Final answer:
Voltage, current, and power go through zero 120 times per second in a 60-Hz AC circuit due to the sine wave's two zero crossings per cycle, resulting in 120 crossings with 60 cycles per second.
Step-by-step explanation:
Voltage, current, and power go through zero 120 times per second for 60-Hz AC electricity primarily due to the nature of sine wave cycles.
A single cycle of a sine wave for AC power consists of both a positive and a negative excursion from the zero line, which means that the wave will cross the zero line twice during one cycle. Since we're dealing with 60-Hz AC electricity, we have 60 cycles per second.
Each cycle has two zero crossings (once when going from positive to negative, and once when going from negative to positive), resulting in a total of 120 zero crossings per second.
Furthermore, these characteristics of AC power result in a fluorescent light bulb brightening and dimming at a rate of 120 times per second, as the current flows back and forth through the filament.
This is also why power, which is the product of voltage and current (P=IV), fluctuates with the alternating current and voltage, showing zero power at the instances of zero current and voltage.