Those labels should not be the same. Instead, the 'GND' label at device2, should be called something different, like GND2.
Perry --- your thread article is excellent already. And your definition '0V for device 1' and '0V for device', and also 'gnd for device1' and 'gnd for device2' looks good too. I can attach some diagrams where I use the pics to explain how connecting GND1 and GND2 together allows a particular voltage at any selected node of both devices to be the same voltage (with respect to both local grounds).For the case where GND1 and GND2 are disconnected from each other ------- device1 has no means to apply 5V to the input of device2 (relative to GND2). Device1 is able to generate 5V with respect to GND1. But is unable to make the voltage at input2 equal to 5V with respect to GND2. So device1 has no control of device2 when GND1 and GND2 are disconnected.There may be holes and gaps in my diagrams and notes. But just contributing too. Once again ----- excellent thread you made Perry. An excellent sticky.
How old are your kiddies ? Tell them well done for the pictures
I drew them myself BSB ...... I'm a big kiddy hahahaha. Ok ..... a kiddy at heart. And thanks for the kind comment! The colours turned out ok hehehehe. I used 'paint.net' (that's what the paint package is called .... paint dot net ..... it's free).
Lets stick with the kids and save your reputation LOL.
Why can the current from device 1 not simply flow off into ground of device 2? I understand that there must be a current flow. But why does the current have to flow back to the SAME ground? For batteries, I imagine that there could be some kind of capacity/limit of how much current can be dumped into their ground. But if the power supply uses a power socket ground, the need for common ground is completely mysterious to me. For circuit 1, why does it matter if some current is lost along the way? For circuit 2, why does it matter if its ground has to sink a little more than the + side of the power supply provides
Circuit common should be called that and only called "ground" if it is connected to EARTH. We need a new, agreed upon symbol for circuit common / 0V, maybe we could have a contest and...