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Why does the impedance of inductor change with frequency? 
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#1
Oct1411, 05:03 AM

P: 2

The power stored in an inductor is E=0.5LI^{2} which is independent of frequency
However, the impedance is ωL. So, what makes the impedance increase with frequency? 


#2
Oct1411, 08:12 AM

P: 38

It helps to look at an inductor's response to a step input in the time domain to get a physical "feel" for it behaves. An inductor initially looks like a high resistance (open circuit) that drops to a low resistance (short circuit) over time. So, for a low frequency signal, which takes a lot of time, it behaves more like a short circuit. For a high frequency input, it looks more like a high resistance component.
A capacitor is the exact opposite, a low resistance (short circuit) that becomes a high resistance (open circuit) over time. There's more to it of course but once you get a physical feel for these components, you can use the equations freely without wondering why they're valid. 


#3
Oct1411, 11:23 AM

P: 2

I understand the behavior of inductor but i just puzzle what is opposing to the change of current in high frequency.
Originally, I guess it is because there are more energy stored in form of magnetic field in high frequency range. But the formual just says the opposite. The energy remains the same regardless of the frequency. So, what matter plays the tricks? 


#4
Oct1411, 11:34 AM

Sci Advisor
PF Gold
P: 1,776

Why does the impedance of inductor change with frequency?
An inductor works off of the principal of Lenz's Law. A changing current induces a changing magnetic field that works to oppose the change in the current. The stronger the change in current, the stronger the changing magnetic field (and viceversa). Obviously if we were to increase the frequency of the current through an inductor we would see an associated increase in the strength of the induced magnetic fields that work to oppose the change in the currents. This translates to an increase in the apparent impedance of the inductor with frequency.



#5
Oct1411, 02:19 PM

P: 38

You can analyze this mathematically and experimentally and it gives the correct results, along with a much more reasonable physical picture. Personally, I think it removes a bunch of the "magic" in electronics (the change in xxxx mysteriously opposes a change in yyyy, etc.), allowing one to use the higher level equations with confidence in their validity. 


#6
Oct1611, 09:01 PM

Sci Advisor
PF Gold
P: 1,776

There isn't any magic in Lenz's Law here. Lenz's Law simply states that as a consequence of Faraday's law of induction that, if I may borrow the expression from Wikipedia, given a magnetic flux through N coils of wire [tex] \mathcal{E} =  N \frac{\Delta \Phi}{\Delta t} = N \frac{ \partial \Phi}{\partial t}[/tex] A very basic inductor, which is actually how most inductors are made, is simply a coil of wire (usually with a ferrite core to magnify the effect). We know that a solenoid gives rise to a magnetic field through the center of the coils and thus we see that an inductor creates its own magnetic field. For an ideal solenoid, the magnetic field produced is [tex] B = \frac{\mu NI}{\ell}[/tex] where [itex]\ell[/itex] is the length of the solenoid. And we note that the EMF is the negative voltage and that the flux [itex]\Phi[/itex] is the area of the coil times the magnetic field B (assuming constant field through the coil's area) we thus see [tex] V = N \frac{\mu N A}{\ell} \frac{\partial I}{\partial t} = L \frac{\partial I}{\partial t} [/tex] So we can easily recover the circuit equations for an inductor by looking at the selfinductance of a solenoid. So as the change in current increases in time (ie: increasing frequency) we expect that the back voltage due to the inductance increases in strength. That's why we find that the transformed complex impedance scales with frequency. 


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