GND on Arduino

Has GND on Arduino a 0V potential or can it be different?

Measuring voltage needs 2 points; calling it "potential" is just a short-hand for "potential difference".

If you happened to put the black probe of your meter on the pin labelled 5V and the red one on the pin labelled GND, it would read -5V and you would say it has a potential of minus 5. All voltage measurements depend on the relative positions of the 2 measuring probes.

So if you put the black probe on Gnd it's 0V by definition as much as anything else.

But since there's nothing on the board that would read lower than 0V if you used the Gnd as your black probe point, then yes you could say it's 0V... but it's always relative.

[ There is no sensible notion of absolute potential - there's a text book definition involving moving a unit charge from infinity, but clearly that's neither usable or desirable - the earth may be at megavolts compared to the rest of the solar system, we'd never normally know down here under the ionosphere as it shields us, but its pointless to call any voltage in our circuit absolute, the simplest is relative to the local ground.

0V is a convention, and normally we treat circuit ground or protective mains earth as 0V, and sometimes they are the same even, but don't have to be.

In case anyone's wondering electric potential is the integral of electric field (or you can say that the electric field is the gradient of the electric potential). Potential and differences in potential are measured in volts and informally termed voltages. Electric potential is potential to perform work on a charge, just as gravitational potential is for mass. The work done depends on the change of potential times the charge that moved. ]

Back to the Arduino - you could for instance connect a 5V supply to the Arduino where the +ve terminal was connected to local earth, then the Arduino's ground would have to be at -5V and the Vcc at 0V, that's unsual,
but nothing basically wrong with it so long as you remember not to connect Arduino ground to other grounds
which are at the local earth potential. You could do it, but without a compelling purpose its not useful and
might lead to expensive mistakes.

Ok so basically the 5V output on Arduino is 5V measured grom GND?

Yes. Voltages are ALWAYS relative to some point. That's why your multimeter needs two connections to measure anything.

Steve

Ok so basically the 5V output on Arduino is 5V measured [f]rom GND?

YES, it's +5V relative to IT'S ground.