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Chapter 20 Electric Forces and Fields Topics: • Electric charge • Forces between charged • • objects The field model and the electric field Forces and torques on charged objects in electric fields Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-1 Conceptual Example Problem All charges in the diagrams below are equal magnitude. In each case, a small positive charge is placed at the blank dot. In which cases is the force on this charge: A. to the right? B. to the left? C. zero? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-28 Checking Understanding 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 largest? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-29 Answer 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 largest? C Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-30 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 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 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? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems - Prepare (also identify key physics) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Solving Problems (continued) Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Using Mastering Physics If Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Coulomb’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-15 Example Problem 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? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-33 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 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Three charges in a line Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Two hanging cans You and a friend are doing the laundry when you unload the dryer and your friend wants to get some idea of the amount of charge that causes static cling. You immediately take two empty soda cans, which each have a mass of 120 grams, from the recycling bin. You tie the cans to the two ends of a string (one to each end) and hang the center of the string over a nail sticking out of the wall. Each can now hangs straight down 30 cm from the nail. You take your flannel shirt from the dryer and touch it to the cans, which are touching each other. The cans move apart until they hang stationary at an angle of 10º from the vertical. Assuming that there are equal amounts of charge on each can, you now calculate the amount of charge transferred from your shirt. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 The Electric Field Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-34 The Electric Field of a Point Charge Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-35 Checking Understanding Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-36 Answer Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest? D Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-37