The Secret Behind AC Phase Shift

Added: 2026-05-27

[00:00:00]Here we have three AC circuits, one resistive, one capacitive, and one inductive. Now let's compare the source voltage and circuit current in each one.

[00:00:12]In the resistive circuit, voltage and current rise and fall together. They stay perfectly synchronized. But once we add a capacitor or an inductor, things change. The voltage and current no longer line up perfectly in time. one waveform shifts relative to the other.

[00:00:32]Now let's look at this more carefully.

[00:00:35]When the circuit contains only resistance, the voltage and current waveforms line up perfectly. They rise together, fall together, and cross zero together. But when we introduce a capacitor, something interesting happens. The current waveform shifts ahead of the voltage waveform. In other words, the current starts changing before the voltage reaches the same point in its cycle. So we say in a capacitor, the current leads the voltage. You can think of the capacitor as constantly charging and discharging almost like it's trying to respond instantly to changes in voltage.

[00:01:17]Now look at the inductive circuit. When we introduce inductance, the opposite happens. This time the current waveform shifts behind the voltage waveform. The voltage changes first and the current responds slightly later. So we say in an inductor the current lags behind the voltage. That delay happens because inductors resist changes in current. The magnetic field inside the inductor needs time to build up and collapse. So whenever a circuit contains capacitance or inductance, the current can shift relative to the voltage. This behavior is called phase shift. Phase shift describes how much one waveform shifts ahead of or behind another waveform along the time axis. In AC circuits, it usually refers to the timing difference between the voltage and current waveforms. You can think of it as one wave reaching its peaks and zero crossings slightly earlier or later than the other waveform.

[00:02:22]Phase shift is usually represented by the Greek letter fi.

[00:02:26]So to summarize, in a resistive circuit, voltage and current stay in phase. In a capacitive circuit, current leads voltage. In an inductive circuit, current lags behind voltage. This phase shift happens in AC circuits because the voltage and current are constantly changing over time.