RSMTC Foundations Practice Exam

The capacitor reactance Xc is given by Xc = 1/(2π f C). Which statement is true about Xc as frequency increases?

Xc increases with increasing frequency Xc decreases with increasing frequency Xc is independent of frequency Xc increases with the square of frequency

Correct Answer:

Xc decreases with increasing frequency

Explanation:

Capacitive reactance decreases as frequency increases. The magnitude is Xc = 1/(2π f C) = 1/(ωC); as frequency f rises, ω grows, so Xc gets smaller. Physically, a higher frequency means the voltage changes faster, giving the capacitor less time to charge each cycle, which lowers its impedance to the AC current. At DC (f = 0) it would be infinite (open circuit), and at very high frequencies it tends toward zero (acting more like a short). So the statement that Xc decreases with increasing frequency is the correct description. The other possibilities would contradict the inverse relationship with frequency.

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