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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 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 Three charges in a line Two charged particles, with charges q1= +q and q2 = 4q, are located at a distance d = 2 cm apart on the x axis. A third charged particle, with charge q3= +q , is placed on the x axis such that the magnitude of the force that charge 1 exerts on charge 3 is equal to the force that charge 2 exerts on charge 3. Find the position of charge 3 when = 1.0 For equilibrium (not asked) there are 3 conditions • Charges must be on x-axis • Fe,1=>3 = Fe,2=>3 • The two forces must point in opposite directions 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 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