[HWR_14]

So, there is a true value for 0?

Does that mean when you have electronics and use multiple dc-dc converters all the inputs and outputs share the same ground, it's not just the values for that pair of wires?

And if I want to use a telephone on an incorrectly wired 48dc circuit, I could switch the positive and negative wires, as long as the circuit in the telephone is isolated and never touches ground?

Thanks. Somehow I got in my head that all circuits were just about the delta from neutral and therefore nothing outside them mattered.

[wat10000]

There is a true zero potential. You can detect this because two charged objects with zero delta between them will still repel each other.

I think a circuit should mostly care about the deltas, but when you’re talking about things like phone lines, the earth becomes part of your circuit. You can’t influence its potential (it’s almost exactly neutral because any charge imbalance gets removed by interaction with the interplanetary medium) so everything else is going to end up being determined by what you need for their relative potential to that.

[the_cosmos]

Voltage is _always_ relative; you only ever measure differences between two points. Voltmeters have two probes. Ground is just a convenient reference, not a universal zero. And while Earth can act as a reference or return path in some systems, it’s not perfectly neutral or fixed...it’s just large enough to approximate that in practice. (The interplanetary medium aka aether doesn't exist)

Objects don’t repel because they’re at the same potential. Electrostatic force comes from electric fields due to charge. If two objects truly have zero potential difference and no field between them, there’s no force.

You’re correct that circuits care about voltage differences. After all, all work requires a force gradient of some kind.

[wat10000]

Can't you define zero as the limit of the potential at infinity? And neutral charge bodies in empty space are generally close to that.

The interplanetary medium absolutely exists. I'm not talking about aether. I'm talking about the soup of dust, gas, and particles that fills space in the solar system. It contains a lot of charged particles, which is what keeps Earth extremely close to neutrally charged. Any deviation from neutral starts attracting positively charged particles and repelling negative, or vice versa, which equalizes the charge.

I didn't say objects repel because they're at the same potential. I said that objects at the same potential will still repel each other if that potential isn't zero.

Seriously, what is this reply? Aether? Objects repelling because they're at the same potential? You seem to have read a comment very different from what I wrote.

[the_cosmos]

Yeah, I did mistake what you were trying to say. We’re just coming at this from two different sides... physics does have the convention that absolute zero potential is defined at infinity. I was looking at it strictly from a practical circuit perspective, where 'zero' is just wherever we stick the ground probe, and we only ever calculate for the relative deltas. And we generally don't have wires that can make it to the interplanetary medium or the edge of the infinite ground plane :)

[wat10000]

Space is only ~60 miles away, that's not that much wire. Getting one end up there is left as an exercise for the electrician....

I think it all ties together. Ground potential as in sticking things in the actual ground is in practice pretty much the same as that theoretical potential at infinity, because space has enough charged particles flying around to equalize the imbalance. But it doesn't really matter from a practical perspective when making circuits.

[bonzini]

> all the inputs and outputs share the same ground, it's not just the values for that pair of wires?

No, it depends on the converter. There are converters that leave 160V on the DC power rail for a 110V AC input, and 155V on the DC "ground" rail.

They are economic and you could find then when galvanic isolation is at least in theory not important, but they're terribly unsafe when used on PCBs that people might muck with.

If you have some "normal" converters and some of this kind, sharing the ground would be quite dangerous.

[HWR_14]

I have done some projects that needed some generic dc-dc converters from aliexpress (eg stepping down 12v to 5 or 3.3) I alway treated the output of each step down as a pair of wires that share no ground. It sounds like that would be overkill if they were reputable but it's probably best to not try tying the grounds together.

I figured any happenstance from the multimeter that the grounds match was transitory and not to be trusted.