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. 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. 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. Uniform Circular Motion Slide 6-13

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

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Circular motion vs. Angular (Rotational) motion Slide 6-14

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. Ski Ramp - SPS Problem Slide You are working over winter break to design a new skiing ride at Cliff’s Amusement Park. The ride consists of a ramp that acts like a beginners ski hill. The ramp has 2 parts. Both parts are inclined at 15 degrees to the horizontal. The skiers will start at the top of the upper part of the ramp and ski down both parts of the ramp. The upper part of the ramp is made from a low-friction (friction is negligible) surface so that skiers pick up speed going down the upper part of the ramp. The skiers should be at 40.0 MPH at the end of the upper part of the ramp. Assume the skier has a mass m = 75 kg. The lower part of the ramp, which is m long, has just enough friction force so that the riders maintain constant speed over this part of the ramp. At the end of the 2 nd part of the ramp is a horizontal braking area. As part of the design of the ramp, determine the following: How long the upper part of the ramp has to be so that the skiers reach 40.0 MPH starting from rest. What is the magnitude and direction of the friction force of the lower ramp on the skier so that skiers on the lower ramp stay at constant velocity. How long would it take skiers to reach the bottom of the ramp

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. Peter Piepan Demo 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. 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