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Formative assessment questions for teaching physics

A2L Item 197

  • Description: Indicate the situation in which a moving charge in a magnetic field has the largest displacement in time T.
  • Goal: Reasoning with magnetic forces
  • Source: UMPERG-283-626
  • Keywords: Displacement, Electromagnetism, Magnetic Field, Right-Hand Rule

The question for students:

In the following situations a charge q moves in a uniform magnetic field. The strength of the magnetic field is indicated by the density of field lines. In all cases the speed of the charge is the same. For which situation(s) will the charge q have the largest displacement in a given time T.

Item197_fig1.gif

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  6. 1 & 3
  7. 2 & 4
  8. 1, 2, 3 & 4
  9. 1, 2, 3, 4 & 5
  10. Cannot be determined

Commentary for teachers:

Answer

(5) Since the speed cannot change, the greatest displacement will occur when the path is a straight line. Some students may answer #10 thinking that the time matters.