Let q be the angle between dl and the
displacement vector r According to Biot-Savart’s law, the
magnitude of the magnetic field dB is proportional to the current
I, the element length |dl|, and inversely proportional to the square
of the distance r Its direction* is perpendicular to the plane
containing dl and r Thus, in vector notation,
d
I d
r
B
r
∝
×
l
3
           =
×
µ0
3
4π
I d
lr
r
 [4