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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

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Thought for the day If you don’t have time to do things right, when will you have time to do it over. Benjamin Franklin Slide 20-3

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Strategic Problem Solving Strategy (PSS) When solving problems, it is important to show how you reasoned from the information given in the problem and key physics ideas to your final answer. The correct final answer with units is only worth 1-3 points. The remainder of the points (70-90% of credit) are awarded for the quality of your solution. You are expected to include the following to receive full credit: Prepare Identify the Physics: State explicitly which physics’ principle(s) apply to the problem situation and that you will use to solve the problem Drawing a Picture: Draw at least one picture to visualize the physics of the problem and define your variables and constants. For motion problems this could be a motion diagram, motion graph, or pictorial diagram Collecting Necessary Information: State all the information given in the problem with correct units. Include preliminary calculations such as unit conversions Assume/Observe: State assumptions or observations that would be useful Solve Start with key equation(s) in symbol form Solve for the unknown quantity in symbols before numeric calculations Then substitute numbers with units and calculate the numeric answer Assess Check to see if your answer is reasonable Does it answer the question that was asked Does it have the right units?

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems - Prepare (also identify key physics)

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems (continued)

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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 20-16 Checking Understanding

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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 20-17 Answer

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Field How do non-contact forces work? How do we get action at a distance without contact? Need concept of a field Field is something that has a value at every point in space Field is the influence a mass or charge has at every point on the space around it Slide 20-3

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

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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

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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 20-14 Examples

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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

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

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Van de Graff Generator From Worksheet 2, Problem 9: What happens to Mike Piepan?

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

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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

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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 20-27 Checking Understanding

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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 20-28 Answer

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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

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Three charges in a line Slide 20-3

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

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