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. This is also why inductors show up in motor-starting and relay-coil circuits - the same property that shifts the phase also means an inductor resists sudden current changes, which is useful for smoothing switching transients.
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. Capacitors show up constantly in power factor correction for exactly this reason - deliberately introducing a capacitive phase shift to counteract the inductive shift that motors and other inductive loads already put on a system.
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. Under exam pressure, working the phase relationship out from first principles takes longer than it should - the mnemonic exists specifically so you don't have to.
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. Resonance isn't just a theoretical curiosity - it's the same principle behind tuned filter circuits, and an unintentional resonance in a system with both significant inductance and capacitance can produce unexpectedly high currents at a specific frequency.
Why the mnemonic outlasts the formula
Most people who've been away from theory questions for a while can still recall ELI the ICE man long after XL = 2πfL has faded. That's the point of a mnemonic like this one: it survives the gap between when you studied it and when you actually need it on the exam or in a troubleshooting conversation about why a motor circuit's current and voltage readings don't line up the way a simple resistor would predict.
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.