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G5A

ELECTRICAL PRINCIPLES

- Reactance; inductance; capacitance; impedance; impedance transformation; resonance

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G5A011 of 12

What happens when inductive and capacitive reactance are equal in a series LC circuit?

Why In a series LC circuit the inductive and capacitive reactances are opposite in sign, so when XL = XC they cancel exactly. What remains is only the small resistance of the components, so the total impedance drops to a minimum and current peaks. That condition is series resonance, at f = 1/(2*pi*sqrt(LC)).
Watch out Very high impedance at resonance describes a parallel LC circuit (a tank), where the circulating current between L and C makes the circuit look like a large impedance to the source. The mean-of-L-and-C choices are nonsense; impedance is not an average of henries and farads.
Series resonance = Small impedance, big current. Parallel resonance = the opposite.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A022 of 12

What is reactance?

Why Reactance is the AC opposition produced by energy storage in capacitors (electric field) and inductors (magnetic field), symbol X, measured in ohms. Unlike resistance it depends on frequency: inductive reactance XL = 2*pi*f*L rises with frequency, while capacitive reactance XC = 1/(2*pi*f*C) falls with frequency. It opposes current without dissipating power as heat, because the stored energy is returned to the circuit each cycle.
Watch out The choice about direct current and resistance describes plain resistance instead; reactance only shows up with changing (alternating) current, and the two 'reinforcement' choices are backwards since reactance opposes rather than helps current flow.
Reactance = AC resistance from L and C. Measured in ohms, but stores energy rather than burning it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A033 of 12

Which of the following is opposition to the flow of alternating current in an inductor?

Why In an AC circuit an inductor opposes changes in current by generating a back EMF, and that frequency-dependent opposition is called reactance, specifically inductive reactance XL = 2*pi*f*L, measured in ohms. Unlike resistance it does not dissipate power; energy is stored in the magnetic field and returned to the circuit each cycle. Capacitors show the same kind of opposition, XC = 1/(2*pi*f*C).
Watch out Reluctance is the tempting one because it sounds magnetic, but it describes a magnetic circuit's opposition to magnetic flux, not current. Conductance and admittance are the reciprocals of resistance and impedance, so they measure how easily current flows, not opposition.
Reactance opposes AC in inductors and capacitors; reluctance opposes magnetic flux in a core. Words ending in -ance that start with 'ad-' or 'con-' mean easy flow.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A044 of 12

Which of the following is opposition to the flow of alternating current in a capacitor?

Why In an AC circuit a capacitor opposes current changes through capacitive reactance, Xc = 1/(2πfC), measured in ohms. Unlike resistance, reactance stores and returns energy instead of dissipating it as heat, and it changes with frequency: higher frequency or larger capacitance means less opposition. Inductors have reactance too (XL = 2πfL), so reactance is the general term for AC opposition from capacitance or inductance.
Watch out Conductance and admittance are the reciprocals of resistance and impedance, that is, measures of how easily current flows rather than opposition, and reluctance is opposition to magnetic flux in a magnetic circuit, not to current.
Reactance = the AC-only ohms of capacitors and inductors. Xc = 1/(2πfC).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A055 of 12

How does an inductor react to AC?

Why Inductive reactance is XL = 2πfL, so reactance is directly proportional to frequency: double the frequency and you double the opposition. Physically, a faster-changing current produces a larger back-EMF in the coil, which opposes the current more strongly. Reactance depends only on frequency and inductance, not on how large the voltage or current happens to be.
Watch out The choice saying reactance falls as frequency rises describes a capacitor, where XC = 1/(2πfC); the amplitude-based choices are wrong because reactance is independent of signal level in a linear component.
XL = 2πfL: inductors go UP with frequency, capacitors go DOWN. Amplitude never enters the formula.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A066 of 12

How does a capacitor react to AC?

Why Capacitive reactance is Xc = 1/(2πfC), so frequency sits in the denominator: double the frequency and the reactance is halved. Physically, a capacitor has to charge and discharge, and at higher frequencies the polarity reverses so quickly that more current flows for the same voltage, which is the same as saying the opposition is lower. Note that reactance depends only on frequency and capacitance, not on how large the applied voltage is.
Watch out The choice saying reactance rises with frequency describes an inductor, where XL = 2πfL. The amplitude choices are wrong because reactance is independent of signal level in an ideal component.
C and f are both on the bottom of 1/(2πfC): higher frequency, lower Xc. Capacitors pass highs, block DC.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A077 of 12

What is the term for the inverse of impedance?

Why Impedance Z is the complex opposition to AC, combining resistance and reactance. Its reciprocal, Y = 1/Z, is admittance, measured in siemens, and it tells you how readily current flows for a given applied voltage. Admittance itself splits into a real part (conductance) and an imaginary part (susceptance), just as impedance splits into resistance and reactance.
Watch out Conductance is only the reciprocal of resistance, the real part of the story, and susceptance is the reactive part of admittance; reluctance is a magnetic-circuit term, the opposition to magnetic flux, not an AC electrical quantity.
Z and Y are the whole-package pair: impedance flips to admittance. R flips to G, X flips to B.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A088 of 12

What is impedance?

Why Impedance is the AC generalization of resistance, and like resistance it is defined by Ohm's law as voltage divided by current: Z = V/I. It combines resistance and reactance into one complex quantity measured in ohms, and it also carries a phase angle describing how much the current leads or lags the voltage. Doubling the applied voltage across a fixed impedance doubles the current, keeping the ratio constant.
Watch out The current-to-voltage ratio is the reciprocal, which is admittance (measured in siemens), not impedance. Multiplying voltage by current gives power in watts, not ohms.
Z = E/I, same shape as R = E/I. Ohms come from volts over amps, never amps over volts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A099 of 12

What unit is used to measure reactance?

Why Reactance is the opposition that capacitors and inductors present to alternating current, and like resistance it is the ratio of voltage to current, so it is measured in ohms. Formulas bear this out: inductive reactance XL = 2(pi)fL and capacitive reactance XC = 1/(2(pi)fC) both work out to ohms. The difference from resistance is that reactance stores and returns energy rather than dissipating it as heat, but the unit is the same.
Watch out The farad is the unit of capacitance itself, not of the opposition that capacitor shows at a given frequency; the siemens is the unit of conductance or admittance, the reciprocal of ohms.
Resistance, reactance, impedance: all three are volts per amp, so all three are in ohms.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A1010 of 12

Which of the following devices can be used for impedance matching at radio frequencies?

Why Impedance matching just means transforming one impedance into another so power transfers efficiently, and several components do that at RF. A transformer changes impedance by the square of its turns ratio (a 2:1 turns ratio gives a 4:1 impedance ratio). A Pi-network of two capacitors and an inductor, the classic tube amplifier output network, transforms the plate impedance down to 50 ohms and filters harmonics at the same time. A length of transmission line also transforms impedance, the best known case being the quarter-wave matching section where Z0 equals the square root of the two impedances being matched.
Watch out Picking only the transformer is the usual trap because it is the familiar low frequency example, but at RF the LC network and the quarter-wave line section are just as standard.
Turns ratio, LC network, or a piece of coax: all three can transform impedance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A1111 of 12

What letter is used to represent reactance?

Why Reactance, the opposition to AC offered by inductors and capacitors, is symbolized by X, as in X_L = 2*pi*f*L and X_C = 1/(2*pi*f*C). It combines with resistance R to form impedance: Z = R + jX. Reactance is measured in ohms, but unlike resistance it stores energy rather than dissipating it.
Watch out Z is impedance, the total opposition including both R and X; Y is admittance (the reciprocal of impedance) and B is susceptance (the reciprocal of reactance), which show up in parallel circuit math.
R and X add up to Z. Flip them over and you get G, B and Y on the admittance side.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G5A1212 of 12

What occurs in an LC circuit at resonance?

Why At resonance the inductive reactance XL = 2πfL and the capacitive reactance XC = 1/(2πfC) are equal in magnitude, and because they act in opposite directions (one leads, one lags by 90 degrees) they cancel each other out. That leaves the circuit looking purely resistive, with no net reactance and zero phase angle between voltage and current. The frequency where this happens is f = 1/(2π√LC).
Watch out Resistance is not cancelled at resonance; only the two reactances cancel, and whatever resistance is present is exactly what remains to set the impedance and limit the current.
Resonance = XL = XC, reactances cancel, only R is left. Resistance never cancels.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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