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 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. 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. 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. 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. Dryer Sheet Problem - SPS Problem Slide You and a friend are doing the laundry when you unload the dryer and the discussion comes around to static electricity. 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. Dryer Sheet Problem - SPS Problem Slide You and a friend are doing the laundry when you unload the dryer and the discussion comes around to static electricity. 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. Van de Graff Generator Slide 20-3

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. How to make an object move in a circle Slide 6-14 Consider coordinate system with radial, tangential, and z components Consider Force Diagrams for Ball on String on table Ball with plastic circle Ball on string hanging Ball on string in vertical circle

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Uniform Circular Motion Slide 6-13

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Forces in Circular Motion Slide 6-21

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Circular Motion There is an acceleration because the velocity is changing direction. Slide 3-35

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Circular Motion Old vinyl records are 12" in diameter, and spin at 33⅓ rpm when played. What’s the acceleration of a point on the edge of the record? Two friends are comparing the acceleration of their vehicles. Josh owns a Ford Mustang, which he clocks as doing 0 to 60 mph in a time of 5.6 seconds. Josie has a Mini Cooper that she claims is capable of a higher acceleration. When Josh laughs at her, she proceeds to drive her car in a tight circle at 13 mph. Which car experiences a higher acceleration? Slide 3-36

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. For uniform circular motion, the acceleration A.points toward the center of the circle. B.points away from the circle. C.is tangent to the circle. D.is zero.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. For uniform circular motion, the acceleration A.points toward the center of the circle. B.points away from the circle. C.is tangent to the circle. D.is zero.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Acceleration The average acceleration of a moving object is defined as the vector As an object moves, its velocity vector can change in two possible ways. 1.The magnitude of the velocity can change, indicating a change in speed, or 2. The direction of the velocity can change, indicating that the object has changed direction.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Tactics: Finding the acceleration vector

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Tactics: Finding the acceleration vector

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Uniform Circular Motion