A2L Item 232

Goal: Reason regarding the Biot-Savart law

Source: 283-650 B from an infinite wire.

If B = μ0I/4πD at point P which is a distance D from
the end of a “half-infinite” wire, then what would be the magnetic field
at P if the wire went to infinity in both directions?

  1. It would be 0 because the left-portion of the wire would cancel the
    magnetic field from the right-portion of the wire.
  2. It would be twice the answer above.

  3. It is not 0, but it is also not twice the answer above – you have some
    cancellation and you need to figure it out putting in the appropriate
    sines and cosines.



(2) This question is only of value if one has obtained the field at the
end of a semi-infinite wire by direct integration. It is important for
students to recognize situations where the Biot-Savart Law must be used
versus cases where Ampere’s Law can be used.