Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric charge Forces between charged objects The field model and the electric field Forces and torques on charged objects in electric fields 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. Charge Model Electric forces can be attractive or repulsive There are two kinds of charges, positive (protons) and negative (electrons). In solids, electrons are charge carriers (protons are 2000 time more massive). A charged object has a deficit of electrons (+) or a surplus of electrons (-). Neutral objects have equal numbers of + and - charges Objects with the same sign of charge repel each other Objects with the opposite sign of charge attract each other Neutral objects are polarized by charged objects which creates attractive forces between them Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. How do charged objects and neutral objects interact? Part A A small ball with zero net charge is positively charged on one side, and equally negatively charged on the other side. The ball is placed near a positive point charge as shown. Would the ball be attracted toward, repelled from, or unaffected by the positive point charge? Explain. Is your answer consistent with Coulomb's Law? Is it consistent with what you observed in the tape lab? Explain. Through careful observations of physical phenomenon, scientists develop models or mental pictures to account for what is observed. These scientific models can also be used to predict physical behavior. From observations of electrical we can develop a model for electric charge. Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Polarization model Insulator and Conductor simulation Colorado Phet and_Circuits and_Circuits Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging objects with insulators and conductors Demonstrations Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Charges and Forces, Part III Slide 20-10

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding Two spheres are touching each other. A charged rod is brought near. The spheres are then separated, and the rod is taken away. In the first case, the spheres are aligned with the rod, in the second case, they are perpendicular. After the charged rod is removed, which of the spheres is: i) Positive ii) Negative iii) Neutral Slide 20-13

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging an object Rubbing nets_and_Circuits nets_and_Circuits Conduction (see previous slide) Induction Electroscope example Charging two spheres example You have two conducting spheres. How can you charge them with opposite charges without touching either one with a charged object? (Anything else is fair game) 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