Diagnostic · Charge & Coulomb
Static Electricity Diagnostic
Maps sixteen bands across charge, force, field, polarisation and conductors, anchored on attraction against repulsion and field against force, and shows which to address first.
IB DP · AP · A-Level · GCSE / IGCSE
1
Forms
30
Questions
~30
Minutes
16 + 3 lenses
Bands
FundaFirst HS diagnostics have run in classrooms at 10 schools · 9 countries · 3 continents
The wrong ideas come from physics education research, which has recorded them since the 1970s: Viennot, Trowbridge & McDermott, Arons, Hestenes, Maloney. The physics comes from Arons, Knight, Moore, and Chabay & Sherwood.
What it maps
Sixteen misconception bands across five families, plus three cross-cutting lenses.
CHG-1
Charge Is a Signed Imbalance
A body is charged when its positive and negative counts are unequal. Charging moves existing carriers and creates nothing. Charge is not a fluid that can be made or run out of.
CHG-2
“Neutral” Is Not “No Charge”
A neutral object holds equal positive and negative charge in balance and still takes part electrically. It polarises and is attracted. Close to a core band.
CHG-3
Carriers Transfer, Protons Stay
Rubbing transfers charge and does not create it. Protons stay bound. The mobile carrier depends on the material, with electrons in a solid and ions in solution.
FOR-1
Attraction Is Ambiguous, Repulsion Proves Charge
The core band. Attraction occurs between unlike charges, or between a charge and a neutral object it has polarised. Only repulsion requires both objects to carry charge of the same sign.
FOR-2
Coulomb’s Law: Magnitude, Inverse-Square
For point or spherical charges the magnitude scales as 1/r². Halving the separation quadruples the force. Direction is settled separately from the signs.
FOR-3
Third Law for a Charged Pair
The forces on the two charges are equal and opposite however different the charges are. Their accelerations differ in size only when their masses differ, because a = F/m. A bridge to Newton’s laws.
FLD-1
A Field Exists With No Test Charge
The source sets up a field at every point in space. A test charge reveals it and does not create it. Asked once, so a lower-confidence band.
FLD-2
Field Is Not Force: E = F/q
The core band. The field is force per unit charge. The field at a point is not the force on a particular charge, and it belongs to the source, whatever the probe.
FLD-3
Field Direction Set by the Source
The field points away from positive source charge and toward negative source charge. A negative source’s field points inward. Asked once.
FLD-4
Fields Add as Vectors
Each source contributes its own field and the total is the vector sum. In a vacuum nothing shields or uses up a contribution. A nearer charge does not use up another’s field.
FLD-5
An Arrow Is a Vector, Not a Path
A field arrow shows direction and strength at a point. It does not show the path a charge will follow. Asked once, so a lower-confidence band.
DIP-1
A Net-Zero Object Can Make a Field
A dipole has zero net charge but a real field, and it feels a net force in a non-uniform field. Induced attraction needs no net charge.
DIP-2
Polarised Is Not Charged
Polarisation shifts bound charge a little. The net charge stays zero, so an attracted or polarised object has not gained charge. The closing band.
CON-1
Conductor or Insulator Is About Mobility
Carriers move freely in a conductor and stay bound in an insulator. The amount of charge present does not decide it. Asked once.
CON-2
Interior Field Zero Only at Equilibrium
At electrostatic equilibrium, excess charge sits on the surface and the field inside the conductor is zero. The conductor’s own mobile charges stay spread through it. Zero field means charges arranged to cancel, and not charges absent.
CON-3
Grounding Is a Path, Induction Is Ordered
Grounding is a conducting path to a large reservoir. Charging by induction is a definite ordered sequence, in which the ground is removed before the inducing charge, and needs no contact.
L1
A Name for the Operation
Naming the charge or the field is taken as explaining it. A cross-cutting lens, read from the pattern of options chosen and reported as a percentage of the class. It is never scored as a band.
L2
The Test Charge for the Source
The field read off the test charge instead of the source: its direction, value, or existence tied to the probe. The lens most often paired with a flagged FLD-2.
L3
A Rule Outside Its Conditions
A point-charge or equilibrium result applied where its conditions do not hold. A cross-cutting lens, read from the pattern of options chosen.
A question from it
Students can recite the rules of electrostatics and still hold these ideas.
The scrap jumped, so it was charged
A charged rod is brought near a small paper scrap and the scrap jumps to it. Asked whether the scrap is charged, many students say yes. It was attracted, so it must be. A neutral scrap is pulled in just the same. Attraction cannot settle the question. Only repulsion requires both objects to carry like charge. The rest of the topic rests on this one distinction.
Double the test charge, double the field
A small test charge feels a force. Asked what happens to the field if the test charge is doubled, many students double the field with it. To them the field is the force they measured. The field is force per unit charge. It belongs to the source and does not change when the probe changes. Reading E = F/q as bare algebra is the second core misconception.
A neutral object has no charge in it
Asked what is inside a neutral object, many students say nothing, because neutral means no charge. A neutral object is full of charge held in balance, equal positive and negative. That is why a neutral scrap still responds to a charged rod and why an insulator polarises. Reading neutral as empty shuts off induction, polarisation and induced attraction.
Equal and opposite cancels to nothing
Two equal and opposite charges sit a small distance apart. Asked what they do at a distance, many students cancel them to zero. A dipole has zero net charge, yet it sets up a real field and feels a net force in a non-uniform field. The same reasoning lets a neutral object be attracted at all. A system with zero net charge is not an inert one.
The field is whatever the test charge does
Across several questions the same habit shows: students read the field off the test charge instead of the source. Its direction is set by the sign of the probe, and its value changes when the probe changes. Some students need a charge present before the field is real. The diagnostic tracks this habit as a cross-cutting lens, flagged where it damages the field idea most.
How it runs
Students use one shared link and need no logins.
30 questions, about 30 minutes, no calculator required, single sitting.
The report arrives within 48 hours of your class finishing.
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Arons and Knight document the misconceptions. Moore and Chabay & Sherwood supply the physics treatment.