The reactance of a capacitor $C$ at frequency $f$ in an a.c. circuit is:

1. Capacitive reactance is the opposition a capacitor offers to alternating current. 2. It is given by $X_C=\dfrac{1}{2\pi fC}$. 3. So reactance falls as frequency rises: a capacitor passes high frequencies more easily. 4. For $C=1\ \mu\text{F}$ at $f=1000$ Hz, $X_C\approx159\ \Omega$, and the voltage follows from $V=IX_C$. 5. Writing $X_C=2\pi fC$ inverts the relation and would wrongly predict that a capacitor blocks high frequencies. _Source: NECTA ACSEE 2023 Physics 131/1, Question 8: Current Electricity_