Diagnostic · Electricity
Current Electricity Diagnostic
Maps what your students think a current, a potential difference and a cell do in a circuit, and shows which wrong idea to address first.
IB DP · AP · A-Level · GCSE / IGCSE
1
Forms
24
Questions
~27
Minutes
16
Bands · 12 reported
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 reasoning difficulties are measured. Twelve are reported at class level.
CUR-1
Current is conserved
The same current leaves a component as enters it. What a bulb takes from the circuit is energy. This is the core idea in the topic, and most of the network reasoning rests on it. Three questions.
NET-1
Series and parallel come from connections
Two components are in parallel when they span the same two points, however the wires are bent to get there. Tested on an unfamiliar drawing, on a redrawn circuit, and at a node where the current divides. Three questions.
NET-5
A circuit is one system
A change anywhere is settled by solving the whole circuit again. Solving the circuit may show that some quantities stay the same. It is not a rule that effects travel only one way. Two questions.
NET-4
An open gap is not a short
In the circuit used for these two questions, removing the component opens the current path. No current flows, and the source’s terminal potential difference appears across the gap. The two questions share one circuit, shown together on one page.
POT-2
Potential difference drives current
The potential difference is the cause and the current is the result. This holds even when the drop is worked out by multiplying current and resistance. Two questions.
BAT-1
The source holds potential difference, not current
An ideal source keeps a fixed potential difference across its terminals, and a real one comes close at small currents. The current that flows depends on the circuit attached to the cell. Two questions.
POT-1
Potential difference is between two points
Voltage is a difference between two points. A single point does not have a voltage of its own. Only differences are physical, and the zero is a choice. Asked once.
CUR-4
The carriers are not always electrons
In a metal the carriers are electrons. In an electrolyte both positive and negative ions move. Asked once.
PWR-1
A rating is not what a bulb always dissipates
A rating states what a component does at its rated operating point. It does not state what the component dissipates wherever it is connected. Asked once.
RES-2
Not every load obeys Ohm’s law
Where a fixed power is delivered, the current follows from the power and the potential difference. A resistance held constant gives the wrong answer. Asked once.
NET-3
A parallel branch lowers resistance
Each parallel branch is another route for the current. The combination has less resistance than the smallest single resistor. Asked once.
NET-2
A wire is a circuit element
A plain wire across a component is a path of zero resistance. The current goes round the component, which then contributes nothing between those two points. Asked once.
The other four are scored but not reported, because the evidence behind them does not support a statement about a class. The count behind every reported figure is given in the cohort summary.
A question from it
A student can pass a circuits test and still hold each of these five wrong ideas.
The bulb uses the current up
Put an ammeter on each side of a bulb and ask what the two meters read. Many students say the second reads less, because the bulb has used some current. The current leaving a bulb equals the current entering it. What the bulb takes from the circuit is energy. Almost every later argument about a circuit rests on this.
Series and parallel are read off the drawing
Two components are in parallel when both ends join the same two points. How the wires are drawn does not matter. Many students decide from the shape of the picture instead. Components drawn end to end are called series. A circuit redrawn with the same connections is called a different circuit. A student can recite the definition and still get an unfamiliar drawing wrong.
The cell supplies a fixed current
A cell holds a potential difference across whatever is connected to it, and the circuit decides how much current flows. This holds for an ideal cell, and nearly so for a fresh cell driving a small current. Many students treat the cell as a supply of fixed current. They expect halving the resistance to change nothing, so their predictions about a changed load come out wrong.
An effect travels with the current
Add a resistor part way round a series circuit and ask what changes. Many students expect only the parts after the resistor to change, because they picture the current carrying the effect along. A circuit is one system. Change one part and the whole circuit settles again, including the parts the current reaches first.
The rating is what the bulb always does
A rating says what a bulb dissipates at its rated voltage. Put a 60 W bulb and a 100 W bulb, both rated for the mains, in series. The same current passes through both. The 60 W bulb has the higher resistance, so it is the brighter one. The comparison says which bulb is brighter. It does not say by how much. Most classes are surprised.
How it runs
Students use one shared link and need no logins.
24 questions, about 27 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.