1. Electrical Energy and Capacitance Concept of potential difference and potential Potential and potential energy for point charges Potentials and charged conductors  Equipotential surfaces to capacitance

2. Clicker question I If the distance between two negative charges is decreased by a factor of 3, the resultant force between the two charges changes by what factor? A. Decreases to 1/9 B. Decreases to 1/3 C. Increases by 9 D. Increases by 3

3. A question A suspended object A is attracted to a neutral wall. It is also attracted to a positively charged object B. Which of the following is true? A. The object A is uncharged B. It has a positive charge C. It has a negative charge D. It may be either charged positively or negatively E. It may be either charged or uncharged

4. Polarization Forces A charge near a neutral object can move other charges. Like charges move away and unlike move front. All resulting in a net attraction. A weak force -q -Q +Q

5. Chapter 16:Lecture II We have talked about Electric Potentials. V = U/q = Scalar (not a vector) Adds like numbers. -q 1 q2q2 q3q3 -q 4

6. F E U V

7. The Electron Volt The electron volt (eV) is defined as the energy that an electron gains when accelerated through a potential difference of 1 V Electrons in normal atoms have energies of 10’s of eV Excited electrons have energies of 1000’s of eV High energy gamma rays have energies of millions of eV 1 eV = 1.6 x 10 -19 J

8. Equipotential Surfaces An equipotential surface is a surface on which all points are at the same potential No work is required to move a charge at a constant speed on an equipotential surface The electric field at every point on an equipotential surface is perpendicular to the surface

9. Equipotentials and Electric Fields Lines – Positive Charge The equipotentials for a point charge are a family of spheres centered on the point charge The field lines are perpendicular to the electric potential at all points

10. Equipotentials and Electric Fields Lines – Dipole Equipotential lines are shown in blue Electric field lines are shown in gold The field lines are perpendicular to the equipotential lines at all points

11. Application – Electrostatic Precipitator It is used to remove particulate matter from combustion gases Reduces air pollution Can eliminate approximately 90% by mass of the ash and dust from smoke Recovers metal oxides from the stack

12. Application – Electrostatic Air Cleaner Used in homes to reduce the discomfort of allergy sufferers It uses many of the same principles as the electrostatic precipitator

13. Application – Xerographic Copiers The process of xerography is used for making photocopies Uses photoconductive materials A photoconductive material is a poor conductor of electricity in the dark but becomes a good electric conductor when exposed to light

14. The Xerographic Process

15. Application – Laser Printer The steps for producing a document on a laser printer is similar to the steps in the xerographic process Steps a, c, and d are the same The major difference is the way the image forms on the selenium-coated drum A rotating mirror inside the printer causes the beam of the laser to sweep across the selenium- coated drum The electrical signals form the desired letter in positive charges on the selenium-coated drum Toner is applied and the process continues as in the xerographic process

16. Capacitance A capacitor is a device used in a variety of electric circuits The capacitance, C, of a capacitor is defined as the ratio of the magnitude of the charge on either conductor (plate) to the magnitude of the potential difference between the conductors (plates)

17. Capacitance, cont Units: Farad (F) 1 F = 1 C / V A Farad is very large Often will see µF or pF V is the potential difference across a circuit element or device V represents the actual potential due to a given charge at a given location

18. Parallel-Plate Capacitor The capacitance of a device depends on the geometric arrangement of the conductors For a parallel-plate capacitor whose plates are separated by air:

19. Parallel-Plate Capacitor, Example The capacitor consists of two parallel plates Each have area A They are separated by a distance d The plates carry equal and opposite charges When connected to the battery, charge is pulled off one plate and transferred to the other plate The transfer stops when V cap = V battery

20. Electric Field in a Parallel- Plate Capacitor The electric field between the plates is uniform Near the center Nonuniform near the edges The field may be taken as constant throughout the region between the plates