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Elecricity problem

  1. Aug 2, 2006 #1
    A conducter with two diameters d1 and d2 (one single piece), carries a current.having lengths L.
    What is same
    n=free electrons in one cubic mtr
    e=constant electronic charge=1.6X10^-19columbs
    Vd=drift velocity


    I solved as

    Let total electrons in rod A be N
    therefore I=N*e*Vd*A/A*L
    Let total electrons in rod B be N'
    Therefore I'=N'*e*Vd*A'/A'*L
    THEREFORE I is not equal to I'
    BECAUSE DRIFT VELOCITY REMAINS CONSTANT
     
  2. jcsd
  3. Aug 2, 2006 #2

    berkeman

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    Staff: Mentor

    I'm having a hard time understanding the question. Is it asking about comparing the physical characteristics of current in two different diameter metal rods? If so, I don't necessarily agree that the drift velocities will be the same. If the currents are equal, then the number of electrons flowing past a cross section of the rod per second will be equal. A larger cross sectional area means that the electrons have to be moving slower in order to get the same number per second. What does the question ask exactly?
     
  4. Aug 2, 2006 #3
    This was a multiple choice question.to select one option out of four
    1.electric field current and drift velocity will be same
    2.Only current will be same
    3.Only drift velocity will be same
    4.None of the above
     
  5. Aug 3, 2006 #4

    berkeman

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    I'm still having to guess a bit at the question, but if it is a conductor formed by two rods in series (one rod larger than the other), then there is definitely just one correct answer out of the 4 you have listed.

    To help you figure out the answer, how is the drift velocity defined? How is the current defined?
     
  6. Aug 4, 2006 #5
    current is the charge passing through a cross section of a conductor per unit time
    and drift velocity is the distance travelled by the electron in unit time
     
  7. Aug 4, 2006 #6

    berkeman

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    Correct. So if the density of electrons per unit area stays the same, and you have more cross-sectional area, what has to happen to the drift velocity in order to keep the same current value in the two conductors? (they are connected in series after all, so the current value will be the same in both)
     
  8. Aug 4, 2006 #7
    drift velocity has to reduce
     
  9. Aug 4, 2006 #8

    berkeman

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    Correct. The current will be the same, since the two conductors are in series. Does that make sense? And since the current is constant, the drift velocity has to be less in the larger diameter conductor in order to keep the same number of charges passing by per unit time.
     
  10. Aug 4, 2006 #9
    so the corect answer is none of these. right?
     
  11. Aug 4, 2006 #10

    berkeman

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    What's wrong with -2-? Please summarize what is wrong or right about each of the 4 options.
     
  12. Aug 4, 2006 #11
    Sir,pl confirm
     
  13. Aug 4, 2006 #12

    berkeman

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    Staff: Mentor

    Confirm what? Can you please try answering my question:
     
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