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A2L Item 140

Meta-Information
Description: 
Indicate which charge configuration contains least potential energy.
Goal: 
Link potential energy with work needed to assemble a charge configuration.
Level: 
Undergraduate
Keywords: 
Electromagnetism
Keywords: 
Energy
Keywords: 
Potential Energy
Source: 
283-460 Lowest potential energy

Which of the following charge distributions has the lowest potential energy?

Commentary

Answer

(2) Encourage students to reason to the answer rather than write formal expressions for each case. They should be able to perceive that cases #1, #3 and #5 all have positive PE. Situation #4 has zero energy as can be seen by assembling subunits, then moving the two positive charges along the zero equipotential of the charges on the y-axis. Finally, situation #2 is clearly negative.

A good follow-up question is to ask students to order the cases according to increasing potential energy.