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Resistance & Current in a wire

  1. Jun 6, 2014 #1
    1. The problem statement, all variables and given/known data

    The resistivity of gold is 2.44 x 10-8 Ω m at a temperature of 20°C. A gold wire, 0.5 mm in diameter and 44 cm long, carries a current of 380 ma. The number of electrons per second passing a given cross section of the wire, is closest to:

    A) 2.4 × 10^18
    B) 1.2 × 10^22
    C) 2.8 × 10^14
    D) 2.4 × 10^17
    E) 6.3 × 10^15

    2. Relevant equations

    R=ρL/A

    3. The attempt at a solution

    R=(2.44 x 10^-8) (.44)/∏(.00025^2)
    R= .0547
    then i multiplied the resistance by the current (.38) to get .021 for a voltage.
    then i divided that by (1.6 x 10^-19) and got 1.3 x 10^17 which isn't right but is close to the answer which is A
     
  2. jcsd
  3. Jun 6, 2014 #2

    mfb

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    Staff: Mentor

    Why did you divide a voltage by the electron charge? Do you expect the right unit "per second" as result of that calculation?

    You are given the current. What does a current of 1 A (example value) mean?
     
  4. Jun 6, 2014 #3

    rude man

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    Something's off by a factor of 18.5 and you call that 'close'?

    Learn that people are going to give you problems with a lot of irrelevant information!
     
  5. Jun 6, 2014 #4
    so i need to use the fact that 6.241 x 10^18 electrons/sec pass through a circuit which is 1 A 

    when i use that number though i still have it wrong so i'm not really sure where else my calculations are messed up.
     
  6. Jun 6, 2014 #5
    But the current isn't 1 A.
     
  7. Jun 6, 2014 #6
    oh my goodness so most of the information in this problem is irrelevant? all i need is to do a proportion between 1A and the 38 mA in the problem?
     
  8. Jun 7, 2014 #7

    NascentOxygen

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    If that gets you closer to the right answer, go for it! :smile:

    380 mA would be even better ... :wink:
     
  9. Jun 7, 2014 #8

    mfb

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    Yes (apart from the typo (?) NascentOxygen pointed out).
     
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