1. Capacitive Reactance

2. 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 rvA i C

3. An AC source provides a sinusoidal voltage. 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

4. Phase Shift • Voltage and current are not in phase through a capacitor. • Current leads voltage: ICE man

5. The amplitude of voltage and current through a capacitor are related. The ratio gives an expression like Ohm’s law. This is the capacitive reactance. Measured in ohms Almost Ohm v

6. Resistance in a resistor is the same for all frequencies. Capacitive reactance depends inversely on frequency. High reactance at low frequency Low reactance at high frequency Frequency Dependent XC f

7. 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

8. Extreme Behavior • 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 next