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Chapter 20 Electric Forces and Fields Topics: • Electric charge • Forces between charged • • objects The field model and the electric field Forces and torques on charged objects in electric fields Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-1 Newton’s Laws of Motion Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Questions on P151 Force & Motion Review Note sheet for Mastering Physics? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Coulomb’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-15 Charge & Forces 1. Draw individual and net forces acting on object B for the four situations below. 2. Calculate the magnitude and direction of the net force on object B. Be sure to state your assumptions Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Two hanging cans You and a friend are doing the laundry when you unload the dryer and your friend wants to get some idea of the amount of charge that causes static cling. You immediately take two empty soda cans, which each have a mass of 120 grams, from the recycling bin. You tie the cans to the two ends of a string (one to each end) and hang the center of the string over a nail sticking out of the wall. Each can now hangs straight down 30 cm from the nail. You take your flannel shirt from the dryer and touch it to the cans, which are touching each other. The cans move apart until they hang stationary at an angle of 10º from the vertical. Assuming that there are equal amounts of charge on each can, you now calculate the amount of charge transferred from your shirt. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Introduction to Electric Field 1. Find the Electric force from a 9.00e-6 C charge on the following charges at 1m, 2m, and 3 m. a) 3.00e-6 C b) -4.00e-6 C c) -10.0e-6 C Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Introduction to Electric Field 1. Find the Electric force from a 9.00e-6 C charge on the following charges at 1m, 2m, and 3 m. a) 3.00e-6 C b) -4.00e-6 C c) -10.0e-6 C 2. Find the force per charge from a 9.00e-6 C charge on the charges above at 1m, 2m, and 3 m. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Nature of Electric Field E-field Applet 1 http://physics.weber.edu/schroeder/software/EField/ What observations can we make about E-fields? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Nature of Electric Field • Test charge is a small positive charge to sample the E-Field • Charge of test charge is small compared to source charges (source charges are the charges that generate the field) • E-field vectors • E-field is the force per charge • E-field vectors points away from + charges • E-field vectors point towards - charges • E-field for point charges gets weaker as distance from source point charges increases • For a point charge E = Fe / q = [k Q q / r2] / q = k Q / r2 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Find the Electric Field Given the following forces that a positive test charge feels if placed at these three points, find the E-field vectors at these points. A B C E D Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Find the Electric Field Given the following forces that a positive test charge feels if placed at these three points, find the E-field vectors at these points. A B C E D How would the Force vectors and E-field vectors change at point 3 for the following changes: • Replace the positive test charge (+q) with a negative test charge (-q) • Replace the positive test charge (+q) with a test charge twice as large (+2q) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Electric Field Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-34 The Electric Field of a Point Charge Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-35 Checking Understanding Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-36 Answer Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? D Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-37