Chapter 19. Capacitors, Resistors and Batteries

Transcription

1 Chapter 19 Capacitor, Reitor and Batterie

2 Capacitor: Charging and Dicharging Experiment 1 Experiment 2

3 Capacitor: Contruction and Symbol The capacitor in your et i imilar to a large two-dik capacitor D There i no connecting path through a capacitor

4 Electron current Capacitor: Dicharge

5 Capacitor Dicharge The fringe field of the capacitor plu the electric field of the charge on the urface of the wire drive current in a way to REDUCE the charge of the capacitor plate. Recall that we derived an expreion for the fringe field by conidering the uperpoition of two charged dik eparated by a ditance. We ignored the mall amount of charge on the exterior of the capacitor. See Lecture 8 Part 1 note.

6 How i Dicharging Poible? E Poitive and negative charge are attracted to each other: how can they leave the plate? Fringe field i not zero! Electron in the wire near the negative plate feel a force that move them away from the negative plate. Electron near the poitive plate are attracted toward it.

7 Capacitor: Charging Fringe field of a capacitor rie until E= in a wire tatic equilibrium. Fringe field oppoe the flow of current!

8 Capacitor: Charging Why doe current top flowing in the circuit? Ultimately, the fringe field of the capacitor and the field due to charge on the wire are uch that E= inide the wire. At thi point, i=.

9 The Effect of Different Light Bulb Thin filament Thick filament Which light bulb will glow longer? Why? 1) Round i brighter capacitor get charged more? 2) Long bulb glow longer capacitor get charged more?

10 Effect of the Capacitor Dik Size Ue two different capacitor in the ame circuit Initially, which capacitor will caue the bulb to produce more light? Which capacitor will make the light bulb glow longer? Fringe field: E 1 Q / A 2 R

11 Effect of the Capacitor Dik Separation Initially, which capacitor will caue the bulb to produce more light? Which capacitor will make the light bulb glow longer? Fringe field: E 1 Q / A 2 R

12 Effect of Inulator in Capacitor Inulator Initially, which capacitor will caue the bulb to produce more light? Which capacitor will make the light bulb glow longer? Fringe field: E 1 Q / A 2 R Edipole

13 The capacitor hown are initially uncharged. When connected to identical circuit, after.1 of charging: A) The fringe field of each capacitor i the ame. B) The fringe field of the maller capacitor i greater. C) The fringe field of the larger capacitor i greater. R 1 R 2 E fringe Q / A ( 2 R )

14 Conider two capacitor whoe only difference i that capacitor number 1 ha nothing between the plate, while capacitor number 2 ha a layer of platic in the gap. They are placed in two different circuit having imilar batterie and bulb in erie with the Capacitor. In the firt fraction of a econd - A) The current decreae le rapidly in the circuit containing capacitor 1. B) The current decreae le rapidly in the circuit containing capacitor 2. C) The current i the ame in both circuit.

15 Parallel Capacitor Initial moment: brighter? Will it glow longer? Fringe field: E 1 Q / A 2 R Capacitor in parallel effectively increae A

16 An Iolated Light Bulb Will it glow at all? How do electron flow through the bulb? Why do we how charge near bulb a - on the left and + on the right?

17 Capacitor in a Circuit I Charging time E batt E heat time Energy conervation E cap time time

18 Capacitance Electric field in a capacitor: E Q / A -Q +Q V f E dl V E i E V Q / A Q A V In general: Q ~ V Definition of capacitance: Q C V Capacitance Capacitance of a parallelplate capacitor: C A

19 Capacitance Q C V Unit: C/V, Farad (F) Michael Faraday ( )

20 Exercie The capacitor in your et i equivalent to a large two-dik capacitor How large would it be? D =1 mm C A A A 1 F.1 m C C 2 /N m 2 A m 2 D ~ 1 km (6 mile)

21 Exercie A capacitor i formed by two rectangular plate 5 cm by 3 cm, and the gap between the plate i.25 mm. What i it capacitance? C A C A C 2 /N m 2.15 m m F 5.4 nf

22 A Capacitor With an Inulator Between the Plate No inulator: Q / A E With inulator: E Q / K A D V V E Q / Q A A V V E Q / V K K Q A A V C A A C K