Resistance in an AC circuit
The relationship V = IR applies for resistors in an AC circuit, so
In AC circuits we'll talk a lot about the phase of the current relative to the voltage. In a
circuit which only involves resistors, the current and voltage are in phase with each
other, which means that the peak voltage is reached at the same instant as peak
current. In circuits which have capacitors and inductors (coils) the phase relationships
will be quite different.
Capacitance in an AC circuit
Consider now a circuit which has only a capacitor and an AC power source (such as a
wall outlet). A capacitor is a device for storing charging. It turns out that there is a 90°
phase difference between the current and voltage, with the current reaching its peak
90° (1/4 cycle) before the voltage reaches its peak. Put another way, the current leads
the voltage by 90° in a purely capacitive circuit.
To understand why this is, we should review some of the relevant equations,
including:
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 Fall '10
 DavidJudd
 Physics, Current, Resistance, Alternating Current, Electric charge, power supply, 90°

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