Chapter 25 Capacitance 25-1 WHAT IS PHYSICS?
Physics Bprovide the basic science for practical devices designed by engineers.
Capacitor Ba device in which electrical energy B stored.
The batteries B in a camera store energy in the photoflash unit by charging a capacitor.
The batteries B supply energy at only a modest rate B too slowly for the photoflash unit to emit a flash of light.
25-2 Capacitance
Chapter 25 Capacitance
Chapter 25 Capacitance
FIG.25-3 B (b) As the field lines show B the electric field B the charged plates is uniform in the central region between the plates. The field Bis not uniform at the edges of the plates, as indicated by the
"fringing" of the field lines there.
FIG.25-3 B (a) A parallel-plate capacitor B made up of two plates of area A separated by a distance d. The charges on the facing plate surfaces have the same magnitude q but opposite signs.
Chapter 25 Capacitance
Plates are conductors, B equipotential surfaces Ball points on a plate B at the same electric potential.
The charge B q Û V B proportional to each other Ð
The proportionality constant B C B the capacitance of the capacitor.
The value of capacitance B depends only on the geometry of the plates and not on their B q Û V
The capacitance B measures B how much charge the plates to produce B potential difference between them:
The greater the capacitance, B the more charge is required.
The SI unit of capacitance B from Eq. 25-1 B coulomb per volt B farad (F) Ð
1 farad = 1 F = 1 coulomb per volt = 1 C/V. (25-2)
Farad B very large unit B microfarad (1 µF = 10-6 F) B Pico farad B (1 pF = 10-12 F) B convenient units in
Chapter 25 Capacitance Charging a Capacitor
Charging a Capacitor B place it in an electric circuit with a battery.
Electric circuit B a path through B charge can flow.
Battery B a device maintains B potential difference between B capacitor terminals
h Û l B higher Û lower B potentials B + Û - FIG.25-4 (a) Battery B B switch B S Û plates h Û l of capacitor C, connected in a circuit.
Chapter 25 Capacitance
The circuit in Figs. B 25-4a Û b B incomplete B S B open;
S closed, B the circuit B complete B charge B flow through B S to the wires.
The circuit of Fig. 25-4 B completed B electrons B driven through B the wires by an E B the battery sets up in the wires
E B drives electrons B from capacitor plate h to the FIG.25-4 (b) A schematic diagram with the
circuit elements represented by their symbols.
Chapter 25 Capacitance
Plate h, B losing electrons B positively charged.
The field drives B many electrons from the negative terminal of the battery to capacitor plate l B plate l, gaining electrons B negatively charged
When the plates B uncharged BV B between them B zero.
The plates oppositely charged, BV increases B equals BV between the terminals of the battery. B C B fully charged BV Û q Brelated by Eq. 25-l.
