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Finding induced emf of solenoid

  1. Mar 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A 800-turn solenoid, 18 cm long, has a diameter of 2.9 cm. A 15-turn coil is wound tightly around the center of the solenoid.
    If the current in the solenoid increases uniformly from 0 to 3.1 A in 0.28 s, what will be the induced emf in the short coil during this time?

    2. Relevant equations


    3. The attempt at a solution
    I keep getting the wrong answer. Originally I just plugged in 800 and 15 for the n's, but then I used turns per meter. Both were wrong. I think I may be using the wrong equation. Any help would be appreciated!
  2. jcsd
  3. Mar 5, 2009 #2


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    Hi number14,

    What did you do to get your answers? (what equations did you use, and what numbers did you plug into them)
  4. Mar 6, 2009 #3
    I used B=(mu_0 *N*I)/l delta_flux = B*pi*r^2 and
    Emf=-N * delta_flux/delta_t

    N=15, I=3.1, l=.18, delta_t=.28 , r=.0145

    and I got: 3.6 * 10^-5
  5. Mar 6, 2009 #4


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    I'm not getting that answer from using those numbers. What are you getting for just the B part by itself?
    Last edited: Mar 6, 2009
  6. Mar 6, 2009 #5
  7. Mar 6, 2009 #6
    Wait, this time i got 1.2*10^-5 . I'm using a calculator that is very finicky.
  8. Mar 6, 2009 #7


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    I think here you are using N=15; but the B is the magnetic field from the solenoid, which has N=800.
  9. Mar 6, 2009 #8
    thanks so much for the help, i got the right answer finally!!!
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