←All concepts
Concept · Electromagnetism
Electric 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)
Switches you to Physics
Named traps
12 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- 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.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
FoundationsPhysics 2 breadthMechanics foundationsAdvanced mechanics support