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I was thinking about when current can and can't flow to ground. I came up with that current will only flow to ground when we have at least two grounds in the circuit, since then we have a complete circuit and current can flow. I can't however, think of why current will not flow to ground if we only have one ground in the circuit? I would say that it is because the we do not have a closed circuit through which current can flow. But, current, does not need a closed circuit to flow (like with a capacitor). So please can you explain why current does not flow to ground if we only have one ground (or if current does flow to ground if we have only one ground why)?

When I say ground I mean actually physically attached to the earth rather then a conductor (I say this because the earth has so many more electrons that a electrons flowing to/from it would have a negligible effect on the overall number of electrons in it) does this make a difference??

The following circuit is one where I think current should flow to and from ground (sorry for the poor diagram my computer is slow so I can't draw a proper one) enter image description here

Quantum spaghettification
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    Current very rarely flows to what you've defined as "ground" in most circuits. Usually current flowing to earth ground is a failure mode in high voltage devices. – Ignacio Vazquez-Abrams Jan 11 '15 at 19:48
  • @IgnacioVazquez-Abrams I have added a diagram where I think current should flow to ground, irrespective of the voltage or the values of the resistors – Quantum spaghettification Jan 11 '15 at 19:53
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    Your diagram changes nothing. The connections to earth ground short out the resistor, but no current would flow into earth proper. – Ignacio Vazquez-Abrams Jan 11 '15 at 20:01
  • @IgnacioVazquez-Abrams but current flows through the earth??, Also why does current rarely flow to ground expect at high voltages? – Quantum spaghettification Jan 11 '15 at 20:03
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    Note that since there are connections to ground on both sides of it, the resistor at the bottom-right of your diagram has no real effect on anything. Unless you specify an isolated ground all ground connections are normally the same, so your two ground connections form a short across that resistor. – Jerry Coffin Jan 11 '15 at 20:31

2 Answers2

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There is nothing "magic" about ground. It is just another route for current to get to its destination.

In most small signal circuits like you have shown the ground symbols are just a way of connecting points together without actually drawing the wires. The ground symbols also act as a reference point against which other voltages can be measured.

For instance, these two circuits are identical:

schematic

simulate this circuit – Schematic created using CircuitLab

When you have real earth in there, the circuit is slightly modified to be more like:

schematic

simulate this circuit

The key point is that the current flows from one point of the circuit, through ground, then back into the circuit.

With only one connection to ground there is no circuit for the current to flow through. It can't flow "to" ground, because there is nowhere for it to flow to. There's no difference between ground and a wire dangling in the breeze.

Electricity flowing to ground in high voltage systems has nothing to do with the fact that they're high voltage. It's purely to do with there being a second connection to ground elsewhere in the circuit (usually at the sub-station) which forms the circuit.

enter image description here

You can read more on the different earthing systems on Wikipedia.

Majenko
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  • And on https://en.wikipedia.org/wiki/Ground_(electricity). Also, the earth is not the magic sink of electrons the OP thinks it is. Locally you can well have https://en.wikipedia.org/wiki/Earth_potential_rise – Fizz Jan 11 '15 at 20:57
  • @RespawnedFluff Would it be possible to explain why earth is not a 'magic' sink/source of electrons, or provide me with a relevant source, thanks – Quantum spaghettification Jan 12 '15 at 11:15
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    @Joseph I thought my answer did just that. The magic word you need to really understand: *circuit*. Without that there is no current, and thus no flow of electrons. – Majenko Jan 12 '15 at 11:20
  • But why can't the current just flow directly into the earth, and stay there, I think in wire this is stopped due to a build up of electrons creating an electric field preventing more electrons from flowing, but this build up would take a very long time in the earth and thus an current should flow? – Quantum spaghettification Jan 12 '15 at 11:25
  • There is no reason for it to flow into the ground. Why would it? The ground *doesn't form part of the **circuit**.* – Majenko Jan 12 '15 at 11:32
  • Would they not flow (say into the circuit) because they could get to a higher potential then they would be at in the earth? – Quantum spaghettification Jan 12 '15 at 11:40
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    Current never ever ever ever flows *to* earth. It only ever flows *through* earth. Earth is just another wire - albeit a planet sized one. Electrons have to flow from one side of the power source back to the other. They can never flow out and never return. – Majenko Jan 12 '15 at 11:43
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    @Joseph: You can get *measurable* transients such as electrostatic discharges through circuits that "are not" loops. I put "are not" in quotes, because that's an approximation. Every two points in the known universe are connected. When lightning strikes Earth's surface, it may seem that you only have a circuit that's like a line/wire with no return path, but that's only an approximation. How did the charges get into the clouds, relative to the earth? Over time through "the other half" of the circuit... which is not easily seen... so we tend to forget about it. [continued in the next comment.] – Fizz Jan 12 '15 at 12:55
  • @Joseph: Another thing worth recalling here is that [lumped components](https://en.wikipedia.org/wiki/Lumped_element_model) are also only an approximation; the return path can be extremely [distributed in space](https://en.wikipedia.org/wiki/Distributed_element_model), as it is in the case of lighthing. – Fizz Jan 12 '15 at 12:57
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    I'm hesitantly adding this here because it might cause more confusion than help, but in a more general physics context, the conservation stated by KVL isn't true because it's based on assumptions that don't hold. Cosmic rays can [actually produce current](http://www.ewh.ieee.org/r6/scv/rl/articles/ser-050323-talk-ref.pdf) [via ionization etc.] when they strike atoms inside circuits on earth. Sometimes this effect cannot be ignored, such as in the case of RAM; that's why they make ECC memory, etc. – Fizz Jan 12 '15 at 13:27
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    @Fizz: I think you've lost the plot. ;^) We're talking about a circuit with three resistors, drawn with PaintBrush (or similar). The OP has a much more basic question that probably doesn't need mention of cosmic rays. Keep it down to earth (pun intended). – Transistor Feb 25 '16 at 18:10
  • This was something I didn't understand before this picture. Thank you so much! – Saketh Malyala Jul 31 '20 at 04:54
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Ground is a conductor, and not a very good one. As a conductor it equalizes electrical potential. In case of static electricity, only one connection is needed for current. In most cases 2 connections are needed. If many applications use ground as a conductor, the ground may not be at 0V. Actually if a hydro wire falls close a person, best advice is not to walk, but rather jump on one leg away, because there could be a potential. To maintain a current into a ground, there has to be a potential and that requires 2 points to complete the circuit.

sparky Al
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    You are just making stuff up here. Ground (the 'Earth') is a great conductor as long as the electrical system is properly grounded the impedance is near 0 ohms. That's why most lightning strikes hit the earth (or a tree with water in it) (or a metal building made of aluminum or steel). Hopping on one leg does absolutely nothing except increase your chances of falling and getting into more serious trouble. You must be assuming that one leg is going to make some super-magical ground whilst the other leg somehow makes direct connection to the circuit HOT line. – Nick May 10 '16 at 20:55
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    Good luck getting a ground impedance anywhere near 0 ohms with any real-world ground rod. Here in the UK, the maximum recommended for a house supply is 200 ohms. – Simon B Mar 31 '17 at 22:24
  • Late to the party but I suspect 200 ohms is hardly considered a great impedance when dealing with mains levels. – RichieHH Sep 19 '19 at 11:54