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I am calculating the following Biot-Savart integral according to the suggestion of @Ulrich Neumann in a previous post called Biot-Savart Integral:

(*p[\[Theta]] represents the path to map the curved portion of the \
saddle coil*)

p[\[Theta]_] := {d/2 Cos[\[Pi]/3 - \[Theta]], 0, 
   d/2 Sin[\[Pi]/3 - \[Theta]]};

(*FP is the final point at which I want to calculate the field.*)

FP = {x, y, z};
R = FP - p[\[Theta]];

(*The unit vector from R*)
ar = R/Sqrt[R.R];
(*Bx, By,Bz*)
{Bx, By, Bz} = \[Mu]0/(4 \[Pi])*
  Integrate[
   Cross[D[p[\[Theta]], \[Theta]], ar]/(R.R), {\[Theta], 0, 
    2/3 \[Pi]}, GenerateConditions -> False]

The formula implemented by the code is:

enter image description here

where the vector R connects the element of current to the final point:

enter image description here

(the \theta angle in figure is not the same present in the code) If FP is for example the point of coordinates FP={0,h/2,0} (centre of the saddle coil), the code returns the solution quite fast. However, If I set FP={x,y,z} to find the magnetic field at a generic point of coordinates {x,y,z} it runs for several minutes (~>10) and then returns the following answer:

{(1/(4 \[Pi]))\[Mu]0 Integrate[(d y Sin[\[Pi]/6 + \[Theta]])/(
   2 (y^2 + (z - 1/2 d Cos[\[Pi]/6 + \[Theta]])^2 + (x - 
        1/2 d Sin[\[Pi]/6 + \[Theta]])^2)^(3/2)), {\[Theta], 0, (
    2 \[Pi])/3}, GenerateConditions -> False],....}

which makes me thinking that Mathematica performs the derivative D function but it is not doing the integral. Is there a way in which I can perform the integral? p.s.: The code runs very smoothly and produces the correct answer even for FP={x,y,z} for the case for which the path is a linear segment of the form d[s]:=(1-s)A+sB rather than acurved path p[[theta]] as reported above.

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  • $\begingroup$ A few thoughts: (1) It seems entirely plausible to me that there is just not a closed form for this integral, which is why Mathematica can't find one. You could always use NIntegrate if numerical answers are OK. (2) Mathematica might be able to find the answer if you integrate over the full circle rather than just an arc, since there are various integration techniques that can work over a full circle but not an arc. (3) Providing Assumptions about the parameters d, x, y, z may help; by default, Mathematica will assume they're complex numbers... $\endgroup$ Commented Nov 18, 2021 at 16:30
  • $\begingroup$ ... (4) Is it intentional that you're integrating along an open curve rather than a closed loop? Using the Biot-Savart law for open curves is problematic. $\endgroup$ Commented Nov 18, 2021 at 16:30
  • $\begingroup$ The path is on the surface of a cylinder (A saddle coil). So I am breaking it down over the contribution of the curved and linear paths. I was looking, possibly, for a general solution which depends on the actual geometry of the coil which is basically the radius and height of the corresponding cylinder. $\endgroup$ Commented Nov 18, 2021 at 16:44
  • $\begingroup$ I was unfamiliar with the concept of a saddle coil, but from Googling I assume you mean something like this? With a geometry as complicated as that, I suspect you're going to need numeric solutions. $\endgroup$ Commented Nov 18, 2021 at 16:49
  • $\begingroup$ Yeah, that is precisely the saddle coil shape. By symmetry what I think I need is only the curved integral. My goal if I can find the expression B={Bx,By,Bz} is to calculate the flux of the magnetic field through the surface portion of the cylinder represented from the saddle coil. I have calculated the field B at the centre of the coil at position {0,h/2,0} and used that value in the flux calculation. I think that already gives a close estimate but it is an overestimation I believe. $\endgroup$ Commented Nov 18, 2021 at 16:57

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