when using Biot-savorts law, when do I use uI/2pir and uI/4pir and what is the difference between them

Question

 I have been having trouble distinguishing when to use either or. Does it matter about the shape? is it dependent on the length of the wire?

Answer 1

Look up the formula for the Biot-Savart law and see that it has a factor of μ₀/(4π).