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Homework Help: Cancelling the Earths magnetic field

  1. Nov 12, 2012 #1
    1. The problem statement, all variables and given/known data
    A researcher would like to perform an experiment in a zero magnetic field, which means that the field of the earth must be cancelled. Suppose the experiment is done inside a solenoid of diameter 1.0{\rm m} , length 5.0{\rm m} , with a total of 9000 turns of wire. The solenoid is oriented to produce a field that opposes and exactly cancels the field of the earth.
    What current is needed in the solenoid's wires?

    2. Relevant equations
    We use this for B=μ0*I*(N/L) for solenoids.
    L=5.0 m

    3. The attempt at a solution
    I=(B*L)/μu0*N)=(5E-5*5)/(9000*5) = 0.02209=2.2E-2
    Mastering Physics says this is incorrect. What did I miss?
  2. jcsd
  3. Nov 12, 2012 #2


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    The formula for I looks good. Trouble with the substitution, I think. Looks like the number for u0 didn't go in and there is an extra 5.
  4. Nov 15, 2012 #3
    That was my bad. My actual equation looks like:

    (5E-5*5)/(1.257E-6*9000)=0.02209 which is wrong according to Mastering physics. Doesn't like 2.2E-2 either.
  5. Nov 15, 2012 #4


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    Earth's field varies greatly from place to place. Is this the value you were told to use?
  6. Nov 15, 2012 #5
    I talked with the instructor and found my error -- I just needed to adjust for units the question required. I usually catch this, but not this time. Thanks for your replies anyways. It's always good to know there is a safety line!
  7. Nov 15, 2012 #6


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    OK, I did wonder about the units. In your post it says e.g. "diameter 1.0{\rm m}". LaTex problem?
    PS - I guess you mean the units the answer was required in, like mA?
  8. Nov 16, 2012 #7
    Not sure what a La Tex problem is -- but you are correct on the PS
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