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Simple lab calculation (Magneticfield vectors)

  1. Aug 1, 2009 #1
    1. The problem statement, all variables and given/known data

    I = 2.89amps, r = 0.035m, resultant vector angle (55), proporptionality constant (2 x 10^-7)

    2. Relevant equations

    B = K X I / r , trigonometry, vector divisions.

    3. The attempt at a solution

    Hi, I'm just pasting this off my lab to make sure I've done it correctly. Thanks in advance.


    A rod is put, parallel to the earths magnetic field (due north)with a compass needle 0.035m below it. A flow of 2.89amps is switched on and the compass needle deviates 55 degrees due east as the current creates a magnetic field of its' own.

    Question - Calculate the earths magnetic field density (B) using these vectors.

    Using the formula above the field density for the X component is

    (2 x 10^-7) x 2.89amp
    ------------------------------ = 1.651428571 x 10 ^- 5 Tesla

    To find the resultant vector (B-res), I used

    Cos(35) = B-x/B-res, B-x/Cos(35) = B-res,

    1.651428571 x 10 ^- 5 T

    ----------------------------------- = B-res

    B-res = 2,016022035 x 10^-5 T

    And finally with Pythagoras you can find the vector of the Y-component,
    \/B-res^2 – B-x^2 = 1,156342737 x 10^-5 T

    ~ 12microT
    Since the compass stays in the resultant vector (Bres),
    Last edited: Aug 1, 2009
  2. jcsd
  3. Aug 1, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks good. You could also use the fact that

    tan(35 deg) = ____ / ____ ?

    which gets you from Bwire to Bearth in a single step, without the need to calculate Bres.

    p.s. welcome to PF :smile:
  4. Aug 1, 2009 #3
    Thanks. :)
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