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This is probably a really silly question, but I can't seem to find the answer anywhere.
Say I have two totally separate DC power supplies running from AC power, and another piece of equipment that requires 12v DC. Why is it not possible to take 12v from one supply, to the device that needs powering, and then take the return to the ground on the second supply? What is it that stops the current flowing?
Sometimes you can, if they're both grounded power supplies which have a low-impedance connection to a common earth point.
If they aren't, then "ground" on one is not at all connected to "ground" on the other, and there is no loop for current to flow.
Sometimes they're both supposed to be ground, but are actually at slightly different potentials, which causes all sorts of problems. Especially to audio systems ("ground loop"), resulting in everything from annoying hums to arcing and damage.
You mean ...
simulate this circuit – Schematic created using CircuitLab
There is no potential difference established between the +ve of V2 and the -ve of V1. voltages are not absolute values, they are differences between two points.
If you measure the voltage with respect to the unconnected end you would see that there is no voltage across R2 - hence no current will flow through it.
If there is a "hidden" connection between the negative sides of the DC supplies, then a potential difference exists across R2.
What is it that stops the current flowing?
Air, probably. Most DC power supplies are galvanically isolated. That means they pump charge between their two terminals and aren't attached to anything else.
Consider these two circuits:
simulate this circuit – Schematic created using CircuitLab
V3 is able to develop a current, because it can pump charge around the circuit and through the load.
V1 and V2 are not able to develop any current, because they are pumping charge into the end of a wire. There's no place for current to go. The current doesn't flow for the same reason that it doesn't come shooting out of the power supply terminals with nothing connected: air is a poor conductor.
If your DC adapter has only two plugs, then it's going to be isolated if it meets safety standards. Many don't, and they're quite dangerous.
Enough home outlets have the neutral and hot wires incorrectly reversed that it's a massive safety problem to assume neutral is ground.
DC adapters with three prong plugs, however, usually tie the DC side ground to the ground pin on the outlet. Usually.
In circuits theory we know that the current only flows in a CLOSED circuit. If you do not close, you cannot make the power supply to work. A simple way to understand is to think in a simple battery (chemical reaction as we see in school). If just one side supplies current, you cannot maintain the chemical reaction so your voltage still working.
