Physics chapter 27 1

Capacitance and Dielectrics

Chapter 27

Physics chapter 27 2

Capacitor Any two conductors separated by an

insulator Used in electronics Usually have equal but opposite charges,

so the net charge is zero If a capacitor has a charge Q, that means +Q on

one conductor and -Q on the other

Physics chapter 27 3

Capacitance Ratio of the magnitude of the charge on

either conductor to the magnitude of the potential difference

Units of farad (1 F = 1 C/V) Often use mF (10-6) or pF (10-12)

V

QC

Physics chapter 27 4

Circuit diagrams Capacitors represented as

Physics chapter 27 5

Parallel plate capacitors Two parallel conducting plates separated

by a distance that is small compared to their dimensions.

Nearly all the electric field is between them, and it is uniform

d

A

V

QC 0

Physics chapter 27 6

Example The plates of a parallel-plate capacitor are

4.00 mm apart and 1.00 m2 in area. A potential difference of 6 000 V is applied across the capacitor. Find The capacitance The charge on the capacitor

Physics chapter 27 7

Capacitors in series

Charge is the same Voltage is not Equivalent capacitance

...1111

321

CCCC

Physics chapter 27 8

Example Three capacitors, C1 = 10pF, C2 = 25 pF,

and C3 = 5 mF, are connected in series. Find the equivalent capacitance of the series.

Physics chapter 27 9

Capacitors in parallel

Voltage is the same Charge is not Equivalent capacitance

...321 CCCC

Physics chapter 27 10

Example Exercise 27-7

Physics chapter 27 11

Potential energy of charged capacitor

C

QCVQVU

22

1

2

1 22

Physics chapter 27 12

Dielectrics Solid, nonconducting material between

capacitor plates Helps maintain plate separation Increases capacitance

Physics chapter 27 13

Dielectric constant, K

0C

CK

C, V, and e are with the dielectric C0, V0, and e0 are without it K is always greater than 1

V

VK 0

0

K