance is  infinite so that the current it draws from the circuit is  negligible  b. Ammeter measures currents  i. It is designed to measure the flow of current through a  particular portion of a circuit ii. If an ammeter has a finite resistance the presence of the meter  in the presence of the meter in the circuit will alter the current 1. An ideal ammeter would be one with zero resistance 


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