General G5A: Reactance, impedance and resonance
These are the 12 official questions in group G5A of the 2023–2027 General question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- G5A01Question 1
What happens when inductive and capacitive reactance are equal in a series LC circuit?
- AResonance causes impedance to be very high
- BImpedance is equal to the geometric mean of the inductance and capacitance
- CResonance causes impedance to be very low Correct
- DImpedance is equal to the arithmetic mean of the inductance and capacitance
Why: At series resonance the reactances cancel, leaving very low impedance.
- G5A02Question 2
What is reactance?
- AOpposition to the flow of direct current caused by resistance
- BOpposition to the flow of alternating current caused by capacitance or inductance Correct
- CReinforcement of the flow of direct current caused by resistance
- DReinforcement of the flow of alternating current caused by capacitance or inductance
Why: Reactance is opposition to AC caused by capacitance or inductance.
- G5A03Question 3
Which of the following is opposition to the flow of alternating current in an inductor?
- AConductance
- BReluctance
- CAdmittance
- DReactance Correct
Why: An inductor's opposition to AC is reactance.
- G5A04Question 4
Which of the following is opposition to the flow of alternating current in a capacitor?
- AConductance
- BReluctance
- CReactance Correct
- DAdmittance
Why: A capacitor's opposition to AC is reactance.
- G5A05Question 5
How does an inductor react to AC?
- AAs the frequency of the applied AC increases, the reactance decreases
- BAs the amplitude of the applied AC increases, the reactance increases
- CAs the amplitude of the applied AC increases, the reactance decreases
- DAs the frequency of the applied AC increases, the reactance increases Correct
Why: An inductor's reactance increases as frequency rises.
- G5A06Question 6
How does a capacitor react to AC?
- AAs the frequency of the applied AC increases, the reactance decreases Correct
- BAs the frequency of the applied AC increases, the reactance increases
- CAs the amplitude of the applied AC increases, the reactance increases
- DAs the amplitude of the applied AC increases, the reactance decreases
Why: A capacitor's reactance decreases as frequency rises.
- G5A07Question 7
What is the term for the inverse of impedance?
- AConductance
- BSusceptance
- CReluctance
- DAdmittance Correct
Why: The inverse of impedance is admittance.
- G5A08Question 8
What is impedance?
- AThe ratio of current to voltage
- BThe product of current and voltage
- CThe ratio of voltage to current Correct
- DThe product of current and reactance
Why: Impedance is the ratio of voltage to current.
- G5A09Question 9
What unit is used to measure reactance?
- AFarad
- BOhm Correct
- CAmpere
- DSiemens
Why: Reactance is measured in ohms.
- G5A10Question 10
Which of the following devices can be used for impedance matching at radio frequencies?
- AA transformer
- BA Pi-network
- CA length of transmission line
- DAll these choices are correct Correct
Why: Transformers, LC networks and transmission line sections can all match impedances. All the choices are correct.
- G5A11Question 11
What letter is used to represent reactance?
- AZ
- BX Correct
- CB
- DY
Why: Reactance is written with the letter X.
- G5A12Question 12
What occurs in an LC circuit at resonance?
- ACurrent and voltage are equal
- BResistance is cancelled
- CThe circuit radiates all its energy in the form of radio waves
- DInductive reactance and capacitive reactance cancel Correct
Why: At resonance, inductive and capacitive reactance cancel.