If I understand this right; A negative voltage is when there are more electrons present in a circuit than the voltage draw from the positive end. phone lines kept at a -48VDC to prevent corrosion to the lines. I'm currently an electronics engineering student at my local community college and attempting to wrap my head around the electron flow model. in order for there to be current there has to be some sort draw from to the positive terminal from the negative. so how a negative voltage is achieved I'm not entirely sure, but this is my best guess at the moment.

 

Not again.

How does electron flow direction alter the voltages in this battery circuit?


Voltages need a reference, exactly the same batteries can provide either polarity defending on the reference point. (Gnd or common in this circuit)

 

A negative voltage at some point in a circuit merely means that it is at a lower potential than whatever point we arbitrarily chose to call zero volts.

So the positive terminal of a 9 V battery is at +9 V relative to the negative terminal, but the negative terminal is at -9 V relative to the positive terminal.

To get a phone line (or some other line) to be -18 V relative to earth ground, you simply take a 48 V power source and connect the positive terminal to ground and the negative terminal to the line.

 

Voltage is a measure of difference in potential. It is always measured between two points.

It is like measuring distance.
You cannot ask "What is the distance at New York City?"
That is a meaningless question.

Instead, you can ask "What is the distance from Miami to New York City?"

Now you could also ask "What is the distance to Miami from NYC?"
Depending on one's perspective, one answer could be a positive value while the other is a negative value, depending on your starting point.