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Batteries, Ground & Static Electricity

  1. Aug 19, 2009 #1
    Hi all,

    If I take a battery (lets assume a car battery) and connect the +ve end to Earth Ground and leave the -ve end floating. Why is there no charge transfer from the battery to ground?

    I've read up on a variety of threads regarding this question on this forum. Haven't quite found my answer yet. Here is my confusion:

    I understand that you need a close loop back to the battery -ve for the circuit to be complete. But what I don't understand is why (when the battery is wired up as I mentioned) there is no charge transfer as we see in static discharges. I mean if the battery +ve terminal has more protons than the Earth Ground, should I not see some charge transfer? Some people might respond by saying the the battery +ve terminal is at an unknown voltage potential to ground, etc. I don't really care about what potential it is as long as it's got a charge different from Earth Ground.

    I'm pretty sure the answer has something to do with electro-chemistry of the internals of the battery and that is probably what's confusing me. Please clarify or point me to the right direction. Thanks!
     
  2. jcsd
  3. Aug 19, 2009 #2

    Danger

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    I've had a lot of beers, so I might not be interpreting the question properly. If I am, however, the simple answer is that you don't have a constant conductive path between the terminals. Air is a very good insulator. Even if you touched the negative end to the ground, dirt is not really a great conductor. You have an open circuit.
     
    Last edited: Aug 20, 2009
  4. Aug 19, 2009 #3
    lets say I dig a deeeeep hole and put in a conductive metal rod. This will provide my Earth Ground.

    Now if I connect my +ve battery terminal to this wonderful metallic connection (while leaving the -ve terminal floating) ... will I have charge transfer?

    Our home power outlets ... their negative return line is grounded to Earth at the station in the same way as I mentioned ... deep hole with long metal rod. Am I correct or totally wrong here?
     
  5. Aug 19, 2009 #4

    Danger

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    Power stations are a different matter. If you could output anything like the power that they do, then your system would work and you would be rich. The main point here is that your opposite terminal is, in your words, 'floating'. That implies that it isn't attached to anything. You have to have something to complete the circuit.
    I also just noticed that the title of the thread mentions static electricity. That is an entirely different critter that has nothing to do with your question. You can scuff your feet on a carpet, touch a doorknob, and release about 100,000 volts of static. You might feel a millisecond or so of discomfort, but you sure can't run a motor from it.
     
  6. Aug 20, 2009 #5

    vk6kro

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    If you put the battery in a puddle on the ground, the plastic case would act as a dielectric of a capacitor with the battery innards as one plate and the ground as the other.

    You could charge this capacitor to 1000 volts by having a 1000 volt power supply between battery and the ground puddle. Why you would do this escapes me, but if you then removed the power supply, you would have a capacitor charged to 1000 volts.

    You could short out this capacitor and get a satisfying "splat" but the battery would not contribute any of its original charge to this. This is only available between the terminals of the battery, not to ground.
     
  7. Aug 21, 2009 #6
    I understand this. My question is:
    Why does the circuit loop have to be completed back to the -ve terminal for it to work? If the +ve end of the battery has a deficiency of electrons, why would the electrons NOT flow from Earth ground to the +ve terminal when connected? I mean I understand how static electricity works, but what makes it so different from the way charge is sent into or out of the batter? I mean isn't it just movement of electrons at the end of the day?

    Someone in another post in this forum mentioned something about ions inside the battery have to flow too, not just the electrons from the plate - therefore you have to connect -ve and +ve for the reaction in the battery to take place. I don't have a complete picture of the situation yet and am still pretty confused.
     
  8. Aug 21, 2009 #7
    I'm not so sure you do. Static electricity as an accumulation of charge, not necessarily a difference in potential. In other words, static electricity is a result of a collection of charged molecules (lots of electrons) coming into close proximity of molecules with neutral or positive (lack of electrons) charge. Electrons flow because because the source has more electrons than the sink. This is not what happens in a battery or any kind of electrochemical device.

    In a battery, current flows due to a potential difference or a difference in activation energy between materials (electrolytes and metals). In other words, there is no sink or source like with static electricity, only electrochemical reactions that can only take place if current has a path to travel. If you take away that path by leaving one of the electrodes floating then the reactions can't happen. The amount of electrons contained in a battery remains constant when current is flowing. This is would not happen if it operated on the same principles as static electricity. I would advise you to read up on how batteries work for a better explanation.

    http://en.wikipedia.org/wiki/Battery_(electricity)#How_batteries_work
     
  9. Aug 25, 2009 #8
    There is. It just doesn't last very long.
    The chemical reactions inside the battery make sure that there is always a certain potential difference between the two terminals.
    Let's say the battery has a voltage of 2V. Then that means there is always a potential difference of 2V between +ve and -ve. Even if you connect the battery to a 100,000V static generator. If you connect -ve to +100,000V then the negative terminal of your battery will be at 100,000V relative to ground and +ve will be at 100,002V.
    If +ve has a positive potential relative to ground and you ground +ve then electrons will flow into +ve until it is neutral and at the same time the battery pumps electrons into -ve until it is at a potential of -2V. No matter what you do to the battery - as long as it is not empty or shorted out, the potential difference between the 2 terminals always stays at 2V.
     
  10. Aug 30, 2009 #9
    In battery system, we usually forget the battery electrolyte media. Indeed positive and negative terms in battery are relative conceptions. In fact, in a separated battery bank after electrochemical stability, the charge transmissions between electrolyte media and both of two electrodes will be stopped and opposite transmission current will be impossible. After stability there are no any free charge in positive or negative pole and battery electrolyte, therefore each type of flow current between poles and electrolyte or poles and ground is impossible.
    For similar conceptual question you can refer to General elec. Riddle No.2 from http://electrical-riddles.com
     
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