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Capacitors

CIRCUIT SYMBOL

How capacitors work.

Two parallel metal PLATES separated by an INSULATING gap.

The insulating gap

is called a

DIELECTRIC.

They store electric charge

which can be used again

later.

1

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Capacitance

We define capacitance as the charge stored

per volt, measured in farad (F).

1 farad is the capacitance of a conductor, which

has potential difference of 1 Volt when it carries

a charge of one coulomb

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A capacitor

A parallel plate capacitor as a device that consists of two

oppositely charged conducting plates separated by a

small distance. It is used to stores electric charge.

+

+

+

-

-

-

• Charges will

…………… on

the plates

• positives

………………..

to negatives

• A ……………

electric field will

be established

between the

plates.

+

+

+

-

-

-

x

• If the cells are

removed and a

device (x)

connected –

the charge can

flow through

the device and

“……………….”

the capacitor.

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A capacitorCharging Discharging

Voltage (

V)

Curr

ent

(A)

Voltage (

V)

Curr

ent

(A)

Time (s)Time (s)

Time (s)Time (s)

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A capacitorCharging Discharging

Initially the capacitor has no charge and it gains a

charge rapidly. As the charge builds up the rate of

charging decreases because the charge on the

capacitor opposes the potential difference on the

battery and resists the build up of further charge.

Initially the capacitor has a high charge and so

can deliver a high current which causes the

voltage to drop which in turn decreases the

current.

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Dielectric• Inside the capacitor is a dielectric

(non conducting substance)

• The dielectric function is to reduce the

electric field between the plates so that more electrons can crowd onto the plates, thereby increasing the charge that can be held.

0. r

AC

d

A is the area of each plate (shared) (m2)D is the distance separating the plates (m)

o is the relative permittivity of free space. (always used)

r is the relative permittivity of the dielectric material only needed if a dielectric is used.

A = area

(m2 )Shared – 1

plate only

d

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Example

• 0.2μ farad capacitor has its plates separated by

0.1mm of perspex. What will the area of its

plates need to be?

7

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A capacitor in a DC circuit.

• Capacitor is used in a

circuit for smoothing

• As the circuit is

connected the capacitor

is charged

• If the potential in the

circuit is disrupted the

capacitor will discharge.

• The discharge from the

capacitor momentarily

restores the current flow.

Circuit

Circuit

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And

sometimesCAPACITANCE

shared

SURFACE AREA

distance (m)

FARADS (F)

Permiativity of free

space (O )

Relative Permiativity

(r) (given)

PARALLEL

PLATES

Coulombs (C)

Volts (V)

Measured in

Made of Measured in

VACUUM or AIR

Filled with

DIELECTRICMeasured in

CHARGE (Q)

CAPACITORSVOLTAGE (V)

ratio

CHARGE

VOLTAGE

d

AC r .0

O =8.85 x 10-12 C2 N-1 m-2.

9

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Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain a lot more more slides and other resources, are freely

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WarneScience