In a purely resistive AC circuit, voltage and current rise and fall together, perfectly in step. Add an inductor or a capacitor and that stops being true - each one shifts the timing between voltage and current, and they shift it in opposite directions from each other. Keeping the two straight is one of the most reliably tested AC theory concepts, which is exactly why the mnemonic exists.
Inductive reactance: voltage leads
An inductor - a coil of wire - stores energy in a magnetic field and opposes changes in current. That opposition is called inductive reactance, and it increases with frequency: XL = 2πfL, where L is inductance in henrys. The practical effect: in a purely inductive circuit, voltage peaks before current does. Voltage is out in front, leading the way.
Capacitive reactance: current leads
A capacitor stores energy in an electric field instead, and it opposes changes in voltage. Capacitive reactance decreases with frequency: XC = 1 / (2πfC), where C is capacitance in farads - the opposite relationship from an inductor. In a purely capacitive circuit, current peaks before voltage does. Current leads this time.
ELI the ICE man
The mnemonic packs both relationships into five letters each:
- ELI: in an inductor (L), voltage (E) comes before current (I) - E, then L, then I.
- ICE: in a capacitor (C), current (I) comes before voltage (E) - I, then C, then E.
It looks almost too simple to be useful, but it's the fastest way to avoid the single most common AC theory mix-up: which component leads which.
Where this leads - impedance and resonance
In real circuits with resistance, inductance, and capacitance together, total opposition to current is impedance (Z), and for a series RLC circuit: Z = √(R² + (XL - XC)²). AC's version of Ohm's Law then reads I = V / Z. When XL and XC happen to be equal, they cancel each other out entirely - that's resonance, where impedance drops to its minimum (pure resistance) and current hits its maximum.
RMS voltage, frequency and period, and the full reactance and impedance formulas are covered in the Study Guide, with practice problems in the Exam Companion.