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Capacitors. A device storing electrical energy. – – – – – – –. + + + + + + +. –q. +q. Capacitor. A potential across connected plates causes charge migration until equilibrium. Charge stored q = C D V C = capacitance Unit = C/V = henry = H. D V. A. C 2. N m 2.

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capacitors

Capacitors

A device storing electrical energy

capacitor
– –

– –

– –

+ +

+ +

+ +

+

–q

+q

Capacitor

A potential across connected plates causes charge migration until equilibrium

Charge stored

q = CDV

C = capacitance

Unit = C/V = henry = H

DV

parallel plate capacitance
A

C2

N m2

e0 = 8.8510–13

Parallel Plate Capacitance

Plate area A, separation d

d

Capacitance = Ae0/d

circuit element symbols
+ –

DV

  • Conductor
  • Capacitor

or

  • Resistor
Circuit Element Symbols
  • Potential Source
at equilibrium
DV

+ –

C

+ –

DV

At Equilibrium
  • Capacitor charges to potential DV
  • Capacitor charge Q = CDV
energy in a capacitor
DQ

slope = 1/C

DV

area = W

Q

Energy in a Capacitor
  • C = Q/DV so DV = Q/C
  • Work to push charge DQ W =DVDQ = (Q/C)DQ
energy in a capacitor1
DV

Q/C

Q

Energy in a Capacitor
  • Work to charge to Q is area of triangleW = 1/2 Q(Q/C) = 1/2 Q2/C
  • Work to charge to DVW = 1/2 DV (CDV) = 1/2C(DV)2

CDV

parallel components
Parallel Components
  • All have the same potential difference
  • Capacitances add
  • (conceptually add A’s)
series capacitors
Series Capacitors
  • All have the same charge separation
  • Reciprocals are additive
  • (conceptually add d’s)
gauss s law
e0 = 8.8510–13

C2

N m2

Gauss’s Law
  • Electric flux through a closed shell is proportional to the charge it encloses.

FE = Qin/e0

field around a point charge
R

q

q

1

q

kq

e04pr2

if k =

=

=

e0A

4pe0

4pe0 r2

r2

Field around a Point Charge

Shell Area = 4pr2

FE = q/e0 = EA

+q

E =

=

field around infinite plate
s

1

sA

FE =

, so E =

e0

2

e0

Field Around Infinite Plate

With uniform charge density s = Q/A

= E(2A)

infinite plate capacitor
–q

–q

+q

1/2 s/e0

0

0

+q

s/e0

1/2 s/e0

Infinite ||-Plate capacitor

Individually

Together

parallel plate capacitance1
Field E =

=

s

Q

e0

Ae0

Qd

  • Potential DV = Ed =

Ae0

Q Ae0

Ae0

  • Capacitance Q/DV =

=

Qd

d

Parallel Plate Capacitance
  • Plate area A, plate separation d
ad