# I How realistic is the term "ground"

1. Aug 30, 2016

### Samson4

Does it really mean the earth or exceedingly large bodies like the ocean are useful sources of charge? What would happen if you built a very large Van De Graaff generator under the ocean? If the capacitance of the earth is only 710 µF, could we infact measure a rise in potential at distances around said ocean?

2. Aug 30, 2016

### Staff: Mentor

Your Van De Graff generator would have to have its base on one planet and its top on another planet. having to and bottom of the same planet does not change the planets net charge.

More important, you should not try to find absolute truths in science or engineering. It is useful to define ground as zero voltage. The usefulness is all the justification we need.

3. Aug 30, 2016

### Samson4

I wasn't suggesting the charge of the planet to change; but, the surface charge of the ocean would be influenced. Does your statement still apply?

4. Aug 30, 2016

### FactChecker

The term "ground" usually just refers to a reference voltage that will be considered 0 and is easily (or made easily) connected to from all points of an electrical system. Any two connections to "ground" should have the same voltage. You will find out that even a direct connection to the Earth is not necessarily properly "grounded". Two "grounds" to the Earth on different sides of a house should be connected with a heavy gauge conductor (wire or metal water pipe) so that the resistance of the soil will not cause a potential difference. An airplane in the air has a ground that is not the same as the Earth ground.

Regarding your example, I am not sure if the salt water in the ocean would ever present the same problem. It is a good conductor. But the test is whether a close lightning strike would cause electrical problems when two separated points are directly grounded in the ocean.

5. Aug 30, 2016

### David Lewis

Ground describes any circuit conductor whose:
1. Voltage is arbitrarily called (for convenience) zero, and
2. Voltage drop is negligible over its normal operating current.

Circuit ground may not be literal ground; i.e., circuit ground does not have to be at zero volts with respect to the Earth. Circuit ground may be the negative terminal of a battery or power supply, for example.

The term originated because a conductor with a low resistance connection to the ground cannot store electric charge.

Last edited: Aug 30, 2016
6. Aug 30, 2016

### Staff: Mentor

I believe that what the OP was after was the concept of ground being identically zero voltage. (Apologies if I'm wrong.)

In real life, we can only measure voltage differences. Voltmeters always have two leads, not one. Therefore, which voltage we call zero voltage is arbitrary. The only absolute voltage definition requires moving a charged particle in from infinitely far away; that's not real life.

7. Aug 31, 2016

### CWatters

The problem is that the earth and oceans are quite good conductors so the capacitor you are trying to charge is effectively "shorted".

Not sure what you mean by useful? Are you asking if you could use an ocean like a capacitor to store energy?

I remembered reading that the sunlit surface of the moon becomes charged to around 100V wrt the dark side by the action of the solar wind. Google found a calculation that puts the capacitance of the moon at around 100-200uF. The energy stored on a capacitor is 0.5CV^2 so the energy stored is perhaps less than 1 Joule. Your average mobile phone battery holds perhaps 5000-7000 Joules?

The capacitance of a capacitor is proportional to A/d where A is the area of the plates and d is the separation between them. Using an ocean or continent gets you large A but perhaps on balance it's easier or more useful to minimise d?

8. Aug 31, 2016

### Staff: Mentor

No. What I meant was that it is very convenient and useful to have a common reference when analyzing and discussing circuits. We call that ground and we arbitrarily assign it zero volts. Often (but not always) the reference is tied to the physical earth ground. Even then, we arbitrarily assign it the value zero volts, ignoring the potential difference between ground at our location and other places on the Earth's surface. I was not thinking of any physical, or safety, or noise suppression kinds of usefulness.

9. Aug 31, 2016

### FactChecker

Soil is not as good a conductor as one might think. That is why separate "grounds" to Earth on two sides of a house should be connected by a large gauge conductor (wire or pipe). A great danger near a lightning strike is if a camper is sleeping on a mat with his head and feet touching the ground. The electricity goes through the person easier than through the soil and can kill him. That is also why hundreds of reindeer in a field were killed by lightning the other day http://www.nytimes.com/2016/08/31/science/lightning-strike-dead-reindeer.html?_r=0. In a separate incident, 19 cows were killed similarly. http://www.kltv.com/story/32870795/lightning-strike-kills-19-cows-taking-shelter-under-tree

10. Aug 31, 2016

### Egebo

Mmm, no. Earth neither sinks nor sources a charge. Earth can be one electrode of a charged capacitor, but the charge would have to come from some generator.

In reality, it would be shorted, but in principle, it could pump a current through the water.

Since salt water is not a perfect conductor, you can get a potential difference if you drive current through it.

Re, 710 µF: Where do you get that value? What is the other plate? Earth is not a capacitor, it is a conductor.

Hans

11. Aug 31, 2016

### CWatters

So where does your Van De Graaff generator come in to all this?

12. Aug 31, 2016

### Staff: Mentor

Not mine, the OP's. The messages are very garbled in this thread, I'm afraid my posts included. Sorry.

13. Sep 1, 2016

### Beanyboy

I'm very new to Science. I liked your quote about the utilitarian value of scientific knowledge being its justification, rather than wasting time looking for absolute truths. Would you think it's possible to say the same of pi? i.e. we don't know exactly what it is, but we know enough to build bridges, airplanes, satellites, treat cancer, etc.

14. Sep 1, 2016

### Beanyboy

That's a really neat explanation, even for a beginner like me, so thanks.
May I ask your response to the following definition of voltage: the difference in electric potential a charge could experience between two locations, as measured in Joules per Coulomb of charge.

15. Sep 1, 2016

### bsheikho

I would say voltage is equivalent to how tired a group of electrons become after they pass this point. Such that when they start their path theyre full of energy but when they reach the end of their path, all their energy is gone. You know, work life, without any weekends.

16. Sep 1, 2016

### FactChecker

I wouldn't say that the definition of pi is the same as the definition of a ground voltage. A ground voltage is a convenient reference to compare other voltages to. You can consider it to be 0 for most uses. Things get trickier when several electrical systems, each with it's own "ground", are combined. The grounds may have different values. But pi is defined as a specific value that can not be changed (in a flat space). It is the ratio of a circle's circumference to it's diameter. We would expect a Martian to use exactly the same value as we do, accurate to as many decimal places as we care to compare.

17. Sep 1, 2016

### Beanyboy

Perhaps I misled you. Let me try that again. It was said that what's sought after in Science and Engineering is not necessarily absolute truths, but rather truths that are useful to us, e.g. ground ought to be considered as zero voltage. For practical purposes, this works. Similarly, you can sit around for the rest of your life trying to define pi, with billions and billions of decimal places. However, for the purposes of Science and Engineering, a more practical value of say, three or four decimals places will suffice.

18. Sep 1, 2016

### Beanyboy

Very kind of you to pitch in here. Thanks. Two questions, if you have time to spare:
1. Would you characterize the definition I quoted as misleading?
2. What "point" are you referring to when you say, "after they pass this point".

I should also add that intuitively, I feel more comfortable with your definition. There's something about the personification of the electrons that seems to resonate with me, So, thanks again.

19. Sep 1, 2016

### rcgldr

or possibly zero net charge per unit mass. This could be considered "neutral", but not necessarily ground.

In the USA, 3 phase AC generators only use 3 lines for distribution of electricity. The earth is used for the "return" line, and in that sense is considered to be "ground".

20. Sep 5, 2016

### David Lewis

Electric potential I think is same thing as voltage.
Units of measure aren't necessary to define a physical quantity.
The definition I like is energy divided by charge.

21. Sep 6, 2016

### kuruman

Not quite the same. As the definition says, voltage is the difference of electric potential between two points in space. Saying that electric potential is the same as voltage is like saying that position is the same as displacement.

22. Sep 6, 2016

### Beanyboy

OK, so, now excuse me while my head explodes:)

I'm off to have a nice cup of tea, to cogitate on all of this. Yours etc, The Patron Saint of Ignorance.

23. Sep 6, 2016

### Beanyboy

Ok, think I've got it now. You're saying voltage is a statement of the magnitude of the difference between two points?

24. Sep 6, 2016

### kuruman

Bingo.

25. Sep 6, 2016

### Beanyboy

It's not that I now feel more intelligent; I just feel less stupid :)

Just saw this. Great demo I thought on why it's the amps that kill you, not the volts.

Thanks again.