Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric potential energy Electric potential Conservation of energy Chapter 21 Electric Potential Topics: 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

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A Conductor in Electrostatic Equilibrium Slide 21-27

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Potential and a Conducting Sphere Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Outside the Sphere (Just like a point charge) E = k|q| / r 2 V = kq / r Inside the sphere (not like a point charge) E = 0 Delta V = 0 => V = constant

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Exercise What is Q 2 ? Slide 21-28

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Equipotential Maps (Contour Maps) Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 5. At which point is the magnitude of the electric field the greatest? 6. Is it possible to have a zero electric field, but a non-zero electric potential? 7. Is it possible to have a zero electric potential, but a non-zero electric field? 1.Describe the charges that could create equipotential lines such as those shown above. 2.Describe the forces a proton would feel at locations A and B. 3. Describe the forces an electron would feel at locations A and B 4.Where could an electron be placed so that it would not move?

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 3D view Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

E-field lines and Equipotential lines Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. E-field Lines Go from + charges to - charges Perpendicular at surface of conductor or charged surface E-field in stronger where E-field lines are closer together More charge means more lines Equipotential Lines Parallel to conducting surface Perpendicular to E-field lines Near a charged object, that charges influence is greater, then blends as you to from one to the other E-field is stronger where Equipotential lines are closer together Spacing represents intervals of constant  V Higher potential as you approach a positive charge; lower potential as you approach a negative charge

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Two last Points Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electron Volt - unit of energy The energy an electron gains as it goes through Delta V = 1 V PE e = qV = (1.6e-19 C)(1 V) = 1.6e-19 J 1.6e-19 J = 1 eV Path Independence Delta V does not depend on path Delta V = 0 around any closed path

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Capacitance and Capacitors The charge ±Q on each electrode is proportional to the potential difference ΔV C between the electrodes: Slide C = Q/  V c

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Charging a Capacitor Slide 21-30

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. The Capacitance of a Parallel-Plate Capacitor Slide 21-31

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Dielectrics and Capacitors Slide 21-32

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Dielectric Constant With a dielectric between its plates, the capacitance of a parallel-plate capacitor is increased by a factor of the dielectric constant κ: Slide 21-33

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Light the Bulb Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Can you light a bulb when you have 1 battery 1 Bulb 1 wire A - yes B - no

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Light the Bulb Slide Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Can you light a bulb when you have 1 battery 1 Bulb 1 wire A - yes B - no

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. Slide ∆ V bat = =  W chem q ____

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Properties of a Current Slide 22-8

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Definition of a Current Slide 22-9

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Kirchhoff’s Laws Slide 23-11

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Using Kirchhoff’s Laws Slide 23-12