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Subelement G5
Electrical Principles
Section G5A
Reactance; inductance; capacitance; impedance; impedance matching
What is impedance?
  • The electric charge stored by a capacitor
  • The inverse of resistance
  • Correct Answer
    The opposition to the flow of current in an AC circuit
  • The force of repulsion between two similar electric fields

(C). Impedance is the opposition to the flow of current in an AC circuit. Impedance is composed of resistance and reactance (both capacitive and inductive).

Note: Think that "impedance" is going to "impede" or get in the way of current flow.

For more info see Wikipedia: Electrical Impedance

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What is reactance?
  • Opposition to the flow of direct current caused by resistance
  • Correct Answer
    Opposition to the flow of alternating current caused by capacitance or inductance
  • A property of ideal resistors in AC circuits
  • A large spark produced at switch contacts when an inductor is de-energized

(B). Reactance is the opposition to the flow of alternating current caused by capacitance or inductance. Reactance changes with both the capacitance and inductance of the current to act along with resistance as components of the impedance.

Note: Reactance (either from changes in capacitance and/or inductance) is going to make the circuit "react" and block (oppose) current flow in the AC circuit.

For more info see Wikipedia: Reactance

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Which of the following causes opposition to the flow of alternating current in an inductor?
  • Conductance
  • Reluctance
  • Admittance
  • Correct Answer
    Reactance

(D). Reactance is the factor listed which causes opposition to the flow of alternating current (AC) in an inductor. Both inductive (from an inductor) and capacitive (from a capacitor) reactances act with resistance to oppose the flow of current as components of impedance.

For more info see Wikipedia: Electrical Reactance, Inductor

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Which of the following causes opposition to the flow of alternating current in a capacitor?
  • Conductance
  • Reluctance
  • Correct Answer
    Reactance
  • Admittance

(C). The Reactance is the factor which causes opposition to the flow of alternating current (AC) in a capacitor. Both capacitive (from a capacitor) and inductive (from an inductor) reactances along with resistance combine as the impedance causing the opposition to the flow of AC current through the circuit.

For more info see Wikipedia: Electrical Reactance, Capacitor

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How does an inductor react to AC?
  • As the frequency of the applied AC increases, the reactance decreases
  • As the amplitude of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance decreases
  • Correct Answer
    As the frequency of the applied AC increases, the reactance increases

Inductive reactance changes proportionately with the frequency and the inductance. As frequency increases, the inductive reactance also increases.

Notice that the equation for inductive reactance is defined with frequency, not amplitude:

\[X_L = 2\pi{f}L\]

\begin{align} X_L & = \text{Inductive reactance}\\ \pi & = \text{pi (3.14159...)}\\ f & = \text{Frequency}\\ L & = \text{Inductance}\\ \end{align}

The amplitude of the applied AC has no effect on inductance, eliminating two distractors.

For more info see Wikipedia: Electrical Reactance, Inductor

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How does a capacitor react to AC?
  • Correct Answer
    As the frequency of the applied AC increases, the reactance decreases
  • As the frequency of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance increases
  • As the amplitude of the applied AC increases, the reactance decreases

(A). As the frequency of the AC current applied to a capacitor increases, the reactance of the capacitor decreases.


The capacitive reactance is inversely proportional to the frequency. The higher the frequency of the AC current, the less charge can accumulate in the capacitor, and so the opposition to the current decreases.

Given:
\(\pi\) = pi (3.14…)
\(f\) = frequency
\(C\) = Capacitance

\[\text{Capacitive Reactance }(X_C) = \frac{ 1 }{ 2\pi{f}C }\]

From the equation, one can see that as the frequency increases, the reactance of the capacitor decreases.


From Wikipedia:
Reactance is the opposition of a circuit element to a change of electric current or voltage, due to that element's inductance or capacitance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current. The notion of reactance is similar to electrical resistance, but they differ in several respects.

For more info see Wikipedia: Electrical Reactance, Capacitor

Silly Hint: Pretty much the same as in Lucille Ball's funniest skit: as the frequency of the candies increased, Ethyl and Lucy's capacity to react decreased! https://m.youtube.com/watch?v=8NPzLBSBzPI#

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What happens when the impedance of an electrical load is equal to the output impedance of a power source, assuming both impedances are resistive?
  • The source delivers minimum power to the load
  • The electrical load is shorted
  • No current can flow through the circuit
  • Correct Answer
    The source can deliver maximum power to the load

(D). When the impedance of an electrical load is equal to the internal impedance of the power source, the source can deliver maximum power to the load. When the impedances are equal they are said to be "matched". For both AC and DC currents matching the impedances causes the reactance of the system to be negligible or ideally, zero. This allows for maximum flow of current.

For more info see Wikipidea: Impedance matching

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What is one reason to use an impedance matching transformer?
  • To minimize transmitter power output
  • Correct Answer
    To maximize the transfer of power
  • To reduce power supply ripple
  • To minimize radiation resistance

(B). One reason to use an impedance matching transformer is to maximize the transfer of power. This type of transformer alters the current and voltages, which changes the impedances between the power source and load. Matching the impedances allows for maximum power transfer, so this is one component which may be used for the function.

For more info see Wikipedia: Impedance matching, Impedance Matching Transformer

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What unit is used to measure reactance?
  • Farad
  • Correct Answer
    Ohm
  • Ampere
  • Siemens

(B). The ohm is the unit used to measure reactance. The ohm is also the unit for electrical impedance and resistance, as these are all related properties which impede the flow of current in an AC circuit.

For more info see Wikipedia: Ohm, Electrical Reactance

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Which of the following devices can be used for impedance matching at radio frequencies?
  • A transformer
  • A Pi-network
  • A length of transmission line
  • Correct Answer
    All these choices are correct

All of the choices are correct. All of the listed devices are ones that can be used to match the impedances of the circuit frequency. Impedance matching is important as it allows for maximum transfer of power from the source to the load.

For more info see Wikipedia: Impedance matching

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Which of the following describes one method of impedance matching between two AC circuits?
  • Correct Answer
    Insert an LC network between the two circuits
  • Reduce the power output of the first circuit
  • Increase the power output of the first circuit
  • Insert a circulator between the two circuits

(A). One method of impedance matching two AC circuits is to insert an LC network between the two circuits. In an LC network (also called a tuned circuit), L refers to an Inductor, and C refers to a Capacitor. Together they act to "tune" the circuit to match the impedances.

For more info see: Impedance matching, LC network

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