- ELI the ICE man Definition: ELI the ICE man is a mnemonic that helps remember the relationship between current and voltage in inductors and capacitors.
- ELI: In an inductor, the voltage (E) leads the current (I), known as ELI.
- ICE: In a capacitor, the current (I) leads the voltage (E), known as ICE.
- Capacitor Function: In a capacitor, the current must lead the voltage to charge it, causing an increase in voltage.
- Inductor Function: In an inductor, the voltage resists changes in current, causing the current to lag behind the voltage.
What is ELI the ICE man?
ELI the ICE man is a memory phrase for sine-wave current and voltage in an inductor and a capacitor. ELI: voltage [E] leads current [I] in an inductor [L]. ICE: current [I] leads voltage [E] in a capacitor [C].
It is a mnemonic for AC phase, not a circuit law. Real coils and capacitors have resistance, so the angle is often a little under 90°.
In short:
- ELI: Voltage [E] leads current [I] in an inductor [L]
- ICE: Current [I] leads voltage [E] in a capacitor [C]
The same idea in waves:
- In a pure inductive (L) circuit the voltage (E) sine wave peaks before the current (I). ELI: voltage (E) leads current (I) in an inductor (L). Add series resistance and the lead shrinks.
- In a pure capacitive circuit the current (I) sine wave peaks before the voltage (E). ICE: current (I) leads voltage (E) in a capacitor (C). Series resistance shrinks that lead too.
A capacitor is a two-terminal passive part. It stores energy in an electric field between its plates. Capacitance is the ratio of charge to voltage.
An inductor (coil, choke or reactor) is a two-terminal passive part. Current through it stores energy in a magnetic field.
On a capacitor, voltage is proportional to electrical charge (Q = CV). Current equals C times the rate of change of voltage, so current peaks while voltage crosses zero. Current therefore leads voltage on an ideal capacitor.
An inductor opposes change in current. Voltage is L times the rate of change of current, so voltage peaks while current is still crossing zero. Current therefore lags voltage on an ideal inductor.
In AC, capacitor or inductor current and voltage do not peak at the same instant. Phase difference is how far apart those peaks sit, in degrees (or radians) of the cycle.
For a linear series RL or RC circuit that angle sits between 0° and 90°. One common sign rule quotes the angle by which voltage leads current.
Under that sign rule an inductive circuit has a positive angle: voltage leads, current lags.
A capacitive circuit then has a negative angle: current leads voltage. ELI the ICE man is the memory check for those two signs.
ELI the ICE man Examples
An ideal inductor on an AC source has a 90° phase difference. A real coil has winding resistance, so the angle is a little less than 90°.
Voltage leads current by that angle. ELI: in an inductor (L), E is ahead of I.
An ideal capacitor on an AC source also sits at 90°. A real capacitor has some series resistance, so the angle is a little less than 90°.
Voltage lags current. ICE: in a capacitor (C), E is behind I.
