lecture1_rlc - passive components

8/19/2019 Lecture1_RLC - Passive Components

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The Capacitor• The basic idea is pretty simple

– Imagine you have two parallel metalplates, both of which have equal andopposite excess charges

–

Plates are separated by an insulatinglayer (air, glass, wood, etc)

• The charges would love to balance out

• Insulator blocs them (!ust as the groundblocs you from falling into the center of the

earth)


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The Capacitor• If you were to connect a resistive

wire to the plates – Charges would "ow through the wire

• Charge "ow is current

• #nergy has been released as heat

•


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The Capacitor• %emember that a voltage is the

electrical potential between twopoints in space

•

&ere, we have an imbalance of charge,and thus an electric 'eld, and thus avoltage – ield strength is dependent on number and

distribution of charges as well as materialproperties

– ield length is dependent on sie of capacitor

– Capacitor sie and material properties lumped

into single *capacitance+ C

V=Q/C


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The Capacitor

• Thus, if you connect a voltage sourceto the plates – ie charges will move to get away from

the source• Charge "ow is current

• Current will stop once charges reachequilibrium with voltage source, i-e-

• #nergy has been stored

•

. / . / . /


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The Capacitor

. / . / . /

ots of current

0ero 1C

0ero current

1C213

ots of current

&igh 1C

0ero of current

0ero 1C


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Symbol:

Units: Farads (Coulombs/Volt)

Current-Voltage relationship:

or

Note: v c must be a ontinuous !untion o! time sine the

harge stored on eah plate annot hange suddenly

Capacitor

+

v c

–

i c

dt

dvC

dt

dQi cc ==

C C

(typial range o! "alues: # pF to # F$ !or %superapa-

itors& up to a !e' F)

+

Electrolytic (polarized) capacitor

C

These have high capacitance andcannotsupport voltage drops of the wrongpolarity


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4ode 1oltage withCapacitors

+

v c

–

i c

dt

dvC

dt

dQi c

c

==


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lecture1_rlc - passive components

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