Stick with me...
So the voltage question you raise has to do with the differential between negative vs positive DC voltage. The only difference is what way the electrons will flow. If your device requires negative DC voltage then the electrons will simply flow in the opposite direction than if the same device required positive dc voltage. Flowing meaning.....as they pass through the device.
In DC you have 2 poles, the power(+) and the return(-). The power(+) sends electrons outward, through the device and the excess returns to the battery via the (-negative pole)/return.
Think of it like cars on a road traveling down a highway bumper to bumper with other cars. If the path ahead is not available(circuit not completed) the cars cannot move down the road and it effects every car behind it and they are unable to move. Once the path is completed (positive and negative are connected) the electrons can move and draw each other along the circuit.
The reason for the negative voltage or running current in the opposite direction:Meat and potatoes
Understand that as electrons moves down a wire, they travel on the skin of the wire in a spiraling pattern, this creates a magnetic field as the electrons move along and around the wire.
Run them in one direction(positive config), they generate a magnetic field that pulls toward the wire, run them in other direction(negative config) the magnetic field pushes away from the wire.
Back in the days of laying copper cable in the ground, it was realized very quickly that if you ran a charge through the ground in a positive config, it would attract minerals and quickly build deposits on the line, but sending power in the opposite(negative config) would actually push away minerals and keep the wires relatively sound and buildup low. This also assists in oxidation resistance in high salinity air environments. This standard continued into the COs and into everywhere else in telecom/networking world.
