Chapter 21 Electric Potential Topics: Sample question: Electric potential energy Electric potential Conservation of energy Sample question: Shown is the electric potential measured on the surface of a patient. This potential is caused by electrical signals originating in the beating heart. Why does the potential have this pattern, and what do these measurements tell us about the heart’s condition? Slide 21-1

Class Format Question More traditional lecture method with some clicker questions Advantages More comfortable More examples to adapt to homework problems More content coverage (one chapter every 2-4 days) Disadvantages More content coverage Less learning of Physics concepts No improvement in exam scores, possible decline unless exam problems are exactly like homework problems Encourages memorizing vs. learning physics Not as much development of useful thinking and problem solving skills Will not raise grades Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Class Format Question More conceptual approach with in-class activities (modified to include more examples per unit) Advantages Better learning of Physics concepts Encourages learning physics vs. memorizing Less content coverage (more time on harder chapters until last month, then normal pace - one chapter every 2-4 days ) Development of useful thinking and problem solving skills Disadvantages Less comfortable Less content coverage Improvement in exam scores requires practicing doing problems using concepts and less reliance on memorizing examples Possibly more frustration on homework Learning to think and problem solve new ways is not easy Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Class Format Question More traditional lecture method with some clicker questions Cover more topics - learn less More conceptual approach with in-class activities (modified to include more examples per unit) Cover fewer topics - learn more Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

What to do to do well in this class Focus on key physics concepts May seem like basics but will help you solve even complex problems Focus on principle rather than recipes Need to have a functional understanding of key concepts Express key equations as sentences Know where they come from and what they mean Know how and when to apply them Know which equations are general and which are special cases Must know when not to apply special cases Look at a problem after a good physics diagram and maybe a good physical diagram and know what key physics concepts apply in that problem Memorize key concepts so you can look at a problem, say that’s Newton 2, and know the associated equation in a snap Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

What to do to do well in this class Focus on key physics concepts How to do this When you look at problems, mentally group problems by the physics rather than the physical situation After each class or at least each week, create a notesheet to organize a structure of the new key concepts for each chapter and note how they fit in with previous key concepts Use the note sheet to do homework problems (a) do as many homework problems as you can just using this sheet. (b) then go to your notes and the textbook for your missing pieces Use flash cards to memorize key concepts - include the concept description, relevant equations, diagrams, and what types of problems benefit from using that concept Pay close attention to examples done in class and note the physics and assume/observes in each example and how these are used Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Chapter 21 Key Equations (1) Key Energy Equations from Physics 151 Definition of Work Work- Energy Theorem (only valid when particle model applies) Work done by a conservative force (Fg, Fs, & Fe) Also work done by conservative force is path independent Conservation of Energy Equation Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Chapter 21 Key Equations (2) Key Energy Equations from Physics 152 Electric Potential Energy for 2 point charges (zero potential energy when charges an infinite distance apart) Potential Energy for a uniform infinite plate For one plate, zero potential energy is at infinity For two plates, zero potential energy is at one plate or inbetween the two plates Electric Potential V and Change in Electric Potential => V Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Example: Electric Potential Energy A cart on a track has a large, positive charge and is located between two sheets of charge. Initially at rest at point A, the cart moves from A to C. Draw qualitative force diagrams for the cart at positions A, B and C. Draw qualitative energy bar charts for the cart when it is at each position A, B and C. List the objects that make up your system: c. How would your force and energy diagrams change (if at all) if the sheet to the right were also positively charged? Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Changes in Electric Potential Energy PEe For each situation below, identify which arrangement (final or initial) has more electrical potential energy within the system of charges and their field. Initial Final Greatest PEe (a) (b) (c) (d) Hydrogen Atom Slide 21-16

Changes in Electric Potential Energy PEe For each situation below, identify which arrangement (final or initial) has more electrical potential energy within the system of charges and their field. Initial Final Greatest PEe (e) (f) (g) Slide 21-16

Changes in Electric Potential Energy PEe Answers for a positive test charge: a) 0; b) negative; c) positive; d) negative; e) zero; f) negative; g) zero (Instructor should state whether moving charge is positive, negative, or zero.) Is the change ∆PEe of a charged particle positive, negative, or zero as it moves from i to f? (a) Positive (b) Negative (c) Zero (d) Can’t tell Slide 21-11

Chapter 21 Key Equations (3) Key Points about Electric Potential Electric Potential increases as you approach positive source charges and decreases as you approach negative source charges (source charges are the charges generating the electric field) A line where V= 0 V is an equipotential line (The electric force does zero work on a test charge that moves on an equipotential line and PEe= 0 J) For a point charge For very large charged plates, must use Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Electric Potential and E-Field for Three Important Cases Slide 21-25

Checking Understanding b) 3, 1 = 2 c) 3, 2, 1 d) 1 = 2, 3 = 4 e) 1, 2, 3 Rank in order, from largest to smallest, the electric potentials at the numbered points. Slide 21-14

Example A proton has a speed of 3.5 x 105 m/s at a point where the electrical potential is 600 V. It moves through a point where the electric potential is 1000 V. What is its speed at this second point? Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

A Topographic Map Slide 21-12

Graphical Representations of Electric Potential Slide 21-13

Slide 21-15 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Example A proton has a speed of 3.5 x 105 m/s at a point where the electrical potential is 600 V. It moves through a point where the electric potential is 1000 V. What is its speed at this second point? Slide 21-16 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

Electric Potential Energy Slide 21-9

Electric Potential Slide 21-10