Simple A.C. Circuits
Physics · 48 practice questions
Explanation of a.c. current and voltage; peak and r.m.s. values; a.c. source connected to a resistor, capacitor (capacitive reactance) and inductor (inductive reactance); R-L-C circuits; vector diagrams, phase angle and power factor; resistance and impedance; effective voltage; resonance and resonant frequency.
- Differentiate between peak and r.m.s. values of a.c. and determine the phase difference between current and voltage.
- Interpret R-L-C circuits and analyse vector diagrams.
- Calculate effective voltage, reactance and impedance.
- Determine the resonant frequency and the instantaneous/average power and power factor in a.c. circuits.
Try a few questions
- Practice question 1
In a purely capacitive a.c. circuit, the current
- A.lags behind the voltage by a phase angle of 90°
- B.is exactly in phase with the voltage
- C.is exactly 180° out of phase with the voltage
- D.leads the voltage by a phase angle of 90°
Show the answer
D. leads the voltage by a phase angle of 90°
A defining characteristic of a purely capacitive circuit is this 90° phase lead of current ahead of voltage.
- Practice question 2
An a.c. supply has a peak voltage of 340V. Its r.m.s. value is approximately
- A.480 V
- B.170 V
- C.240 V
- D.340 V
Show the answer
C. 240 V
Vrms = Vpeak/√2 = 340/1.414 ≈ 240 V.
- Practice question 3
Capacitive reactance is given by the formula Xc = 1/(2πfC), showing that it
- A.decreases as the frequency or capacitance increases
- B.increases as the frequency or capacitance increases
- C.remains completely constant, regardless of frequency or capacitance
- D.is unrelated to both frequency and capacitance
Show the answer
A. decreases as the frequency or capacitance increases
Since Xc is inversely proportional to both f and C, increasing either one reduces the capacitive reactance.
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