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So I work on motherboards and understand ground is both a reference point (acts as 0 when testing against) and its "where electrons carrying charge want to go." but why?

Since a battery separates electrons and protons to opposite ends (and please correct me if I'm wrong, no electronics background here), you have a build up of electrons carrying negative charge at one end (positive terminal for conventional). Those want to get to negative terminal where there's both a lack of electrons and build up of protons --basically to be balanced.

And since short circuits involve an unintended path to ground - example cap becoming wire to ground, is ground in a circuit nothing more than an easy return path to the other end of the battery terminal where electrons want to go?

Again I'm just asking about circuit ground not earth ground. Thank you for your help.


First thanks for all the answers I really appreciate it. So am I correct to state the following:

Voltage isn't a set number or "thing" but rather the difference between two points in a circuit? Example a battery really isn't a voltage source but an introduced abundance of electrons built up at one terminal while doubling as the object that completes the loop for electrons to "flow?" Since there's a lack of electrons at the opposite terminal they rush through the circuit to get back to this "lack of electrons" to become balanced.

Ground is a point, deemed by engineers, to measure all voltages against aka reference point. When designing they measure voltages against the same point and list those in the schematic. So ground isn't really 0 it's just acting as our 0 reference point to measure another point against?

And by return path do you mean the other battery's terminal is connected and allows electrons to flow back through it? Thanks again.

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Ground is something we define to be ground.

Voltages don't exist on a node. They exist between nodes - it describes the difference in potential between two points - a measure of how eager electrons want to flow from one to the other. A positive difference means they want to flow towards that node, a negative difference means they flow away from that node.

Because it would be clumsy to always talk about two nodes, we often define something as "ground". We say that this is the node to which all other voltages will be measured.

In many single-supply circuits, ground is also the final return path for currents - the conventional current starts out in the most positive point a from their flows to the most negative point. Electrons, however, are negative charges and flow from the negative to the positive. They come out of the ground terminal of your battery and go into the positive terminal.

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Ground, or more specifically "ground potential" is simply a reference node within your circuit.

Voltages are not absolute things, but rather a difference in potential between two nodes in a circuit. In other words a voltage is always measured as the difference between two points in a circuit.

To make circuit analysis easier we tend to use a single node in the circuit (usually the point of lowest potential, but not always) and measure all voltages relative to this zero point as it makes calculation easier. This 0V point is what we call ground.

Current will flow through ground (assuming there is a complete path), but it's not necessarily the "return path" - current can flow into ground, yes, but it can also flow out. It is simply the point in the circuit that we call 0V.


Since a battery separates electrons and protons to opposite ends

Batteries don't really collect protons at one end and electrons at the other, although you are right that there is a build up of electrons at one end.

A battery uses redox chemical reactions which oxidise the anode (anode material loses electons) and reduce the cathode (cathode material gains electrons). The process generates a voltage because as you say you have an excess of electrons at one end and a deficiency of electrons at the other.

When you connect a circuit between the anode and cathode, the excess of free electrons at the anode flow through your circuit to the cathode (remember, cathodes are pussitive =^_^= ) to equalise the charge. This is why a chemical battery runs down - the reaction runs out of material to redox.

And since short circuits involve an unintended path to ground

Not necessarily. A short circuit is simply an unintended current path between two nodes in a circuit. One of the nodes could be ground, but it doesn't have to be.

For example you could short out the 5V and 12V rails on a computer PSU. This wouldn't entail a short to ground, but it would still be damaging none-the-less. In this example the electrons flowing through this short don't want to go to ground per say, they simply want to find a route to a point of lower potential and flowing from 12V to 5V will do just that.

Of course in the end current only flows in circular paths, so in the end the electrons that enter a circuit will always be equal to the electrons leaving the circuit.

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  • \$\begingroup\$ Tom first, thank you very much! To keep it short my biggest problem with finding answers on this subject is no one can seem to give a clear one and I'm finding different definitions everywhere. Very clear and informative and you actually answered my questions. One last one if you don't mind...I understand the definition of voltage being difference in potential between two points. Do you mean difference in electrons between two points? \$\endgroup\$ – Stephen Sep 20 '17 at 22:38
  • \$\begingroup\$ @Stephen, think of it like this. Suppose there are two buildings, one is 150m high and the other 50m high. Now the difference in the heights of the two buildings is 100m. Voltage is similar, it's the difference of two potentials; here the potentials being analogous to the heights 150m and 50m. These two heights are measured with respect to the ground we stand on, this can be considered as the ground of a circuit and the two potentials mentioned earlier are the two 'heights'. \$\endgroup\$ – noorav Jun 11 at 9:09
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It is both a return path and a shield. Any closed (or nearly closed) metallic surface will reflect electromagnetic waves. Thus it helps to shield sensitive signals from external interference, or prevents noisy traces on the board from radiating them.

Looking at the return path just as a path where the electrons flow back, is oversimplifying things quite a bit. The path electrons flow depends on more then just where you connect the power supply. If you have high frequency (where as "high" starts as low as a couple of kHz) signals, the return path will not follow the "shortest" route back, but will actually be concentrated on the ground plane beneath the wire. There was a nice two part article on ground planes and the current flow on EDN a while back:

http://www.edn.com/design/analog/4394761/Successful-PCB-grounding-with-mixed-signal-chips---Part-1--Principles-of-current-flow

http://www.edn.com/design/analog/4395817/Successful-PCB-grounding-with-mixed-signal-chips---Part-2--Design-to-minimize-signal-path-crosstalk

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    \$\begingroup\$ Don't confuse return path with electrons. Electrons flow FROM ground. It's the current return path.. \$\endgroup\$ – Trevor_G Sep 20 '17 at 18:02
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    \$\begingroup\$ I would argue electrons flow from low potential to high potential. Ground is then whatever we define it to be. Granted the OP is asking about a two-terminal system, but still I think it might be usefull to make a clear point about it \$\endgroup\$ – Joren Vaes Sep 20 '17 at 18:07
  • \$\begingroup\$ +1 for shield, but I don't think reflect is the right term. Maybe just say the shield excludes E and B fields. \$\endgroup\$ – George Herold Sep 20 '17 at 18:43
  • \$\begingroup\$ Reflect is the right term, at least for EM waves. E and B fields are not excluded. E fields would be completely reflected, if the plane would be an ideal conductor. But it isn't so it gets just damped... How much depends on the frequency and the properties of the field. B fields get only damped, as the permeability of copper is rather low. \$\endgroup\$ – Attila Kinali Sep 20 '17 at 18:49
  • \$\begingroup\$ @AttilaKinali, So with a metal box in a static (low frequency) E field, I don't see any 'reflection' of the field. Charges build up on the surface such that the field inside is zero. (At least that's how I see things.) At higher frequencies you can talk about reflection and skin depth, then you don't even need a closed box to have exclusion of the field. \$\endgroup\$ – George Herold Sep 20 '17 at 19:04

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