# What's ground? Is it the point defined as 0 V of potential or voltage? If the latter, is it because we measure its voltage with respect to itself?

I have some questions regarding the definition of ground that some people use. One question leads to the other so I've organized them below.

Some people say that "ground is the point/node defined as 0 V". Is this definition correct? If no, what is your definition of ground? If it's correct, keep reading. If they mean 0 V of electric potential, then just clarify me this and I have no further questions. But if they mean 0 V of voltage, then I have two thoughts:

1. The definition of ground ("the point defined as 0 V of voltage") is meaningless because voltage is the difference between the (electrostatic) potential of two points, yet in the definition of ground, they aren't specifying with respect to which point the ground has 0 volts. An example of this is like saying "I'm 10 cm taller" without comparing yourself to anyone; that phrase has no meaning. If you agree with me, do you have a more precise definition of ground? If you disagree, please explain why.

2. Ground is 0 V not because we define it like so, but because we measure its voltage with respect to itself. If that's true, then the definition of ground (point defined as 0 V of voltage) should be changed, since any point/node has 0 V when you measure its voltage with respect to itself. So in a strict sense, according to the definition of ground, all points/nodes are grounds! If you agree with me, do you have a more precise definition of ground? If you disagree, please explain why.

Note: this question is not the same as mine, because they asked about the ground in circuit simulators (which is just a reference node when performing nodal analysis to solve the circuit) and about ground symbols. My question is about the conceptual definition of ground.

### Edit Sep./22/2021

The following image helps clarify the structure of my questions. (I'm adding it in case someone reads these questions in the future.)

I think that what The Photon defined as ground (the node with respect to which we measure the nodal voltages), should actually be the definition of reference node, and that we should define ground as the node/point/region in space defined as zero potential (not zero voltage because that doesn't make sense, as I said above), besides the other uses of ground in electricity (ground of planet Earth as in where we stand, to ground as a verb to connect something to Earth ground, ground busbar, ground rod, ground/grounding wire, neutral/grounded wire, ground loop, the different ground symbols, etc.)

• Comments are not for extended discussion; this conversation has been moved to chat. Commented Oct 26, 2020 at 3:18

Ground is 0 V not because we define it like so, but because we measure its voltage with respect to itself.

Ground is the point we choose to measure all voltages in the circuit with reference to.

So in a strict sense, according to the definition of ground, all points/nodes are grounds!

No, because we didn't choose them as the reference for measuring other voltages.

On a different day, we might choose some other node as the reference. That doesn't mean those nodes are the reference today.

As a comparison, just because any player on a soccer team is capable of acting as captain on any given day doesn't mean that all of them are captain today.

• "No, because we didn't choose them as the reference for measuring other voltages." This reply if true if we use the definition of ground you used ("Ground is the point we choose to measure all voltages in the circuit with reference to.") But if we use the definition of ground I said ("ground is the point/node defined as 0 V"), then what I said is true. Commented Oct 23, 2020 at 3:54
• @AlejandroNava, the choice of which node to define as 0 V is an arbitrary choice. We can only choose one node at a time to have that function. If we choose node "A" as the one defined to be 0 V, then nodes "B", "C", and "D" are not ground. Commented Oct 23, 2020 at 3:55
• @AlejandroNava, yes, and it wouldn't change the current flowing through any branch in the circuit. It wouldn't be a very convenient choice when it comes to analyzing the circuit because it would mean the whole (physical planet) Earth would have an oscillating voltage of 120 or 240 V in your analysis, but it would be perfectly valid from a theoretical point of view. Commented Oct 23, 2020 at 3:59
• @AlejandroNava It's not wrong, but it's circular. 0 V is defined as "equal to ground", so of course ground is at 0 V. Commented Oct 23, 2020 at 11:23
• With respect to ground. See? It’s a circular definition. Commented Oct 24, 2020 at 7:56

I think the term we use for the zero volt point in a circuit should really be called "reference point" or "Common", rather than "Ground", but this misuse of the term "Ground" is so deeply embedded in electronics that we're stuck with it.

The point we call "Ground" in a circuit simulator, in your battery-powered circuit on a breadboard, or in a car, is simply the point we choose to call Zero Volts, and will use as a reference when measuring voltages elsewhere in the circuit. It has no magic properties, and is no different than any other point in the circuit except that we've chosen to call it "zero volts".

In a circuit I might decide to call the negative terminal of the battery "Ground", while in the same circuit you might call the positive terminal of the battery "Ground". Both decisions are equally valid, but we'd get confused while discussing the circuit with each other.

In some systems, like AC power distribution, and some radio antenna systems, "Ground" does really mean "A connection to the Earth".

• I agree with what you said, including to stop using the word "ground" when we actually mean "reference node". I disagree in calling the reference node also as "zero volts (of voltage)", since that's misleading as I explained in my question. Commented Oct 23, 2020 at 5:39
• "is simply the point we choose to call Zero Volts" // Zero volts of what, of voltage or of electric potential? I think the correct definition for ground would be only the phrase you said right after the previous one, namely "and will use as a reference when measuring voltages elsewhere in the circuit." Commented Apr 25, 2021 at 5:21

There is no one definition of 'Ground'.

'Ground' is an over-used word that gets used to label several related but subtly different concepts.

The best way perhaps to think about it is in an operational or contextual way. That is 'if you want to do thing, then is something like ground useful to you here, and if so, how should it be defined?' The definition of 'ground' is the answer to that question.

a) If you want to refer to all the voltages in a circuit, is it useful to pick one of them, and define it as 0 V?

Absolutely.

b) If you have a circuit, and want to do (a), does it matter which point you choose to define as 0 V?

Yes and no. If you have a battery driven circuit operating at DC with no external connections, then it doesn't matter which terminal you define as 0 V. However, the circuit topology may treat one connection as common to many components, and it will be easier to design and analyse the circuit if that conductor is chosen to be ground.

If you have inputs and outputs referred to that common conductor, then you'd be making life difficult for yourself if you chose any other node as ground, but the circuit would still work regardless.

If it's an RF circuit, then the existence of a very common conductor means that things like stray capacitances are not equal to/from all nodes, but that the capacitance of any node will tend to be dominated by that to the common node. Here, you do need to choose that as ground, and connect it to the case, because with some RF designs, the strays have to be as designed in order for the thing to work.

So in a strict sense, according to the definition of ground, all points/nodes are grounds!

Are capable of being called ground, but cannot all be grounds at the same time. Once you've chosen one as ground, then the others, by definition, cease to be.

Both potential, and potential difference, are measured in volts. However, potential is also often called voltage. Don't let the plethora of names confuse you to what is actually being measured or referred to.

In much the same way, an altitude, and a difference of altitudes, are both measured in metres. A metre is very definitely a distance between two things. But how often do you hear a mountain being referred to like this? 'Everest is 8848 metres'. The altitude that's marked on a map is usually above a datum of mean sea level, just as those 132 kV powerlines are measured with respect to the large brown conductor the pylons are built on. However, if you're climbing a summit, the datum your legs care about is the height of your base camp. It depends what you want to use the potential or the potential difference for, for where you base it or how you describe it.

• "However, potential is also often called voltage." But that's wrong, right? Electric potential is one thing, and voltage (also known as electric potential difference or electric tension) is another thing. Commented Oct 23, 2020 at 17:53
• You said "If you want to refer to all the voltages in a circuit, is it useful to pick one of them, and define it as 0 V? Absolutely", which is the definition of ground I quoted. If you meant 0 V of voltage (and not of potential), then I find that definition misleading. The problem in that definition is the phrase "and define it as 0 V". Since voltage is difference of potential, i.e. you need to talk about two points, with respect to which other point the ground has 0 V? Commented Oct 23, 2020 at 18:12
• I make the analogy with altitude, height and metres to show you that although confusing potential and voltage is technically wrong, it's silly to insist on not understanding what is meant, because the context of each is clear. 'Voltage difference' and 'potential difference' are both phrases that mean the work required to take one Coulomb of charge between two general points, and 'voltage' and 'potential' are both words that mean the energy required to take 1 C of charge between a point and a reference point. Language is sloppy, the science is precise, get over it. Commented Oct 23, 2020 at 18:39
• So, the definition of ground I quoted is wrong? Commented Oct 23, 2020 at 18:54
• @AlejandroNava It's not that your definition is wrong, it's your attitude that this or that particular wording of a definition is right or wrong that's not useful to you. I would recommend you read Richard Fyenman on Knowing Something. It sounds like you're spinning your wheels comparing similar true-in-context definitions asking which is wrong, rather than finding the truth they all tell. You must find it very tiring and frustrating. Let go of the words and find the physics. Look at my use of 'energy' in my last comment. Commented Oct 24, 2020 at 3:38