Magnetic field by a finite section of wire?

helpmeplz!

hey guys i have a question. i solved the problem but i dont understand how to do it.

Two perpendicular straight wires join in the ends of a semicircular loop of radius a = 11 cm, as shown in the figure above. If the current I =6 A, what is the resultant field at the center of the circular section?

http://gauss.vaniercollege.qc.ca/web...DpQdlatGJalrsW

i know how to get the magnetic field due to the semi circle, but for the two finite wires, i know from the integral we get uo I/ 4piR (sin phi1 + sinphi2). But for me when i look at each section of wire, I see the angles going from 0 to 90 degrees, so phi one is 0 degrees and phi 2 is 90 degree. Doing that gives 0. But that is obviously not right. Why do we use 45 degrees i just dont understand it? Please anyone!!

tiny-tim

hi helpmeplz!!

your picture is not visible (without logging in) …

can you please explain the shape (particularly the 45°)?