Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Coulomb Force The field model and the electric field Chapter 20 Electric Forces and Fields Topics: Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Slide 20-1

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 = F e / q = [k Q q / r 2 ] / q = k Q / r 2

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 D E 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. A set of electric field lines is directed as below. At which of the noted points is the magnitude of the field the greatest? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A set of electric field lines is directed as below. At which of the noted points is the magnitude of the field the greatest? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Are the Fields 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)

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagram below are of equal magnitude. In each of the four cases below, two charges lie along a line, and we consider the electric field due to these two charges at a point along this line represented by the black dot. In which of the cases below is the net field to the right? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagram below are of equal magnitude. In each of the four cases below, two charges lie along a line, and we consider the electric field due to these two charges at a point along this line represented by the black dot. In which of the cases below is the field to the right? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagram below are of equal magnitude. In each of the four cases below, two charges lie along a line, and we consider the electric field due to these two charges at a point along this line represented by the black dot. In which case is the magnitude of the field at the black dot the largest? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagram below are of equal magnitude. In each of the four cases below, two charges lie along a line, and we consider the electric field due to these two charges at a point along this line represented by the black dot. In which case is the magnitude of the field at the black dot the largest? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Electric Field Slide 20-30

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Electric Field of a Point Charge Slide 20-31

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 1.Determine the magnitude and the direction of the electric field at point A. 2.Determine the individual forces and the net force on charge B for each of the following cases. Slide E-field Superposition Examples

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Field Vectors and Electric Field Lines E-field Applet 2 What observations can we make about E-field lines?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Nature of Electric Field E-field Applet 1 What observations can we make about E-fields?