Sometimes people get confused just by the many definitions of the word.
ground
noun
- the solid surface of the earth; firm or dry land: to fall to the ground
- Often, grounds. the foundation or basis on which a belief or action rests; reason or cause: grounds for dismissal.
In the context of electronics, sometimes ground means sense 1 above. Earth is, after all, approximately a \$6\cdot10^{24} kg\$ ball of iron. Like everything else, it exists at some electric potential, and if you stick a long conductive rod in Earth, you can make other things connected to that rod at approximately the same potential:
Of course, Earth is really big. Not all of it is at the same potential. In fact, not even close. Earth's huge magnetic field is constantly changing, and induces currents all over Earth. Other people have their own rods stuck in Earth and put currents in Earth. Lightning moves tremendous current in Earth. Since Earth is not a perfect conductor, and by Ohm's law any current through any resistance much be accompanied by a voltage, the potential between two points on Earth is not the same, unless you are lucky, or the points are very near each other.
And, if you've ever operated a battery powered device, you know that electronic devices can function perfectly well without a connection to Earth. Yet, these devices do have a ground. So, this is probably not the sense of ground you should use for your basis of electrical understanding. The other sense, the basis on which a belief rests, is probably a better start.
It's a very astute observation that your confusion involves voltage, as well. Ground is, simply put, \$0V\$. But to understand what this really means, one must really understand voltage. Many people fall into the trap of thinking that since ground is \$0V\$, then ground is where there is no voltage. Thus, there must be voltage everywhere else. But, once you understand voltage, you see this can't be true.
So what is voltage? The more rigorous term for it is electric potential difference. All three words are part of the understanding of voltage. Electric is obvious.
What about potential? Potential has specific meaning in physics. Potential energy is the capacity for some arrangement of things to do work. For example, a compressed spring, a stretched bow, or a high-pressure tank of gas have the potential to do work, if released.
Imagine a ball at the top of a ramp. If the ball is allowed to roll down the ramp, at the bottom, it will be moving quite fast. It acquired this kinetic energy from the potential energy it had at the top of the ramp. If there were no other losses (friction, for example), then the kinetic energy gained by the ball is equal to the potential energy it lost, by the law of conservation of energy.
That's potential energy. Just potential by itself has a different definition: it is potential energy per unit of stuff at some point in a system. Obviously, a massive ball at the top of the ramp has more potential energy than a small ball at the top of the same ramp. So, the two balls have different potential energies at the top of the ramp, but they are at the same potential.
The relevant kind of stuff it is depends on the kind of potential. For gravity fields, the stuff is mass. For electric fields, the stuff is charge. Potential energy is measured in joules. Gravitational potential is measured in \$J/kg\$. Electric potential would then be measured in joules per coulomb (\$J/C\$), which actually, is exactly the definition of the volt.
So earlier we said voltage is electric potential difference. What's the difference? Imagine again our ramp. If you assume that gravity is equally strong anywhere on Earth (this is only approximately true, but is a valid simplifying assumption for much practical engineering), then does the location of the ramp matter? It could be in Death Valley or on Mount Everest: the ball, after rolling down the ramp, will have at the end the same kinetic energy. The potential at the top and bottom of the ramp is irrelevant; the important thing is the difference in potential between the top and the bottom. If we are assuming that Earth's gravity field is the same wherever we might take this ramp, than just the height of the ramp is relevant.
So, since voltage is a difference, we need two points to have a voltage. If we say some node in a circuit is \$5V\$, then we are saying it's \$5V\$ more than some other point. Ground is that other point, unless context says otherwise.
A similar convention exists with height. If I say the height of Mt. Everest is \$8848 m\$, you will assume I mean its height is \$8848m\$ more than sea level. I can also override this reference with explicit context. For example, I can say Mt. Everest is \$237m\$ higher than K2. The default reference can change, also. For example, if I say that Olympus Mons is \$21229 m\$, you probably don't assume this is above sea level, but instead some equivalent datum on Mars. There is no universal reference for elevation.
This is why ground is \$0V\$, just as sea level is \$0m\$. It's not that ground has no voltage, or sea level has no elevation: it's that these things are differences, and the difference between a thing and itself is \$0\$. Thus, there is no magic about ground. It doesn't do anything. It's just a node in the circuit, just like any other. It's only by definition that it is also \$0V\$, and this definition exists just as a convention to simplify our discussion of a circuit. There is no universal ground or \$0V\$ until we define something as such. Usually, it's just whatever we decide to stick the ground symbol on. We can put it anywhere we like, but we usually put it where it makes calculations easiest and discussion simplest.
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