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Formative assessment questions for teaching physics

A2L Item 232

  • Description: Identify the correct assertion regarding the field due to a current segment.
  • Goal: Reason regarding the Biot-Savart law
  • Source: 283-650 B from an infinite wire.
  • Keywords: Electromagnetism, Magnetic Field

The question for students:

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.

Commentary for teachers:

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.