ConceptElectromagnetism
← All conceptsElectric potential & work
potential φ=Ep/q (relative), potential difference UAB=φA-φB (absolute), work by electric force W=qU=qEd (path-independent in uniform field), equipotential surfaces, capacitor C=Q/U (foundational)
12 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- C=Q/U rearranged wrongSolves C=Q/U incorrectly, e.g. writes Q=U/C or C=U/Q.→
- Capacitance depends on Q or UReads C=Q/U as a proportionality, concluding that charging the capacitor changes its capacitance rather than C being fixed by geometry.→
- E=U/d invertedUses E=Ud or U/E for the field between parallel plates instead of E=U/d.→
- Field vs potential confusedTreats electric field E and potential φ as the same quantity, e.g. uses E=kQ/r2 to compute a potential.→
- PE/work sign ignores charge signIgnores the sign of the charge in W=qU or Ep=qphi, getting the wrong sign for the work or potential energy of a negative charge.→
- Potential (relative) vs potential difference confusedTreats a single point's potential as an absolute fixed value rather than reference-dependent, or conflates φ with the difference UAB between two points.→
- Potential sign near a source wrongThinks potential is highest near a negative charge (most negative) is treated as largest, mis-ordering potentials around source charges.→
- Sign of work vs direction of potential changeAssumes a positive charge moving to higher potential gains kinetic energy (or that the field always does positive work along its own direction), mismatching W=qU with the sign of U.→
- W=qEd uses wrong displacementUses the total path length, or a distance not along the field, for d in W=qEd in a uniform field.→
- Work along equipotential nonzeroAssigns nonzero work to moving a charge along an equipotential surface, where the work is zero.→
- Work depends on path in E fieldBelieves the work done moving a charge between two points depends on the path taken, even in a uniform field where it does not.→
- Zero field means zero potentialConcludes potential is zero wherever the field is zero (or vice versa), ignoring that they are defined independently.→
