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 Week 3 Day 1 => 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. 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. The Charge Model Slide 20-11

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. Introduce concept of polarization

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. Charging objects with insulators and conductors Demonstrations Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging by Contact Charges on an insulator do not move. Charges on a conductor adjust until there is no net force on any charge. Slide We call this electrostatic equilibrium.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging by Contact Slide 20-18

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. 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 Examples

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 20-13

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 Slide 20-13

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

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Soda Can Electroscope Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging by induction How can we charge an object without rubbing, touching it with a charged object, or by conduction? Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Van de Graff Generator Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 C is placed on sphere A on the left and a charge of 6.0 x 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%? Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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 C is placed on sphere A on the left and a charge of 6.0 x 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) Force2) Speed3) Acceleration Choices: a) Increase b) Decrease c) Stay the same d) Can’t tell Slide 20-3

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

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

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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? Conceptual Example Problem Slide 20-28

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 largest? Slide Checking Understanding

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 largest? Slide Answer

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

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