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Is current reduced in a resistor?

  1. Dec 22, 2012 #1
    Everyone knows the classic series circuit rule that states that current is the same everywhere, but is this also true in the resistors or a load, like a light bulb filament? I would think that the resistance caused by the electrons smashing against the atomic nucleii in the conductor would cause a reduction in current. The reduced electron velocity would be transfered to the nucleii in the conductor, causing heat.
     
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  3. Dec 22, 2012 #2
    Resistance does cause a reduction in current but the effect is for the entire circuit. It is a similar effect to water in a pipe. If I put a restriction in the pipe I will reduce the water flow but I cannot have more water flowing toward the restriction then away from it because the water would have no place to accumulate.
     
  4. Dec 22, 2012 #3

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    Hi infomike!

    With any component, the electrons that go in must be equal to the electrons going out (on average).
    If they didn't, charge would heap up, which is not something that can go on indefinitely.

    So if electrons were diverted somewhere, other electrons will have to jump in, effectively making sure that the number of electrons that leave a power source is the same as the number of electrons that return to a power source (on average).

    And yes, you will have losses in the form of heat, so those losses will have to be compensated by energy supplied by the power source, and electrons may have to be pulled in from somewhere else.
    No one said that each electron has to make a full circuit.
     
  5. Dec 22, 2012 #4

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    The total number of electrons flowing into the filament per unit time must be equal to the number of electrons flowing out; otherwise charge would build up in the filament. So the current out is the the same as the current in.

    You are right that the electrons want to slow down as they interact with the atoms of the conductor (not the nuclei - they don't get anywhere near the nucleus, but that's not relevant here). But if an electron slows down, the electron behind it gets closer and the one in front moves farther away, and as electrons repel one another the effect is to push the laggard forward harder. So the electrons keep on moving through even as energy is transferred to the atoms of the filament.
     
  6. Dec 22, 2012 #5
    So would it be accurate to say that the velocity or energy lost by the electrons in a resistor is responsible for the voltage or potential drop across the resistor?
     
  7. Dec 22, 2012 #6

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    Yes.
    The energy loss is the voltage drop times the charge transmitted.
     
  8. Dec 27, 2012 #7
    Re-think your question from the perspective of charge conservation: If a certain amount of charge enters the resistor, it cannot go extinct, also not by collisions to nuclei (if we disregard nuclear reactions for the moment).
    Of course, the current has a cause: an electric field aka a voltage difference.
    Then, by definition, the (ohmic) resistance determines the current via I = U/R assuming a lot of things like linearity etc. But that's just how we define an ohmic resistor - it cannot have a different behaviour, see also here.
     
  9. Dec 27, 2012 #8
    Thanks to all for all the good responses.
     
  10. Dec 28, 2012 #9
    That's true, but I was refering to the current in a resistor, separate from the current in the rest of the circuit. In your example, the increased resistance will reduce the current everywhere in the circuit. I had mistakenly thought that current could be different in a resistor as opposed to the rest of the circuit.
     
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