electrical shock question

Discussion in 'Homework Help' started by fran1942, Jul 19, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
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    Hello, regarding an auto chassis ground system.
    If a person touches the chassis whilst standing on the ground next to a car, why does that person not receive a shock ?

    I am thinking that the current will flow from the chassis through the person's body and into the earth.

    Can someone please explain why this does not happen ?

    Thanks kindly.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I actually receive frequently shocks, when I get out of the car , depending on air humidity, clothes I'm wearing... I use to touch the metal of the car key, and with the metal I touch the car chassis.

    If you were talking about the high voltage generated for spark plugs, you don't receive any shocks because it's a closed loop inside the car. from battery to high voltage coil to the spark plug through the motor block back to the battery...
     
  3. fran1942

    Thread Starter Member

    Jul 26, 2010
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    thanks, but doesnt your body also create a closed circuit between the chassis you touch and the earth you are standing on ?

    Thanks - I am just trying to grasp the concept here.
     
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    There are a couple of reasons:

    1) The chassis of a car is often ground. If you stand on the ground and touch the grounded car, there is no voltage difference, and hence no current flow.
    2) Some cars are positive grounded, which means the chassis is connected to the positive terminal of the battery. This way there is a voltage difference, but 12v is generally not enough to cause an electric shock. If you were wet or had a cut, were grounded, and came in contact with the chassis of a positive-grounded car, you could theoretically receive a shock.

    If you are talking about static electricity, as praondevou was talking about, you can get a shock. When it is cool and dry outside, static charges are very common.

    The ignition circuit is specially designed and insulated to prevent current from passing through the wrong parts of the car. If a spark plug wire was in bad shape and was near enough to the chassis, you could definitely get a shock.

    Der Strom
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    If the ignition system was your question have a look at this picture from wikipedia. [​IMG]

    In this example the current goes from the spark plugs through ground back to the battery. No way you can get a shock here touching the car's chassis.

    Totally non-related but interesting though, they say you're safe inside a car if a lightning happens to strike your car, because it's a Faraday cage. This is something I would rather not want to experience.:)
     
  6. DerStrom8

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    Feb 20, 2011
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    You have to remember, though, that there is a gap on the spark plug. This creates a LOT of resistance, so if the wire is worn and is close enough to something (smaller distance than on the plug), current will take the easiest path--through the part to the chassis. If you are close enough to the wire and are touching (or very near to) grounded metal, you could receive a shock.

    I have seen a car get struck by lightning before. Since the people inside were not grounded, the electricity flowed around the outside of the car to ground. The static charge, however, could fry sensitive components in, for example, the car's computer. This is what happened to this vehicle that I saw--It was struck by lightning which caused it to stop, but the people were unharmed.

    NOTE: Found this video that is very much like what happened to the car I saw: http://www.youtube.com/watch?v=lUUOdO6eEZA
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I see your idea, but in the case you described, even if the high voltage spark went to the chassis, the chassis is connected via a low resistance path to the minus pole of the battery...

    I think I'll have to test this out to prove my own theory, or maybe I ask my neighbor to give me hand.:D
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There is in fact usually no closed circuit, since the car stands on tyres which are pretty much insulating. You may discharge any accumulation of static charge on the chassis, but once that has happpened there would be no further current flow.

    Even if another part of the bodywork were grounded, for example by a static discharge chain, the potential difference between that point and your hand would normally be too small to drive appreciable current through the resistance of your body.

    The resistance between two points on a car chassis would typically be a small fraction of an ohm: to develop sufficient potential between them to give anyone a shock would require a huge flow of current.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You will create a lower path of resistance for the spark plug wire than the .070 inch spark gap of the plug, if you had one hand on the wire and one hand on chassis ground.
     
  10. Adjuster

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    Dec 26, 2010
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    AARGH! The OP might actually try that - and they could get a real hard belt if there was even the slightest chink in the insulation. Some modern lead sets are so shrouded that you might not get anything at all, but I would not be willing to try it.

    @fran1942: Please don't fondle ignition cables, they are inclined to bite!
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Accidental contact with spark plug wire is enough to remind someone of the lethal potential.

    Just remember this..... the a$$ you save may be your own.
     
  12. someonesdad

    Senior Member

    Jul 7, 2009
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    Everyone who works on cars sooner or later gets bit by a spark plug wire -- it's almost a rite of passage. It's an excellent example of negative feedback -- once you get one, you don't want another one.

    In the 80's I was on a trip back east and talking to my wife from my hotel room. I was looking through the screen door to the balcony (I was on the second floor) over the pool at the other side of the building and I saw a lightning bolt hit the building. It sounded like a gunshot because there was no acoustic dispersion at all. Scared the crap outta me. Lots of sparks, but not as 4th-of-Julyish as that van getting hit. On the plane the next day, I was talking to a lady sitting next to me who had lived there all her life (near Boston) and she said it was the worst thunderstorm she had ever seen.

    Since the charge/current is going to stay on the outside of a metal vehicle, I doubt it's static electricity that gets the sensitive electrical components -- I'll bet it's the induced voltages from the magnetic field from the large current flow. Lightning strikes can have currents in the range of around 10 to 100 kA.
     
  13. DerStrom8

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    Feb 20, 2011
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    Yeah, I guess I didn't phrase it very well earlier. What I meant was that if someone came close enough to he wire and were touching the chassis, they would receive a shock. However, if it sparked to the chassis, and you were touching the chassis, you would NOT get a shock (as you said :D).

    Feel free to test it, anyway :D:p
     
  14. DerStrom8

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    Feb 20, 2011
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    That is a good point. You are probably right--I didn't think about that :p
     
  15. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, I'll post the results later.. or maybe not [​IMG]
     
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