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# Capacitive Reactance - PowerPoint PPT Presentation

Capacitive Reactance. Flow in a pipe can be blocked by an elastic membrane. Provides restoring spring force A capacitor blocks the flow of direct current. Gap between plates The charge provides a restoring potential. AC current can flow. Rubber Sheet. r vA. i. C.

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### Capacitive Reactance

Provides restoring spring force

A capacitor blocks the flow of direct current.

Gap between plates

The charge provides a restoring potential.

AC current can flow

Rubber Sheet

rvA

i

C

Capacitor matches

One loop circuit

The capacitor charge varies with the voltage.

The current is related to the rate of change of the charge.

AC Driver

v

C

• Voltage and current are not in phase through a capacitor.

• Current leads voltage: ICE man

The ratio gives an expression like Ohm’s law.

This is the capacitive reactance.

Measured in ohms

Almost Ohm

v

Capacitive reactance depends inversely on frequency.

High reactance at low frequency

Low reactance at high frequency

Frequency Dependent

XC

f

Find the capacitive reactance of a 0.100 mF capacitor at an audio frequency of 100 Hz and at an FM-radio frequency of 100 MHz.

The formula for capacitive reactance depends on frequency.

Frequency f in Hz

Find the reactance in ohms.

1/(2p 102 Hz 10-7F)= 16 kW

1/(2p 108 Hz 10-7F)= 16 mW

Note that there is virtually no resistance at radio frequency.

Audio Cutoff

Extreme Behavior audio frequency of 100 Hz and at an FM-radio frequency of 100 MHz.

• At low frequency the capacitive reactance is high.

• Like high resistance

• Open circuit element – capacitor plates

• At high frequency the capacitive reactance is low.

• Like low resistance

• Closed circuit element – ideal conductor

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