What is the relationship between the AC current through a capacitor and the voltage across a capacitor?

In a purely capacitive circuit, the relationship between voltage and current is defined by the way capacitors store energy as an electric field. When an alternating sine wave voltage is applied, the current is highest when the voltage is changing most rapidly, which occurs at the voltage zero-crossing points. Because of this dynamic, the current waveform reaches its peak 90 degrees before the voltage waveform reaches its peak. A universally used memory aid for this relationship is the phrase 'ELI the ICE man'. The 'ICE' portion of the mnemonic stands for Current (I) leads Voltage (E) in a Capacitor (C). Therefore, the correct fundamental relationship is that current leads voltage by exactly 90 degrees.