Answer .
Instantaneous voltage is the voltage between two points at a particular moment in time, the voltage at a specific instant in time. Consider a 115 volt 60 cycle-per

…-second (cps) AC sine wave like the one at a wall outlet where you might plug in a household toaster. Starting at zero, the voltage increases up to some maximum value, then decreases back to zero. What follows is a negative excursion. It's just like the positive one, only the polarity is reversed. During the second half of the cycle, the voltage goes below zero, down to some maximum NEGATIVE value, then back up to zero. One complete cycle sees the voltage go up, reach some maximum, drop back down, cross zero and go on down reaching some maximum negative value and then come back up to zero. Got it? A 60 cps sine wave makes one complete cycle in 1/60th of a second. Okay? Now check this out. At half of the 1/60th of a second, or 1/120th of a second, the voltage has gone up, reached its peak, and then come back down to zero. It's exactly at the middle of its cycle. The instantaneous voltage at that point in time is zero volts. The instantaneous voltage of a 60 cps sine wave at half a cycle (at time = 1/120th of a second) is zero volts. When does the voltage reach its positive peak? At 1/4th the time of one cycle, or at 1/240th of a second. A 115 volt AC sine wave has a maximum peak voltage of about 163 volts. Yes, that's correct. (The explanation is a separate question.) The voltage starts at zero and then goes up to 163 volts over a period of 1/240th of a second, then comes back down to zero again over the next 1/240th of a second. The instantaneous voltage of a 115 volt 60 cps AC sine wave at 1/240th of a second is 163 volts. Hope that you can read through the explanation to see the idea. ( Full Answer )