Electronics > Beginners
Some noob questions
Mr D:
According to the attached schematic, having both the pos. and neg. terminals attached to ground will let the current flow.
But what if the two connections to ground are 10cm or 1km from each other? Does it matter?
Is it the case that the Earth is electrically neutral (or at least much more neutral then a power source), so current will flow from pos. into ground, and from ground towards neg.?
So in fact a closed circuit isn't needed?
Would current flow if the two wires (from pos. & neg.) where attached to two different but identical planets, 100 million kilometers apart?
james_s:
You're confusing "ground" with "earth", they are not necessarily the same thing. When you see ground symbols on a schematic it is normally indicating "these nodes all tie together at that same point" and that point happens to be called ground. It's the common reference point for any voltage measurements shown for the circuit. Schematics are drawn this way for clarity as it reduces the rat's nest of wires in the drawing. Often you'll have various points for Vcc or various voltages which may be drawn in numerous points around the circuit. Again those symbols mean those nodes connect together. The "+5v" symbol drawn on a dozen ICs means each of those ICs power pin connects to the same +5V rail, it's neater than drawing a wire that goes all over the whole circuit.
Yes the earth has some resistance, so does a wire, everything (except for superconductors) has resistance, current can still flow.
rstofer:
--- Quote from: Mr D on August 17, 2018, 02:37:19 pm ---According to the attached schematic, having both the pos. and neg. terminals attached to ground will let the current flow.
But what if the two connections to ground are 10cm or 1km from each other? Does it matter?
Is it the case that the Earth is electrically neutral (or at least much more neutral then a power source), so current will flow from pos. into ground, and from ground towards neg.?
So in fact a closed circuit isn't needed?
--- End quote ---
The earth being 'neutral' or not has nothing to do with anything. You can model the resitance between any two points on the earth as millions or billions of parallel resistors forming a grid. There are test instruments for measuring earth resistance.
http://support.fluke.com/find-sales/Download/Asset/2633834_6115_ENG_A_W.PDF
On high voltage substations, we build a 2'x2' mesh of 4/0 wire under the entire area and we tie all of the metal structures to that grid. A few Ohms of ground resistance doesn't sound like much until you drop 100,000 amps into it and the voltage goes sky high. This is a really big deal! Just ask the cows that die when they stand on the ground near a HV tower that experiences a fault. The voltage difference across their legs is enough to kill them. The grid extends a few feet out from the fence line to ensure that a person touching the fence is standing over the grid. There's a lot of engineering in this stuff.
--- Quote ---Would current flow if the two wires (from pos. & neg.) where attached to two different but identical planets, 100 million kilometers apart?
--- End quote ---
In the perfect vacuum of space? Probably not. But the vacuum isn't perfect so probably yes.
The force is so tiny it couldn't be measured and is swamped by the gravitational force. Not important to EEs
https://www.quora.com/Is-there-any-electromagnetic-attraction-between-planets-and-the-sun
Mr D:
Thanks for trying but, urrrghhhh, it's not making any sense to me.
Just when i thought i had my head around Ohm's law (which i do), this ground thing looms it's ugly head.
Let me try to precisely express what i'm not understanding:
In the attached schematic, both the pos. & neg. terminals lead to this undefined "ground". Apparently according to James S this is not real earth, but, for example the metal case of my device.
So the current will travel from pos., down the circuit, into the metal case, then back into the neg. terminal, right? So, we have a complete circuit.
But in such a device, the case is often earthed, right? So why would the current choose to travel back into the neg. terminal when there's an easier path (with virtually no resistance) to real earth?
Or to turn the question round the other way: I take a long nail, wet my hand and jam into the positive hole of a wall socket. I'm likely to get killed, right?
But why? The current can flow into my body, but not back into the negative terminal, so there's no complete circuit so no current flow, right?
Until now i've been dealing with simple circuits: a battery, one or two resistors, and that's it. Can you guys not at least partly understand how this idea of a ground leading somewhere undefined (at least in my mind) might be somewhat confusing? Is it the case of the device? Or real earth. Or an earthed case ........ or.....?
Cheers! ......... D
Mr D:
But if i was to stop trying to intuitively understand it, is this a correct summary of what i need to know about ground:
Every time i see a ground symbol in a schematic, i just have to imagine a wire travelling from that symbol to a single bus, with that bus (that has zero resistance) leading back to the negative terminal?
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