The potential difference between two points can be calculated along any path between them. I take a parallel plate capacitor and consider a small positive charge on the surface of the negatively charged sheet. I move it outside the sheet without doing any work as the net field inside a conductor is zero. Now, behind the sheet, the fields due to the two plates cancel each other perfectly. I move the charge away parallel to the plate, infinitely far away, when any fringes would've decayed away to nothing and move it to the other side. I repeat, bringing back the charge to the other plate. The two plates are seemingly at the same potential when they clearly are not. At what point of my process did I do work? What exactly is the potential at all points in space due to this system?

  • $\begingroup$ There are field lines "behind" the plates of a real capacitor too, they don't cancel out due to the other plate. $\endgroup$
    – Triatticus
    Commented Jun 26, 2020 at 2:55

1 Answer 1


Two infinite plates have perfectly parallel uniform field lines. For them, the field exactly cancels outside. But you can't go around infinite plates.

Finite plates at a large distance are like a dipole.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.