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Formative assessment questions for teaching physics

A2L Item 212

  • Description: Identify the distributed charge configuration having the larges electric field at the origin.
  • Goal: Reason with electric fields
  • Source: 283-370 E due to circular rods
  • Keywords: Distributed Charges, Electric Field, Electromagnetism

The question for students:

All of the curved charged rods shown in the image below have the same radius and linear charge density (though some are positively charged and others are negatively charged). For which configuration would the magnitude of E at the origin be greatest?

Item212_fig1.gif

Commentary for teachers:

Answer

(6) This problem constitutes a good exercise for students learning the vector nature of the electric field. There are many good followup questions, such as; Which configurations have zero field at the origin?, Order the configurations by increasing magnitude of electric field at the origin. Stress the value of symmetry for reasoning to the answer. A negative distribution in a quadrant is equivalent to a positive distribution in the opposite quadrant, which means that distributions #5 and #7 are equivalent (for purposes of finding the E field at the origin).