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lecture 21 2
Impedance
• AC steady-state analysis using phasors allows us to express the relationship between current and voltage using a formula that looks likes Ohm’s law:
V = I Z
• Z is called impedance (units of ohms, )
lecture 21 3
Impedance
• Resistor: V = I R
– The impedance is ZR = R
• Inductor: V = I jL
– The impedance is ZL = jL
lecture 21 5
Some Thoughts on Impedance
• Impedance depends on the frequency, f
• Impedance is (often) a complex number.
• Impedance is not a phasor (why?).
• Impedance allows us to use the same solution techniques for AC steady state as we use for DC steady state.
lecture 21 7
Impedance Example
• How do we find VC?
• First compute impedances for resistor and capacitor:
ZR = 20k= 20k 0
ZC = 1/j (377·1F) = 2.65k -90
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Impedance Example
Now use the voltage divider to find VC:
0k2090-k65.2
90-k65.20 10VCV
4.82- 1.31VCV
lecture 21 21
Impedance Summary
Element Impedance
Capacitor ZC = 1 / jC = -1/C 90
Inductor ZL = jL = L 90
Resistor ZR = R = R 0