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Shock from automotive battery

  1. May 26, 2015 #1
    It is said that with dry hands, you can touch the poles of a car battery and it will not shock you. I understand this is because dry hands have a very high resistance.

    Now if you have wet hands, or even worse if you have needles stuck into your skin that are attached to the battery posts, this should reduce the resistance and could shock you.

    Someone told me that the second scenario (wet hands, or needles in fingers) would not be dangerous. They cited the example of putting a small 9V on your tongue. In this case you only feel a slightly uncomfortable buzz on your tongue. Therefore, because a car battery has only slightly higher voltage, it would be only a slightly worse than this and not deadly.

    Does the fact that the car battery has so much more capacity than the small 9V not affect this situation? If you connected an automotive battery across your tongue the same way as the small 9V, would it be only slightly worse of a shock?

    If this is true, then I can see how the important factor is just voltage and not the amount of energy available.
     
  2. jcsd
  3. May 26, 2015 #2

    phinds

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    Current capability is irrelevant to your question. You could put a the terminal of a 1000 amp, but 3 volt, electric welder on you tongue and not feel a thing. It's the voltage that matters, assuming at least a small current capability so 1000 volts with the capability of delivering something like 80 milliamps will kill you but a 50,000 volt Van De Graff generator will just make your hair stand on end.
     
  4. May 26, 2015 #3

    davenn

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    only partly

    in a PP9 9V battery, the 9V potential can only a small amount of available current
    A 12V car battery can supply 100's of Amps ... across a low resistance connection ... spanner handle. wet conductive skin or its sub layers
    significant current can flow


    the 9V battery across dry skin ... nothing ... it cannot overcome the high resistance, across the wet tongue good current flow as can be strongly felt
    I would consider it stupid and dangerous to try that with a 12V car battery ... its likely to result in significant burns

    Dave
     
  5. May 26, 2015 #4
    Ok thanks. One thing I found confusing is in circuits class, there would be circuit examples with a constant current source instead of a constant voltage source. So that had me confused about what is the driving force in a circuit.
     
  6. May 26, 2015 #5
    What about the danger of wet hands on a car battery? That was my original question to the person I am discussing it with. Wikipedia suggests wet skin could reduce resistance to 1000 ohms. That gives 12 mA across whatever path the current takes, and that seems dangerous to me. He maintains that in general, a car battery has too low of a voltage to pose any real threat to a person even with wet hands (I am referring strictly electrical danger, not danger from shorting the terminals with something metal and getting burned), and that the energy stored is irrelevant.
     
  7. May 26, 2015 #6

    davenn

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    That is possible ... Im not willing to try. you need enough voltage to overcome the skin resistance
    from just a surface of the skin point of view its not an issue for once outside that wet area you are back to normal
    skin resistance between the hands. IF the moisture has had time to penetrate the skin layers on each hand and provides a low resistance path
    to the inner flesh etc then you will be at a point where there is a low resistance path between the hands and its possible that you will receive a shock and possibly burns

    its not irrelevant once the skin resistance breakdown voltage has been reached
    and reducing the resistance, reduces the breakdown voltage value

    Dave
     
  8. May 26, 2015 #7
    Car batteries are completely safe to touch. Your body has too high of an internal resistance to cause any dangerous current.
    Only at higher voltages does it get dangerous.
     
  9. May 26, 2015 #8

    Nugatory

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    Real physical constant current sources work by varying their voltage to maintain a constant current. For example, if I connect a 1 amp current source across a potentiometer dialed to ten ohms, the source will adjust its output voltage to ten volts and one amp will flow. Turn the potentiometer down to 5 ohms, and the source will notice that the current is increasing and adjust its voltage down - it will stabilize at five volts and one amp pretty quickly.
     
  10. May 26, 2015 #9
    Ah, ok. All that information was hidden behind a little circle symbol that said "5 amps" (or maybe I didn't read my text book thoroughly).
     
  11. May 26, 2015 #10

    marcusl

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    The curent path also matters. Passing current from one side of your tongue to the other is unpleasant but harmless, while passing the same amount of current through your heart is dangerous. It is good practice when working around electrical wiring to keep your left hand behind your back so there`s no chance of passing current arm-to-arm (and through the heart).
     
  12. May 26, 2015 #11

    phinds

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    That's going to be a good trick. I usually find I need one MORE arm, not one less, when dealing with wiring/connectors.
     
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