Lecture 7

Capacitors

Micro-economists are wrong about specific things, and macroeconomists are wrong about things in general.

Yoram Bauman

The potential at 1 is

Gre

ater

than

a...

Les

s than

at 2

The s

ame a

s 2

33% 33%33%

V=0 V=+400 V

1

2

1mm

3mm

1. Greater than at 22. Less than at 23. The same as 2

The potential at point 2 is

0

100 V

200 V

300V

400V

Non

e of t

he ab...

17% 17% 17%17%17%17%1. 02. 100 V3. 200 V4. 300V5. 400V6. None of the above

The electric field strength at 1

Is gr

eater

tha..

.

The s

ame a

s at..

.

Is le

ss th

an a.

..

Can

not te

ll fr.

..

25% 25%25%25%1. Is greater than at 22. The same as at 23. Is less than at 24. Cannot tell from

diagram

The electric field strength at 2 is

0 v/m

400 v

/m

300 v

/m

100 v

/m

Nee

d mor

e info

...

20% 20% 20%20%20%1. 0 v/m2. 400 v/m3. 300 v/m4. 100 v/m5. Need more

information

QUIZ #3

• A proton has been accelerated through a potential difference of -1000V. What is its speed?

Electric Field from Potential

• Finding E from ΔV:

• Direction perpendicular to equipotential surfaces and from higher to lower potential

VE

d

Capacitors

• Can store electric charge• Two conductors, one with +Q, the other with

-Q and voltage difference ΔV• Capacitance defined by

• Units C/V = 1 Farad = 1F

QC

V

Plane Parallel Plate Capacitor

• Recall E between plates is

•

• Special case of plane parallel plate capacitor• If d = 1mm, what is A?• If square plates, what is L? A=L2

0

QE

A

0ACd

Put dielectric spacer between plates

• Dielectric = insulator

• is the dielectric constant. See table 21.2 page 709

• How would you make C large while keeping physical size of capacitor small?

0ACd

Energy stored in capacitor

• U is the energy stored, book uses UC

• Energy stored in demonstration capacitor?

221 1

2 2C

QU CV

C