ConceptElectromagnetism
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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)
14 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- 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.→
