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Homework Help: Reply quickly, quiz in a few hours, very simple magnetic field question

  1. Aug 2, 2008 #1
    1. The problem statement, all variables and given/known data

    At a certain location, Earth has a magnetic field of 0.70 10-4 T, pointing 75° below the horizontal in a north-south plane. A 8.5 m long straight wire carries a 12 A current.
    (a) If the current is directed horizontally toward the east, what are the magnitude and direction of the magnetic force on the wire?

    horizontal and directed due west
    15° above the horizontal in the northward direction
    15° below the horizontal in the southward direction
    75° below the horizontal in the southward direction
    75° above the horizontal in the northward direction
    horizontal and directed due east

    2. Relevant equations

    (Force) = (BsinΘ)(I)(L)

    In English...(magnetic force) = (magnetic field perpendicular to current)(current)(length)

    3. The attempt at a solution

    I know the answer, it is...
    F = (0.70 10-4 sin 90) (12 ) (8.5) = (7.14 e -3) N
    15° above the horizontal in the northward direction

    But I don't understand that the ( ' sin 90 ' ) bit. The magnetic field is 75° below the horizontal and the current is directly on the horizontal. So shouldn't it be sin 75°? I don't understand how can I apply the right hand rule 1 to this situation?

    I don't understand what the question is asking, is the magnetic field going into the page (if so, then the force being 15° above the horizontal makes sense).
  2. jcsd
  3. Aug 2, 2008 #2
    OK,... i hope this be helpful before your quiz!

    the point is the B field is North-South but the wire is East-West... so take a paper as your horizon and draw a line on it as your wire in East-West. now take your pencil and push it trough the middle od the line and hold it 75 degree North South.... what do you see??? the line and the pencil are 90 degree apart! because one is N-S and the wire is E-W... the B field is making 75 with the paper... but who cares!!! we want to use the angel between the wire and the B field...

    so sin(teta)=sin(90)=1

    about the direction if F:

    keep the same paper...! ;) have your 4 fingers in the direction if current on the wire ... not rotate your hand slowly to be able to close your fingers in the same direction as B field rotating... (which is inside as you said) so your tumb is heading North!!!!......

    good luck!
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