Definition
In alternating current (AC), one complete cycle consists of two alternations: the positive alternation, during which current flows in one direction and voltage rises from zero to a peak and back to zero, and the negative alternation, during which current reverses direction and voltage drops from zero to a negative peak and back to zero. Together these two alternations make up one full cycle of AC.
Plain English
AC electricity flows back and forth. Each full back-and-forth is one cycle, and that cycle has two halves: one half where the current goes one way (positive), and one half where it goes the other way (negative). Those two halves are the alternations.
Context Anchor
Seen in basic aircraft electrical-system study, especially when learning how AC power, generators, and electrical waves are described.
Derivation
Alternation comes from the Latin alternare, meaning 'to do by turns.' In AC, the current literally takes turns flowing one way then the other, so each turn is called an alternation.
Why Pilots Care
Aircraft AC systems (such as those powering certain instruments and avionics) rely on this back-and-forth flow. Understanding that one cycle contains two alternations is the foundation for understanding frequency (cycles per second, or hertz), which directly affects how AC equipment operates.
Analogy
Picture a swing moving forward and backward. One full back-and-forth motion is like one cycle; the forward half is like the positive alternation, and the backward half is like the negative alternation.
Grounding Statement
Picture a swing going forward and back: forward is one alternation, back is the other, and the full forward-and-back motion is one cycle.
Intuition Check
Positive and negative do not mean good and bad here. They mean the electrical value is on one side of zero, then on the other side of zero.
Example Sentence 1
On an oscilloscope, the technician could clearly see the positive alternation rise above the centerline and the negative alternation fall below it, completing one cycle of AC.
Example Sentence 2
An uneven negative alternation may signal a problem in the AC power supply.