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?
- It would be 0 because the left-portion of the wire would cancel the
magnetic field from the right-portion of the wire. -
It would be twice the answer above.
-
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.
Commentary:
Answer
(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.