Why is static electricity being introduced to this discussion?
because it's a real phenomenon that can cause trouble, and because it's mentioned in earlier posts.
Does a normally operating loop (circuit) that is (let's just say) hovering in mid-air, somehow create a build up of charge that would be attracted to the Earth below?
Not so long as things are going well. But rotating machinery like motors moves a lot of air to cool itself and static electricity can build up wherever there's moving parts and insulators. Consider resemblance of a conveyor belt to Van de Graff generator.
I don't really understand this because it seems to me that whatever leaves the source, has to come back on the return... If every electron that left the source has to return, which ones are being attracted to the earth? Where did they come from?
You're restating Kirchoff's Current law. Kirchoff describes electricity
flowing in a closed system.
Static electricity appears to violate Kirchoff. But i told my students to think of it this way -
for the case of static electricity Kirchoff will accept a temporary delay. Static means it's temporarily not flowing.
Example - Lightning is electrons that were carried up from the Earth's surface by water or dust molecules. It's returning to Earth with a bang. Same thing when you walk across a carpet in wintertime up north and touch a doorknob - electrons pulled out of the carpet by your leather shoe soles make a spark to the doorknob, then work their way back via the hinges and doorjamb to the carpet, giving you that shock.
The electrons do get back, eventually.
So your question is valid - where does the potential come from?
It comes from either an excess or a shortage of electrons on the surface of the un-earthed loop. Its value is literally undefined.
That potential
should be small, but Murphy's Law says if you are depending on it to be small you will get embarassed. What if your circuit recently suffered a lightning stroke that deposited a zillion electrons there?
So we design to make sure it's small.
that's my attempt at a word picture.
In any capacitor Q = CV
and your loop has some capacitance to earth
so potential to Earth in volts V is equal to excess (or shortfall) of charge Q divided by capacitance to Earth C.
Usually Q is small and so is V.
But don't bet your life on it.
ever wonder about Earth itself?
Does Earth have an excess of shortfall of electrons?
What is Earth's potential with respect to the sun?
Earth has an electric field a hundred volts per meter or so.. search on Earth potential gradient
It has a magnetic field. As if a current were flowing around equator.
Would excess charge riding on the surface give those two effects?
Maybe that's discussed in astronomy forum i don't know, just i have always been curious.
Will peruse over there.
old jim
