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. Clicker Question 1.A charged rod is brought near a neutral metal sphere. Which of the following is true? A. There is an attractive force between the rod and sphere. B. There is a repulsive force between the rod and sphere. C. There is no electric force between the rod and sphere. Slide 20-2

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Clicker Question 1.A charged rod is brought near a neutral metal sphere. Which of the following is true? A. There is an attractive force between the rod and sphere. Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charge Model: Electric forces can be attractive or repulsive 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 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 F e gets weaker with distance: F e α 1/r 2 F e between charged tapes are > F e between charged tapes & neutral objects Rubbing causes some objects to be charged by charge separation Charge can be transferred by contact

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Triboelectric series => Triboelectric Effect When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged. MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE

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. 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 what you observed in the tape activity? 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.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. How do charged objects and neutral objects interact? Part B Use your model for electric charge to account for the electrical attraction between a charged tape and an uncharged metal ball. As part of your answer, draw a sketch of the charge distribution on the tape and ball both before and after they are brought near one another.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charge Diagrams: Polarization, Grounding, and Charging Insulator and Conductor simulation Grounding Simulation Shocked by the door – John Travoltage Grounding a Positively charged-electroscope Grounding a negatively charged-electroscopes Balloons and Charge Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A Question of Charge If I give you an object, how can you tell if it is positively or negatively charged? (assume a top tape is + charged.) Suppose a newspaper article claims that scientists in California have found a third type of charged object. What experiments would you perform to check their claim? 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. Question for Wednesday Charging 2 spheres 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. Einstein’s Big Idea Nature of Science What do scientists do? How are scientific ideas developed? Why do we believe them? What is a theory in science? How is this different from how most people think about the word? How is this different from say journalism or religion? Slide 20-3