How do i get this equation,(adsbygoogle = window.adsbygoogle || []).push({});

$$\frac{db}{dl}=(\hat{B}.∇)\hat{b}$$

This equation is a vector whose direction is towards the centre of the circle which most closely approximates the magnetic field-line at a given point, and whose magnitude is the inverse of the radius of this circle.

However I have no idea how to obtain the above and the source basically state it as by definition. Where is this equation obtained from?

Source: https://ocw.mit.edu/courses/nuclear...a-physics-i-fall-2003/lecture-notes/chap2.pdf page 8

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# I Calculating the curvature drift in a plasma

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