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Homework Help: Electrodynamics question

  1. Oct 1, 2004 #1
    I was working through the following problem:

    The negative termincal of a 12V car battery is connected to the car frame which can be regarded as 'ground', at a potential of 0V. What is the potential of the other terminal?

    The answer is +12V, but I cannot understand why. Could someone give me an explanation to this?

    Also, what does it mean that the car frame can be regarded as 'ground'?

    Any help would be greatly appreciated, I'm so lost!
  2. jcsd
  3. Oct 1, 2004 #2

    Chi Meson

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    The 12 V battery actually has a negative terminal at -6V and positive terminal at +6 volts. The 12 is actually the total difference in the potential between the two poles. Since potential difference is all that really matters, it is easier to simply call the negative "zero" and the positive "+12."

    In a dc circuit, "ground" refers to the lowest potential of the circuit, which means the negative end of the battery. The conventional current of electricity "flows from high to low potential." ( I hope you already know that the actual flow of electrons is from negative to positive, but we still talk about "conventional current")

    Therefore, in a car, you don't really need to run two wires to every electrical device, since the car's metal frame (which is attached to "ground") can be the "return wire" from practically every part of the car.
  4. Oct 1, 2004 #3
    Thanks for explaining that!
  5. Oct 1, 2004 #4


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    Open the hood of your car, look at the battery, and trace where the black wire goes. There's nothing but copper wire between the negative terminal and the car frame (virtually no resistance). Therefore, both have the same voltage level. This saves a lot of money on wiring. The ground wire from all your electrical equipment simply runs to the car frame.

    By the way, this is why you're always supposed to disconnect the negative (black) cable first when removing the battery. If you accidentally touch a piece of the engine or the frame with your wrench, nothing happens since all are at '0' volts. If you disconnect the positive (red) cable first and accidentally touch the frame, you have a 12 Volt difference with no resistance. Very exciting experience. Your tool has a permanent burn mark, as does whatever piece of the car you touch (in my case, the radiator cap). If you panic and leave the wrench touching both the terminal and grounded frame, your battery could explode.
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