Questions tagged [reactance]

The opposition of a circuit element to a change of electric current or voltage, as happens in capacitors and inductors.

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175 views

About power dissipation of capacitors and inductors & I-V phase difference

For instance, The Art of Electronics (by Horowitz & Hill) states that ideal capacitors cannot dissipate power, even though current can flow through them, because the voltage and current are 90° ...
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Voltage drop across a Power Transmission Line

For a short power transmission line (<80 km), the shunt capacitances/admittances are ignored, i.e. the transmission line model can be modelled using only the line resistance and line reactance. The ...
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How to relate resistance/reactance to parallel/series equivalent circuits?

Problem 7A of The Electronics of Radio asks the reader to find the relationships between resistance and reactance values in any arbitrary series/parallel circuit and their counterparts in a parallel/...
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63 views

What is the capacitive reactance in this case?

Capacitive reactance in ohms is defined as: $$X_C(\omega) = \frac{1}{2\pi f C}$$ where the unit of C is farad. Focusing on the frequency \$f\$ in the equation: If the input signal to the capacitor is ...
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How can you do "impedance matching" on USB or other serial communication lines?

I was told that that impedance matching is important when going to high speeds on USB. I assume that the "impedance" matching in practice means "reactance" matching. But, how can ...
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Why reactance is imaginary? [duplicate]

In AC circuit, cartesian impedance formula is Z = R + jX (Z = impedance, R = resistance, X = reactance). Why is reactance imaginary (for capacitors and inductors)? I cannot find a good explanation ...
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189 views

Proof of capacitive reactance equation

I have a doubt about the ratio of potential difference and current of capacitor $$X_c = \frac{v}{i} = \frac{1}{\omega C}$$ If I have a sinusoidal potential difference $$v = V_Msin(\omega t)$$ and the ...
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Why is an infinite reactance an open circuit?

I'm writing up a summary about classic impedances (resistors \$Z_R=R\$, capacitors \$Z_C=1/j\omega C\$ and inductors \$Z_L = j\omega L\$) for students starting out with AC networks. I was about to ...
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Coupling coefficient in function of reactance

If there are two coupled inductances L1 and L2, we may want to write k in function of the reactances, that is, in function of Z_L1 and Z_L2. How would you do so?
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Are impedance calculations only specific to sinusoidal inputs?

All impedance formulae we've derived in my course and calculations e.g. combined impedances in series have all assumed sinusoidal steady state. My question is, does the notion of "impedance" ...
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67 views

Building intuition on the behavior of inductors and capacitors with both voltage and current

I understand that an inductor cannot change current instantaneously, however I am not sure how that relates to the voltage. In a high pass RL circuit, for example, I know that the inductor presents a ...
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Current limiting rectifier with capacitive reactance (isolated)

[UPDATE] I have answered my own question with links to experimental data and theory. Please expand on that answer if you can for the reference of future readers. Thanks for replies so far and ongoing....
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Reactance to varying DC

Input signal (a) is AC. Input signal (b) is varying DC. As (b) has a frequency but does not change direction, does the capacitor's reactance have a different effect than input signal (a)?
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Keeping a cheap, DIY transformer safe

I want to design a cheap but safe, low-wattage, always-on heater to keep a small space above freezing. (A small underground tunnel that the water pipe for an automatic horse waterer goes through.) I'...
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Effect of a capacitive or inductive divider on noise

I am used to thinking about the effect of an attenuator on a system's noise in the following way: there is some system noise (and signal) at the attenuator's input, coming from the preceding part of ...
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What unit is expressing the effect of this capacitor in a common emitter amplifier?

I'm working through this handout I3.PDF in my attempt to understand negative feedback in amplifiers. The following formula appears, in which the author is substituting \$v_{out}\$ and \$v_{in}\$ to ...
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Why does impedance decrease in an MLCC as frequency increases?

I understand that the impedance of any capacitor decreases as frequency reaches the resonance frequency, following the formula Xc = 1 / ( 2 pi f C ), but why does the impedance of an MLCC such a ...
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949 views

Why does wire not have large capacitive reactance?

For capacitive reactance, I am presented this formula: $$X_C = -\frac{1}{2\pi fC} $$ Where clearly, the capacitive reactance is inversely proportional to the capacitance. This confuses me. We usually ...
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How do you calculate phase shift induced by a shunt capacitor?

Given a simple loaded-line shunt-capacitor phase shifter, how can I calculate the phase shift and insertion loss at S21 caused by the shunt capacitor (varicap in my case, but for any cap) assuming I ...
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What is Negative Inductance?

I am experimenting with a stripline square spiral inductor in Sonnet and the graph is showing both a positive and negative inductance. What does it mean to have a negative inductance? Why does it ...
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853 views

Capacitive reactance related to frequency

Why does capacitive reactance decrease with the increase of the frequency of the applied signal? It is easy to prove why capacitive reactance decreases with increased capacitance. The more we increase ...
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In a RCL circuit; where V_C > V_L is the phasor angle leading or lagging?

Is this example correct in stating that the phasor angle is leading by 38.323deg?
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Why can't i calculate the Inductive reactance in this way?

Question: \$V_1=40sin(1000t)\$,what is the value of Inductive reactance? Solution : \$Z=\frac{V_1}{I}=\frac{40}{8}=5\$,so \$5=\sqrt{R^2+X^2_L}=\sqrt{4^2+X^2_L}\$,so \$X_L=3\$ But i want to ask ...
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What is the reactance when the frequency is 80 kHz?

I recently had a test with the following question: What is the reactance of a 6.8µF capacitor at 80 kHz I found the reactance to be: \$X_c=\frac{1}{j \cdot 2\pi \cdot f\cdot C}=\frac{1}{j \cdot 2\...
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Thevenin resistance in a fault current

In the reactance diagram, why is j0.20 (as circled in the picture) in series with j0.20 + j0.10 + j0.17. Why isn't j0.20 (circled) in parallel with j0.20 + j0.10 + j0.17 and j0.20 + j0.1333 + j0.24.
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Series Resistance-Capacitance Circuit

I have an optocoupler and I connect it to 230V, the datasheet tell me the optocoupler has a forward voltage: IF = ± 5 mA VF - 1.5 V, so, for limiting the current I need an resistor of 4.7K - 11wats, ...
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Current when switching on with inductance. Role of Reactance?

When switching on the current through a coil it can be described as: \$ I(t) = \frac{U}{R} \cdot (1 - e^{(-x\cdot R/L)}) \$ For a sinusiodal current the complex resistance for a circuit with ...
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287 views

Heating vacuum tube filament via capacitor

As I understand vacuum tubes (like triode / pentode) usually are designed to be heated with certain current through the filament. And usually it is about low-voltage and comparatively high-current. E....
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Electrical RL circuit problem

So I am given u(t)=325sin(314t+90°)V i(t)=14.14sin(314t+30°)A I have to find the impedance Z∠θ, value for the resistor and inductor, inductive reactance Urms∠θ = 325/sqrt2 ∠90° Irms∠θ = 14.14/...
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How to calculate reactance from an impedance analyzer circuit?

Background I have an impedance analyzer that uses the below circuit: Source One can measure impedance of the device under test (DUT) by using the following formulas: ...
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237 views

Foster's Reactance Theorem - Reactance Increasing with Frequency?

I'm currently reading a text on analog filters (Zverev) that discusses Foster's Reactance Theorem in relation to passive circuits and I was thrown a little by the text that says: "Physically, this ...
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Total phase inductance of three phase reactor

Consider a three phase reactor (inductor) using E-I or E-E laminations with phase winding on each limbs. Now inductance of R phase is given by: Lr=Lrr+Mry+Mrb Lrr: Self inductance of R-phase Mry: ...
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232 views

The resistance and magnetising reactance of the core

I did an experiment where I varied the test voltage and determine the values of the resistance and magnetising reactance of the core. What I noticed is that their values actually vary (not constant). ...
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240 views

Under open-circuit conditions, does a transformer behave more like a resistor, capacitor, or inductor?

I have the below schematic showing the equivalent circuit of a transformer under open-circuit conditions. Will the transformer behave like a resistor, inductor, or capacitor? And will this behaviour ...
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Crockcroft-Walton Voltage Multiplier Large Voltage Loss

I have a CW voltage multiplier that uses 30kV, 1nF capacitors and doubled 20kV, 5mA, 100ns diodes (for a total of 40kV). My ac source is a sine wave (viewed on the primary side of the transformer ...
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Are my calculations wrong for the capacitance?

In an earlier post i discussed about the effects of voltage derating on a capacitor. From here i measured the output with DC and AC applied to a highpass filter and found the cutoff frequency with the ...
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How to calculate the reactance of a capacitor with an input signal that is 2 sin waves of difference frequencies added together?

I am trying to calculate the reactance of a capacitor with an input signal that is not a single frequency. For example, x = sin(4t)+sin(5t), or a capacitor that is used in an audio circuit like the ...
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263 views

what does low reactance mean physically

What does "lower" reactance (higher susceptance) mean for a physical system, and when is it "good"? I see that it has less opposition to a change in voltage or current, but what does that mean? Lower ...
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Peak Current vs RMS Current when calculating Reactance

I am having a hard time getting a grip on AC circuit analysis. I have been taking my peak voltage/current and converting it to RMS current and RMS voltage so I can use ohms law and things like that to ...
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139 views

Voltage in RC circuit with unknown frequency

We were given the task of finding the voltage drop across R1 of the circuit below with the following alternatives: 5.8V 4.8V 4.0V 10.0V All given information is contained within the image, i.e. no ...
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Capacitor Reactance/Impedance Confusion

I always thought that capacitors (when used in phasor analysis) just had an impedance of $$1/jwc$$. I understand that impedance $$Z=R+jX$$where R is resistance and X is reactance. Now, in one book I ...
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Nonzero current at voltage zero-crossing when consuming reactive power

In a circuit that consumes some reactive power, the current waveform indicates current flow even when the supply voltage is at the zero crossing. This seems paradoxical since there cannot be current ...
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992 views

Does reactance increase noise?

Impedance is made of resistance and reactance which is capacitance and inductance. I read that increasing resistance increases noise, but what if capacitance or inductance is increased, does the noise ...
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Trying to analyze the audio attenuation.. Is this analysis correct?

I am learning about impedance. So sorry any stupid question here... Consider this circuit: I have an audio input, from 20 to 20 kHz. I want to know how the AC voltage over AB will behave as the ...
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Why does voltage always lead current by 90 degrees in an inductor?

I have learnt that in an inductor voltage leads current by 90 degrees. However, I do not fully understand why it is 90 degrees. I have been looking everywhere for more information on why this is so. ...
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389 views

For an ideal capacitor, when is the current leading and when is the voltage leading by 90 degrees?

Say I have a purely resistive load as a reference in parallel with an ideal capacitive load. I know that the current through the capacitor is said to be leading by 90 degrees. Specifically, though, ...
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Rectangular Waveguide H-Plane Step Discontinuity - Equivalent Reactance for Change in Height

I have been reading Pozar's Microwave Engineering, and in Section 4.6, he goes through the analysis to determine the reactance of a discontinuity between two coupled waveguides. In that chapter, he ...
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What's the relationship between susceptance and reactance?

I was trying to find the relationship between these two online, but I found out two completely opposite answers, and I'm wondering which one is the right one. Is B = -1/X or is B = 1/X ?
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785 views

Why inductive reactance has two formulas

We know inductive reactance XL will have two formulas. One is XL = 2piFL and other is XL = V/I (Ohm's law). Why we have 2 formulas for inductive reactance. Is the solution ( inductive reactance value)...
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What is the difference between Instantaneous current (i) and current (I) in a purely inductive circuit?

I was reading about AC inductor circuits on All About Circuits: Link: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/ac-inductor-circuits/ Consider this circuit of a purely ...