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Question about my short circuit argument

  1. Jul 12, 2006 #1
    So I had an argument with a friend, more like a discussion where we disagreed.

    It led to some interesting questions, or at least they are to me.

    I made a simple electromagnet using a battery, a nail, and an insulated copper wire wrapped around the nail many times with the ends of the copper wire being bare and connected to the battery terminals.

    Now we finally agreed that this was a short circuit...but what the hell does that really mean? So what if you connect the two terminals on a battery?

    Now he said the heat that I felt on my fingers, since my fingers were holding the bare ends of the wire to the terminals (I had a glove on), were simply because heat was produced from the short circuit. I told him the heat was definitely due to resistance.

    What's the truth? Are we both right?

    So to sum it up, what was the true cause of the heat and what on earth does a short circuit mean?

    Similar question about the short circuit, why is it that when you put a (+) and (-) end of a live wire (used to do this all the time with a battery charger), that it sparks or just goes berserk?

    Thanks guys.
  2. jcsd
  3. Jul 12, 2006 #2
  4. Jul 13, 2006 #3

    Claude Bile

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    Science Advisor

    There are multiple definitions of a short-circuit, most of which are covered by the wikipedia link.

    An ideal short circuit is simply a zero-resistance connection between two nodes of a circuit. In practice though you rarely have zero resistance - thus connections which have a very low resistance are referred to as short circuits also, as they closely approximate an ideal short circuit.

    The reason you are feeling heat is because a low resistance connection will draw a large current. Because the wires have a small, but finite resistance, they will generate heat due to the current flow.

    The sparking you see with the live wire is due to the potential difference between the wire and earth (or ground). If the potential is high enough, or the distance between the wire and earth is small enough, the electric field becomes strong enough to start ripping electrons from the air molecules. This causes a temporary conductive path to form and charge will flow between the wire and earth causing the spark (or arc).

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