Field of a circular loop around its axis issue

[Amaelle]

Good , i want to derive the formula for field of a circular loop around its axis

but i was stucked with the following point:
they consider dl perpendicular to r ! why is that??
many thanks in advance!

 

[TSny]

It might help to construct a 3D model. Draw a circle on a piece of paper. Let the paper rest on a horizontal surface. Hold a pencil vertically with one end at the center of the circle. The pencil represents the axis of the circle. Choose an arbitrary point C on the circle and hold another pencil such that one end is at C and the other end touches the axis of the circle. This pencil represents r. Imagine a little element dl of the circle at point C. Can you see that r is perpendicular to dl?

 

[Chandra Prayaga]

Because dl is perpendicular to the page (either coming out of the page or going into it), and r is in the page.

 

[Amaelle]

thanks a lot both of you, now i can see :)