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Magnetism - Quick solenoid question

  1. Jan 13, 2009 #1
    1. The problem statement, all variables and given/known data
    A 200-turn solenoid having a length of 25cm and a diameter of 10cm carries a current of 0.29A. Calculate the magnitude of the magnetic field B inside the solenoid.


    2. Relevant equations
    B = [tex]\mu[/tex]0ni


    3. The attempt at a solution
    Well, I plugged the numbers into the equation and got 7.28x10^-5 T as the answer. However, this is wrong. The correct answer is meant to be 0.3 mT. I can't see where I've gone wrong because the magnetic field inside a solenoid is meant to be independent of its dimensions, right?

    EDIT: I found the number of turns per unit length, then put that into the equation and still get the wrong answer. I'm confused.

    B = 4[tex]\pi[/tex]x10-7 Tm/A x (200/0.25) x 7.5A = 7.28x10^-5.

    I've no idea why I get that. Maybe the diameter has some significance that I haven't managed to see yet.
     
    Last edited: Jan 13, 2009
  2. jcsd
  3. Jan 13, 2009 #2

    Doc Al

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    Staff: Mentor

    Show exactly what you plugged in for each factor.
     
  4. Jan 13, 2009 #3

    Doc Al

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    Staff: Mentor

    ...
    Looks to me like you are using the wrong current.
     
  5. Jan 13, 2009 #4
    Oops. I apologise for wasting time. I used a current from a different question by accident :).
     
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