Amateur Extra E5B: Time constants, phase angles and admittance
These are the 12 official questions in group E5B of the 2024–2028 Amateur Extra 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.
- E5B01Question 1
What is the term for the time required for the capacitor in an RC circuit to be charged to 63.2% of the applied voltage or to discharge to 36.8% of its initial voltage?
- AAn exponential rate of one
- BOne time constant Correct
- COne exponential period
- DA time factor of one
Why: One time constant charges to 63.2% or discharges to 36.8%.
- E5B02Question 2
What letter is commonly used to represent susceptance?
- AG
- BX
- CY
- DB Correct
Why: Susceptance is written B.
- E5B03Question 3
How is impedance in polar form converted to an equivalent admittance?
- ATake the reciprocal of the angle and change the sign of the magnitude
- BTake the reciprocal of the magnitude and change the sign of the angle Correct
- CTake the square root of the magnitude and add 180 degrees to the angle
- DSquare the magnitude and subtract 90 degrees from the angle
Why: Take the reciprocal of the magnitude and change the sign of the angle.
- E5B04Question 4
What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel?
- A55 seconds
- B110 seconds
- C440 seconds
- D220 seconds Correct
Why: In parallel: 440 µF and 0.5 MΩ, and 0.5 MΩ × 440 µF = 220 s.
- E5B05Question 5
What is the effect on the magnitude of pure reactance when it is converted to susceptance?
- AIt is unchanged
- BThe sign is reversed
- CIt is shifted by 90 degrees
- DIt is replaced by its reciprocal Correct
Why: Converting a pure reactance to susceptance takes its reciprocal.
- E5B06Question 6
What is susceptance?
- AThe magnetic impedance of a circuit
- BThe ratio of magnetic field to electric field
- CThe imaginary part of admittance Correct
- DA measure of the efficiency of a transformer
Why: Susceptance is the imaginary part of admittance.
- E5B07Question 7
What is the phase angle between the voltage across and the current through a series RLC circuit if XC is 500 ohms, R is 1 kilohm, and XL is 250 ohms?
- A68.2 degrees with the voltage leading the current
- B14.0 degrees with the voltage leading the current
- C14.0 degrees with the voltage lagging the current Correct
- D68.2 degrees with the voltage lagging the current
Why: arctan((250 − 500) ÷ 1,000) ≈ −14°: capacitive, so the voltage lags.
- E5B08Question 8
What is the phase angle between the voltage across and the current through a series RLC circuit if XC is 300 ohms, R is 100 ohms, and XL is 100 ohms?
- A63 degrees with the voltage lagging the current Correct
- B63 degrees with the voltage leading the current
- C27 degrees with the voltage leading the current
- D27 degrees with the voltage lagging the current
Why: arctan((100 − 300) ÷ 100) ≈ −63°: capacitive, so the voltage lags.
- E5B09Question 9
What is the relationship between the AC current through a capacitor and the voltage across a capacitor?
- AVoltage and current are in phase
- BVoltage and current are 180 degrees out of phase
- CVoltage leads current by 90 degrees
- DCurrent leads voltage by 90 degrees Correct
Why: In a capacitor, current leads voltage by 90 degrees.
- E5B10Question 10
What is the relationship between the AC current through an inductor and the voltage across an inductor?
- AVoltage leads current by 90 degrees Correct
- BCurrent leads voltage by 90 degrees
- CVoltage and current are 180 degrees out of phase
- DVoltage and current are in phase
Why: In an inductor, voltage leads current by 90 degrees.
- E5B11Question 11
What is the phase angle between the voltage across and the current through a series RLC circuit if XC is 25 ohms, R is 100 ohms, and XL is 75 ohms?
- A27 degrees with the voltage lagging the current
- B27 degrees with the voltage leading the current Correct
- C63 degrees with the voltage lagging the current
- D63 degrees with the voltage leading the current
Why: arctan((75 − 25) ÷ 100) ≈ 27°: inductive, so the voltage leads.
- E5B12Question 12
What is admittance?
- AThe inverse of impedance Correct
- BThe term for the gain of a field effect transistor
- CThe inverse of reactance
- DThe term for the on-impedance of a field effect transistor
Why: Admittance is the inverse of impedance.