# impedence

Discussion in 'Physics' started by harikanaidu, Dec 1, 2014.

1. ### harikanaidu Thread Starter Member

Nov 28, 2014
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Can any one tell me what is the use of measuring impedance in a circuit or a component?What actually this parameter represents?

2. ### studiot AAC Fanatic!

Nov 9, 2007
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Why does a plumber measure the size of a pipe before he repairs it?

Why does a grocer measure the weight of a bag of sugar before he sells it?

Why do you measure the time of day when you are intending to catch a bus?

3. ### ericgibbs Senior Member

Jan 29, 2010
2,413
369
hi,
The impedance of a circuit is the 'resistance' of the circuit to a Alternating Current input, its a frequency dependent impedance.
The impedance can be due to the capacitive reactance of the circuit, inductive reactance or a combination of both.
XC = 1/[ 2* pi *F * C] ... XL = 2 *pi *F *L

The total impedance value will have a 'real' part and 'imaginary' part component.
E

4. ### harikanaidu Thread Starter Member

Nov 28, 2014
76
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Then what exactly it represents in the circuit?

5. ### harikanaidu Thread Starter Member

Nov 28, 2014
76
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Does this mean that circuits powered with ac only will have impedence?

Nov 9, 2007
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7. ### BR-549 Well-Known Member

Sep 22, 2013
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The reason you need to know the impedance is because the impedance of the circuit is what you are working against. It's the force you need to overcome. Weather you need to know the impedance depends on the purpose or function of your circuit.
The parameter that it represents is inertia.

8. ### #12 Expert

Nov 30, 2010
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All circuits that actually work (except maybe superconductors) have impedance. Sometimes it is only the real part and sometimes it includes the imaginary parts. Otherwise, they just short everything out or blow a fuse. Even a circuit driven with DC will have an impedance with real and imaginary parts at the moment of start-up. That's because parasitic inductance and capacitance will see the application of DC voltage as 1/2 of a fast pulse. Only after the time of first transient response passes will the circuit behavior resolve to only the real part of the impedance (resistance). Impedance is just a bigger word for resistance that reminds you to, "Pay attention to the imaginary part of the response and what frequency range you're working in".

Powered only with AC? Impedance has three parts: inductance, capacitance, and resistance. You can power a purely resistive circuit with AC. If there are no imaginary parts, the current will merely respond to the resistive component.

Jul 18, 2013
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One other practical example is a coil in an AC relay, at switch on the current is limited only by the resistance of the coil wire, which is usually relatively low, once this high current inrush has occurred, the coil will posses a certain inductive reactance or impedance in Ohms, based on the inductance of the coil (H) and frequency of the AC , this will appear to be in series with the coil resistance, therefore limiting the current to a practical level.
If you apply an AC voltage to a DC relay which posses a higher coil wire resistance, the inductive reactance that now appears in series with this coil, makes it very inefficient as now the total resistance of wire and inductive reactance (ohms) creates a much lower magnetic effect.
Also read up on tuned circuits.
Max.

Oct 2, 2009
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