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Question on current in series

  1. Aug 20, 2015 #1
    Take this question as an example ImageUploadedByPhysics Forums1440066974.082733.jpg
    They asked for the current flowing through the bulb and the answer is 6A
    I know that it's 12V divided by 2 ohms(2 ohm is the resistance of resistor X) , but why? I thought the current is supposed to be 12V divided by the resistance of bulb and resistor X(total resistance of resistors) since its arranged in series?
     
  2. jcsd
  3. Aug 20, 2015 #2

    olivermsun

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    As stated, the voltage across the resistor X is measured at 12 V, so the other parts of the circuit shouldn't matter.
     
  4. Aug 20, 2015 #3

    NascentOxygen

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    Staff: Mentor

    Yes, current would be 12/(rtotal), where rtotal is the sum of the series resistances.

    I'm not able to agree or disagree with you on the correctness of the textbook's answer, because you haven't disclosed the exact wording of the question. The figure you selected to use as an example is not a particularly good choice---because it examplifies a fundamental wiring "mistake".
     
  5. Aug 20, 2015 #4

    davenn

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    for a idea circuit, yes you could say that. But if it was a real world circuit, that is no longer true. The light globe will have significant resistance
    which will change as it heats up

    indeed

    @Lim Y K, can you recognise the problem ?


    Dave
     
  6. Aug 20, 2015 #5

    olivermsun

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    I realize that is the case for a real incandescent lightbulb, but since the heated filament resistance, wasn't given in the problem, I'm assuming that the student isn't expected to incorporate that information in the solution.
     
  7. Aug 20, 2015 #6

    davenn

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    maybe but they need to be informed that it isn't like that in the real world and you didn't give that clarification :smile:

    the poor guy is already getting confused enough with incorrectly drawn circuits ....
     
  8. Aug 20, 2015 #7

    NascentOxygen

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    Staff: Mentor

    This is not the case in the figure I'm seeing.
     
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