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Current reference direction confusion

  1. Jun 7, 2013 #1
    1. The problem statement, all variables and given/known data

    I have two questions.

    1) The ends of a length of wire are labeled a and b. If the current i from a to b is negative, are electrons moving toward a or b?

    2) We have a circuit element with terminals a and b. The voltage and current from a to b are both positive. If electrons carry the charge, which terminal do they enter?


    2. Relevant equations
    None, this is a conceptual problem.



    3. The attempt at a solution
    For question 1), I concluded that a negative current from a to b means that electrons flow from a to b. This makes sense and is the correct answer.

    For question 2), I concluded that electrons flow from b to a since the positive current goes from a to b, but this is not the answer in the book.


    Is the book wrong? Or am I missing something?
     
  2. jcsd
  3. Jun 7, 2013 #2

    berkeman

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    Staff: Mentor

    I agree with your answers, since electrons move in the opposite direction of the "positive current". But question 2 is worded a bit loosely, so maybe that is where the disconnect lies.

    The statement "The voltage and current from a to b are both positive." is not precise or correct. Voltage from a to b does not make sense. "Voltage drop from a to b" would be precise and correct. Maybe there is some strange voltage inversion because of the imprecise statement? Maybe it's a voltage source instead of a sink like a resistor?
     
  4. Jun 7, 2013 #3

    CWatters

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    Science Advisor
    Homework Helper

    For 2) I believe the circuit element must be a battery or voltage source.

    If "b" is the +ve terminal of the source and "a" the negative then it makes sense.

    The voltage a -> b is +ve.
    The current flowing within the source/battery is also from a -> b.

    ..so I agree with your answer. Electrons flow from b to a.
     
    Last edited: Jun 7, 2013
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