Question

Which of these are not equipotential surfaces? (Dotted lines represent the surfaces in question)

• A. 2 infinitely large parallel charged plates as shown
  
• B. Point charge
  
• C. Infinitely long charged rod
  
• D. None of these

Answer 1

The correct option is C Infinitely long charged rod


The equipotential surfaces should always be perpendicular to the field lines coming out of the charges.
For option (a) the field lines will be parallel to each other and perpendicular to the plates. So the surfaces which are perpendicular to those lines are equipotential. This is not our option.
For option (b) the field lines would be radially outward from the charge. So any spherical surface with the charge at the centre is equipotential. This is not our option.
For option (c) the field lines would be radially outward from the line and hence equipotential surfaces are cylinders with the rod at the central axis. The surfaces drawn are not equipotential. Option (c) is the answer.