Can earth ground draw current?

Discussion in 'General Electronics Chat' started by kelumptus, Jan 17, 2013.

  1. kelumptus

    Thread Starter New Member

    Nov 29, 2012
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    Although i have now spent a considerable amount of time studying electronics at a hobby level I am still stumped by the concept of earth ground.
    My question is, should there be a potential difference between a positive battery terminal and earth ground?
    After searching the internet for an answer to no avail i set fourth into my backyard carrying a metal dog stake (to use as my ground stake) a multimeter, a 6 V battery and an alligator clip connector to a banana plug lead. I then performed the following experiment:
    1) I tested that the stake would conduct current by performing a continuity test with my DMM (this revealed that the resistance of the stake was only a few ohms).
    2) I tested the battery pack worked by hooking it up to my breadboard and powering up a circuit I had previously built.
    3) I connected one end of the alligator clip to the dog stake and the other end to the negative terminal of my multimeter.
    4) I connected the positive battery terminal to the positive terminal of my multimeter.
    5) I set my autoranging multimeter to volts. At this point i expected to see a potential difference between positive battery terminal and ground (such as 3V for instance). The potential difference seemed to fluctuate between 0 to 100mV however. This is not what i expected. Is my fundamental understanding of earth ground incorrect?
    I also tried a similar experiment using my kitchen sink instead of the dog stake. Again, there was no appreciable potential difference. I also tried using my multimeter as an ammeter and saw that there was no current between the positive terminal and the kitchen sink. Just for shits and giggles I even tried the negative battery terminal and still nothing.
    My understanding of ground is that it is like a 0V battery terminal. Is this not the case?
    If the problem is in my method of testing the ground connection, then what is the correct method to use?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Please post a drawing of your setup. Where is the negative terminal of battery connected?
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    I have an ominous feeling about this. But let's hear what he says.
     
  4. BReeves

    Member

    Nov 24, 2012
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    Ya, not sure what he is getting at..

    You need to run a wire from the negative battery terminal to another ground rod. The earth will conduct electricity depending on the soil mosture but Earth Ground is normaly just used for grounding lightning strikes.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Unless we are talking about lightning, or RF current received by an antenna, current will only flow in a circuit that is a closed loop (even in these, there is a closed loop, but it is not so obvious), and has some sort of voltage source in the loop. From what you described, you are leaving one terminal of your battery floating, so no current can flow.

    See the attachment.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Yes, you have a fundamental misunderstanding of what "ground" is.

    The first thing to realize is that there is nothing magical assocaited with saying that something is 0V. All that means is that we have chosen to measure other things relative to that point. It's like the saying that the elevation of something is 0ft -- we are free to pick ANY point and declare it to be at an elevation of 0ft and then measure the heights of everything else relative to it. We do this all the time when we talk about a building being 300ft tall -- we never mean that it is 300ft tall relative to the center of the Earth or (very seldom, anyway) relative to see level. Instead, we mean that it is 300 feet tall relative to the ground on which it is standing and, thus, we are implicitly declaring the ground to be 0ft.

    Voltage is, by definition, a measure of the difference in electrical potential between two points. We often use an analogy to gravity, but, like any analogy, that can't be pushed too far.

    Consider two batteries, a 6V battery and a 12V battery. All the 6V and 12V means is that, on the 6V battery, the positive terminal has an electrical potential that is 6V higher than the negative terminal and, similarly, that the 12V battery's positive terminal is 12V higher than that battery's negative terminal. We know absolutely nothing about the potential difference between any point on the 6V battery to any point on the 12V battery -- they are "floating" relative to each other. If you take your multimeter and measure the voltage between the positive terminals of the two batteries, you will see that it is a small value (ideally zero). But, at the same time, you have just established a relationship between the two batteries by connecting them together via the 10MΩ resistance (or whatever) of the meter. If you could now find a much higher resistance meter (or use a galvanometer, for instance), you would find that the negative terminal of the 12V battery is 6V lower than the negative terminal of the 6V battery because, with no current flowing, the two positive terminals are at the same potential.

    Basically, you get to connect any two floating structures together at any one point and that forces the potential difference between those two points to be the same (provided the connection is sufficiently low resistance).

    This is what is happening with your dog stake experiment. Your meter is establishing a reference point between them.

    So if we connect the positive terminal of the 6V battery to the negative terminal of the 12V battery, the positive terminal of the 12V battery is now 18V higher than the negative terminal of the 6V battery. But they are still floating with respect to the physical ground. You are free to call the point at which the batteries are connected 0V and now you have +12V and -6V relative to your "ground" (and "ground" used this way is nothing more than a label used to refer to whatever node you have declared to be your 0V refarence point). By convention, if we happen to connect a point in our system to physical earth we choose to use that as our reference point and call it our "ground" and declare it to be 0V.
     
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  7. kelumptus

    Thread Starter New Member

    Nov 29, 2012
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    Thanks WBahn for your great explanation. I think this brings me closer to understanding how ground works.
    To answer Ron H and kubeek's question, I am indeed leaving the negative terminal of the battery unconnected.

    My original hypothesis was that current would flow between the positive terminal of the battery and ground because the positive terminal would be of a higher electric potential than ground. This was why I did not connect the negative terminal of the battery.

    If I understand WBahn correctly, the reason I am not seeing an appreciable voltage on my multimeter is because the voltage is equalizing between the two points (the dog stake and the positive battery terminal) as current flows through the meter.
    I experimented with checking the voltage between the positive terminals of two batteries of different voltages and indeed i see 0V and this stands to reason since I have connected the two terminals by way of my multimeter thus bringing the voltage of each terminal to an equilibrium.
    What I do not understand however is that when I connect the positive and negative terminals of a single battery, why do I then see a voltage reading on my meter?
    If potential difference is all about relative difference in electric potential then shouldn't the difference between +3V and +9V (i.e. the positive terminals of two different batteries) be the same as the difference between -3V and +3V (i.e. the positive and negative terminals of a single battery)?
    Why is it that I see a voltage reading between a negative and positive terminal of a single battery and yet I do not see a voltage reading between two positive terminals of two batteries with different positive voltages?
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Need to be a bit more specific with your description, here. I assume you mean connect the meter across the positive and negative terminals of a single battery, and not connect the positive and negative terminals of a single battery together with a wire.

    Let's step back and talk about the two battery situation with the meter connected between the positive terminals. What I said before about them being floating relative to each other bears a little bit closer examination. If you were to use a sensitive electric field strength meter and map the space between the two batteries, you would almost certainly find that you could actually calculate a small voltage difference between the two. You could then calculate how much charge would have to be moved from one to the other in order to cancel out this voltage. When you connect them together (via a wire or via a large resistor called a DMM), it takes almost no time for that tiny amount of charge to flow from one to the other and that places both of them at the same potential, a situation that you could confirm using your sensitive electric field meter to remap the space between them and see how it has changed.

    But with the two terminals of the same battery, there is an electrochemically active substance connected between the two terminals that maintains the voltage difference between the two terminals. This chemical reaction happens spontaneously as long as the potential difference between the two terminals is less than some value (the battery's nominal voltage) but the reaction increases the voltage difference and once the difference reaches the nominal voltage the reaction stops. If you move charge from one terminal to the other (electrons from the negative terminal to the positive terminal), the voltage difference does drop between the two terminals, but now the chemical reaction starts again and the terminal voltage is restored. Not surprisingly, this is a very simplistic explanation, but the broad brush strokes are there.

    Again, the +3V means ONLY that the positive terminal of THAT battery is 3V higher than the negative terminal of THAT battery. Imagine a bunch of cubic blocks and each block has a value written on it. A block with a 2cm means that the top of the block is 2cm higher than the bottom of the block while a block with a 6cm on it means that the top of that block is 6cm higher than the bottom of that block.

    So what is the height difference between the tops of the two blocks? Before you say 4cm, think about the assumption you are making. You are assuming that the bottoms of the blocks are at the same level (or that the negative terminals of the battery are tied together).

    What if the 6cm block is sitting on a table in the basement and the other block is sitting on the floor upstairs? You have no idea because they are, in essence, floating relative to each other.

    What if we glue the two blocks together such that the tops are at the same level? Now the bottom of the 2cm block is 4cm higher than the bottom of the 6cm block. Now relate that to when you tie the positive terminals of the two batteries together.

    What if we stack one block on top of the other, which is saying nothing more than we are forcing the top of one block to be at the same height as the bottom of the other block? What is the voltage of the top of the top block relative to the bottom of the bottom block? Relate this to connecting the negative one battery to the positive of another.

    Hopefully you can answer that now, at least to some degree, for yourself.
     
    kubeek likes this.
  9. tiredtech

    New Member

    Mar 3, 2013
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    I just joined today. I'm going to love this place. All of the good theory aside, BReeves post is the easy way to show results. If you use a big enough battery you have a worm shocker if you need fish bait.
     
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