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Field Strength Of Pulse

  1. May 22, 2009 #1
    I'm new to the forum, so hello!

    How can i calculate the magnetic field strength of the pulse of an air core coil, the number of turns is 20 & the height of the coil is 20mm, not single or multi layered just random wound.
    The pulse is 10V & 1Amp, the Inductance is 200uH.

    Not sure how to go about this?
  2. jcsd
  3. May 22, 2009 #2
    There are several web-based inductance calculators, but they all give lower inductances than you measured. Once the inductance can be calculated approximately, the B field inside the coil can be estimated. You did not provide the coil diameter, which would be useful.
    For now, the very approximate B field inside the coil using the solenoid formula is

    B =u0 N I/z Tesla, where
    u0 = 4 pi x 10-7 Henrys per meter
    N = 20 turns
    I = 1 amp
    z = 0.02 meters

    So B = 1.25 x 10-3 Tesla = 12.5 Gauss

    One can also use the short coil formula from integrating the Biot-Savart Law yielding

    B = (u0/2) N I/R (where R = radius)
    = 1.25 x 10-3 Tesla = 12.5 Gauss using R = 1 cm (0.01 meters)
  4. May 22, 2009 #3
    Thanks Bob S,

    The coil Radius is 150mm--300mm Diameter, i am very interested to know how the diameter affects the field strength.

    I see you mention the B field "inside" the coil, is there a way to calculate the strength of the B pulse at distance away from the coil, say 25mm 50mm 75mm etc?
    Thanks Again, your post is very usefull!
  5. May 22, 2009 #4
    Sorry Bob


    I am missing something,
    u0 = 4 pi x 10-7 Henrys per meter, = Permeability of Air------Correct?

    Say L Inductance is 300uH or 400uH instead of 200uH how does L fit into the equation?
    Last edited: May 22, 2009
  6. May 23, 2009 #5
    The integral of the Biot-Savart Law for a circular loop relates the current in a current loop of radius R carrying a currrent I to the magnetic field anywhere along the axis of the loop.

    B = (u0/2) I R2/(x2+ R2)1.5
    where units are meters, amps and Tesla. x is the distance along the axis from the center of the loop. uo is the permeability of free space.

    Use the web calculator http://www.66pacific.com/calculators/coil_calc.aspx
    to calculate inductance.
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