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Week 3 Day 1 => 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 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 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Soda Can Electroscope Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 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. After the charged rod is removed, which of the spheres (A & B) is: i) Positive ii) Negative iii) Neutral A => Sphere A is + and sphere B is – B => Sphere A is – and Sphere B is + C => Spheres A and B are both + D => Spheres A and B are both – E => Spheres A and B are both neutral Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-13 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 second case, they are perpendicular. After the charged rod is removed, which of the spheres (C & D) is: i) Positive ii) Negative iii) Neutral A => Sphere C is + and sphere D is – B => Sphere C is – and Sphere D is + C => Spheres C and D are both + D => Spheres C and D are both – E => Spheres C and D are both neutral Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-13 Van de Graff Generator Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and a charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. A. Draw Free-Body Diagrams for the pucks and spheres B. How do the Coulomb forces acting on spheres A & B compare? (Use a ratio) C. Which sphere has the greater acceleration? How would your answer change if the mass of the puck under sphere A was reduced by 50%? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0 x 10-8 C is placed on sphere A on the left and a charge of 6.0 x 10-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. D. As the two spheres get farther away from one another, how would (if at all) the following quantities change? 1) Force 2) Speed 3) Acceleration Choices: a) Increase b) Decrease c) Stay the same d) Can’t tell Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-3 Coulomb’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-15 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 20-29 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 20-25 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? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-26 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 The Electric Field Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-30 The Electric Field of a Point Charge Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 20-31 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