February 10

1CAPACITORS

A. GreatB. GoodC. Not So GoodD. BadE. Don’t turn your back!

Today◦ Examination I◦ Complete Capacitors

Friday DC Circuits

Play with batteries, wires and light bulbsWatch for WebAssign...Next Week – More of the same

Two pieces of metalclose to one another but they don’t touch.

RealWorld

Charge has no placeto go!

C

QC

V

Single Piece of Metal. Not connected to

anything

electrons

-------

electrons

+++++++

-------

+++++++

electrons

-------

+++++++

-------

+++++++

q -q q -q

Charge is the same for every capacitor in a series “string”

-------

+++++++

-------

+++++++

q -q q -q

Wires are equipotentials … sort of.

C1 C2

-------

+++++++

-------

+++++++

q -q q -qC1 C2V1 V2

VT

1 2 TV V V

1 2

11

22

Important:

V

T

TT

V V V

qV

C

qV

C

q

C

1 2

1 2

1 1 1T

T

q q q

C C C

C C C

VT

1 2 31 2 3

TT

T T

Q Q QQC C C C

V V

One Single Conductor

Model

C1

C2

Calculate the combined capacitanceC1= C2= C1||c2=

Measure=__

(C1+C2)series=

Determine the equivalent capacitance between A and B for the group of capacitors in the drawing. Let C1 = 14 µF and C2 = 5.0 µF.

A capacitor which acquires a charge of 1 coulomb on each plate with the application of one volt is defined to have a capacitance of 1 FARAD

One Farad is one Coulomb/Volt

CVq

orV

qC

d

AC

sod

AVEAAq

V

qC

0

00

The capacitance of a

parallel plate capacitor depends only on the Area and separation between the plates.

C is dependent only on the geometry of the device!

mpFmF

andm

Farad

Voltm

CoulombVoltCoulombm

Coulomb

Joulem

Coulomb

Nm

Coulomb

/85.8/1085.8 120

2

2

2

2

0

DIELECTRIC

Some LOCAL ordering Larger Scale Ordering

- +Net effect REDUCES the field

qqq

and

A

qE

E

A

qqE

A

qE

'

'

0

0

0

00

C0 = Vacuum or air Value

C = With dielectric in place

C=C0