How do capacitors behave at low and high frequencies?

• A. Open at low frequencies; short at high frequencies
• B. Short at low frequencies; open at high frequencies
• C. Infinite impedance at all frequencies
• D. Short at all frequencies

Answer: (D)

A capacitor’s impedance changes with frequency, because Xc = 1/(2π f C). At very low frequencies (including DC), the impedance is very large, so it blocks current and looks like an open circuit. At high frequencies, the impedance becomes very small, so it allows the signal to pass and behaves like a short circuit. This means capacitors block DC but pass AC more easily as frequency increases. A handy way to picture it is that a DC, steady current charges the capacitor until no more current flows (open), whereas rapid changes in voltage don’t give the capacitor enough time to charge fully, so current flows more easily (short at high frequency).