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Conducting wire and moment

  1. Jul 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A conducting wire runs directly over a horizontal compass in the north-south direction. When a current of magnitude I passes through the wire, the compass needle swings 20° to the west and stays in that position.

    When a current of 2I passes through the wire, the compass needle swings α° to the west and stops. What condition does α fulfill?


    2. Relevant equations
    I used M(couple moment) = BIANcosθ 


    3. The attempt at a solution
       I am not sure that at does the moment between the 1st event (when I passes) and the 2nd event (2I passes) equal???
    I guess it is not because I my answer is wrong when I assume like this... But what is the relationship between 1st and 2nd event for solving this problem...

    Help is appreciate..
    Thanks in advance :)
     
  2. jcsd
  3. Jul 13, 2012 #2

    TSny

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    Gold Member

    The compass needle points in the direction of the net B field which is the vector sum of the field of the earth and the field of the wire.

    Think about the direction of these two fields and draw a sketch showing their vector sum. Make a similar sketch for the case where the current is doubled. Identify the angle of 20o in the first sketch and the angle α in the second. Use an appropriate trig function to relate the two angles.
     
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