Diagnostic · Potential & reference
Electric Potential Diagnostic
Maps what potential, potential difference and the reference point mean to your students: energy per charge, signs, path independence. It shows which band to address first.
IB DP · AP · A-Level · GCSE / IGCSE
1
Forms
22
Questions
~25
Minutes
12 + 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
Twelve misconception bands across four families, plus three cross-cutting lenses.
POT-1
Potential Is Energy per Charge
The potential belongs to the point, fixed by the source charges. The energy of a charge placed there is qV. Doubling the charge doubles the energy and leaves the potential unchanged.
POT-2
Potential Energy Belongs to a System, and Is Signed
Potential energy belongs to an arrangement of charges, and not to one charge alone. For two charges, with zero potential energy at infinite separation, attraction gives negative potential energy and repulsion gives positive potential energy.
POT-3
Potential Is a Scalar
Contributions add as ordinary signed numbers with no direction. The total does not depend on any test charge placed at the point. Asked once, so a lower-confidence band.
FLD-1
The Value of the Potential Does Not Fix the Field
The core band. The field comes from how fast the potential changes, and not from its value. Zero potential can sit in a strong field, a point of zero field need not be at zero potential, and a constant potential means zero field.
FLD-2
Field Strength from Equipotential Spacing
On an equipotential map the field is strong where the lines are close together and weak where they spread out. Strength comes from the spacing, and not from the labelled value.
FLD-3
Field Direction from the Potential
The field points down the potential, from high to low. It crosses the equipotentials at right angles.
FLD-4
The Signed Link Between Field and Potential
The field is minus the slope of the potential. The potential difference is minus the signed area under the field, taken in the direction of travel.
REF-1
Only Potential Differences Are Physical
A potential value means nothing until a zero reference is named. Moving the zero changes the number and not the physics. Potentials can be negative.
SGN-1
The Sign of the Charge Governs Energy and Motion
Positive and negative charges feel electric forces in opposite directions in the same field. The sign of qV depends on both the charge and the potential.
PTH-1
Potential Difference Is Path-Independent
In a conservative field the potential has one value at each point. The difference between two points depends only on the endpoints, and the change around any closed loop is zero.
CFG-2
A Dielectric with the Charge Fixed
With the charge held fixed, a dielectric weakens the field in the gap and the voltage falls. There is no conservation of potential. Asked once, so a lower-confidence band.
CON-1
Inside a Conductor at Equilibrium
At equilibrium the field inside is zero, so the potential is constant through the interior. Constant, and not necessarily zero. Asked once, so a lower-confidence band.
L1
Potential Read as Energy, or Given a Direction
Potential is treated as energy, or given a direction. 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
A Result Used Without Its Condition
A constant-potential, reference, equilibrium or fixed-charge result used with its condition dropped. Read from the pattern of options chosen.
L3
A Sign Lost or Inverted
A sign dropped or flipped in the link between field and potential, in the energy, or in the motion. The most populated lens by design, most often paired with a flagged field band.
A question from it
Students can recite the definitions of potential and still hold these ideas.
Zero volts means zero field
At the midpoint between a positive charge and an equal negative charge the potential is zero, yet the field there is strong. The field comes from how fast the potential changes, and not from its value. The opposite mistake happens too. Inside a conductor at equilibrium, the potential is constant and the field is zero. That constant need not be zero. The rest of the topic rests on this distinction.
Potential is the energy
A charge sits where the potential is V. Asked for its potential energy, many students answer V. Asked what the potential becomes if the charge is doubled, many double it too. Potential is energy per unit charge, fixed by the source charges. The energy of a charge placed there is qV. Doubling the charge doubles the energy it carries and leaves the potential, and the field, unchanged.
Drop the minus sign
The field is the slope of the potential, with a minus sign. It points from high potential to low. Asked to read the field off a graph of potential against position, many students keep the sign of the slope. Their field then points uphill, toward higher potential. The minus sign matters. Dropping it inverts the field direction and the energy argument together.
Fifty volts is fifty volts
A point is at a potential of fifty volts. Asked what that means physically, many students treat it as an absolute fact about the point. Only potential differences are physical. The fifty volts is measured from a chosen zero. Moving the zero to the other plate, to earth, or to infinity changes the number without changing any field, force or motion. A potential can be negative.
The sign looks after itself
Across several questions the same habit shows: a sign dropped or flipped. The field sent uphill, a pair’s negative potential energy made positive, or a charge sent the way the opposite sign would go. The diagnostic tracks a sign lost or inverted as a cross-cutting lens. It is the most populated lens by design, and it is flagged where it does the most harm.
How it runs
Students use one shared link and need no logins.
22 questions, about 25 minutes, no calculator required, single sitting.
The report arrives within 48 hours of your class finishing.
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Knight documents the misconceptions. Moore and Chabay & Sherwood supply the physics treatment.