What is the least amount of time required for a Voltage source to change polarities.?

If you consider gamma ray electromagnetic radiation that has a frequency of approximately 3 x 10^19 Hz, then the time for one cycle would be 1/f = 33.3 x 10^-21 seconds, it will change from positive to negative in 16.7 x 10^-21 seconds.

Hope this helps,

Newton1Law

Then your question was indeed non sense because, polarity switching as you seem to propose it, may not ideally have a minimum time and could be zero time. However, so far as I know at this point, we are constrained by the physical universe around us with the existing laws of physics as we know them. Based on the condition of the existing laws, the natural radiation identified as gamma waves have the highest frequency and thus are the electromagnetic wave I chose as an example of a voltage source that changes polarity as rapidly. Thus at the end on one half cycle, its polarity changes. The maximum rate of change would be given by dv/dt of the driving function, Sine or Cosine time the peak value of the wave form. So if the wave form is 100,000,000 volts peak, 100M*Sin(wt), the derivative sets the per unit change in time and the 100M sets the slope or change per unit time.

If you meant as it applies to a particular circuit, you needed to state those conditions. You do not state under what conditions, so I gave a general answer to an ambiguous question. With your experience on this web site, I would have expected a better framing of the question.

THIS IS A NON SENSE QUESTION. FOR DC POWER SOURCE, POLARITY WOULD NOT CHANGE FOREVER.

FOR AC POWER SOURCE, IT DEPENDS ON ITS FREQUENCY RATE THAT IT CARRIES, AS HIGH AS MICROWAVE POWER SOURCE, TIME IS JUST SMALL AS IN PICO SECOND.