Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric charge Forces between charged objects 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. A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the largest? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the largest? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0e-8 C is placed on sphere A on the left and a charge of 6.0e-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. A.Draw Free-Body Diagrams for the pucks and spheres B.How do the Coulomb forces acting on spheres A & B compare? (Use a ratio) C.Which sphere has the greater acceleration? How would your answer change if the mass of the puck under sphere A was reduced by 50%? Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0e-8 C is placed on sphere A on the left and a charge of 6.0e-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. D.As the two spheres get farther away from one another, how would (if at all) the following quantities change? 1) Force 2) Speed3) Acceleration Choices: a) Increase b) Decrease c) Stay the same d) Can’t tell Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Charge Model Slide 20-11

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Visualizing Charge Charges on an insulator do not move. Charges on a conductor adjust until there is no net force on any charge. We call this electrostatic equilibrium. Slide 20-12

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. If a charged plastic rod is brought near an uncharged metal rod on an insulating stand, an uncharged metal ball near the other end of the metal rod is attracted to this end of the rod. Explain the motions of charges that give rise to this force. Follow-up: Describe a procedure by which you could give two identical metal spheres exactly equal charges. Slide Examples

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. How does an electroscope work? Three cases A. When you put charge on a neutral electroscope B.When you bring a charged object near a neutral electroscope C.When you bring a charged object near a charged electroscope Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Van de Graff Generator

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Coulomb’s Law Slide 20-15

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Two 0.10 g honeybees each acquire a charge of +23 pC as they fly back to their hive. As they approach the hive entrance, they are 1.0 cm apart. What is the magnitude of the repulsive force between the two bees? How does this force compare with their weight? Slide 20-29

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagrams below are of equal magnitude. In each case, a small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the smallest? Slide Checking Understanding

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. All charges in the diagrams below are of equal magnitude. In each case, a small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the smallest? Slide Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 20-3