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Propagation sequencing in series circuit?

  1. Jan 30, 2009 #1


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    This came up at mythbusters, a question about the propragation of currrent from a switch that is a long distance from a DC power source. Assume the wires are sufficiently large (capcitance wise):

    1 mile long wire connected to positive terminal of DC power source at one end, open switch at other end, with DC LEDs in series as part of the wire

    1 mile long wire connected to negative terminal of the same DC power source as above at one end, to the same open switch as above at other end, with DC LEDs in series as part of the wire.

    Assume that the state of the system has stabilized, and an idealized DC power source. The postive side of the switch has positive potential (insufficient electrons), the negative side of the switch has negative potential (excess electrons). Now close the switch.

    Does the potential at the switch result in immediate current flow at the switch, propagating back via both wires to the battery, so that the LEDs light up in sequence at the rate of propagation?
  2. jcsd
  3. Jan 30, 2009 #2
    As you might know, the current don't travel very fast in a wire, but the electromagnetic fields around the wire move near light speed. When the fields change in some part of the wire, the local current changes. So the electrons don't move very fast, but the local current changes do. These changes in current propagate just as fast down a wire as the surrounding fields.

    Anyway, if the two legs of the loop are isolated from one another, so that their fields don't overlap much, a signal propagates down both wires, originating at the switch, back to the battery.

    How did the electrical engineering geniuses at Mythbusters manage to measure when an LED turned on?
  4. Jan 30, 2009 #3


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    They didn't it was just an abstract exercise.
  5. Jan 30, 2009 #4
    I wonder how sharp an edge one could get out of closing a mechanical switch. Compared to the speed of light, it happens very very slowly. Then, some current will flow before contact, as the capacitance of the switch decrease before the current punctures the dialectric, or whatever it is that happens.
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