p2020chap04 - PHYS-2020: General Physics II Course Lecture...

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Unformatted text preview: PHYS-2020: General Physics II Course Lecture Notes Section IV Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3 Abstract These class notes are designed for use of the instructor and students of the course PHYS-2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille. IV. Direct Current Circuits A. Sources of emf . 1. Direct current means that the current travels only in one direc- tion in a circuit = ⇒ DC for short. 2. The source that maintains the constant current in a closed circuit is called a source of “ emf .” a) One can think of such a source as a “charge pump” = ⇒ forces electrons to move in a direction opposite the E-field, hence current, inside a source. b) The emf, E , of a source is the work done per unit charge = ⇒ measured in volts. c) A battery is a good example of a source of emf. 3. Since batteries have internal resistance, r , the potential difference across the terminals of a battery are Δ V = E - Ir . (IV-1) a) E is equal to the terminal voltage when the current is zero. b) From Ohm’s law, the potential difference across the exter- nal resistance R , called the load resistance , is Δ V = IR , so IR = E - Ir or E = IR + Ir = I ( R + r ) . (IV-2) c) If r lessmuch R , then E ≈ IR (as we have been assuming to date). IV–1 IV–2 PHYS-2020: General Physics II B. Resistors in Series. 1. The current through all resistors in a series circuit is the same. I = I 1 = I 2 = ··· (IV-3) R 1 I 1 Δ V 1 R 2 I 2 Δ V 2- + Δ V I I 2. The reduced circuit above is R eq I eq- + Δ V I I where Δ V = I eq R eq = IR eq Δ V = Δ V 1 + Δ V 2 = I 1 R 1 + I 2 R 2 = IR 1 + IR 2 IR eq = IR 1 + IR 2 . Donald G. Luttermoser, ETSU IV–3 3. As such, we can express the equivalent resistance as R eq = R 1 + R 2 (IV-4) for two series resistors or more generally ( e.g. , for more than 2 resistors in series) as R eq = N summationdisplay i =1 R i , series circuits (IV-5) where N is the total number of resistors. 4. Once the equivalent resistance is found, one can then calculate I from Δ V using Ohm’s law. C. Resistors in Parallel. 1. When resistors are connected in parallel, the potential difference across them are the same: Δ V 1 = Δ V R 1 I 1 Δ V 2 = Δ V R 2 I 2- + Δ V I I ⇓ IV–4 PHYS-2020: General Physics II R eq I eq- + Δ V I I Δ V = Δ V 1 = Δ V 2 . (IV-6) 2. The current entering a parallel circuit is equal to the sum of all currents traveling through each resistor: I = I 1 + I 2 (IV-7) in our 2 resistor diagram above....
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This note was uploaded on 03/21/2012 for the course PHY 2020 taught by Professor Staff during the Fall '08 term at University of Florida.

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p2020chap04 - PHYS-2020: General Physics II Course Lecture...

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