Physics II week 3- Ch. 21
Physics II week 3- Ch. 21 1012
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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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