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Homework Help: What exactly is current, and how come it is constant in a circuit?

  1. Jan 19, 2010 #1
    1. The problem statement, all variables and given/known data

    This is not exactly a homework problem, but it relates to homework problems. I have trouble understanding why exactly current in a circuit is the same everywhere.

    Current is the rate of flow of charge, or "charge per second". Why wouldn't charge flow faster in some areas and slower than other areas? Why wouldn't passing through a resistor slow down the charge? Why wouldn't the charge speed up when the resistance decreased?

    2. Relevant equations

    I = V/R

    3. The attempt at a solution

    In the equation, if the resistance is lower, current should be higher. If there is variable resistance in a circuit (different types of wires say, or resistors), why wouldn't current be different in different places?
  2. jcsd
  3. Jan 19, 2010 #2


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    The whole circuit "feels" the effect of that resistor. It's kind of like a traffic jam. You can't have electrons all flowing at different rates and not expect collisions. These "collisions (more like coulomb interactions)" make sure that the current within a circuit is the same everywhere within that circuit.

    If you had a circuit the size of light years, this may change...but usual circuits are small, and the effects are nearly instantaneous.
  4. Jan 19, 2010 #3
    I can sort of see it, but not quite. In a traffic jam analogy, shouldn't the cars be able to theoretically all speed up together and increase the rate of flow? Or all slow down together and decrease the rate of flow?

  5. Jan 19, 2010 #4


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    Here's a way of thinking about it. Suppose that at point A of a circuit, the current is I. Suppose that at point B, which is downstream from A, the current is I+0.1. That means that the wire between A and B would gain 0.1C of electrons every second. This can't happen, or else the wire segment would eventually have infinite charge.

    To continue the traffic jam analogy, suppose cars can't leave the road or pass in between other cars. If 3 of the cars try to go faster than the other cars, they'd collide with the cars in front of them. If they try to go slower, they'd collide with the cars behind them. The cars can all change speed together, just as the current in a circuit can change depending on its resistance.
  6. Jan 20, 2010 #5


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    Electrons can slow down in parts of a circuit, but wherever that happens they will be bunched closer together. The number of electrons passing by a point will be the same everywhere in the circuit loop, and that is the current.
  7. Jan 20, 2010 #6
    I would compare electric current in a circuit to the water flowing in a pipe.
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