What is Impedance: Definition and 505 Discussions

In electrical engineering, electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.
Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of the sinusoidal voltage between its terminals, to the complex representation of the current flowing through it. In general, it depends upon the frequency of the sinusoidal voltage.
Impedance extends the concept of resistance to alternating current (AC) circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude.
Impedance is a complex number, with the same units as resistance, for which the SI unit is the ohm (Ω).
Its symbol is usually Z, and it may be represented by writing its magnitude and phase in the polar form |Z|∠θ. However, cartesian complex number representation is often more powerful for circuit analysis purposes.
The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law.
In multiple port networks, the two-terminal definition of impedance is inadequate, but the complex voltages at the ports and the currents flowing through them are still linearly related by the impedance matrix.The reciprocal of impedance is admittance, whose SI unit is the siemens, formerly called mho.
Instruments used to measure the electrical impedance are called impedance analyzers.

View More On Wikipedia.org
Recent contents Top users
  1. I

    Understanding Impedance Matching with Audio Transformers

    I have a question about impedance matching with transformers. If I have a source impedance of 20 K ohms, to step up the voltage does the primary have to match 20K ohms with a secondary of 40K ohms for a step up of 1:2. Or can I change it so the primary is 50 Ohms with a secondary of 40K ohms...
  2. V

    Impedance - Current phasor

    Impedance -- Current phasor In a series circuit of two elements, the voltage and current, for w = 2000 rad/s, are V = 150|-45° and I = 4,74|-116,6°.That variation in the frequency of the source would result in a phasor current of 6 amps? Assuming an unlimited variation in the frequency, what is...
  3. F

    Simulation of a double layer capacitance and a warburg impedance with comsol

    Hallo, i'm trying to implement a comsol simulation of a impedance spectroscopy experiment. But unfortunatly I'm an absolut beginner in Comsol and i have many problems with this work. First of all i want to say that I'm using the AC/DC module, more precisely the transient analysis. At the...
  4. N

    Improve Surge Impedance Loading: Methods for Boosting Performance

    what are the methods of improvement of surge impedance loading
  5. L

    Find I_1, I_2, V_1, V_2: Reflection Impedance Explained

    For the circuit attached, I'm first asked to find I_1, I_2, V_1, V_2 by using the appropriate loop equations. I got these answers (which I checked and are correct): \overline{I_1} = 1.556A\angle61.4^\circ \overline{I_2} = 1.111A\angle-100.25^\circ \overline{V_1} = 3.92V\angle139.8^\circ...
  6. fluidistic

    Impedance of free space/wave impedance

    If I'm not wrong, empty space (vacuum) has the greatest impedance amongst any material. Because \eta = \frac{1}{n} \sqrt {\frac{\mu _0}{\epsilon}} if I'm not wrong and n=1 (smallest value possible) for free space. What does this mean that free space has a big impedance? It makes the greatest...
  7. 9

    How to Calculate Impedance in an AC Circuit with Series Components?

    Homework Statement A 25-Ω resistance is connected in series with a 30-mH inductance and a 12-µF capacitor and are connected to a 90-V (rms) ac. If the frequency of the ac source is 500 Hz, calculate (i) the current in the circuit. (ii) the voltage across each element (iii) the phase...
  8. J

    Will current take the path of least impedance in this circuit?

    Homework Statement See the circuit in the attached image. Homework Equations N/A Consider the circuit given in the attached image, then please answer the following questions: 1. When the current will come out of the current source, will all that current will go through the path...
  9. R

    Engineering Plotting Impedance of Parallel RLC Circuit

    Homework Statement Plot |z| vs. f(Hz) of the circuit. R=100, C=600 pf = 6E-12 F, L=10E-6 H . All elements of the circuit are in parallel. Also identify the resonant frequency. Homework Equations ZR=R ZL=jwL ZC=1/(jwC)=-j/(wC) w=2(pi)f w0=1/sqrt(LC) f0=1/(2(pi)sqrt(LC)) G=1/R...
  10. I

    Characteristic impedance and transfer function

    I have to find the absolute value of the transfer function for the circuit shown in the attachment. However, I have capacitors instead of inductors in my circuit and inductors instead of capacitors. So, the cross circuit will have L in the upper and lower branch and two C in the cross. I...
  11. L

    Capacitor impedance from ExH

    Homework Statement A cylindrical capacitor(2 round plates with dielectric between them) with radius a, height hc and volume V. A sinusoidal voltage V0 is applied to the capacitor plates. Find the E-field inside the cap, Mag field Hs on the cap side wall(i.e at r=a) Use poynting theorem...
  12. 0

    High impedance vs frequency

    I'd like to measure a Wheatstone bridge with a lock in. The bridge has something like 20kOhms resistance. I would like to measure with some fairly high frequency like 50kHz but my cable capacitance is causing trouble and low passing at 10kHz or so. Two questions: 1) What would you...
  13. I

    Non-Matching Impedance: Effects & Distortions

    Besides the obvious inefficiency (waste of energy) of a non-matching impedance between the transmission line and the antenna for a RF transmitter circuit, what are the additional effects? Is it safe to say the reflected portion of the signal returning to the source will also cause distortion...
  14. S

    High impedance circulating current scheme(calculate stabilising resistor)

    Hi, I am going through some texts but it was not very clear.I hope some experts here can help me on this. The max voltage across the relay and stabilising resistor is given by; (If/N)(Rs+Rb) where If - max short circuit rating, A N - current transformer, CT ratio Rs - CT internal...
  15. W

    Understanding Impedance of Free Space and Its Significance for Antenna Design

    Hi All, I understand that the impedance of free space, according to the formula E/H, is 377 ohms. I understand that if the impedance is high, a given electric field is associated with a smaller magnetic field but what does this really mean? What is being impeded? Also, what is the...
  16. M

    Engineering Finding impedance in an ideal transformer circuit

    Homework Statement This is a review question for my test, but I can't get the right answer, the problem is to find the impedance ab. Homework Equations Zin = 1/a^2 * ZL The Attempt at a Solution The 10 ohm resistor and the capacitor are in parallel and then add the 50 ohm resistor...
  17. fluidistic

    Impedance of an RLC series circuit when in resonance

    I don't find my notes right now, I'll try to use my memory on this. According to my notes, the impedance of an RLC series circuit is given by \frac{1}{\sqrt{R^2+\left ( \omega L-\frac{1}{\omega C}\right )^2 }}. So when in resonance, Z=\frac{1}{R} instead of Z=R. Also if I recall well, for...
  18. S

    Calculating impedance for parallel circuit

    Hi there, Below shows the solution to the circuit. However i am confused about how to solve the eq in red box. http://img208.imageshack.us/img208/2797/123vv.jpg do i need to solve it by calculator or more work needs to be done? If it's by calculator i am using sharp and i couldn't get the...
  19. S

    Expressing Impedance in Polar and Complex Number form HELP

    Expressing Impedance in Polar and Complex Number form HELP! Homework Statement A Single Pahse, 50Hz, A.C. Generator is to be used to supply two single phase A.C. induction motors in parallel. The no-load terminal voltage of the generator is 400V. And it has an output resistance of 0.2ohms...
  20. D

    Calculating impedance in complex number and polar form

    Homework Statement I have a motor with a full load output rating of 2.8 kW with an effciency off 70% and at 80% of full load runs with a Power Factor of 0.8 lagging. A second motor has afull load output of 4.2kW with an effciency of 85% and at 80% of full load runs with a PF of 0.75...
  21. B

    What is the equation for the impedance of an inductor in a Wheatstone bridge?

    Homework Statement Two inductive elements that are wired into a Wheatstone bridge. Upon displacement of the sensor, each inductor will change by delta L one increasing, the other reducing. What is the equation for the impedance of an inductor? Determine an expression for the ratio of the...
  22. A

    Calculating Impedance, verify.

    Homework Statement Calculate the circuit's impedance Z between input and ground for an applied sinusoidal voltage with frequency f = 1000Hz, and express Z in the form re^{i\theta} There is a picture that goes with it, it's just a function generator (makes a sinusoidal AC voltage) connected...
  23. F

    Impedance of voltage source, and tank ckt

    Can someone please tell me whether the following statements are wrong or right? this is not homework: 1. Voltage sources, like batteries, have a low impedance (ideally zero) 2. A good car battery's impedance (even with alternating current going through the battery) is typically far less than a...
  24. J

    Impedance (Inductor-Capacitor)

    Statement: If we have a circuit consisting of an inductor and capacitor in series, the impedance is defined by the following: Z = L + C = \sqrt{(X_{L} - X_{C})^{2}} = \sqrt{(X_{C} - X_{L})^{2}} My question: Howcome the impedance (or reactance) isn't defined by the following equation: Z = L + C...
  25. B

    How do you pronounce X and Z (reactance and impedance)?

    Are they Latin or Greek characters? Do I say "chi" and "zeta" or "eks" and "zee"? Thanks
  26. O

    Some subtleties regarding Impedance

    Hello. Homework Statement I am taking a first year electricity and magnetism course. In class, the following was taught: a. What is the impedance of an inductor? b. What is the total impedance of 2 components connected serially? But only at home I realized that I don't understand...
  27. K

    Impedance Spectra of Dye-Sensitizedd Solar cells

    Can anyone tell me how can I take the impedance spectra of a dye-sensitized solar cell ? Do I need to irradiate (i.e. AM 1.5, 100 mW/cm2) the solar cell during impedance measurement? or I can measure the spectra without irradiating light ? More specifically, how can I get the comparison between...
  28. A

    How can I get absolute value of impedance?

    Here's the thing. I have 4,7 nF capacitor and AC: 1 kHz < f < 10 MHz. I measure real and imaginary part of capacitance C*=C'+iC" (C'=C'(f) and C"=C"(f)). I did 2 measurements - in first capacitor is directly attached to measuring device, in second I use 1 m coaxial cables to attach it. I...
  29. V

    Understanding Frequency Effects on Impedance: Resistors, Capacitors & Inductors

    While working in a lab this summer, I've been reading up on impedance and AC circuits as much as I can, considering I have to present at the symposium here in August. I'm really confused about a few...parts of impedance, for lack of a better term. How does frequency affect impedance, in...
  30. B

    Calculating the effective impedance of a current source

    I'd like to calculate the gain of a common emitter transistor circuit I'm looking at that has a a constant current source as the emitter load. Now I know the gain is going to be -gm(Re||RL) but how do I calculate what the effective resistance of the current source is? The current source I'm...
  31. B

    Calculating impedance and transfer function of arbitrary length ladder filter?

    Hi everyone - I'm trying to figure out this "tack hammer" method of determining the impedance and transfer function of an arbitrary length RCL ladder filter. This method is briefly explained in the second chapter of Wes Hayward's book "Introduction To Radio Frequency Design," however it is gone...
  32. M

    How Do You Calculate Impedance in a Resonant RLC Circuit?

    Homework Statement Quick one guys, I am very confused about the impedance of a RLC circuit. Say when you have more than one resistor or more than one capacitor or inductor in different combination, you get a impedance with two parts(imaginary which is the reactive part and the resistive...
  33. B

    Adding capacitance to an impedance

    Homework Statement If I have an impedance of: 10+j4 ohms and was told to add a capacitance of -j15 in parallel to this, would this make the impedance 10-j11 ohms??
  34. N

    Connect a antenna with 75Ω line impedance to VNA

    Connect an antenna with 75Ω line impedance to VNA Hello, how can i connect an antenna with 75Ω line impedance if vna has 50Ω input impedance. thanks
  35. F

    Calculating Impedance for a Resonator: Tips and Strategies

    I am trying to find the impedance of a resonator, but I am not quite sure where to begin. The resonator contains three LCL circuits which are then connected to a large capacitor. Should I calculate the impedance of each element in the resonator or should I use Kirchhoff's Laws to analyze the...
  36. C

    Thevenin impedance with mutual inductance

    Homework Statement I have a part of circuit with 2 inductors (L1, L2) of given values having a given mutual inductance, and a (capacitor+inductor L3) parallel to L2. Please find attached the circuit.Homework Equations Find the impedance of this part of the circuit.The Attempt at a Solution (all...
  37. L

    Terminators / Impedance Matching

    I am not sure how the use of a terminator with an oscilloscope works. I have a signal going into the input of an oscilloscope and improved this signal by attaching a terminator to the input of the scope. The way I understand it is, that the scope has a large impedance, larger than the...
  38. C

    Impedance matching transformer question

    1. It is required to pass a current of between 6.0A and 6.5A through an impedance of (1.6 + 1.2i) ohms. With only a 230V supply available, it is proposed to use a transformer for impedance matching. A transformer is available with a turns ratio of 16:1. A laboratory test showed that with the...
  39. C

    Impedance mismatch at the universes edge

    Hey all As is my way I pondered what would happen to light (and other emr) when it reaches the edge of the universe. First one must assume the universe has an edge. Anyways I figured there is possibly two cases 1./ The universe is expanding at the speed of light so nothing ever...
  40. D

    How to Design a Matching Network for 10-j35 ohm Load Impedance at 1 GHz

    Homework Statement A matching network consists of a length of T-line in series with a capacitor. Determine the length(in wavelength) required of the T-line section and the capacitor value needed (at 1 GHz) to match a 10-j35 ohm load impedance to the 50 ohm line
  41. B

    Impedance Matching Transformer

    Hi All, I am building a circuit that i need to get a better understanding of before jumping in head first, it says it requires an Impedance Matching Transformer. I don't get this with the Attached Circuit, is it the whole Primary circuit has to match the whole Secondry Circuit Impedance...
  42. M

    Speaker Impedance: Effects on Sound Quality & Power

    what is the effect of speaker's impedance on the qualiy and the power of the sound? for example: tow speakers one has 10 ohms impedance and the other has 50K impedance, which one is better? which one gives a louder voice?
  43. A

    Understanding the Derivation of Complex Impedance in AC Circuits

    I have a quick question regarding how the complex impedance of an AC circuit is derived. The voltage and current in an AC circuit are given by the equations V=V_{m}cos(\omega t) = V_{m}Re(e^{i\omega t}) I=I_{m}cos(\omega t-\phi) = I_{m}Re(e^{i(\omega t-\phi)}) The impedance is given by...
  44. K

    What is the Impedance of a Particular Inductor with Appreciable Resistance?

    Homework Statement A particular inductor has appreciable resis- tance. When the inductor is connected to a 19.7 V battery, the current in the inductor is 4.38 A. When it is connected to AC source with an rms output of 19.7 V and a frequency of 58 Hz, the current drops to 1.65 A. Find...
  45. K

    How Do You Calculate Impedance in a Series RLC Circuit?

    Homework Statement A 1170 resistor, a 1.46 μF capacitor, and a 2.7 H inductor are connected in series across a 350 Hz AC source for which the maximum voltage is 230 V. Calculate the impedance of the circuit. Homework Equations Z=sqrt(R^2+(XL-XC)^20 XL=2*pi*f*L XC=1 /...
  46. E

    Engineering AC circuit, mystery impedance

    Homework Statement http://img159.imageshack.us/img159/1867/es100wi8.th.png http://g.imageshack.us/thpix.php a) Write an expression for V1 and V2 both in time domain and phasor domain (Solved) b) Write an expression for the current I both in time domain and phasor domain (Solved) c) Calculate...
  47. G

    Impedance matching/purely resistive

    is this true? http://en.wikipedia.org/wiki/Impedance_mismatch To prevent all reflections of the signal back into the source, the load (which must be totally resistive) must be matched exactly to the source impedance (which again must be totally resistive) what if you...
  48. Battlemage!

    Deriving a formula for Impedance for an AC circuit

    Homework Statement Find the impedance of the circuit (R and L in series, and then C in parallel with them). A circuit is said to be in resonance if Z is real; find ω in terms of R, L, and C at resonance. Here is the circuit: http://img382.imageshack.us/my.php?image=g...ccircuitgf5.png...
  49. H

    Understanding Impedance Matching for Optimal Energy Transfer

    Hi, I'm having a little trouble understanding the concept of impedance matching to maximise energy transfer. I understand that the reactance of a capacitor is X_c= \frac{1}{\omega C } and that it is always 90 degrees lagging of the current in the complex plane which leads to a capacitors...
  50. J

    Engineering Understanding Total Impedance in a Parallel Resistor-Inductor Circuit

    Im tryin to figure...In a circuit having a parallel resistor and inductor, as the applied voltage frequency increases, the total impedance...increases?
Back
Top