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Formative assessment questions for teaching physics

A2L Item 231

  • Description: Identify the current configuration that gives maximum magnetic field at a specified point.
  • Goal: Reason regarding the fields due to current elements
  • Source: 283-650 Maximum B field
  • Keywords: Current, Electromagnetism, Magnetic Field

The question for students:

In all cases the wire shown carries a current I. For which situation is the magnitude of the magnetic field maximum at the point P?

Item231_fig1.gif

Commentary for teachers:

Answer

(4) #4 is larger than #3 because contributions from the two half circles reinforce in #4 rather than oppose each other as in #3. #4 is larger than #1 because #4 has a half loop at half the radius. Situation #2 having a finite length of wire is a distracter. The field in #2 is smaller than #3 and would be even if the wire in #2 were infinite. An interesting follow-up is to ask students to well order the cases according to the strength of the field.