What is the capacitive reactance in an ac circuit?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → It is the effective resistance offered by the capacitor.
Option (1) It is the effective resistance offered by the capacitor. — Correct. In a purely capacitive circuit the current amplitude is \( i_m=v_m/(1/\omega C) \). Compared with \( i_m=v_m/R \), the quantity \( X_C=1/\omega C \) plays the role of resistance. It limits the current, has the dimension of resistance and is measured in ohm.
Option (2) It is the effective wattage. — Incorrect. Wattage is power, measured in watt, while reactance is measured in ohm. An ideal capacitor consumes zero average power.
Option (3) It is the effective voltage. — Incorrect. The effective (rms) voltage is \( V=v_m/\sqrt{2} \), measured in volt. Reactance is the ratio of voltage to current, so it is measured in ohm.
NCERT (2026-27):
AC Voltage Applied to a Capacitor – “Comparing it to im = vm/R for a purely resistive circuit, we find that (1/ωC) plays the role of resistance. It is called capacitive reactance and is denoted by Xc,”
\( X_c=1/\omega C \qquad (7.17) \)