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I'm not entirely sure how to convert from spherical coordinates to Cartesian coordinates, I believe the way to do it is by using the following code:

TransformedField["Spherical" -> "Cartesian", B, {r, θ, ϕ} -> {x, y, z}]

Where B is

    -((0.0003*Cos[θ][Sin[θ]]^2)/r^2)

Then I want to take the -Grad of this equation (after it's converted into cartesian)

MP0 = -Grad[-((0.0002906250000000001*(z/Sqrt[x^2 + y^2 + z^2])[Sqrt[x^2 + y^2]/
      Sqrt[x^2 + y^2 + z^2]]^2)/(x^2 + y^2 + z^2)), {x, y, z}]

Then the result is a vector which I want to plot using VectorPlot3D, however it won't plot, it just gives me a blank output

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  • $\begingroup$ Please format the code and define B so we can run your code and see if "something is wrong". If you include what you expect and what you actually get maybe we can help you. $\endgroup$
    – tsuresuregusa
    Commented Apr 19, 2016 at 23:47

1 Answer 1

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How about this:

B = -((0.0003*Cos[θ] (Sin[θ])^2)/r^2);

f[x_, y_, z_] = Grad[Simplify[TransformedField["Spherical" -> "Cartesian", B, {r, θ, ϕ} -> {x, y, z}]], {x, y, z}]

VectorPlot3D[f[x, y, z], {x, 0, 1}, {y, 0, 1}, {z, 0, 1}]

enter image description here

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