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Simple DC voltage question

  1. Mar 30, 2009 #1
    This is probably a very easy question but I have been wondering for a while and can't seem to find a definitive answer anywhere.

    If you touch one hand to the positive lead on a 12v car battery and are grounded on something not connected to the negative terminal on the same battery, will you get a shock?

    My initial thought was that if you are standing on the ground which has a voltage of around 0(correct?) and initiate a potential difference by touching the 12v lead, current will result which produces a shock. This just doesn't seem right though for some reason. Please clarify this.

    Thanks.
     
  2. jcsd
  3. Mar 30, 2009 #2

    mgb_phys

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    Without a circuit there is no current and you can't get shock.
    It's wrong to say that the potential of the positive terminal of a car battery is 12V without saying relative to what. It's only 12V relative to the negative terminal.

    Relative to the ground the battery could have any voltage you like. You can see this sometimes with static on a car when you touch the '0v' body of the car you get a shock because the bodywork is actually at a much higher voltage than the ground.
     
  4. Mar 30, 2009 #3
    mgb's answer is technically just fine....but it also helps to note that tires on an auto do a good job of insulating the car from ground....hence on dry days you can get a shock just climbing out of a car via static electricity without going near the 12 volt terminal.

    Typically in a boat, where negative IS firmly grounded, tt's difficult to get a shock from 12 volrs dc because skin to terminal is generally not a good contact and because the body has a fairly high resistance...I forget typical figures....to the extent that amperage is very,very small...and you need some POWER to feel a shock....
    but I think I have gotten a tingle when I have had damp fingers working on positive wires in a boat....whatever it was I have jumped my hand away involuntarily.....without any damage....
     
  5. Mar 30, 2009 #4

    Lok

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    U could get a very small charge upon you, that is a very small current while you charge just like a capacitor. Given the high resistance of the body and the large distance from the other battery pole, and the very small cross-section of the poles, you will not feel anything. Even with a perfect grounding connected to your feet, as the same thing applies. As mgb-phys already said you would need a circuit.
     
  6. Mar 30, 2009 #5
    Ok, I have a decent understanding of how a potential difference occurs and what I meant to say in my post was touching the positive terminal on the battery and being grounded to the earth's surface which has a potential difference of about 12v relative to the terminal (right?). But you indicated that there needs to be a complete circuit in order for the flow of current to occur. When you touch a power line while you are in contact with the ground you die, so how do these not correlate?
     
  7. Mar 30, 2009 #6

    mgb_phys

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    No the battery has an unknown voltage relative to Earth.

    The other end of the power line is connected to Earth at the power station
     
  8. Mar 31, 2009 #7

    Lok

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    Powerlines use AC current not DC. The difference is that Direct current tries to push electrons towards the other pole, while Alternative current oscillates the electrons in almost the same place. So DC needs a closed circuit, While AC can shock you through grounding. When you touch a powerline and ground, you act as the conductor of the oscillation. You can get a shock from powerlines even without grounding, as a tiny amount of current flows through you back and forth.
     
  9. Apr 1, 2009 #8
    A battery is a sorta miser when it comes to giving electrons.


    It does not give electrons unless get any, so thought the P.D will exist, the batt will not release electrons.

    Am I right?
     
  10. Apr 1, 2009 #9

    Lok

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    A battery will give away electrons and take a few if u connect it to a capacitor, so it is not imperative to have a closed circuit for that to happen.
     
  11. May 1, 2009 #10
    You will not feel the shock even if you touch the two terminals of the battery because the current through the body depends on body resistance and the applied voltage across that. Since our body is having considerably high rerstance you will not feel any shock while you touch two terminals directly. But you can experience shock by following two manners.
    1. If you are having any cut mark on your hand or if you touch the terminal by some wet and soft portion of your body (e.g. Tongue or leaps) you can feel shock even with 1.5V battery (i.e. the less resistance across the circuit helps in flowing more current )
    2. For getting shock in hand minimum 85 Volt is required across the body. That is why in Electric Arc welding Voltage applied across the electrode is limited to 20volts to avoid shock.(i.e. More voltage helps to flow more current in the circuit even if the resistance is constant)
     
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