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Current in car jump leads

  1. Oct 1, 2014 #1
    When I connected my car with a flat battery to another working car I was told to attach the positive connection to the positive connection but I was told to attach the negative connection from the working car to a piece of metal on the car(preferably in the engine bay) instead of to the negative connection on the flat battery. I may have misunderstood but surely this breaks the circuit flow from battery to battery. If so then how does the flat battery become charged. Alternatively iof this does not break the circuit please explain why and also explin why it is that when you touch any metal on the car you don't get a shock because surely if there is a complete circuit it is flowing through metal in the car and should shock anyone who touches it
     
  2. jcsd
  3. Oct 1, 2014 #2

    anorlunda

    Staff: Mentor

    The battery negative terminal is connected to the car's metal frame, so electrically the frame is the same as the negative terminal.

    The reason you are advised to connect the negative jumper to the frame is that it may spark while connecting. The frame is farther away from the battery so that a spark is less likely to ignite any hydrogen gas that the battery might be emitting.
     
  4. Oct 2, 2014 #3

    CWatters

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    .....on the vast majority of cars that's true. On some old cars the positive is connected to the frame.
     
  5. Oct 2, 2014 #4

    CWatters

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  6. Oct 2, 2014 #5

    jbriggs444

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    http://mgaguru.com/mgtech/ignition/ig104.htm

    Polarity is important in the spark plug, but that polarity has nothing to do with whether the vehicle is positive earth or negative earth.
     
  7. Oct 3, 2014 #6

    CWatters

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    Thanks for that. It hadn't occurred to me that the temperature of the two electrodes in the plug might be different. Makes sense.
     
  8. Oct 3, 2014 #7

    sophiecentaur

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    I think that quote of yours:

    "An additional advantage of positive earth was that a sparking plug sparks better with the earth electrode positive."

    was stated the wrong way round for making the point. It is surely the improved thermionic emission from the hotter, negative centre pin that would help with the spark.
     
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