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In an AC circuit with only a capacitor, how are current and voltage timed?

Current leads voltage by a quarter cycle (90 degrees).

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  1. They peak at the same moment
  2. Current leads voltage by 90 degrees ✔
  3. Voltage leads current by 90 degrees

Why: Charge must flow onto the plates before voltage can build up across them. So current peaks a quarter cycle ahead of voltage. In an inductor it is the other way around.

Huh, didn't know that: A memory trick engineers use is ELI the ICE man: in an inductor (L) voltage E leads current I; in a capacitor (C) current I leads E.

The dad joke
The capacitor's current always shows up early. The voltage is fashionably late by a quarter cycle.
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References

Answer: Current leads voltage by 90 degrees. Charge must flow onto the plates before voltage can build up across them. So current peaks a quarter cycle ahead of voltage. In an inductor it is the other way around.

  1. OpenStax College Physics 2e, 23.11 Reactance, Inductive and Capacitive openstax.org/books/college-physics-2e/pages/23-11-reactance-inductive-and-capacitive
    The voltage follows the current by one-fourth of a cycle
    Checked 2026-10-10.

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