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Question
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.
Question by OpenStax is licensed under CC BY 4.0.
Final Answer

(c)

Solution Video

OpenStax College Physics Solution, Chapter 19, Problem 29 (Test Prep for AP® Courses) (0:44)

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Video Transcript
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 K Q over R. And, this means in any direction, the potential will be according to that. And so, I'd have distance R, which all of these points along this blue line R, will have the same potential. And so, no work is done when traveling along an isoline of potential, so the answer is C.