2019年系列讲座课件Physics110.ppt

Voltage-divider rule “The voltage across any resistor in a series is some fraction of the battery voltage.” 2.7 Parallel DC Networks Two elements are in parallel if they have two terminals in common. Total Resistance Source Current Concept Test For resistors in series, what is the same for every resistor? R, V or I? Answer: I For resistors in parallel, what is the same for every resistor? R, V or I? Answer: V Kirchhoff’s Current Law “The sum of the current entering a junction must equal to the current leaving.” Example Problem 2.28 Using Kirchhoff’s current law, determine the currents I3 and I6 for the system of Figure 2.42 I1 = I2 + I3 I3 = I1 - I2 = 6 A – 2 A = 4 A I3 + I4 = I5 + I6 I6 = I3 + I4 - I5 = 4 A + 6 A – 1 A = 9 A Consider Figure 2.35. E1=100V E2=50V E3=20V R1=10W R2=30W R3=40W What is I? What is V2? Example Problem 2.25 Find V1 and V2 of Figure 2.36 using Kirchhoff’s voltage law. Voltage Sources in Series Current-divider rule “The current through any resistor in parallel with other resistors is some fraction of the source current.” Example Problem 2.26 Determine the following for the parallel network in Fig. 2.40. (a) RT (b) I (c) I2 (d) P3 2.8 Series-Parallel Networks Solve for RT, I, I1, I2, and V1 More on Batteries Positive (+) and Negative (-) terminals Batteries use a chemical reaction to create voltage. Construction: Two different metals and Acid e.g. Copper, Zinc, and Citrus Acid e.g. Lead, Lead Oxide, Sulfuric Acid e.g. Nickel, Cadmium, Acid Paste Batteries “add” when you connect them in series. Circuit Symbol: V=W/Q V = the voltage in volts (V) Q = the amount of charge in Coulombs (C) W = the energy expended in Joules (J) Equation for Voltage Example Problem 2.7 Determine the energy expended by a 12 V battery in moving 20 x 1018 electrons between its terminals. (a) If 8 mJ of energy is expended moving 200 mC from one point in an electrical circuit to another, what is the difference in potential between th