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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 Electric Field Vectors and Electric Field Lines Applet 1: Exploring E-field from point chargeshttp://physics.weber.edu/schroeder/software/EField/ E-field Applet 2: Exploring E-field vectors, E-field lines and trajectories for point charges http://vnatsci.ltu.edu/s_schneider/physlets/main/efield.shtml E-field Applet 3: E-field vector fields for point charges & charged plates (tricky – make notes on how to use) http://www.falstad.com/vector2de/ Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. E-field Superposition Example 1. Determine the magnitude and the direction of the electric field at point A. In your physical diagram, make sure you label your r’s as well as your angles Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Are these Field Vectors Real??? Are either or both of these a possible electric field? Explain the reasoning behind your answer (Focus on the vectors, not the source charges) A) B) C) D) E) A is possible but B is not A is not possible but B is possible Both A and B are possible Neither A nor B are possible It is not possible to tell Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Conductors and Electric Fields Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-55 Consider an infinite sheet of charge h Q E= where h = 2e 0 A • Epsilon nought, e 0 = 8.85 ´ 10 -12 C2 N × m2 is electric permitivity of free space • Electric permitivity is a measure of how well electric field can pass through space or materials Discuss charge density Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Consider two infinite sheets of charge What is the E-field at points A, B, and C ? Case 1: A B C Qleft = +Q Qright = -Q Case 2: Qleft = 2Q Qright = Q Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding Two parallel plates have charges of equal magnitude but opposite sign. What change could be made to increase the field strength between the plates? A. B. C. D. E. Increase the magnitude of the charge on both plates Decrease the magnitude of the charge on both plates Increase the distance between the plates Decrease the distance between the plates Increase the area of the plates (while keeping the magnitude of the charges the same) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Answer Two parallel plates have charges of equal magnitude but opposite sign. What change could be made to increase the field strength between the plates? A. B. C. D. E. Increase the magnitude of the charge on both plates Decrease the magnitude of the charge on both plates Increase the distance between the plates Decrease the distance between the plates Increase the area of the plates (while keeping the magnitude of the charges the same) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Additional Clicker Questions A small sphere is suspended from a string in a uniform electric field. Several different cases of sphere mass and sphere charge are presented in the following table. In which case is the angle at which the sphere hangs the largest? Sphere mass (g) A. 2.0 B. 3.0 C. 2.0 D. 3.0 E. 4.0 Sphere charge (nC) 4.0 4.0 6.0 8.0 9.0 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Answer A small sphere is suspended from a string in a uniform electric field. Several different cases of sphere mass and sphere charge are presented in the following table. In which case is the angle at which the sphere hangs the largest? Sphere mass (g) C. 2.0 Sphere charge (nC) 6.0 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Begin Energy Review (time permitting) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.