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Magnetic Field involving solenoids

  1. Apr 3, 2012 #1
    1. The problem statement, all variables and given/known data

    Two long ideal solenoids (with radii 20 mm and 30 mm respectively) carry the same number of turnes of wire per unit length. The smaller solenoid is mounted inside the larger, along a common axis. It is observed that the magnetic field within the inner solenoid is zero. The current in the inner solenoid must be:

    A. two-thirds the current in the outer solenoid
    B. one-third the current in the outer solenoid
    C. twice the current in the outer solenoid
    D. half the current in the outer solenoid
    E. the same as the current in the outer solenoid

    This one is hard. If you could please provide an explanation with the answer so I could better understand this.
    THanks in advance.

    2. Relevant equations

    n/a

    3. The attempt at a solution

    Right now I'm ruling out answer A.
     
  2. jcsd
  3. Apr 3, 2012 #2

    tiny-tim

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    Hi McAfee! :smile:
    Call the currents I and J.

    Hint: what is the formula for the magnetic field inside a current-carrying solenoid?

    And can we just add the fields?
     
  4. Apr 3, 2012 #3
    The formula is B = μ_o*i*n

    n is the number of turns
    and yes
     
  5. Apr 3, 2012 #4

    tiny-tim

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    yup! :smile:

    so what's the total equation, and is it A B C D or E ? :wink:
     
  6. Apr 3, 2012 #5
    20mm = .02
    30mm = .03

    assuming that I can ignore n because it will be the same for both.
    Would it be E because the radii doesn't play a role in the equation?
     
  7. Apr 4, 2012 #6

    tiny-tim

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    sorry, i missed your post :redface:
    yes the radii don't matter,

    so if (case E) the currents are equal (and opposite), then the magnetic field inside the inner solenoid will be zero :smile:
     
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