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How much work is necessary to keep a positive point charge in orbit around a negative point charge?
  1. A lot; this system is unstable.
  2. Just a little; the isolines are far enough apart that crossing them doesn’t take much work.
  3. None; we’re traveling along an isoline, which requires no work.
  4. There’s not enough information to tell.
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OpenStax College Physics for AP® Courses Solution, Chapter 19, Problem 29 (Test Prep for AP® Courses) (0:44)

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This is College Physics Answers with Shaun Dychko. It takes no work to make a positive charge orbit in a circle around a negative charge, because this positive charge is traveling along an isoline of electric potential, because the electric potential from this negative charge is <i>K Q</i> over <i>R</i>. And, this means in any direction, the potential will be according to that. And so, I'd have distance <i>R</i>, which all of these points along this blue line <i>R</i>, will have the same potential. And so, no work is done when traveling along an isoline of potential, so the answer is C.