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Inductance and solenoid question

  1. Mar 17, 2014 #1
    1. The problem statement, all variables and given/known data

    I am designing a solenoid for a project to meet several specifications. I am attempting to induce an emf of approximately 2V in my circuit by moving a magnet in and out of the coil. I know the magnet has an approximate field strength of 1T. I am trying to determine necessary solenoid specs (#of wraps, length, radius) based on these values to generate the emf.

    However i am unsure how to calculate values such as change of flux over time and change of current over time needed to determine necessary coil characteristics.

    2. Relevant equations

    Faraday's Law of induction

    ε = -dΦ/dt


    L = μN^2A/L

    Self Induced Emf

    ε = -Ldi/dt

    Could the change of flux with respect to time depend based on how fast the magnet is moved in and out of the coil?

    Appreciate any insight!
    Last edited: Mar 17, 2014
  2. jcsd
  3. Mar 17, 2014 #2

    rude man

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    Gold Member

    Yes. That's what emf = -N d(phi)/dt says. Phi = B x area of coil. That goes for every winding in your solenoid. The total emf is the sum of the emf's for each winding since the windings are all in series. You will have to determine dB/dt for every winding. B is the field for a winding and dB/dt is how rapidly you're changing B by moving the magnet in & out of that winding. How were you thinking of moving the magnet?
  4. Mar 17, 2014 #3
    Thanks for the response. The magnet will be attached to a spring loaded platform moving down and up. The spring constant is quite high so the magnet should move at a decent speed
  5. Mar 17, 2014 #4

    rude man

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    OK. You can hope for a very approximate estimate of your emf with the magnet bobbing in & out, including magnitude and waveform shape. Don't expect a nice sinusoid! :-)
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