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Today 3/10 Plates if charge E-Field Potential “Plates of Charge” Due Thursday, 3/13 Lab: “Electric Deflection of Electrons” HW: ++++++++++++++++++ Looking from farther away We call this an “infinite plate.” + + + + + Field lines for a positive plate is the charge on each square meter of the plate. Near the plate the field lines must (by symmetry) point directly away from the plate. How does the field strength at the X’s compare? Same field line density, (+) same strength! Field lines for a plate (+) Field lines for a negative plate (-) Can’t tell if the field is caused by a pos plate below or a neg plate above! Size of the E-field For plates: 0 = 8.8x10-12 C2/Nm2 For point charges: k = 9.0x109 Nm2/C2 E 2 0 Note: no dependence on distance kqs E 2 r Two Parallel Plates (+) (-) Two Parallel Plates (+) (-) Two Parallel Plates Enet (+) (-) Two Parallel Plates Enet Enet (+) (-) Enet = 0 Two Parallel Plates Enet Enet (+) Enet = 0 (-) How big is E? Enet (+) (-) Enet = 0 How big is E? E 2 0 (+) E 2 0 E net Enet = 0 (-) 0 What do the field lines look like? (-) (+) E = 0 outside! Points A and B are located as shown between two large uniformly charged parallel plates. Which of the following correctly describes what happens to the electric fields at A and B if the plate on the right is removed? a. The field at A drops more +Q -Q than the field at B. b. The field at B drops more A B than the field at A. c. The fields at A and B remain that same. d. The fields at A and B drop by the same amount. Charged conducting plate 0 = Q/A A = Area of one side What’s wrong with this picture? Charged conducting plate L = 0/2 R = 0/2 Free charge always goes to surface of conductor. Charged conducting plate L = 0/2 EL = L/20 EL = 0/40 What is the electric field inside the conductor? R = 0/2 Charged conducting plate L = 0/2 R = 0/2 ER = R/20 ER = 0/40 What is the electric field inside the conductor? Charged conducting plate L = 0/2 R = 0/2 The electric field is zero everywhere inside the conductor. What is the electric field inside the conductor?