how does charge interact with earth ground?

Discussion in 'Physics' started by AdamMil, Sep 11, 2009.

  1. AdamMil

    Thread Starter New Member

    Sep 11, 2009
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    I've been trying to understand how earth grounding interacts with charge flow in a circuit. I understand well enough about the concept of a ground being a reference for measurement, and that's not what I'm asking about.

    Say we have a high-voltage zinc/copper battery, comprised of individual zinc/copper cells connected in series. Due to the way the zinc/copper cell works, both terminals of the battery are negatively charged (i.e., they have an excess of electrons), but there's a voltage between them because the zinc terminal is more strongly negatively charged than the copper terminal, so electrons tend to flow from the zinc terminal to the copper.

    The battery powers a load. Now say for safety reasons we decide to connect the "negative" (zinc) electrode to earth ground, like in this picture:

    [​IMG]

    Earth ground is considered to be an infinite charge source/sink that is electrically neutral. I think that means it should have a potential for charge flow from a negatively-charged source, like our battery terminals. How does this connection affect the flow of charge in the circuit? What is preventing charge from simply escaping into the ground?

    Also, what difference would it make (if any) to replace the earth ground with an infinitely large, uncharged (neutral) mass of copper (assuming no other circuits were similarly connected to it)?
     
    Last edited: Sep 11, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Both terminals do not carry a charge of themselves. This is how the cell works - http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html

    As you can see from the electrochemical reaction, the external circuit must return all the electrons that eminate from the zinc plate, or negative terminal. The connection to earth ground only gives a definite reference for the battery. The negative terminal simply assumes the same potential as ground.

    If the earth connection actually supplied electrons (or accepted them in place of the copper plate) the battery would operate differently. We would use a zinc-earth or copper-earth battery instead.

    As to the last - the local quality of the grounding would be improved. And you could feasibly construct zinc-earth batteries. Copper does not have a charge just because it's there. It does have lots of not very well bound electrons that can be induced to carry charge between different voltages. look at the battery example to see how the cell chemistry moves charge internally to create the voltage difference between the plates.
     
    Last edited: Sep 11, 2009
  3. mik3

    Senior Member

    Feb 4, 2008
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    Charge will flow to ground if something touches the hot wire (and touches ground too). Ground also prevents static electricity to built up.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    There will almost alway be a potential difference between earth, and non earth. Even earth is not a homogenous neutral.

    When we work on mobile rigs, we drive ground rods into the ground. The belief is that if the skids become energized, it's not necessarily to drain off the charge to ground, but to bring the localized earth up to the charge, reducing potential difference thereby reducing shock potential for anyone contacting the earth.

    In instrumentation, grounding to earth can present huge difficulties due to currents set up by this potential difference.

    With this being said, mik3 makes a very valid point in regards to static. Static, if I may, is always present in some degree, and will cause current flow. Your circuit is a bit like an antenna, picking up static fields. These fields can be ambient static, or it can be localized noise radiated from other nearby sources. These can cause related flows in your earthing connection, and propogate through your circuit.

    If you look at your circuit specifically, you will realize that your battery generates a potential across it's poles. Either pole can be referenced to something else, but the battery remains true to it's function. If you placed an AC source between your battery and earth, your battery would continue to be true within the circuit, even though your circuit as a whole would follow the AC voltage in respect to ground.

    You suggested that we not consider 'reference', but that needs to be looked at further. Say we introduce a component into your circuit, say a resistor, but we only connect one lead. What effect will it have on the circuit? Obvious?, but what happens when we touch the other lead?

    Again, back to instrumentation, if there is an earth connection and it runs parallel to other wires, it WILL introduce related potential into those wires. That's why twisted pairs are so important, so that induced potential is common to both wires. Shielding is rarely connected to 'ground' on both ends because of this.

    Your thought on a large conductor mass will have an impact compared to a long parallel run, in reducing these induced potentials.
     
  5. Skeebopstop

    Active Member

    Jan 9, 2009
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    Adammil, consider that electrons don't really flow (well they do but in the order of millimeters/second), just that 'potential' propogates (using the same principles of light travel).

    Therefore, when you connect earth ground to the negative lead of the battery, yes there will be a small moment where 'potential' propogates (current as Ohm's law dictates), after which they will settle to an extremely similar value. The 'potential' built up from the chemical reaction of the battery however, is independently generated.

    Imagine this. You could just as easily create a battery circuit with a negative voltage as a positive one by connecting that earth terminal to the positive lead and leaving the negative lead disconnected. This operates on the principle that your reference point is earth, and not the battery terminals per se. It is all relative as Einsteinian physics would say so depends where and how you look.

    Just remember that charge doesn't really flow, rather just gets locally perturbed as the potential propagates, and I think it will clear many things up.
     
  6. Skeebopstop

    Active Member

    Jan 9, 2009
    358
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    O and by the way, kudos for asking such questions. That line of thinking is the difference between really understanding something versus copying application notes.
     
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