Extra exam: electrical principles
Resonance, Q, time constants, phase, complex impedance and power factor. 46 questions, 10 at a time, starting with ones you haven't seen.
Rules to remember10 key electrical principles rulesShow the rules
- Half-power bandwidth = f / Q = 7.1 MHz / 150 ≈ 47.3 kHz. (p. 53)
- One time constant (R × C) is the time to charge a capacitor to 63.2% of the applied voltage or discharge it to 36.8%. (p. 53)
- Susceptance, the imaginary part of admittance, is represented by the letter B. (p. 54)
- Susceptance is the reciprocal of reactance (B = 1/X), so converting replaces the magnitude with its reciprocal. (p. 54)
- Net reactance = 250 − 500 = −250 Ω (capacitive). (p. 54)
- In a capacitor, current leads voltage by 90 degrees, because current must flow before voltage builds up. (p. 55)
- Rectangular notation is R ± jX, with capacitive reactance negative, so pure 100-ohm capacitive reactance is 0 − j100. (p. 56)
- Polar form describes impedance by its magnitude and its phase angle. (p. 56)
- A pure inductive reactance has a phase angle of +90 degrees in polar form. (p. 56)
- For reactive power the current and voltage are 90 degrees out of phase, so energy is exchanged rather than consumed. (p. 58)
Taken from the explanations to checked practice questions; page numbers link to the official handbook.
Sample electrical principles questions
1.How much real power is consumed in a circuit consisting of a 100-ohm resistor in series with a 100-ohm inductive reactance drawing 1 ampere?
- A100 watts
- B200 watts
- C70.7 watts
- D141.4 watts
Show the answer
✓ A. 100 watts
Only the resistance consumes real power: P = I²R = (1 A)² × 100 Ω = 100 W. The reactance consumes none.
2.What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel?
- A220 seconds
- B110 seconds
- C440 seconds
- D55 seconds
Show the answer
✓ A. 220 seconds
Two 220 µF capacitors in parallel make 440 µF; two 1 MΩ resistors in parallel make 0.5 MΩ. τ = RC = 0.5 MΩ × 440 µF = 220 seconds.
3.What is the effect on the magnitude of pure reactance when it is converted to susceptance?
- AThe sign is reversed
- BIt is unchanged
- CIt is shifted by 90 degrees
- DIt is replaced by its reciprocal
Show the answer
✓ D. It is replaced by its reciprocal
Susceptance is the reciprocal of reactance (B = 1/X), so converting replaces the magnitude with its reciprocal.
4.What coordinate system is often used to display the phase angle of a circuit containing resistance, inductive, and/or capacitive reactance?
- AElliptical coordinates
- BMaidenhead grid
- CPolar coordinates
- DFaraday grid
Show the answer
✓ C. Polar coordinates
Polar coordinates plot magnitude and angle, making them a natural way to show a circuit's phase angle.