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Resistors in series - how can current be constant?

  1. Nov 20, 2012 #1
    I can't get my head around this:

    How can the current in a series circuit be unchanged by any amount of resistors?
    1) I understand that there is only one path, and thus;
    2) I understand that the charge isn't used up.

    BUT. If current is the charge passing a point per second, and resistors are something that 'resist' the charge or slow it down, how can the central question be explained?

    Let me explain my reasoning. Take a roundabout and 10 cars, one at the start line and the rest behind each other. Let's say that, when they begin moving, (no acceleration, just constant speed) 5 get to the half way line per second. If I throw in a speed bump a quarter of the way round, this will decrease the cars' speed, and so less cars will pass the half way line per second. The current is decreased?

    I've gotten myself thoroughly confused; can anyone explain this simply? Are my definitions wrong, or my analogy, or something?
     
  2. jcsd
  3. Nov 20, 2012 #2

    Doc Al

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    Staff: Mentor

    Why do you think that adding resistors in series doesn't change the current (for a given voltage source)?
     
  4. Nov 20, 2012 #3
    http://www.youtube.com/watch?v=u-7k8wpl_ZU&list=PLBCCF1129FCAE29DF&index=12&feature=plpp_video

    There are three resistors in a series circuit. The video states that the current through the resistors is the same, or 'I = I1 = I2 = I3'.

    EDIT: Ah, I see what you mean... so, the overall current is reduced, but the individual current readings through the resistors are equal to each other and the overall current?.. Sorry, I'm just writing my realisations..

    If a current is 2A without any resistors, adding two will decrease the current, to say 1A, and this will be the current through the resistor. Is that correct?
     
    Last edited: Nov 20, 2012
  5. Nov 20, 2012 #4

    jtbell

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    The spacing between the cars also decreases.
     
  6. Nov 20, 2012 #5

    Doc Al

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    Sure, the current is the same throughout. (But adding additional resistance in series changes that current.)

    If the current were not the same everywhere, that would mean somewhere electrons are building up. If they were to do so, electrostatic fields would quickly be created to stem that flow so that it is evened out.
     
  7. Nov 20, 2012 #6
    The overall current is reduced, but the individual current readings through the resistors are equal to each other and the overall current?..

    If a current is 2A without any resistors, adding two will decrease the current, to say 1A, and this will be the current through the resistor. Is that correct?

    Ah! Didn't think of that. Thank you, sir.
     
  8. Nov 20, 2012 #7

    Doc Al

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    Right.
    I'd phrase it like this: If a current is 2A and you add resistors in series to double the total resistance, then the current will drop to 1A and that will be the current through the resistors.
     
  9. Nov 20, 2012 #8
    I see. Thank you very much for your time. Circuits have been troubling me for quite a while.
     
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