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## Homework Statement

I'm working through a proof that says the magnetic field of a toroid is circumferential at all points inside and outside the toroid. I can follow most of the proof, but am a bit confused where the first equation comes from.

Here is the figure from the textbook (Griffith's 4th Ed).

http://media.newschoolers.com/uploads/images/17/00/67/76/17/677617.jpeg [Broken]

To begin the proof, Griffiths starts with the field at [itex]\mathbf{r}[/itex] due to the current element at [itex]\mathbf{r}'[/itex].[tex]d\mathbf{B}=\frac{\mu_0}{4\pi}\frac{\mathbf{I}\times\mathbf{\hat{{u}}}}{u^3}dl'[/tex]

## Homework Equations

[tex]\mathbf{B}=\frac{\mu_0}{4 \pi}\int\frac{\mathbf{I}\times\mathbf{\hat{u}}}{u^2}dl'[/tex]

## The Attempt at a Solution

I'm just confused at how Griffiths got from the Biot-Savart law above, into the equation he posted in the question. (I replaced the script r with a u). Where does the cubed term come from?

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