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Concept · Electromagnetism
Electrostatics
charge (two types, conservation, quantization e), Coulomb's law F=k q1 q2/r2 (use |q|, direction from polarity), electric field E=F/q and E=kQ/r2, field lines (do not intersect, tangent = direction, density = magnitude; E source-determined, independent of test charge)
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Named traps
14 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.
- Charge not quantized/conservedAllows a fractional-electron charge or a change in total charge during a transfer/contact process, ignoring quantization and conservation.→
- Charge sharing on contact wrongMis-distributes charge between touching conductors, e.g. fails to split it equally between identical spheres after contact.→
- Charge signs forced into the magnitudePlugs the signed charge values into F=kq1q2/r2 and reports a 'negative force' instead of using |q| for magnitude and stating direction separately from polarity.→
- Coulomb as 1/r not 1/r2Uses an inverse-distance (1/r) dependence for the electrostatic force instead of inverse-square, so halving r is taken to double the force rather than quadruple it.→
- Coulomb forces added as scalarsSums the forces from several charges by adding magnitudes directly instead of adding them as vectors with direction.→
- Field depends on the test chargeReads E=F/q as a proportionality, concluding the field at a point gets stronger or reverses when a larger or opposite test charge is placed there.→
- Field direction at a negative source wrongDraws field lines pointing away from a negative source charge or toward a positive one, reversing the convention.→
- Field lines can crossDraws intersecting field lines, implying two different field directions at a single point.→
- Field strength vs line density reversedReads sparsely spaced field lines as a strong field, reversing the line-density-to-magnitude relationship.→
- Larger charge exerts larger forceClaims the bigger charge pushes harder on the smaller one, violating the equal-and-opposite force pair in Coulomb's law.→
- Like charges attractReverses the interaction rule, expecting like charges to attract and unlike charges to repel.→
- Point-charge field as 1/r not 1/r2Uses E=kQ/r for a point charge's field instead of E=kQ/r2, mismatching how field falls off with distance.→
- Symmetry cancellation missedReports a nonzero net force at a point where symmetric charge components should cancel to zero.→
- Wrong distance in Coulomb's lawUses a radius or diameter instead of the center-to-center separation for the r term in Coulomb's law.→
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 breadth