While under DC current capacitors cause open circuit as time goes to infinity but under AC current they flow through capacitor. As I know Maxwell described this behavior as "Aether current". Is this current under AC source actually caused by microwaves or how is it possible for current to pass through it and how AC current can pass while DC can't?

  • \$\begingroup\$ Do you understand the concept of impedance and how it varies with frequency? Note that DC has a frequency of zero. \$\endgroup\$ Apr 28, 2020 at 15:12
  • \$\begingroup\$ As I know impedence is vectoral sum of resistance and reactance and reactance causes phase shift. But I could not fit these two in my mind to associate with current flowing through capacitor under ac circuit. \$\endgroup\$
    – Nabla
    Apr 28, 2020 at 15:14
  • \$\begingroup\$ @PhilFreedenberg Can you follow up on why impedance creates different behaviors for AC and DC voltage sources for capacitors? \$\endgroup\$
    – user103380
    Apr 28, 2020 at 15:35
  • \$\begingroup\$ Go google "displacement current". \$\endgroup\$
    – Andy aka
    Apr 28, 2020 at 17:18

2 Answers 2


The current does not actually "pass through." Charge flowing into one terminal of a capacitor accumulates and forces charge to flow out the other terminal of a capacitor causing that side to have a deficit of charge. In effect, that is current, but the current stops when the capacitor is charged to a voltage that equals the source voltage. When the source voltage reverses, the accumulated charge reverses direction of flow. With AC current, the process in continuous.


Think about what AC is conceptually. It's a constant change in the direction of current, right? This alternating behavior constantly charges and discharges a capacitor. The current doesn't literally flow through the capacitor with an AC source but the constant charging and discharging of the capacitor makes it seem like current is passing through the capacitor. This doesn't happen with DC because... well... it has no cyclic behavior, hence why it seems like DC creates an open circuit.

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